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thesis entitled

ANALYSIS FOR EFFECTS OF SOME DIFFERENT
OPERATIONAL CONDITIONS ON NEUTRAL-TO-EARTH
VOLTAGE IN A SINGLE-PHASE ELECTRIC POWER
DISTRIBUTION SYSTEM

presented by

Changming LI

has been accepted towards fulfillment
of the requirements for

M.S. Agricultural

de ee in
gr Engineering

 

 

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Major professor

Date Mg /79/

 

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cmmu1

ANALYSIS FOR EFFECTS OF
SOME DIFFERENT OPERATIONAL CONDITIONS
ON NEUTRAL-TO-EARTH VOLTAGE
IN A SINGLE-PHASE ELECTRIC POWER
DISTRIBUTION SYSTEM

By

Changming Li
A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Agricultural Engineering

1991

ABSTRACT

ANALYSIS FOR EFFECT S OF
SOME DIFFERENT OPERATIONAL CONDITIONS
ON NEUTRAL-TO-EARTH VOLTAGE
IN A SINGLE-PHASE ELECTRIC POWER
DISTRIBUTION SYSTEM

By

Changming Li

Neutral-to-earth voltage (NEV) on a dairy farm may affect milk
production of a dairy cow. Computer models were developed to
simulate NEV along a 7.2 kV multi-grounded single-phase electric
power distribution line.

The computer models analyses estimated the NEV changes
arising from high resistance segments with various load in the neutral of
the primary line, phase-to-neutral faults, phase-to-earth faults,
substation ground resistance change, different levels of the neutral
ground connection, primary operating voltage change and secondary
ground faults. Particular attention was paid to ground fault conditions
and the relationship of level of load and abnormal resistance in the

neutral conductor.

The distribution line was simulated as an AC line and compared
to the DC model to determine if the DC model was a valid predictor of
NEV along the distribution line.

Approved:

WCW

Major Professor

..., -

Department Chairperson

ACKNOWLEDGMENTS

I would like to express my deepest gratitude to Dr. Truman C.
Surbrook, advisor and friend, for his academic guidance, moral
encouragement and technical support since 1988. His fruitful
cooperation with Wisconsin Electric Utilities Research Foundation has
made this study possible financially.

I would like to extend my cordial thanks to Dr. Gerald L. Park
and Dr. John B. Gerrish for their helpful academic guidances and
serving on my guidance committee. Special thanks is extended to Dr.
Lawrence J. Giacoletto and Mr. Norman D. Reese for their technical
helps. Special thanks also goes to the faculty, stuff and other graduate
students of the Department of Agricultural Engineering for their
assistance and cooperation in the recent years. .

A very special thanks goes to my parents and all of my family
members for their expectation, encouragement, understanding and
moral support from the remote China.

Special thanks goes to the Ningxia T.V. University, China for
letting me attend the graduate school in the United States, and for its

subsequent moral encouragement of me.

iv

TABLE OF CONTENTS

LIST OF FIGURES ......................................... vii
LIST OF TABLES ........................................... xv
LIST OF SYMBOLS ........................................ xvi
I. INTRODUCTION ...................................... 1
II. LITERATURE REVIEW ....................................... 4
2.1 Stray Voltage and Electric Current Effect on Dairy Cows .......... 5

2.2 Possible Sources of the Farm Neutral-to-Earth Voltage ............ 9

2.3 Field Test and Diagnosis of Farm N eutral-to-Earth Voltage ....... 16
2.4 Mitigation of Farm Neutral-to-Earth Voltage Problem ........... 19

III. OBJECTIVE .......................................... 22
IV. METHODOLOGY ............................................. 24
4.1 The DC Single-Phase Distribution Simulation Model ............. 24

4.2 Simulation of Abnormal Conditions along the Distribution line . . . . 30
4.2.1 Neutral Conductor Resistance Change ................... 30

4.2.2 High Resistance Connection of

Neutral to Transformer Primary ........................ 32
4.2.3 Primary Phase-to-Neutral Fault (or Heavy Load) .......... 32
4.2.4 Primary Phase-to-Earth Fault ........................... 35

4.2.5 Substation Grounding Resistance Change ................ 37

4.2.6 N eutral-to-Earth Resistance Reduction .................. 37

4.2.7 Primary Operating Voltage Level Change ................ 38
4.2.8 Secondary Earth Fault ................................. 38

4.3 Operation of the AC Model .................................. 39

V. RESULT and DISCUSSION ................................... 44
5.1 The DC Model Analysis ...................................... 44
5.1.1 Normal Operational Condition . . . .' ..................... 46
5.1.2 Neutral Conductor Resistance Change ................... 47
5.1.3 High Resistance Connection of
Neutral to Transformer Primary ........................ 60
5.1.4 Primary Phase-to-Neutral Fault (or Heavy Load) .......... 62
5.1.5 Primary Phase-to-Earth Fault ........................... 68
5.1.6 Substation Grounding Resistance Change ................ 82
5.1.7 Neutral-to-Earth Resistance Reduction .................. 87
5.1.8 Primary Operating Voltage Level Change ................ 92
5.1.9 Secondary Earth Fault ................................. 95
5.2 Operation of the AC Model .................................. 114
VI. CONCLUSION ....................................... 132
REFERENCES ............................................ 135

LIST OF FIGURES

Page

Figure 1. DC circuit model for single-phase electrical distribution system

(input voltage 7,200 V) ....................................... 25
Figure 2. DC circuit model for single-phase electrical distribution system

with abnormal neutral conductor resistance simulated ............ 31
Figure 3. DC circuit model for single-phase electrical distribution system

in simulation of bad neutral connection with transformer and

ground rod ................................................. 33
Figure 4. DC circuit model for single-phase electrical distribution system

with primary phase-to-neutral fault simulated .................. 34
Figure 5. DC circuit model for single-phase electrical distributiOn system

with primary phase-to-earth fault simulated ..................... 36
Figure 6. DC circuit model for single-phase electrical distribution system

with secondary earth fault simulated ............................ 42
Figure 7. AC circuit model for single-phase electrical distribution system

(input voltage 7,200 V) ....................................... 43
Figure 8. Profile of the neutral-to-earth voltage along the primary

distribution line from substation (node 1) to 57th node

for the base model ........................................... 45

vii

Figure 9.

Figure 10.

Figure 11.

Figure 12.

Figure 13.

Figure 14.

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with variable
neutral conductor resistance RD33 between node 33

and node 34 and a customer load of 2A at node 33 ..........

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with variable
neutral conductor resistance RD33 between node 33

and node 34 and a customer load of 3A at node 33 ...........

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with variable
neutral conductor resistance RD33 between node 33

and node 34 and a customer load of 5A at node 33 ..........

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with variable
neutral conductor resistance RD33 between node 33

and34(10ALoad).............................. ........

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with load
condition changed in the open circuit (between node 33

and 34) situation ........................................

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with variable
neutral conductor resistance RD32 between node 32

and 33 (2A Load) .......................................

viii

Page

..... 48

51

..... 52

53

..... 56

..... 57

Figure 15.

Figure 16.

Figure 17.

Figure 18.

Figure 19.

Figure 20.

Figure 21.

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 5 7th node with variable
neutral conductor resistance RD32 between node 32

and 33 (5A Load) ...................... ' ...................

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with load
condition changed in the open circuit (between

node 32 and 33) situation ..................................

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with variable bad
neutral connection resistance RC with transformer and

ground rod at node 33 .....................................

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with variable

unground phase conductor to neutral fault at node 416-33 .....

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with variable

unground phase conductor to neutral fault at node 426-53 .....

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 5 7th node with an
ungrounded phase conductor to earth fault at node 416

compared with the normal line ..............................

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with 20 A
phase conductor to earth fault at node 404, 416, or 428

compared with normal line .................................

Page

...59

61

...64

...66

69

..71

...73

Figure 22.

Figure 23.

Figure 24.

Figure 25.

Figure 26.

Figure 27.

Figure 28.

Figure 29.

The primary neutral conductor current for each line segment
from the substation to the end of the line for normal load

compared with normal load plus a 20 A fault at node 416 .......

Phase relationship of normal neutral load current and primary

to earth fault current at the end of the line ...................

The normal operating distribution line grounding electrode

currents from the substation to the end of the line .............

Grounding electrode currents from the substation to
the end of the line with a 20 A phase to earth fault

at node 416 plus normal line loading .........................

Primary ungrounded phase conductor current comparison
from the substation to the end of the line for normal
line load vs. normal line load plus a 20 A phase to earth

fault at node 416 ..........................................

The primary neutral conductor current for each line segment
from the substation to the end of the line for normal load
compared with normal load plus a 20 A fault

at node 404, 416, or 428 ....................................

Substation and transformer current comparison for the normal
load case and for phase to earth fault at node 404, then

at node 416, and finally at node 428 .........................

Primary ungrounded phase conductor current comparison
from the substation to the end of the line for normal
line load vs. normal line load plus a 20 A phase to earth

fault at node 404, 416, and 428 ..............................

Page

..75

76

...77

..79

...80

...81

83

...84

Figure 30.

Figure 31.

Figure 32.

Figure 33.

Figure 34.

Figure 35.

Figure 36.

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with base

case vs. variable substation resistance RS .................

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with base

case vs. ground resistance RFG33 change at node 33 .......

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with base
case vs. ground rod number increasing along the entire

system ................................................

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with
base case 7.2 kV operating voltage vs. 2.4, 4.8 and

26.4 kV operating voltages ..............................

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node
with base case vs. variable secondary ground fault

attached to node 33 (out-of-phase) .......................

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with
base case vs. variable secondary ground fault

attached to node 33 (in-phase) ...........................

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with
base case vs. variable secondary ground fault

attached to node 9 (out-of-phase) ........................

Page

...... 88

...... 91

...... 93

...... 96

...... 99

100

..... 101

Figure 37.

Figure 38.

Figure 39.

Figure 40.

Figure 41.

Figure 42.

Figure 43.

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with base
case vs. variable secondary ground fault

attached to node 9 (in-phase) . . . . . . . . . . . . ................

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with base
case vs. variable secondary ground fault

attached to node 57 (out-of-phase) .......................

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with base
case vs. variable secondary ground fault

attached to node 57 (in-phase) ...........................

Neutral conductor and grounding electrode current with
secondary out-of-phase ground fault attached at node 57

at the end of the distribution line .........................

Neutral conductor and grounding electrode current with
secondary in-phase ground fault attached at node 57

at the end of the distribution line .........................

The ground rod current situation comparison in the distribution
system from substation to end with base case vs. 2.5, 5 and

Page

102

..... 104

107

..... 110

..... 111

10 A secondary in-phase ground fault attached to node 57 ........ 112

The neutral conductor current situation comparison in

the distribution system from substation to end with base case

vs. 2.5, 5 and 10 A secondary in-phase ground fault

attached to node 57 ....................................

xii

..... 113

Figure 44.

Figure 45.

Figure 46.

Figure 47.

Figure 48.

Figure 49.

Figure 50.

Figure 51.

Neutral-to-earth voltage profile along a simulated distribution
line with the DC model on the bottom, AC model with pure
resistance line in the middle and AC model with

resistance and induction line elements on tOp ................

The AC model phase angle profile of the neutral-to-earth
voltage along the primary distribution line from substation

to 57th node (without line inductance effect) .................

The AC model phase angle profile of the neutral-to-earth
voltage along the primary distribution line from substation

to 57th node (with line inductance effect ) ...................

Voltage regulation along the distribution system for the AC

model with the line inductance effect ........................

Voltage regulation along the distribution system for

the DC base model .......................................

The transformer primary side current situation along
the distribution system from substation to the end

of the line in AC model with line inductance effect ............

The transformer secondary side current situation along
the distribution system from substation to the end of

the line in AC model with line inductance effect ..............

The transformer primary side voltage phase angles

along the distribution system from substation to the end of
the line in AC model with line inductance effect

(take voltage phase angle at substation

as zero reference) ........................................

xiii

Page

...116

.. 117

...118

..119

.. 120

.. 122

.. 123

.. 124

Figure 52.

Figure 53.

Figure 54.

Figure 55.

The transformer secondary side voltage phase angles
along the distribution system from substation to the end of
the line in AC model with line inductance effect

(take voltage phase angle at substation ’

as zero reference) .......................................

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with
primary and secondary neutrals bonded and separated
in no fault condition (AC model with

line inductance effect) ...................................

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with
primary and secondary neutrals bonded and separated
in 10 A secondary in-phase fault attached to node 33

(AC model with line inductance effect) ....................

Profile of the neutral-to-earth voltage along the primary
distribution line from substation to 57th node with
primary and secondary neutrals bonded and separated

in 10 A secondary out-of-phase fault attached to node 33

(AC model with line inductance effect) ....................

xiv

Page

125

....127

128

129

LIST OF TABLES

Page
Table 1. Parameters for Base DC Circuit Model in Figure 1 ............... 26
Table 2. Primary Current and Load Resistance, RRL and RFL, for
Different Primary Voltage Simulation .......................... 41

RS
RU
RD

RRL

RSG

RG

REG

RSL

RSU

RSN

RF

RC

TFL

LIST OF SYMBOLS

Substation voltage, volt

Substation grounding mat resistance, ohm

Primary ungrounded phase conductor resistance, ohm
Primary neutral conductor resistance, ohm

DC model residential transformer simulation resistance, ohm
DC model farm transformer simulation resistance, ohm
Residential transformer ground resistance, ohm

Farm transformer ground resistance, ohm

Secondary ground electrode resistance, ohm

Ground electrode resistance between transformers, Ohm
Extra ground electrode resistance between transformers, ohm
Secondary load resistance, ohm

Secondary ungrounded conductor resistance, ohm

Secondary neutral conductor resistance, ohm

Fault simulation resistance, ohm

Neutral contact resistance with transformer and grounding electrode
AC model residential transformer simulation

AC model farm transformer simulation

I. INTRODUCTION

The most prevalent rural electric power distribution systems in the
United States are multi-grounded neutral systems. These are generally
single-phase two-wire or three-phase three-wire or three phase
four-wire systems which are effectively grounded.

The multi-grounded primary distribution system under normal
and abnormal operating conditions will produce neutral-to-earth
voltage all along the distribution line. Some dairy farmers complain
that milk production has been adversely affected by the neutral-to-earth
voltage. In the past decade, research has elucidated neutral-to-earth
voltage behavior under various operational conditions of the
distribution network. Also, the solving of neutral-to-earth voltage
problems has made significant progress. Neutral-to-earth voltage levels
along a distribution line, however, are not clearly understood in relation
to grounding, neutral resistance, primary/secondary neutral bond effect
and ground fault. Guidelines have not been available for diagnosing
distribution line abnormalities which cause elevated levels of
neutral-to-earth voltage. It is important to understand the effect that
various conditions have upon the neutral-to-earth voltage profile along
the line in order to develop effective guidelines for remedying problems.

The scope of this thesis is to: (1) study the neutral-to-earth voltage

profile along a single-phase multi-grounded distribution system as

2

affected by grounding, faults, and abnormal line resistance; and (2)
compare an AC model with a DC model to determine if the DC models
can be a useful tool to study the factors that cause neutral-to-earth
voltage along a single phase primary distribution line.

Computer models were developed to study the neutral-to-earth
voltage profiles produced by different grounding condition, types of
ground faults and neutral resistance under specific operational or
loading conditions. Actual data from an operating distribution line is
important to be considered, but controlled experiments are difficult to
conduct and in some cases may create dangerous conditions. Computer
simulation of a primary distribution system provides the opportunity to
study conditions whose field data are not available or not practical to
obtain.

The following operational conditions were studied to determine
their effect on the neutral-to-earth voltage along a distribution line: (1)
different levels of abnormal resistance in the neutral conductor at a
particular point along the line, and different levels of line loading near
the point of abnormal neutral resistance; (2) a high resistance
connection between the neutral and the primary winding of the
transformer; (3) a 7.2 kV phase-to-neutral fault at different points along
the line; (4) a 7.2 kV phase-to-earth fault at different points along the
line; (5) different levels of resistance-to-earth of the substation; (6)
different levels of resistance-to-earth of the distribution neutral; (7)

different levels of phase-to-neutral voltage; and (8) a secondary

3

ungrounded conductor to earth fault at one point along the distribution

line.

II. LITERATURE REVIEW

Since early this century, extensive research has been conducted
concerning the effect of electrical voltage and current on humans. The
lethal threshold value of continuous alternating current through the
human body was determined by several researchers. Based on the
results of Dalziel’s studies[12], 99.5% of all persons can safely withstand,
without ventricular fibrillation, the passage of a current not exceeding a
magnitude 1B (in amperes) for duration tS (in seconds) determined by
the formula 1B = k tS'O'S. For 50 kg body weight, k=0.116.

Since the 1960’s, extensive research has also found the effect of
the electric voltage and current on animals as well as general indications
of levels of voltage that bothered cows. Phillips (1963, 1969) and
Woolford (1971, 1972) studied the effects of voltage and'current on milk
production (New Zealand). In the past decade, significant research
concerning the farm stray voltage problem has also been conducted in
the United States.

Since the late 1970’s, the major concerns of stray voltage
researchers in the U. S. and other countries were focused on: (1) How,
and to what extent, does the neutral-to-earth voltage affect cows’ milk
production? (2) What is the source of the stray voltage? How is the
neutral-to-earth voltage produced? What is its changing trend under the

various operational conditions of the distribution network system? (3) Is

5

it possible to mitigate or avoid neutral-to-earth voltage problems on
farm? If yes, how? The literature will be reviewed from the three aspects

just mentioned above.
2.1 Stray Voltage and Electric Current Effect on Dairy Cows

Borcherding (1979) reported work by a veterinarian that linked
mastitis in cows to voltage irritation. Britten (1980) reported that a
herd of cows milked faster, with higher production and lower leukocyte
counts after a voltage problem was eliminated.

Appleman and Gustafson (1985) classified the observed effects of
stray voltage on dairy cows into four general areas: effect on milking
performance and behavior; effect on herd health; effect on nutritional
intake; and, effect on production. Nevertheless, it was reported that any
negative effects of electrical shock on milk production or mammary
gland health most likely were not directly related to shock, i.e.,
physiological responses to shock were minimal and milk yield was
generally maintained at normal levels during the shock period.
However, the severe behavioral responses to shock usually caused
management problems. In addition, the degree to which milk
production would be affected depended on how dairy producers would
deal with the abnormal behavior.

Gustafson et al. (1985) indicated that the effect of a stray voltage
on a dairy cow was influenced by many factors: (a) voltage magnitude

and waveform; (b) the resistance of a cow’s body pathway; (c) condition

6

of concrete, soil and metallic conductors affecting resistance to ”true
earth"; (d) resistance of the cow’s contact points; (e) resistance of the
electrical pathway to the cow’s contact points; and (f) impedance of the
source. When combined, these factors determine the current flow
through the cow’s body.

It is known that a current above some level flowing through the
cow’s body directly affects the cow’s behavior or health. A voltage
drives the current in the circuit with the cow body pathways as
resistances.

Norell et al. (1982) reported the electrical resistance data for
eight pathways through a dairy cow. The mean resistances of a cow
ranged from 359 to 877 ohms, with the lowest for a mouth-all hooves
path. Some pathways were selected in order to model the common cow
stray voltage problem on farm. For example: the pathway of
front-to-rear hooves was to model the cows that stood or walked across
an area of the barn or parlor where a floor voltage gradient existed; the
pathway of mouth-to—all hooves was to model the cows that bridged the
gaps between metallic feeders or water bowls and ground; and the
pathway of body-to-all hooves was to model the cows that bridged the
gaps between metal pipeworks connected to the grounded neutral
systems and concrete floor or earth. They found that the hoof-to-earth
contact resistance was a large component of the pathway resistance but
they did not evaluate the hoof-to-earth contact resistance quantitatively.
So far, there has been little research reported on the quantitative

analysis of the resistance.

7

Gustafson et al. (1985) studied 3 dairy cow-body pathways’
behavioral sensitivity to DC and 30 seconds on-off AC current. They
reported that the pathway front-to-rear hooves response rate ( ratio of
responding cows to total test cows) became statistically significant above
2.0 mA AC and 1.0 mA DC. Mouth-to-all hooves response rate became
significant above 2 mA AC and 4 mA DC. Response rate for a
body-to-all hooves pathway with currents from 0-7.5 mA AC and 0-9
mA DC were inconclusive. Appleman and Gustafson (1985) reported
for AC 60 Hz source, the cow’s behavioral sensitivity threshold currents
are different with the different body pathways, ranging from 0.75 to 7.1
mA. Gustafson et al. (1985) reported that the cow’s behavioral
sensitivity threshold to the DC current was about 30 percent higher than
AC.

Drenkard et al. (1985) did the experiment which exposed six
multiparous non-pregnant Holstein cows to electric current to assess its
effects on cow’s behavior, health, milking performance, and endocrine
responses. Three treatments (0, 4 and 8 mA) were applied in a
changeover design over three consecutive one week periods. They
concluded that behavioral responses to regularly applied AC current
treatment decreased in frequency and intensity with time. Changes of
milking performance and milk composition were not significant.
Changes of milking related cortisol responses during 8 mA current
experiment were significant. Oxytocin release was delayed during 8 mA

treatments. Current treatments did not affect prolactin.

8

Aneshansley et al. (1987) reported their experiment which had
fifteen first-calf heifers and fifteen 2nd to 4th lactation cows exposed to
five voltages (04 volts) while drinking. Exposure was continuous for 21
days. They reported no significant difference in water consumption,
feed intake, milk production, or concentration of fat or protein in the
milk. Drinking behavior (number of drinks/day and time/drink) did
change significantly.

Over the past decade, most studies of the cows’ behavioral
sensitivity response have been conducted with continuous, fixed and
steady state current /voltage levels and various duration. However, one
area that has not been adequately studied is dairy cow sensitivity to
short duration currents.

Currence et al. (1987) used three durations ( 1, 10 and 100 cycles
at 60Hz) of AC electrical current with the left front-left rear hoof
pathway to determine the magnitude of short bursts of 60 Hz current
that were required to cause a physical reaction in dairy cows. The
conclusion was that the magnitudes of 60-Hz AC currents required to
cause dairy cows to respond were essentially the same for current
duration of 1.67 and 0.17 seconds (100 and 10 cycles at 60 Hz) with a
value equal to about 3.5 mA (rms). The magnitude required for current
duration of 0.017 second (1 cycles at 60 Hz) was approximately 50%
higher. The similar tests were also given to twelve human volunteers to
determine the magnitude of short bursts of 60 Hz current that were

required to cause their "perception" and "equal level of discomfort".

9

The conclusion was that the difference in mean current magnitudes due
to current duration were statistically insignificant.

Gustafson et al. (1988) conducted the electric strength /duration
experiments to six Holstein cows by using the square current wave-form
(about 11 mA in height and 38 ms in width) to cows’ mouth-to-all
hooves pathways. They processed their experimental data statistically to
verify the model of Pearce et al. in 1982 and their own exponential
model. They concluded that the model of Pearce et al. did not fit their
experimental data as well as might be desired and their own statistical
model was better. Their model was: Is = 11.02 x t'0'16. Where IS was
the current strength of the stimulation needed to evoke the response in
mA and t was duration of stimulation in mS. In the empirical formula
above, the time ranged from 0.1 to 300 m8 and the current strength
ranged within 3 to 14 mA.

2.2 Possible Sources of the Farm Neutral-to-Earth Voltage

Surbrook and Reese (1981) defined the terminologies regarding
the farm stray voltage as follows: (1) Neutral-to-Earth Voltage -- A
voltage difference measured between the neutral of an electrical system
and the earth. Metallic structures and equipment bonded to the neutral
will also be at a difference in potential from the earth. (2) Transient
Voltage -- A voltage that is not constant. It can be a sudden voltage
spike or a gradual rise and fall of the voltage. This voltage is usually

measured between earth and the neutral. (3) Tingle Voltage -- A term

10

sometimes used to describe a very slight voltage that causes a slight
shock or tingle when encountered by a human. A 120 V supply may
only cause a tingle if the person is well insulated. (4) Stray Voltag -- A
general term often used to include all sources of voltages found on the
farm that may be encountered by humans and animals between a metal
object and the adjacent earth on floor. It includes the three previous
voltages encountered.

Surbrook and Reese (1981) traced the origins of farm stray
voltage from two sources -- on-farm and off-farm. They indicated that
common on-farm stray voltage sources were: (1) ground faults on the
farm; (2) voltage gradient across the ground or floor arising from wires
faulted in the earth; (3) electric fencer wires shorting direct to
equipment or inducing a charge in pipes and equipment; (4) grounding
conductor intentionally used as a neutral and a neutral used as a
grounding conductor; and (5) voltage drop on the secondary neutrals.
Typical off-farm stray voltage sources were: (1) voltage drop on the
primary neutral; (2) a ground fault on a neighbor’s property; and (3) a
fault in primary equipment or a problem with primary grounding.

Gustafson and Cloud (1982) indicated that in the field, several or
possibly all of these sources would interact. However, unless the
contribution from each source could be clearly distinguished and
analyzed, successful diagnosis was difficult. So, a good understanding of
the sources and their interaction, the electric nature of the problem, and
the effects of the electrical characteristics of the system were important

to proper diagnosis and solution.

11

Gustafson and Cloud (1982) further proposed the solutions to
stray voltage problems: (a) eliminate or minimize the voltage causing
the problem; (b) isolate the voltage from any equipment in the vicinity
of all potential animal contact points; or (c) install an equipotential
plane that will keep all possible animal contact points at the same
potential. The solution or solutions selected depends on (a) the source
or sources of the stray voltage; (b) the magnitude of the stray voltage;
(c) the cost of alternative solutions; (d) the physical facilities involved;
and (e) the policies of the power supplier. The solutions can be
relatively simple if the problem is clearly diagnosed and the alternatives
evaluated and explained to the farmer.

Several researchers have contributed to the identification of the
sources of the neutral-to-earth voltage in the past decade based on
experimental circuit measurement and circuit calculation by computer

Stetson et al. (1984) developed an analog. model of the
neutral-to-earth voltage in a single-phase distribution system. They
used resistors, conducting wires, switches, and a DC power source to
build up the physical circuit experiment model of a single-phase
distribution system. Their analog used the DC power source to simulate
the substation transformer and resistors to simulate the load
transformers. The analog assumed that each connection between the
neutral conductor and earth ground could be represented by a
resistance between the conductor and true earth. True earth had zero
resistance, and zero potential difference existed between any two true

earth ground connections regardless of their physical spacing. By taking

12

physical measurement of their circuit model, they revealed the
neutral-to-earth voltage phenomenon associated with multi-grounded
single-phase distribution systems.

Their analog could be used to demonstrate the effects on primary
lines of magnitude and location of loads with respect to the substation,
poor neutral connections, and poor grounds. On the secondary lines the
effects of poor connections, poor grounds, and undersized neutrals
could be illustrated with two farmstead loads. These loads could be
connected to illustrate the effects of loads in-phase and out-of-phase
with the primary line, and the influence of one farm on another. The
effect of separating primary and secondary neutrals could be illustrated.

They reported their six demonstrations: (1) The effect of primary
conductor resistance on the neutral-to-earth voltage demonstrated the
influence of neutral wire size. The voltage increased as neutral
conductor resistance increased. For the isolated line segment
represented by the analog, the voltage was a maximum at the ends of
the line and minimum at or near the center. (2) The effect of a high
resistance at some location along the primary or secondary neutral
conductor demonstrated the effect of a high-resistance splice or
connection. The voltage drop across the high-resistance splice or
connection was much higher than across any of the other neutral
segments. (3) The effect of different neutral-to-earth resistance on
neutral-to-earth voltage demonstrated the effect of good and poor
grounding. Ground resistance changes could cause the neutral-to-earth

voltage changes, but the effect was location-dependent. (4) The effect

13

of connecting a farmstead neutral at different locations along the
primary neutral demonstrated that the farm neutral-to-earth voltage
could be location—dependent as well as farm-load-dependent. With no
secondary neutral current, an off-farm source was responsible for the
farm neutral-to—earth voltage. With both on-farm neutral and an
off-farm source, the phase relationship between the primary and
secondary neutral currents and the line location affected the voltage on
farm. (5) With identical neutral current at each farm and farms at the
different taps, the interaction magnitude decreased as the distance
between farms increased. (6) The effect of connecting or disconnecting
the farmstead neutral from the primary neutral demonstrated
connecting the primary and secondary neutrals sometimes reduced and
sometime increased the measured neutral-to-earth voltage. They
summarized that to solve the problem of excessive neutral-to-earth
voltage, an orderly approach was essential because of the difficulties of
analysis necessary to identify its source.

Kehrle (1984) developed the DC circuit model of 7.2 kV
single-phase distribution power system to facilitate the analyses of the
neutral-to-earth voltage on farm. The computer program of circuit
simulation was used to perform the theoretical calculations and analyses
of the neutral-to-earth voltage along the distribution line. The 8
conclusions were reached. (1) The results obtained from the theoretical
analyses showed many similarities to the measurement results of the DC
physical analog model of Stetson et al. in 1984. (2) The effect of a

neutral/transformer grounding resistance was location—dependent. A

14

significant reduction of the grounding resistance at a specific point
resulted in a significant reduction of neutral-to-earth voltage at that
point, and to a less extent elsewhere in the system. Also, the resistance
changes made near the substation did not affect the general
neutral-to-earth voltage over the entire line. However, when changes
were made at the end of the line, net voltage changes at that point were
maximized. (3) The effects of a primary neutral conductor resistance
was also location-dependent. Increasing the resistance of the neutral
conductor in a segment located at the beginning of the line resulted an
increase of the neutral-to-earth voltage at all points along the line.
Increasing the resistance in a segment located at the middle of the line
resulted in a decrease of the neutral-to-earth voltage at some location
ahead of that segment in the direction of the substation. and an
increase of neutral-to-earth voltage resulted at all points behind that
segment in the direction toward the end of the line.) However, an
increase in the conductor resistance in a segment located at the end of
the line did not have a significant effect along the line. (4) Changes
made on the secondary neutral conductor resistance did not affect the
primary neutral-to-earth voltage along the line. (5) Changing the load
at one location resulted in a change of neutral-to-earth voltage at that
point, but the change was much less significant as the distance increased
away from that point. (6) The effect of varying the resistance of the
substation grounding mat was more apparent at the substation and
adjacent locations. (7) The effect of primary ground fault was found to

be location-independent. A sustained primary line to ground fault near

15

the substation, or at the middle, or at the end of the line had the same
effects on the neutral-to-earth voltage along the line. (8) The effect of a
secondary ground fault was found to be location-dependent. The effect
of a sustained secondary line to ground fault. at a location was more
apparent at that location than at the adjacent locations.

Surbrook et al. (1986, 1987) reported that the stray voltage was
caused by the voltage drop and ground faults and might have its origin
in certain parameter changes of the primary electrical distribution
system or in the customer’s secondary electrical system. The voltage
rms value of the neutral-to-earth voltage along a primary distribution
line might be at a value of zero some distance from the substation,
depending on the condition of the conductor resistances and of the
loads.

Gustafson (1985) used the computer program to find the
neutral-to-earth voltages in a DC circuit model. Some of his findings
revealed basic parameters that caused a rise in neutral-to-earth voltage
levels. 3 of his findings listed as follows. (1) Increased neutral wire
resistance resulted in the largest percentage effect (on the
neutral-to-earth voltage) in the central portion of the distribution line.
(2) Poor connection in the primary neutral conductor can dramatically
change the apparent resistance of the conductor. When poor
connection was near the substation, the effect on neutral-to-earth
voltage was largest on those farms near. the substation. The highest
value, at the end of the line, was not changed significantly. When the

additional resistance was placed near the midpoint of the line, some

16

farms ahead of the poor connection had reduced levels of the
neutral-to-earth voltage, while those further along saw increased levels.
(3) As the substation grounding resistance was increased, the voltage
near the substation increased and the zero point along the line was
moved further down the line. The effect diminished with distance from

the substation.
2.3 Field Test and Diagnosis of Farm Neutral-to-Earth Voltage

Beside theoretical analyses of the neutral-to-earth voltage
problem, Several researches have made contributions to the field testing
and diagnosis of farm neutral-to-earth voltage in the past decade.

Soderholm (1982) discussed the possible sources of stray voltage,
measurement techniques to determine their cause, and corrective
measures that could be applied. He indicated that proper choice of a
meter or recording device for measuring stray voltage was essential if
misleading indications were to be avoided. He suggested 6
specifications for voltmeters used for these measurements. (1) The
meter scale should be such that AC voltage levels of 0.1 to 1.0 V can be
observed. (2) The meter must be capable of separating AC and DC. (3)
A low input impedance equivalent to an animal mouth-to-hoof
resistance (approximately 300 ohms) should be used to evaluate
voltages with a low source impedance and avoid misleading
measurements due to stray pick-up. (4) The meter should be capable of

high impedance input (1 megohm or greater) for use in measuring

17

induced voltage. (5) The speed of response should be fast enough to
give an indication of transient voltages. (6) In addition to a voltmeter,
other measurement tools such as graphic recorder, oscilloscope or
clamp-on ammeter capable of measuring current levels from 1 mA to 20
A have been found valuable in monitoring and determining causes of
stray voltage.

Stetson et al. (1982) reported a test method developed to
electrically evaluate an operating primary neutral connector, or splice.
Their method required only a digital voltmeter and was not dependent
upon a constant or given line load. Connections could be evaluated
both before and after adjustment or replacement. They described
specific step by step measurement procedures as follows. ( 1) Proper
safety procedures should be followed. (2) An insulated aerial lift was
the safest and quickest method to reach the connections in question.
(3) All connections and tests should be made using insulated electrical
safety gloves. (4) A digital voltmeter was preferred since precise,
low-level voltage readings were more quickly and easily obtained. (5) A
clamp-on ammeter could be used to determine the level and monitor
any changes in current. (6) Attach the voltmeter leads to the conductor
near the connection with leads approximately 1/3 meter apart and note
the readings. (7) Move to one side or the other of the splice and take a
voltage measurement across a 1/3 meter section of the wire without a
splice. If the voltage reading across the splice was greater than the
voltage reading on the wire without the splice, then there is resistance in

the splice which should be eliminated. (8) In all cases prior to installing

18

any connector, dirt or corrosion on the conductors must be removed
with steel brush, steel wool or emery cloth. They reported their
procedure had been verified by field tests and worked well on primary
neutral or secondary neutral lines with bare conductors.

Surbrook et al. (1988) developed a stray voltage diagnostic
procedure with a minimum number of measurements. They described
the minimum instrumentation needed was voltmeter, reference ground
rod, wire and a 120 V load. Their procedure was: (1) Start at the main
meter location or transformer pole. Measure between the transformer
ground wire and a reference ground at least 15 feet away and not under
the primary right of way. (2) Make the voltage measurement near the
building or area on the farm where the voltage was suspected to be
affecting the animals. (3) Determine if there was an excessive voltage
drop on the neutral wire to any building on the property. (4) Check
ground faults on the farm. If other sources were not found in steps 1
and 3, then it was possible that a ground fault was present. (5) The
power supplier line crew would open the bond between the primary and
secondary neutrals at the farm transformer. At this point, telephone
personnel and cable television personnel must check the grounding of
their lines at the farm to make sure they did not themselves form a bond
from the farm neutral to the electrical power supplier primary neutral.
This was because that if the previous tests did not lead to an
identification of the neutral-to-earth voltage, then it was possible that
the source may be due to a ground fault some place other than on the

farm.

19

Prothero et al. ( 1988) studied the primary neutral-to-earth voltage
levels impacted by various wiring system treatments through taking the
field measurements. The test circuit that they chose was a radial two
phase 3-wire tap, from a three-phase 4-wire multi-grounded neutral
feeder beginning approximately 2.5 miles from the source substation.
Through analyzing the field measurement data, they reported that the
present preferred method of solidly bonding primary and secondary
neutrals was consistent with the goal of minimizing primary
neutral-to-earth voltage on rural feeders. And the large amounts of
supplemental primary neutral grounding, as well as load balancing and
line reconstruction accomplished during their investigation, could not

reduce the primary neutral-to-earth voltage to zero.
2.4 Mitigation of Farm Neutral-to-Earth Voltage Problem

Gustafson (1985) proposed three possible approaches to mitigate
and avoid the neutral-to-earth voltage problem on farm. The first
recommendation was voltage reduction -- by either elimination of the
voltage source ( e.g., by removing bad neutral connections, faulty loads
or improving or correcting wiring and loading ), or by active suppression
of the voltage by a nulling device. (2) When the voltage could not be
reduced to acceptable levels, the suggestion was gradient control -- by
use of equipotential planes and transition zones to maintain the
animal’s step and touch potential at an acceptable level. (3) A final

suggestion was isolation of a portion of the grounding or grounded

20

neutral system accessing the animals, so that they will not be subjected
to objectionable currents due to stray voltages existing on the remainder
of the grounded neutral system.

Surbrook et al. (1989) proposed a number of mitigation
techniques for the farm neutral-to-earth voltage problem. They
indicated that common mitigation techniques can be applied once the
sources were positively identified. On-farm sources usually responded
to one or more of the techniques described in their report. (1)
Elimination of resistance at splices and terminations of the neutral
conductors on the farm was necessary when this was found to be the
source. (2) Increasing neutral conductor size may reduce the voltage.
(3) Reducing the length of feeder conductors to a building was effective
during initial layout of the farm buildings. (4) Balancing the 120 V
loads in a building to maintain neutral current at or near zero was
important. (5) A four-wire feeder, separating the neutral from
equipment grounding conductor, may be installed to a building where
animals may be affected. (6) Elimination of the interconnections
between neutral conductors and equipment grounding conductors in a
building. (7) Providing all electrical equipment with an equipment
grounding conductor that was continuous from the equipment to the
grounding bus of the circuit supply panel increased the safety and
reduced the chances of neutral-to-earth voltage. (8) Elimination of any
fault in equipment or wiring, or any wiring that potentially could cause a
fault from an ungrounded conductor to the equipment or the earth was

extremely important.

21

Surbrook et al. (1989) also indicated that off-farm source
mitigation normally handled by the power supplier included: (1)
separating the primary and secondary neutral conductor; (2) repairing
corroded neutral conductor splices; (3) increasing the neutral conductor
size; (4) increasing the primary line operating voltage; (5) reducing
neutral-to-earth resistance at one or more locations along a distribution
line; and (6) elimination of ground faults on the primary system, or at
another customer location.

They indicated that there were some mitigation techniques that
were an attempt to lower the cow contact voltage regardless of the
source. These mitigation approaches included: (1) Equipotential planes
installed in the floor of milking areas, at watering devices and feeding
areas were intended to put everything within reach of the cow at the
same electric potential so there would be no contact voltage. (2)
Installing an active suppression device counteracting the
neutral-to-earth voltage condition may be effective where the voltage
could not be eliminated.

Althouse and Surbrook (1990) proposed and physically realized
the design of equipotential plane on farm. They reported the step
potential for a cow’s contact could be reduced to below 1 volt when
neutral-to-earth voltage was as high as 5 volts, measured from

equipotential plane to reference ground.

III. OBJECTIVE

The primary goal of this study was to develop an effective
computer model to simulate and analyze the neutral-to-earth voltage
profile along a primary distribution line under some operational
conditions. The computer model developed was then used to
investigate the neutral-to-earth voltage problem on farm which has not
been studied enough to explain in relation to the behavior of real

distribution lines. The specific objectives of this study were as follows:

1. Develop a network model of a single-phase, multi-grounded
primary electrical distribution system which provides the
neutral-to-earth voltage at every grounding electrode and the
current in each neutral, grounding electrode, and other network
element.

2. Simulate the primary circuit network model with the computer
program and analyze the computer numerical solutions of the
neutral-to-earth voltages and some currents under the control of
the following system operating conditions in a network model: (1)
different levels of abnormal resistance in the neutral conductor at
a particular point along the line, and different levels of line
loading near the point of abnormal neutral resistance; (2) a high

resistance connection between the neutral and the primary

22

23

winding of the transformer; (3) a 7.2 kV phase-to-neutral fault at
different points along the line; (4) a 7.2 kV phase-to-earth fault
at different points along the line; (5) different levels of
resistance-to-earth of the substation; '(6) different levels of
resistance-to-earth of the distribution neutral; (7) different levels
of phase-to-neutral voltage; and (8) a secondary ungrounded
conductor to earth fault at one point along the distribution line.

Compare the operations of DC and AC base cases to determine if
a DC model is accurate in studying neutral-to-earth voltage along

a single-phase distribution line.

IV. METHODOLOGY

The electrical network simulation program SPICE running on
Michigan State University mainframe IBM-3090 computer was
employed to simulate the DC base case and other DC cases used to
study effects of some other different operational conditions on the
neutral-to-earth voltage along the single-phase power distribution line.
Then the SPICE computer program was employed to simulate the AC
base model for verifying the effectiveness of the simpler DC model and
for studying primary and secondary neutral separated/bonded effects
on neutral-to-earth voltage. The results from the simulation runs with
the SPICE program were used as the input data to the personal
computer software SUPERCALC-3 to draw the corresponding

neutral-to-earth voltage profiles of all the models and case studies.

4.1 The DC Single-Phase Distribution Simulation Model

A DC base model has been developed for simulation of a
neutral-to-earth voltage of distribution line. Figure 1 shows the circuit
network for this single-phase electrical primary distribution line system
model. The parameter values in the DC base model are shown in Table

1. The following scenario was modeled:

24

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26

Table 1. Parameters for Base DC Circuit Model in Figure 1.

 

 

 

Ungrounded Neutral Earth Load
Wire Wire Ground Resrstance
Symbol Ohms Symbol Ohms Symbol . Ohms Symbol Ohms
RS 0.5
RU2 0.32 RDl 0.08
R02 25.0
RD2 MB
R03 25.0
RD3 0.08
RE04 100 K
RD4 0.08
RR05 15.0 RRLZ 7,200
RU4 0.32 RD5 0.08
RE06 100 K
RD6 0.08
R07 25.0
R07 0.08
R1308 100 K
RD8 0.08
RF09 5.0 RFL4 3,600
RU6 0.32 RD9 0.08
R5010 100 K
RD 10 0.08
R011 25.0
RDll 0.08
REG 12 100 K
RD12 0.08
RRGl3 15.0 RRL6 7,200
RU8 0.32 RD13 0.08
REGl4 100 K
R014 0.08
R015 25.0
RDlS 0.08
REG 16 100 K
RD16 0.08
RF017 5.0 RFLS 3,600
RUlO 0.32 RD 17 0.08
R8018 100 K
RD 18 0.08
R019 25.0
RD 19 0.08
R5020 100 K
RD20 0.08
RR021 15.0 RRLIO 7,200
RU12 0.32 RD21 0.08
R5022 100 K
RD22 0.08
R023 25.0
RD23 0.08
RE024 100 K
R024 0.08
RF025 5.0 RFL12 3,600
RU14 0.32 RD25 0.08
RE026 100 K
R026 0.08
R027 25.0
RD27 0.08
RE028 100 K
RD28 0.08
RR029 15.0 RRL14 7,200
RU16 0.32 RD29 0.08
RE030 100 K
R030 0.08
R031 25.0
R031 0.08

RE032 100 K
RF 033 5.0 RFL16 3.600

RD32 0.08

 

27

 

 

 

Table 1 (cont’d)
Ungrounded Neutral Earth Load
Wire Wire Ground Resrstance
Symbol Ohms Symbol Ohms Symbol Ohms Symbol Ohms
RS 0.5
R018 0.32 R033 0.08
R8634 100 K
R034 0.08
R035 25.0
R035 0.08
R8636 100 K
RD36 0.08
RR637 15.0 RRL18 7,200
R020 0.32 R037 0.08
R8638 100 K
R038 0.08
R639 25.0
R039 0.08
RE040 100 K
R040 0.08
RF041 5.0 RFL20 3,600
R022 0.32 R041 0.08
R8642 100 K
R042 0.08
R043 25.0
R043 0.08
R8644 100 K
R044 0.08
RR645 15.0 RRL22 7.200
R024 0.32 R045 0.08
R8646 100 K
R046 0.08
R647 25.0
R047 0.08
RE048 100 K
R048 0.08
RFG49 5.0 RFL24 3,600
R026 0.32 R049 0.08
R8650 100 K
R050 0.08
R651 25.0
R051 0.08
R8652 100 K
R052 0.08
RR053 15.0 RRL26 7,200
RU28 0.32 R053 0.08
R8654 100 K
R054 0.08
R055 25.0
R055 0.08
RE056 100 K
R056 0.08
RF057 5.0 RFL28 3,600

R057 0.08

 

28

4,267 meters (14,000 feet) 7,200 volts multi-grounded single-phase
distribution system fed fourteen load loops evenly spaced about
304.8 m (1,000 feet) apart. Half of the loads were assumed to be
residential and half of the loads were assumed to be farms. Each
residential load (RRL) was followed by a farm load (RFL).
Ground electrodes were positioned at each farm load (RFG), at
each residential load (RRG), and between transformers including
the substation, node 1 (RG). Between the substation and the first
grounding electrode there was one additional grounding electrode
(R0). In one test condition, extra ground electrodes (REG) were
added uniformly along the primary neutral line to simulate the
effect of increasing ground rod number.

Along the primary neutral conductor line, from the substation to
the last ground rod, 57 equally spaced nodes were selected. The
node numbers were shown in Figure 1.

Primary conductors employed were number 2 AWG, ACSR. The
ungrounded conductor (phase line) resistance (RU) between every
two loads was 0.32 ohm (304.8 m of 2 ACSR) ohm. The neutral
conductor resistance (RD) between every two adjacent nodes
analyzed was 0.08 ohm (76.2 m of ACSR).

The substation (node 1) ground mat resistance (RS) was 0.5 ohm;
each residential load neutral grounding electrode resistance
(RRG) was 15 ohm; and each farm load grounding resistance
(RFG) was 5 ohm. The additional neutral grounding electrode

resistances (RG) were 25 ohm. It was assumed that the all

29

grounding electrodes were connected to the "true earth” whose
resistance was zero.

5. At each residential transformer, the load resistance (RRL) was set
at 7,200 ohm to produce a 1 ampere primary load. At each farm
transformer, the load resistance (RFL) was set at 3,600 ohm to give
a 2 ampere primary load. No secondary systems were connected to
the transformer nodes to model the no load effects on all the

secondary sides of the distribution transformers.

In the initial phase of modeling, the distribution line system was
supplied by a DC voltage source equivalent to an actual AC source; and
the resistance load was equivalent to the transformer load. The
neutral-to-earth voltage values shown were equivalent to rms values.
The negative DC values represented phase reversals in the AC network
model. .

With the DC base model, the eight cases were designed to reach the
research objectives in the simulation of the effects of some different
Operational conditions on the neutral-to-earth voltage along the power
distribution line on farm through the changing of corresponding electric
parameters. In seven of eight cases, secondary system loads were
omitted to model the no load condition; one secondary system was
connected only in the last case to facilitate the study of the
secondary-to-earth fault effect on the neutral-to-earth voltage.

In the field, the effects of these operational conditions are usually

superimposed making the source identification of neutral-to-earth

30

voltage difficult. However, the clear and thorough understanding of
these effects need the individual contribution of each operational
condition to be studied separately. Only one parameter at a time in
each of these nine cases was changed, and all other parameters

remained the same as in the original network.
4.2 Simulation of Abnormal Conditions along the Distribution Line

The single-phase distribution line base model as shown in Figure 1
was used for comparison with each of the abnormal line conditions
studied. The deviation of the neutral-to-earth voltage of the test case as
compared with the base model was considered as significant criteria for

evaluation rather than the actual magnitude of the voltage per se.
4.2.1 Neutral Conductor Resistance Change

The simulated resistances of the primary neutral conductor RD32
and RD33 (located on the two sides of the farm transformer RFL16,
between the two nodes 32 and 33 and nodes 33 and 34 in Figure 2, 0.08
ohm normal) were changed to the following values: 0.08, 0.50, 1.0, 5.0,
20.0 and 100K (open circuit condition) ohms. This was to model the the
effect of a high resistance in the primary neutral conductor.

The effect of neutral conductor resistance change on the
neutral-to-earth voltage under heavy load condition was studied by

increasing the farm transformer load RFL16 (2A normal) for each value

31

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of the resistance RD32 and RD33. The simulated transformer loads
were 2A (normal), 3A, 5A, and 10A. This was accomplished by altering
the simulated load resistance RFL16 in sequence: 3,600, 2,400, 1,440,
and 720 ohms in Figure 2. 1

4.2.2 High Resistance Connection of Neutral to Transformer Primary

Normally the primary neutral line makes a good connection with the
transformer and grounding electrode with a contact resistance RC in
Figure 3 near zero. There are cases in the field where the primary
winding is properly connected to the grounding electrode at the
transformer pole, but there is a high resistance connection to the
primary neutral. The effect of the bad neutral connection with
transformer winding and grounding electrode on the neutral-to-earth
voltage was studied by increasing the contact resistance RC from 0 ohm
respectively to 0.5 ohm, 1 ohm, 5 ohms and 100 kohms at a location

near the middle of the distribution line (node 33).
4.2.3 Primary Phase-to-Neutral Fault (or Heavy Load)

The effect of primary unground phase conductor to neutral fault size
on the neutral-to-earth voltage was studied by inserting a load resistor
in the middle of the distribution line. This was accomplished by
introducing a resistor RF1 from the ungrounded phase conductor node

416 to the corresponding neutral node 33 in Figure 4. RF1 was set at

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the values of 7,200, 1,440, 720 and 360 ohms to model the fault levels of
1 A, 5 A, 10 A and 20 A respectively. A phase to neutral fault had the
same effect as increasing the load on the line at the location of the fault.

The effect of this fault’s location on the neutral-to-earth voltage was
studied by moving the resistor RF1 above from node 416-33 to node
426-53, which was the farthest residential transformer location from the

substation.
4.2.4 Primary Phase-to-Earth Fault

First, the effect of the primary ungrounded-phase-conductor-to- earth
fault size on the neutral-to-earth voltage was studied by simulating a
fault in the middle of the distribution line. This was accomplished by
introducing a resistor RF2 from the ungrounded phase conductor node
416 to earth in Figure 5. RF2 was set at the values of 7,200, 1,440, 720
and 360 ohms to model the fault levels of 1A, 5A, 10A and 20A
respectively.

The effect of this fault’s location on the neutral-to-earth voltage was
studied by moving the resistor RF2 above from node 416 to node 404
and to node 428 respectively. Node 404 was the farm transformer
location nearest the substation and node 428 was the farm transformer
location farthest from the substation. At this time, RF2 was only set at

360 ohms to model a 20 A fault, the most serious fault situation.

36

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37

4.2.5 Substation Grounding Resistance Change

The normal grounding resistance for the substation in this model is
0.5 ohm. The substation grounding resistance change effect on the
neutral-to-earth voltage was studied by increasing substation grounding
resistance RS from 0.5 ohm respectively to 1 ohm, 5 ohms, 10 ohms, 20
ohms and 40 ohms at the substation, node 1 in Figure 1. This was to
model the effect of a poor grounding connection at the substation on

neutral-to-earth voltage.
4.2.6 Neutral-to-Earth Resistance Reduction

The normal grounding resistance in the model for each residential
transformer is 15 ohm and for each farm transformer is 5 ohm. The
neutral-to-earth resistance reduction effect on neutral-to-earth voltage
was studied by reducing the grounding resistance RFG33 from 5 ohm to
3 ohm and then to 1 ohm. Grounding electrode RFG33 was at the
middle of the distribution neutral line (node 33) which was the fourth
farm transformer location from the substation shown in Figure 1. This
exercise was to model how well a low resistance-to-earth electrode could
lower a neutral-to-earth voltage at a farm.

In the base case, the normal grounding resistance for each residential
transformer was 15 ohm, and for each farm transformer was 5 ohm.
There was one grounding electrode at 25 ohm between each

transformer. The effect of lowering the grounding electrode resistance

38

all along the distribution line was studied by changing the extra
grounding electrode resistance, REG, from 100 kohm to 25 ohm. This
resulted in a simulation with a grounding electrode of not more than 25
ohms at every pole along the distribution line. The distribution line
model is shown in Figure 1 and the base case values of grounding

electrode resistance are listed in Table 1.
4.2.7 Primary Operating Voltage Level Change

In the base case, the primary operating voltage level was 7.2 kV. The
effect of the change of primary operating voltage level on the
neutral-to-earth voltage was studied by simulating the other primary
operating voltage levels of 2.4 kV, 4.8 kV and 26.4 kV, respectively. The
secondary load currents for all these cases were maintained the same as
in the base simulation of 30 amperes for residential transformer
secondaries and 60 amperes for farm transformer secondaries. The
power consumed on the circuit remained the same as the base case.
The resistances RRL and RFL were changed to maintain the desired
secondary current. Values of primary voltage, VI, load resistances RFL

and RRL, and primary current are listed in Table 2.
4.2.8 Secondary Earth Fault

The secondary circuit simulation (Figure 6) was a 120/240 volt,

single-phase, three wires grounded system with a balanced load. The

39

grounding resistances of the primary and the secondary were paralleled,
with the resultant resistance remained at 5 ohms. The no load condition
was still assumed. The secondary earth fault effect on the
neutral-to-earth voltage was studied by introduCing the 2.5, 5 .0 and 10 A
secondary ground faults through the resistor RF3 set at 48, 24 and 12
ohms from the ungrounded conductor of the secondary circuit to earth.
The faults of in-phase (only SW3 and SW4 closed in Figure 6) and 180 °
out-of-phase (only SW3 and SW5 closed in Figure 6) on the secondary
side were respectively attached to: (1) node 9 which was the farm
transformer location nearest the substation; (2) node 33 which was the
fourth farm transformer located at the middle of the primary
distribution line; and (3) node 57 which was the farm transformer

location farthest from the substation.
4.3 Operation of the AC Model

In verifying the effectiveness of the DC model, an AC model with the
150/75 kVA, 7,200/120/ 120 V distribution transformer model and the
60 Hz, 7,200 V (rms) sinusoidal voltage source as well as the
transmission line inductance effect was developed. Figure 7 shows the
AC network model. In this AC model, the 60 Hz, 7,200 V (rms)
sinusoidal voltage source replaced the 7,200 DC source; the 150/75
kVA, 7,200/120/ 120 V distribution transformer model replaced each
simulated load resistance; and simulated inductance was inserted along

each segment of ungrounded and neutral conductor ( XL = R on each

40

segment) in the DC base model of Figure 1. On the secondary side of
each residential transformer, a 4 ohm resistor was connected to draw a
30A load. On the secondary of each farm transformer, a 2 ohm resistor
was connected to create a 60A load. ‘

In the AC model with the line inductance established above in Figure
7, along the entire distribution system, the primary neutrals and
secondary neutrals in each of the fourteen residential and farm
transformers were designed into two different connection styles --
bonded and separated. The "bonded" meant that the secondary neutral
was connected with the primary neutral ground system. ”Separated"
meant that each of primary neutrals and secondary neutrals had their
own ground system.

Beside the normal, no fault and balanced operational condition, 10 A
secondary ground faults were introduced through a 12 ohm resistor
placed between the ungrounded conductor of the secondary circuit and
earth. The faults (in-phase and 180 ° out-of-phase) on the secondary
side were attached to node 33 (similar to what had been done to the DC
model in 4.1.10.)

41

Table 2. Primary Current and Load Resistance, RRL and RFL, for Different
Primary Voltage Simulations.

 

 

Distribution Residential Load Farm Load
Voltage Pri. A Sec. A RRL Pri. A Sec. A RFL
2.4kV 3.0 30 800 6.0 60 400
4.8kV 1.5 30 3,200 3.0 60 1,600
7.2kV 1.0 30 7,200 2.0 60 3,600

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V. RESULT and DISCUSSION

The computer simulation results of the eight DC cases and the
AC models were analyzed to determine the level of the neutral-to-earth
voltage produced under various operational conditions.

Although the neutral-to-earth voltage profiles were presented as
if continuous, only the discrete voltage values at each of the 57 node

locations were meaningful.
5.1 The DC Model Analysis

The effects of the different operational conditions on the
neutral-to-earth voltage were studied primarily through the DC model.
The effectiveness of the DC model was verified using an AC model of
the same network.

As mentioned in section 4.1, for the DC cases, the negative values
shown on the voltage profile diagrams signify phase reversals in the AC
model. Some DC voltage profiles were analyzed with the negative signs
changed to positive signs to represent the magnitudes of the rms voltage
values which would be found in the AC model or on an actual

distribution line. Some voltage profiles became confusing when the DC

44

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Figure 8. Profile of the neutral-to-earth voltage along the primary distribution

line from substation (node 1) to 57th node for the base model

46

simulated neutral-to-earth voltages were all converted to positive
values; in these cases, both positive and negative DC voltage values

were shown.
5.1.1 Normal Operational Condition

The profile of the neutral-to-earth voltage along the distribution
line for the base model is shown in Figure 8. The profile shows that the
magnitude of the neutral-to-earth voltage at the substation (5.8 V) was
slightly lower than at the end of the primary neutral line (7.8 V). Note
that the magnitude of the neutral-to-earth voltage decreased as the
distance away from the substation increased, reaching zero and
experiencing a 180 degree phase angle change near node 10. Then the
neutral-to-earth voltage increased as the distance from the substation
became greater, leveling off toward the end of the neutral line.

Along the multi-grounded neutral power distribution systems
which are commonly used in rural areas of the United States, some level
of neutral-to-earth voltage is always present; it is an inherent
phenomenon. Kehrle (1984), Gustafson (1985) and Surbrook et al.
(1986) also obtained theoretical profiles of neutral-to-earth voltage

similar to Figure 8.

47

5.1.2 Neutral Conductor Resistance Change

Figures 9 through 13 present the neutral-to-earth voltage
simulation curves resulting from changing the neutral conductor
resistance RD33 between nodes 33 and 34 (Figure 2) in different load
conditions.

Figure 9 shows the the primary neutral-to-earth voltage profile
with the variable neutral conductor resistance RD33 in the normal load
condition (2A load in the primary side of the farm transformer RFL16).
Note in the base case, when normal neutral resistance RD33 was 0.08
ohm, the voltage drop across this segment was only 0.1 V. (6.3 V
neutral-to-earth voltage at node 33, 6.4 V at node 34). When RD33 was
increased to 0.5 ohm to simulate an abnormal resistance in the neutral,
0.5 V voltage drop occurred between nodes 33 and 34. The
neutral-to-earth voltage decreased by 0.2 V at node 33 toward the
substation side, while it increased by 0.3 V at node 34 toward the end of
the line. When neutral resistance RD33 was increased to 20 ohm, there
was a 3.1 V voltage drop between node 33 and node 34. The
neutral-to-earth voltage decreased by 1.2 V at node 33 and it increased
by 1.8 V at node 34 as compared with the normal RD33 (0.08 ohm)
condition. As the abnormal resistance RD33 was increased, the voltage
drop across this segment also increased. The neutral-to—earth voltage
became lower at nodes toward the substation from the location of the
abnormal neutral resistance, and higher at nodes toward the end of the

line. Note that the effect on neutral-to-earth voltage of an abnormal

Figure 9.

48

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Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with variable neutral conductor

resistance RD33 between node 33 and node 34 and a customer load of
2A at node 33

49

neutral conductor resistance diminished as the distance from the neutral
resistance RD33 increased. In the extreme case, when RD33 increased
to 100 kohm (open circuit simulated), the voltage drop across this
segment reached 3.5 V. The neutral-to-earth voltage decreased by 1.5 V
at node 33 and increased by 2.1 V at node 34. However, the
neutral-to-earth voltage was only changed 0.1 V at the substation and
0.9 V at the end of the line, as compared with the normal RD33 (0.08
ohm) situation (base case).

When a 3A load located at node 33 was chosen to repeat the
simulation, the neutral-to-earth voltage profile of the 3A load
simulation was similar to that of the 2A load simulation, but the voltage
drop across the line segment RD33 was smaller than that of the 2A load
simulation. The voltage drop between nodes 33 and 34 was 2.4 V in the
open circuit condition of this segment, 69% of the 3.5 V voltage drop of
the 2A load simulation in open circuit condition. See Figure 10.

When a 5A customer load was chosen at node 33 to repeat the
simulation, no matter how the neutral resistance RD33 was changed, all
neutral-to-earth voltage profiles were nearly identical. This result is
shown in Figure 11. All these identical neutral-to-earth voltage profiles
were higher than those in the base case with the largest difference 2.5 V
at node 34 and the smallest difference 0.2 V at node 9 (near profile
phase reversal node).

When 3 10A load was chosen at node 33 to repeat the simulation,
the profile of neutral-to-earth voltage increased on the substation side

of neutral conductor resistance RD33 as compared with the 5A

50

customer load. This is shown in Figure 12. As the abnormal RD33 was
increased, the voltage drop across this segment also increased
significantly again. But, unlike cases of the 2A and 3A load simulation,
this time the neutral-to-earth voltages were higher at nodes toward the
substation side and lower at nodes toward the end of the line.
However, similarly, the effect on both sides was diminished rapidly away
from the high resistance segments. In the extreme case, when RD33
was increased to 100 kohm (open circuit simulated), the voltage drop
across this segment reached 4.9 V. But this time the voltage increased
by 2.0 V at node 33 and decreased by 2.9 V at node 34. The voltages
only changed 0.1 V at the substation and 1.2 V at the end of the line, as
compared with the normal RD33 (0.08 ohm) situation.

The profile changing trend in the variable load cases when an
abnormal resistance in series with the primary neutral conductor RD33
was in the extreme open circuit situation (RD33= 100 kohms) is shown
in Figure 13. As the load at node 33 increased, the neutral-to-earth
voltages became higher on the substation side of node 33; and the
profiles were identical from node 34 to the end of the line. The changes
of the neutral-to-earth voltage decreased as the distance increased
along the direction from node 33 to the substation in all load cases. In
those cases of the loads less than 5A, the neutral-to-earth voltages from
node 33 to the substation were lower than those from node 34 to the
end; and the difference between them became smaller as the load
became heavier. In those cases of the loads more than the 5A, the

neutral-to-earth voltages from node 33 to the substation were higher

Figure 10.

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resistance RD33 between node 33 and 34 (10A Load)

54

than those from node 34 to the end of the line; and the difference
between them became larger as the load became heavier.

The theoretical calculation and practical field measurement
indicate that primary neutral-to-earth voltage is highly associated with
the current in the local primary neutral conductor -- higher neutral
current results in higher neutral-to—earth voltage locally and lower
neutral current results in lower neutral-to—earth voltage locally (this will
be discussed in detail in section 5.1.5 and 5.1.9). In Figure 13, since
there was an open circuit between node 33 and node 34 in the circuit
system, and the resistance on the substation side of node 33 was much
lower than the ground rod resistance, most load current took the
pathway in the neutral conductor from node 33 toward the substation
side. In the heavy load condition, this raised the electric potential on
the substation side. The heavier the load, the higher the potential
raised.

Simulation of the neutral-to-earth voltage along single-phase
primary distribution lines reported by Kehrle (1984), Gustafson (1985)
did not examine the effect of level of primary load located immediately
on the substation side of an abnormal resistance in the neutral
conductor. As can be seen from Figure 13, it is possible to have a higher
neutral-to-earth voltage on the substation side of an abnormal neutral
resistance than on the side toward the end of the line.

Figures 14 through 16 present the neutral-to-earth voltage

simulation curves resulting from changing the neutral resistance RD32

55

between nodes 32 and 33 in different load conditions (see Figure 2 for
circuit model).

Figure 14 shows the the primary neutral-to-earth voltage profile
with the variable neutral conductor resistance RD32 in the normal load
condition (2A load). The voltage change trends were similar to those in
the same load condition of the RD33 change shown in Figure 9. When
RD32 was 100 kohm (open circuit simulated), the voltage drop across
the segment of node 32 and node 33 reached 5.4 V. This value was 86%
of that at node 33 in the base case, a decrease by 2.9 V at node 32 and
an increase by 2.5 V at node 33. However, the neutral-to-earth voltage
was only changed 0.2 V at the substation and 1.0 V at the end of the
line, as compared with the normal RD32 (0.08 ohm) situation (base
case).

Figure 15 shows the the primary neutral-to-earth voltage profile
with the variable neutral conductor resistance RD32 and a 5 A load at
simulated transformer, RFL16. The voltage change trends were
different from those in the same load condition of the RD33 change (in
that case all the voltage profile were identical) but similar to those with
the 2 A load condition of the RD32 change. The voltage changed more
dramatically. When the line segment RD32 was 100 kohm (open circuit
simulated), the voltage drop across the segment of node 32 and node 33
reached 9.6 V. This value was 1.52 times of that at node 33 in the base
case with a value of 6.3 V.

The neutral-to-earth voltage profile changing end in the variable

load (RFL16) cases with an abnormal resistance in series with the

56
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Figure 13. Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with load condition changed in the
open circuit (between node 33 and 34) situation.

Figure 14.

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Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with variable neutral conductor
resistance RD32 between node 32 and 33 (2A Load)

58

primary neutral conductor RD32 in the extreme open circuit situation is
shown in Figure 16. As the load became heavier, the neutral-to-earth
voltages became higher from node 33 to the line end (node 57); and the
profiles were not significantly different from node 32 to the substation
node 1). The changes of the neutral-to-earth voltage decreased as the
distance increased along the direction from node 33 to the end of the
line (node 57) in all load cases when RD32 was 100 kohm.

Figure 16 also can be explained by the level of neutral current on
the local line segment. In Figure 16, there was an open circuit between
node 32 and node 33 in the circuit system, and the resistance on the end
side of the line near node 33 was much lower than the ground rod
resistance. Most of the load current took the pathway in the neutral
conductor from node 33 toward the end of the line. In the heavy load
condition, this raised the neutral-to-earth voltage on the direction
toward the end of the line. The heavier the load, the higher the
neutral-to-earth voltage raised. ~

From Figure 9 through Figure 16, it can be seen that the primary
neutral-to-earth voltage changes sharply at the high resistance segment
along the primary neutral line, resulting in the neutral-to-earth voltage
improvement at some distance along the distribution line and worsening
elsewhere along the distribution line. Generally speaking, the higher
the resistance of the segment, the larger the changing effect. But the
effect is both localized and load-dependent. This indicates that along
the primary neutral line, the substantial changes in the neutral-to-earth

voltage may occur in the immediate vicinity of the bad connection, but

59

 

 

 

 

 

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Figure 15. Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with variable neutral conductor
resistance RD32 between node 32 and 33 (5A Load)

60

the effect will be damped rapidly along the line toward both directions.
However, when the segment resistance is beyond some threshold, say,
20 ohm, the changing effect approaches a maximum for given load
conditions. '

Stetson et. al (1984), Kehrle (1984), Gustafson (1985) and
Surbrook et al. (1986) found the profile split phenomenon in
neutral-to-earth voltage caused by a high resistance at some location
along the primary neutral conductor in a normal load condition similar
to Figure 9 and Figure 14. But the effect of high primary neutral
resistance associated with a heavy load condition near the abnormal
neutral resistance on neutral-to-earth voltage has not been documented

prior to this thesis.
5.1.3 High Resistance Connection of Neutral to Transformer Primary

Figure 17 presents the neutral-to-earth voltage Simulation curves
resulting from changing neutral contact resistance RC with
transformer-to-ground rod at node 33 (circuit connection in Figure 3).
In this scenario, the neutral terminal of the transformer primary winding
is solidly connected to ground rod RF G33, but it experiences a high
resistance (RC) connection to the primary neutral at node 33. This
condition has been experienced in the field where the neutral-to-earth
voltages along a distribution line were normal except for a high
neutral-to-earth voltage at the customer transformer location of the

high resistance connection.

Figure 16.

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Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with load condition changed in the
open circuit (between node 32 and 33) situation.

62

Compared with base case, when contact resistance RC was
increased from 0 ohm (base case) to 0.5 ohm, 1 ohm, 5 ohm and 100
kohm, at the poor connection node, neutral-to-earth voltage increased
sharply as the contact resistance increased. NOte in the base case, when
the RC was zero, the value of neutral-to-earth voltage at node was 6.3
V. When the value of RC was increased to 0.5 ohm, the voltage
increased by 0.3 V. When the value of RC was increased another 0.5
ohm ( total RC = 1 ohm ), the voltage again increased by 0.3 V. When
the value of RC was increased to 5 ohms, the voltage increased to-8.0 V
(2.7 V higher than base case), but the increase was only at the
transformer location of bad connection. In the extreme case, when the
value of RC was increased to 100 kohm (open circuit simulated), the
voltage reached 9.9 V, creating an acute local increase of
neutral-to-earth voltage. This was due to all local primary load current
(2 A) flowing through the pathway of ground rod RFG33 (product of 2
A and 5 ohms was 10 V). i

No significant neutral-to-earth voltage changes occurred
elsewhere along the whole line system away from the bad connection.
This means that to locate a bad connection of this type, the

measurements must be at the customer transformer location.
5.1.4 Primary Phase-to-Neutral Fault (or Heavy Load)

Figure 18 presents the neutral-to-earth voltage simulation curves

resulting from different currents through a simulated fault or additional

63

heavy load between primary ungrounded conductor node 416 and
neutral node 33 (circuit connection in Figure 4). Faults in equipment
on a primary distribution line have been reported to have resulted in a
lowering of the neutral-to-earth voltage in a local area when the fault
situation was corrected. Previous research did not explain how this
phenomenon could occur. Analysis of possible primary fault scenarios
revealed that a fault usually as an ungrounded phase conductor to
grounded equipment (neutral) fault. The behavior of a phase to neutral
fault is identical to placing a phase to neutral load at the location of the
fault. Vegetation making intermittent simultaneous contact with the
ungrounded conductor and the neutral will cause an intermittent
increase in the neutral-to-earth voltage as shown in Figure 18.
Compared with the base case, when fault or additional load
current was increased from 0 A (base case) respectively to 1.0 A, 5.0 A,
10 A and 20 A, the neutral-to-earth voltage also increased accordingly
in most segments along the distribution line. Note the most significant
increases of the neutral-to-earth voltage occurred at node 33 where the
fault took place. At this node, in the base case, when the fault current
was zero, the value of the neutral-to-earth voltage was 6.3 V. When the
fault was increased to 1.0 A, the voltage increased by 0.7 V, to 7 V.
When the fault was increased to 5.0 A, the voltage increased by another
2.5 V, to 9.5 V. When the fault was increased to 10 A, the voltage again
increased another 3.1 V, to 12.6 V. When the fault was increased to 20

A, the voltage reached 18.7 V (increased by 2.0 times from the base case

64

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Figure 17. Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with variable bad neutral connection
resistance RC with transformer and ground rod at node 33

65

of 6.3 V). The neutral-to-earth voltage increased almost linearly with
the fault current at a rate of 0.6-0.7 V/A near the fault node.

Toward the end of the line node 57, the neutral-to-earth voltage
increase was less significant. Even so, when the fault was 20 A, the
voltage at node 57 increased to 12.9 V, 165% of the base case value of
7.8 V.

Toward the direction of the substation, the neutral-to-earth
voltage increases first diminished, approaching the values close to those
of base case near node 13. From node 13 to the substation node 1, as
the distance increased, the voltages also increased slightly. Near the
substation, when the fault was 20 A, the voltage increased to 11.9 V,
2.05 times of the base case value of 5.8 V.

Figure 19 presents the neutral-to-earth voltage simulation curves
resulting from changing current through fault simulation bypass from
primary unground conductor node 426 to neutral node 53 near the end
of the line. 4

Compared with the base case, when fault current was increased
from 0 A (base case) respectively to 1.0 A, 5.0 A, 10 A and 20 A, the
neutral-to-earth voltage also increased accordingly in most segments
along the distribution line. Note the most significant increase of the
neutral-to-earth voltage occurred at node 53 where the fault took place.
This voltage increase was similar to that of the fault at node 33 except
that the significant voltage increase occurred at the different location.
At node 53, in the base case, when the fault current was zero, the value

of the neutral-to-earth voltage was 7.8 V. When the fault was increased

66

 

 

 

 

 

 

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Figure 18. Profile of the neutral-to-earth voltage along the primary distribution

line from substation to 57th node with variable unground phase
conductor to neutral fault at node 416-33

67

to 1.0 A, the voltage increased by 0.9 V, to 8.7 V. When the fault was
increased to 5.0 A, the voltage increased by another 3.8 V, to 125 V.
When the fault was increased to 10 A, the voltage again increased one
more 4.8 V, to 17.3 V. When the fault was increased to 20 A, the
voltage reached 26.8 V (increased by 2.5 times from base case 7.7 V).
The neutral-to-earth voltage increased almost linearly with the fault
current at rate 0.9-0.96 V/A near the fault node 53.

Toward the direction of the substation, the neutral-to-earth
voltage increases first diminished, approaching the values close to those
of the base case near node 19. From node 19 to the substation node 1,
as the distance increased, the voltages also increased slightly. Near the
substation, when the fault was 20 A, the voltage increased to 12.5 V, 2.2
times of the base case value 5 .8 V.

From Figure 18 and Figure 19, it can be seen that when a primary
phase-to-neutral fault occurs somewhere in the distribution line, the
neutral-to-earth voltages increased in most segments along the
distribution line. These voltage changes are fault-size and fault-location
dependent. The voltages increased as the fault currents increased. The
most significant voltage increase occurs near the fault location. This is
due to the large neutral current injection from local fault current. The
same conditions would be created by placing a large load at a specific

location along the distribution line.

68

5.1.5 Primary Phase-to-Earth Fault

Figure 20 presents the neutral-to-earth voltage simulation curves
resulting from fault current through a resistance between the primary
ungrounded conductor at node 416 and the earth (circuit connection in
Figure 5). A similar condition has been reported in the literature, but it
is repeated here as a comparison with the data produced with the
primary ungrounded phase conductor to neutral fault condition.

Compared with base case, when fault current was increased from
0 A (base case) respectively to 1.0 A, 5.0 A, 10 A and 20 A, the
neutral-to-earth voltages increased accordingly in some segments near
the substation and decreased some distance away from the substation
toward the end of the line. This is shown in Figure 20. Note the most
significant changes of the neutral-to-earth voltage occurred near node 1
where the substation was located. Near the substation, in the base case,
when fault current was zero, the value of neutral-to-earth voltage was
5.8 V. When the fault was increased to 1.0 A, the voltage also increased
by 0.3 V, to 6.1 V. When the fault was increased to 5.0 A, the voltage
increased by another 1.5 V, to 7.6 V. When the fault was increased to
10 A, the voltage increased 1.8 V, to a level of 9.4 V. When the fault
was increased to 20 A, the voltage reached 13.1 V (increased by 1.26
times from base case 5.8 V). Similar to the primary-to-neutral fault
situation discussed in section 5.1.4, the neutral-to-earth voltage
increased almost linearly with the fault current, but this time at a rate of
0.3-0.37 V/A near the substation node 1.

Figure 19.

69

 

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Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with variable unground phase
conductor to neutral fault at node 426-53

70

This effect of the change of the neutral-to-earth voltage
diminished rapidly as the distance increased from substation node 1
toward the end (node 57) of the line. In a 20 A fault, at the middle
(node 33) of the line, the voltage was 5.1 V, 81% of base case value 6.3
V; at the end node 57, the voltage was 7.3 V, only 93.6% of the base
case value of 7.8 V. It is important to note that a primary ungrounded
conductor to earth fault resulted in a decrease in the neutral-to-earth
voltage from middle to the end of the line.

Figure 21 presents the comparison of the neutral-to-earth voltage
simulation curves resulting from a 20 A simulated fault from the
primary ungrounded conductor to earth introduced at three locations --
node 404, node 416, and at node 428 (absolute values of the DC
voltages were shown). It can be seen that the neutral-to-earth voltage
profiles along the distribution line were identical for all three fault
cases. An observed abnormally high neutral-to-earth voltage near the
substation may be caused by a primary ungrounded conductor to earth
fault at any point along the whole line system.

It is important to note from Figure 20 and 21 that the
neutral-to-earth voltage decreased from node 15 to the end of the line
when a primary ungrounded conductor to earth fault occurred at any
location along the line. Even when the primary fault to earth occurred
near the end of the line (node 428) the neutral-to-earth voltage was
decreased even at the fault node as compared to the base case without a

fault condition.

71

 

 

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Figure 20. Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with an ungrounded phase conductor
to earth fault at node 416 compared with the normal line

72

Figures 22 through 26 present the electric current distribution
situation of the entire system resulting from a 20 A fault current from
primary ungrounded conductor to earth introduced at node 416 near
the middle of the line.

From Figure 22 it can be seen that compared with the base case,
there was an increase in the current flow on the primary neutral in the
fault case due to fault current returning to the neutral by way of the
distribution line grounds. This current increase was associated with the
local neutral-to-earth voltage increase, significant along the line near
substation node 1 and less and less significant in the direction away
from the substation. In this fault case, near the substation, the current
on the primary neutral was 14.3 A, 154% of the base case value 9.3 A;
near the end node 57, the neutral current was 0.5 A, only 125% of the
base case value 0.4 A.

In this simulation of an ungrounded primary conductor to earth,
the grounding electrode current consisted of the load current flowing
between the neutral and the earth and the fault current flowing between
the earth and the neutral. Near the substation, the load current flowing
through the grounding electrodes and the fault current were in phase.
Therefore, the two currents added to cause the increase in
neutral-to-earth voltage near the substation observed in Figures 20 and
21. From node 11 to the end of the line, the load current and the fault
current flowing through the grounding electrodes was 180 degrees out
of phase thus subtracting. From node 15 to the end of the line, the net

grounding electrode current was smaller for the primary ungrounded

Figure 21.

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Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with 20 A phase conductor to earth

fault at node 404, 416, or 428 compared with normal line

74

conductor to earth fault case than for the normal case without a fault.
This resulted in a lower neutral-to-earth voltage for the fault case than
for the normal case. The case of a phase to earth fault located at node
428 at the end of the line is illustrated in Figure 23.

The highest level of phase to earth fault current simulated was 20
amperes. Comparison of Figures 24 and 25 reveals the amount of fault
current returning to the substation by way of the substation grounding
electrode. Subtraction of node 1 current of Figure 24 from node 1
current of Figure 25 reveals that the fault component of the substation
grounding electrode current is approximately 15 amperes. Figure 24
shows the grounding electrode currents along the line for the normal
load case. Figure 25 shows the grounding electrode currents along the
line with normal load plus a 20 ampere phase to earth fault at node 416.
Node 1 on both figures is the current of the substation grounding
electrode.

Note that for nodes 2 through 9 in Figure 25 as compared to
Figure 24, fault current is detected returning to the primary neutral.
For other nodes towards the end of the line, the grounding electrode
currents is actually reduced, but the change is difficult to detect. The
reduction of normal line current due to fault current returning to the
neutral conductor can be seen at nodes 17, 21 and 25 of Figure 25
compared with Figure 24.

It is also important to note from Figures 20, 22, and 25 that it is
not possible to determine that a primary phase to earth fault is actually

present (node 416) by observing a neutral-to-earth voltage or neutral

75

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Figure 22. The primary neutral conductor current for each line segment from the
substation to the end of the line for normal load compared with
normal load plus a 20 A fault at node 416

76

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Figure 23. Phase relationship of normal neutral load current and primary to earth
fault current at the end of the line

77

 

 

 

 

 

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Figure 24. The normal operating distribution line grounding electrode currents
from the substation to the end of the line

78

current abnormally in the area of the fault. The presence of the fault
can be seen by the step reduction in primary ungrounded conductor
current at the location of the phase to earth fault as shown in Figure 26.
Figure 27 represents the current in each Segment of the primary
neutral conductor for the normal load case and for the normal load plus
a 20 ampere primary to earth fault at node 404, 416, and 428. The top
curve of Figure 27 is the same regardless of the location of the primary
to earth fault. Note that at the end of the line, the current flow on the
neutral conductor is only slightly changed by a primary to earth fault at
the end of the line or at any other location along the line. The step
changes in the neutral current of Figures 22 and 27 are due to the
transformer currents and current flowing between the neutral and the
earth at each grounding electrode. The neutral current level change at
the grounding electrode located between transformers is different at the
end of the line as compared with the change near the substation.
Neutral line current flows into the earth at the end of the line while it
returns from the earth onto the neutral conductor near the substation.
Figure 28 shows the current supplied by the substation
transformers and the current of the load transformers along the line for
the normal line case and for the case with a primary to earth fault at
node 404, 416, or at 428. The 20 amperes primary ungrounded
conductor to earth fault increases the substation transformer current
output, but it does not have an effect upon the current of the load
transformers. The changes in neutral current shown in Figures 22 and

27 for the primary fault to earth case as compared to the normal load

79

 

 

 

 

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Figure 25. Grounding electrode currents from the substation to the end of the

line with a 20 A phase to earth fault at node 416 plus normal line
loading

80

 

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305‘...

 

UNGRIIINOEO CONDUCTOR CURRENT

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400 404 408 412 416 420 424 428
NOOE NllIOER

Figure 26. Primary ungrounded phase conductor current comparison from the
substation to the end of the line for normal line load vs. normal line
load plus a 20 A phase to earth fault at node 416

Figure 27.

AMPERAGE(A)

81

 

 

 

 

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NEUTRAL SEGMENT #

The primary neutral conductor current for each line segment from the
substation to the end of the line for normal load compared with
normal load plus a 20 A fault at node 404, 416, or 428

82

case are due to current flow between the earth and the neutral
conductor through the grounding electrodes.

Figure 29 shows the current flow on the primary ungrounded
conductor for the normal load case as compared'to the case where there
was a 20 amperes primary to earth fault at node 404, 416, or at 428.
Note that the primary ungrounded conductor current level is increased
by 20 amperes as compared to the normal load ease up to the point
where the primary to earth fault is located.

From Figure 20 and Figure 21, it can been seen that when primary
phase-to-earth fault occurs somewhere in the distribution line, the
neutral-to-earth voltages increase in some segments near the substation
and decrease some distance away from substation toward the end of the
line. These voltage changes are fault-size dependent and fault-location
independent. The voltage changes increase as the fault currents
increase. The most significant changes of the neutral-to-earth voltage
occur near the substation. The neutral-to-earth voltage profiles at the
three fault locations (the middle and the both ends of the line) are
identical. Similar result was reported by Kehrle (1984), but the reasons

for the results were not explained.
5.1.6 Substation Grounding Resistance Change
Figure 30 presents the neutral-to—earth voltage simulation curves

resulting from changing the substation grounding resistance. Such a

change can occur due to corrosion, water-table fluctuation or nearby

83

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84

 

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400 404 403 412 416 420 424 428
NOOE MEI!

Primary ungrounded phase conductor current comparison from the
substation to the end of the line for normal line load vs. normal line
load plus a 20 A phase to earth fault at node 404, 416, and 428

85

earthwork construction. Low substation resistance to earth may be
difficult to achieve due to soil conditions.

Compared with base case, when substation resistance was
increased from 0.5 ohm (base case) respectively to 1.0, 5.0, 10, 20 and 40
ohms, the neutral-to-earth voltages increased accordingly in some
segments near the substation and decreased some distance away from
the substation toward the end of the line. Note the most significant
changes of the neutral-to-earth voltage occurred near node 1 where the
substation was located. At the substation, in the base case, when the
substation resistance was 0.5 ohm, the value of neutral-to-earth voltage
was 5.8 V. When the substation resistance was increased to 1.0 ohm,
the voltage also increased by 3.4 V, to 9.2 V. When the substation
resistance was increased to 5.0 ohms, the voltage increased by another
8.3 V, to 17.5 V. When the substation resistance was increased to 10
ohms, the voltage increased another 2.3 V, to 19.8 V. When the
substation resistance was increased to 20 ohms, the voltage was only
increased by 1.3 V, to 21.1 V. When the substation resistance was
increased to 40 ohms, the voltage was only increased another 0.7 V, to
21.8 V. Generally, the voltage change was increased near the substation
as the substation resistance was increased; the effect was significant in
the resistance range from 20 -- 40 ohms.

This effect on the neutral—to—earth voltage diminished rapidly as
the distance increased from the substation (node 1) toward the end
(node 57) of the line. With 40 ohms substation resistance, at substation

node 1, the neutral-to—earth voltage was 21.8 V, 3.76 times of base case

86

value of 5.8 V; at the middle node 33, the voltage was 3.8 V, 60% of the
base case value of 6.3 V; at the end node 57, the voltage was 6.7 V, only
86% of the base case value of 7.8 V.

A high value of substation resistance results in a reduced amount
of neutral load current flowing to earth on the grounding electrodes
from the middle to the end of the line. A lowered substation grounding
mat resistance results in a lowered neutral-to-earth voltage.

From Figure 30, it can be seen that when the substation grounding
resistance is increased, the neutral-to-earth voltages increased in some
segments near the substation and decreased some distance away from
substation toward the end of the line. These voltage changes are related
to the distance from and the resistance-value of the substation. The
neutral-to-earth voltages are increased significantly near the substation
and changed less and less significantly as the distance is away from the
substation. Similar results were reported by Kehrle (1984) and
Gustafson (1985) in their simulation. I

From Figure 20 and Figure 30, it may be found that the
neutral-to-earth voltage changing trends of this case and the
primary-to-earth fault case mentioned in 5.1.5 were very similar, except
the voltage changing rates were different. In the case of 5.1.5, the
voltage change rate was quite uniform, almost linear at the rate of
0.3-0.37 V/A near the substation node 1. In the substation-resistance
case, the voltage change rate was not uniform at all, much higher at the
lower resistance levels and much lower at the higher resistance levels.

The explanations given for the phenomenon are as follows:

87

In the case mentioned in 5.1.5, when the ground fault current
increased, most of the current returned to the substation through the
grounding resistance (0.5 ohm), which was much lower than those in
other places. This can cause a voltage increase at the substation
proportionally with the fault current increase. In this substation
resistance case, the substation resistance increase can also cause an
increase in the voltage. But this voltage change across the high
substation ground resistance is constrained by the current that the

system can provide.
5.1.7 Neutral-to-Earth Resistance Reduction

Figure 31 presents the neutral-to-earth voltage simulation curves
resulting from reducing neutral-to-earth grounding electrode resistance
RFG33 at the middle of the distribution neutral line.

Compared with the base case, when the grounding electrode
resistance RFG33 was reduced from 5 ohm to 3 ohm and then to 1 ohm
at node 33, at node 33 and nearby along the distribution line, the
neutral-to-earth voltage decreased as the grounding resistance was
reduced. However, this reduction of neutral-to-earth voltage was
localized. The most significant neutral-to-earth voltage reduction
occurred at node 33 itself. At node 33, in the base case, when RFG33
was 5 ohms, the value of neutral-to-earth voltage was 6.3 V. When the
RFG33 was decreased to 3 ohms, the voltage also decreased by 05 V, to
5 .8 V. When RFG33 was decreased to 1 ohm, the voltage decreased by

100388

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Figure 30. Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with basecase vs. variable substation

resistance RS

89

another 1.7 V, to 4.1 V. The neutral-to-earth voltage decreased more
dramatically at this node when RFG33 was reduced from 3 ohms to 1
ohm than from 5 ohms to 3 ohms.

This voltage reduction effect was less and less significant as the
distance increased from node 33 toward both the substation and the end
of the distribution line. When RFG33 was 1 ohm, at node 33, the
voltage was 4.1 V, 65% of the base case value of 6.3 V; at the end node
57, the voltage was 6.9 V, 88% of the base case value of 7.8 V; however,
at the substation node 1, a very small neutral-to-earth voltage change
was experienced with the voltage at 6.0 V, slightly changing in the
contrary directions to those of nodes 33 and 57, 103.4% of the base case
value 5.8 V.

From Figure 31, it can be seen that when the neutral-to-earth
resistance is decreased somewhere in the middle location of the
distribution line, the neutral-to-earth voltages also decrease accordingly
in most segments along the distribution line. This voltage reduction
effect is localized. The most significant voltage reduction occurs at the
location where the grounding resistance is reduced. The voltage
reduction is less and less significant as the distance increases from that
location in either direction. Similar results were also reported by
Kehrle (1984) and Surbrook et al., (1988) in their simulation.

Prothero et al. (1988) also reported from their field measurement
data that their tests showed a net resistance of less than 0.5 ohm for the
primary neutral network. The measured resistance of farm ground

systems ranged from 2.0 to 5 .0 ohms per farm. The connection of these

90

low resistance farm grounds to the primary neutral network was found
to have a larger influence over primary neutral-to-earth voltage levels
than the additional ground electrodes or counterpoise. This is
consistent with the theoretical analysis in this reSearch.

Figure 32 presents the neutral-to-earth voltage simulation curves
resulting from improving neutral-to-earth grounding by adding an extra
grounding electrode of 25 ohms at every node on the neutral conductor
where the resistance was previously set at 100 kohms. See Figure 1 and
Table 1. For this simulation, there is a grounding electrode of not more
than 25 ohms at each pole along the distribution line.

Compared with the base case, when the grounding electrode
number was doubled, the neutral-to-earth voltage was lowered at most
nodes (from node 11 to node 57) along the distribution line. This
voltage reduction effect was significant from the end to the middle
location of the distribution line. At the end (node 57), the value of
neutral-to-earth voltage decreased by 1.9 V, to 5.8 V, 74.4% of the base
case value 7.8 V; at the middle node 33, voltage was 4.7 V, 75% of the
base case value 6.3 V; however, at the substation node 1, very small
neutral-to-earth voltage change was experienced with the voltage 5.9 V,
slightly changing in the contrary direction to those of nodes 33 and 57,
101.7% of the base case value 5.8 V. Note the "zero point" of the
neutral-to-earth voltage was slightly shifted from node 10 in base case to
node 11 in this case.

From Figure 32, it can be seen that when the grounding electrode

number was doubled uniformly along the neutral line, the

Figure 31.

91

 

 

 

 

 

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NODE(l-57)

Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with base case vs. ground resistance
RFG33 change at node 33

92

neutral-to-earth voltage was lowered in most of the transformer nodes
along the distribution line. This voltage change was insignificant near
the substation.

Reduction of the grounding electrode resiStance at node 33 in the
middle of the line decreased the local neutral-to-earth voltage.
Addition of extra grounding electrodes was equivalent to increasing the
parallel resistance number and decreasing the actual overall resultant
ground resistance.

Prothero et al. (1988) also reported from their field measurement
that the addition of numerous supplemental grounding electrodes, to
the extent that every distribution pole was grounded, had only a slight
effect on reducing primary neutral-to-earth voltage.

Although increasing the number of grounding electrodes can
decrease the neutral-to-earth voltage to some extent, a large number of
ground rods will be costly and the effect may not be as good as
expected. For this simulation, doubling the number of grounding
electrodes along the primary line only resulted in approximately a 25

percent reduction in neutral-to-earth voltage at the end of the line.
5.1.8 Primary Operating Voltage Level Change

Figure 33 presents the neutral-to-earth voltage simulation curves
resulted from changing the primary operating voltage levels. The base
case simulation assumes a primary ungrounded conductor operating at
7.2 kV.

93

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Figure 32. Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with base case vs. ground rod
number increasing along the entire system

94

When the primary operating voltage levels were increased from
2.4 kV respectively to 4.8 kV, 7.2 kV and 26.4 kV with power
unchanged, the neutral-to-earth voltages in all these four distribution
systems decreased correspondingly along the distribution line. These
voltage profiles had the similar shapes and the same positions with the
different proportion scales. All four voltage profiles approached
convergence near node 10.

The neutral-to-earth voltages of all four systems decreased as the
distance away from the substation increased, reaching zero and
changing 180 degree in phase angle between node 9 and node 10. Then
the voltage increased as the distance toward the end of the line
increased until reached the maximum value at node 57. The
neutral-to-earth voltage profile changed sharply at first, then leveled off
toward the end of the neutral line. At node 10, the neutral-to-earth
voltages corresponding to the systems of 2.4 kV, 4.8 kV,7.2 kV and 26.4
kV were 0.4 V, 0.1 V, 0.1 V and 0 V, respectively. At the substation
node 1, the neutral-to-earth voltages corresponding to the systems of 2.4
kV, 4.8 kV, 7.2 kV and 26.4 kV were 16.4 V, 8.6 V, 5.8 V and 1.6 V,
respectively. At the end (node 57), the neutral-to-earth voltages
corresponding to the systems of 2.4 kV, 4.8 kV, 7.2 kV and 26.4 kV were
22 V, 12 V, 7.8 V and 2.1 V, respectively.

Generally, the higher the operating voltage, the lower the
neutral-to-earth voltage. This is due to the constant power situation:

the higher the operating voltage, the lower the current on transformer

95

primary side. The lower transformer primary current leads to lower
current injection to the neutral line and parallel ground path.

This neutral-to-earth voltage change effect due to the primary
operating voltage change was much more significant in the operating
voltage level range from 2.4 -- 4.8 kV than the range from 7.2 -- 26.4 kV.
The neutral-to-earth voltage decreased 20% per kV on the average in
the operating voltage level range from 2.4 -- 4.8 kV and only 3.8% per
kV on the average in the range 7.2 -- 26.4 kV. This implies that striving
for high operating voltage levels may not reduce the neutral-to-earth

voltage as much as expected.
5.1.9 Secondary Earth Fault

Figures 34 through 39 present the neutral-to-earth voltage
simulation curves resulting from normal line loading plus a ground fault
on the secondary side of one of the transformers. The simulation
network used in this case is shown in Figure 6.

When the out-of-phase secondary ground faults of 2.5, 5.0 and 10
amperes were introduced at the transformer between nodes 416 and 33,
at the middle of the distribution line, from Figure 34, compared with the
base case, some changes in the neutral-to-earth voltage were observed.
The most significant voltage changes occurred near node 33 where the
faulted secondary network was attached. At node 33, the voltage
increased significantly as the fault current increased. In the base case,

when the fault current was zero, the value of neutral-to-earth voltage

Figure 33.

VOLTAGE(‘.')

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Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with base case 7.2 kV operating
voltage vs. 2.4, 4.8 and 26.4 kV operating voltages

97

was 6.3 V. When the fault current was increased to 2.5 A, the voltage
increased by 1.6 V, to 7.9 V. When the fault current was increased to
5.0 A, the voltage increased by another 1.6 V, to 9.5 V. When the fault
current increased to 10 A, the voltage increased an additional 2.9 V, to
12.4 V, 197% of the base case value 6.3 V.

This voltage change effect was less and less significant as the
distance increased from node 33 toward both the substation and the end
along the distribution line. When the fault was 10 A, at node 33, the
voltage was 12.4 V, 197% of the base case value 6.3 V; at the end node
57, the voltage was 10.3 V, 132% of the base case value 7.8 V; however,
at the substation node 1, very small neutral-to-earth voltage change was
experienced with the voltage 5.2 V, slightly changing in the contrary
directions to those of nodes 1 and 57, 90% of the base case value 5 .8 V.

When the in-phase secondary ground faults of 2.5, 5.0 and 10
amperes were introduced at the transformer between node 416 -- 33, at
the middle of the distribution line, from Figure 35, compared with the
base case it can be seen that neutral-to-earth voltage changed in a
similar way to the in-phase secondary ground fault situation except the
voltage changes were in the contrary direction. Note in Figure 35 that a
10 ampere secondary in-phase ground fault is subtractive with the
primary load produced neutral-to-earth voltage resulting in a net
neutral-to-earth voltage value near zero at node 33.

When the out-of-phase secondary ground faults of 2.5, 5.0 and 10
amperes were introduced at the transformer between node 404 -- 9, the

first farm transformer location along the distribution line, from Figure

98

36, compared with the base case it also can be seen that there were
changes in the neutral-to-earth voltage. The significant voltage changes
occurred near node 9 where the faulted secondary network was
attached. At node 9, the voltage was increased significantly as the fault
current increased. In the base case, when the fault current was zero, the
value of neutral-to-earth voltage was 0.4 V. When the fault current was
increased to 2.5 A, the voltage also increased by 0.6 V, to 1.0 V. When
the fault current was increased to 5.0 A, the voltage increased by
another 1.3 V, to 2.3 V. When the fault current was increased to 10 A,
the voltage increased an additional 2.6 V, reaching 4.9 V, 12 times of the
base case value 0.4 V.

This voltage change effect was less significant as the distance
increased from node 9 in either direction of the distribution line. When
the fault was 10 A, at node 9, the voltage was 4.9 V, 12 times of the base
case value 0.4 V; at the end node 57, the voltage was 8.4 V, 107.6% of
the base case value 7.8 V; however, at the substation node 1, the voltage
was 3.6 V, changing in the contrary directions to those of nodes 33 and
S7, 62% of the base case value 5 .8 V.

When the in-phase secondary ground faults of 2.5, 5.0 and 10
amperes were introduced at the transformer between node 404 -- 9, the
the first farm transformer location of the distribution line, from Figure
37, compared with the base case it could be seen that the
neutral-to-earth voltage changed in a similar way to the in-phase
ground fault situation except the voltage changes went to the contrary

direction.

Figure 34.

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Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with base case vs. variable secondary
ground fault attached to node 33 (out-of-phase)

100

 

 

 

 

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Figure 35. Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with base case vs. variable secondary
ground fault attached to node 33 (in-phase)

Figure 36.

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Profile of the neutral-to-earth voltage along the primary distn’bution
line from substation to 57th node with base case vs. variable secondary
ground fault attached to node 9 (out-of-phase)

Figure 37.

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Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with base case vs. variable secondary
ground fault attached to node 9 (in-phase)

103

With an out-of-phase secondary ground fault at node 33 in the
middle of the line, most customers experienced an increase in
neutral-to-earth voltage as shown in Figure 34. When there was an
in-phase secondary ground fault at node 33 in the middle of the line,
most customers experienced a decrease in the neutral-to-earth voltage
(Figure 35). But this was not true when the secondary ground fault was
at a location near the substation. Examination of Figures 36 and 37
shows that some customers experience a lowering of the
neutral-to-earth voltage while others experience an increase whether
the secondary ground fault was in-phase or out-of-phase with the
primary load produced neutral-to—earth voltage.

When the out-of-phase secondary ground faults of 2.5, 5.0 and 10
amperes were introduced at the transformer between node 428 -- 57, the
last farm transformer location of the distribution line, from Figure 38,
compared with the base case it still can be seen that there was some
changes in the neutral-to-earth voltage. The most significant voltage
changes occurred near the end node 57 where the faulted secondary
network was attached. At node 57, the voltage was increased
significantly as the fault current increased. In the base case, when the
fault current was zero, the value of the neutral-to-earth voltage was 7.8
V. When the fault current was increased to 2.5 A, the voltage also
increased by 2.7 V, to 10.5 V. When the fault current was increased to
5.0 A, the voltage increased by another 2.5 V, to 13.0 V. When the fault
current was increased to 10 A, the voltage increased by an additional 4.7
V, reaching 17.7 V, 2.3 times of the base case value 7.8 V.

Figure 38.

104

 

 

 

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Profile of the neutral-to-earth voltage along the primary distribution
line from substation to 57th node with base case vs. variable secondary
ground fault attached to node 57 (out-of-phase)

105

This voltage change effect was less significant as the distance
increased from the end node 57 toward the substation node 1 along the
distribution line. When the fault was 10 A, at the end node 57, the
voltage was 17.7 V, 2.3 times of the base case Value 7.8 V; at node 33,
the voltage was 8.7 V, 112% of the base case value 7.8 V; however, at
the substation node 1, very small neutral-to-earth voltage change was
experienced with the voltage 5.6 V, slightly changing in the contrary
directions to those of nodes 33 and 57, 96.6% of the base case value 5.8
V.

When the in-phase secondary ground faults of 2.5, 5.0 and 10
amperes were introduced at the last farm transformer location node 428
-- 57, from Figure 39, compared with the base case it could be seen that
the neutral-to-earth voltage changed in a similar way to the in-phase
ground fault situation except the voltage changes went to the contrary
direction. The net effect of the in-phase secondary ground fault was to
reduce the neutral-to-earth voltage. '

From Figure 34 through Figure 39, it can be seen that for a
ground fault on the secondary side of a transformer, the
neutral-to-earth voltages along the distribution line were changed to
some extent. Generally, this kind of change is fault-size, fault-phase and
fault-location dependent. The heavier the fault currents, the more
significantly the neutral-to-earth voltages change. The voltages change
in contrary directions when the in-phase or out-of-phase fault occurs.
Much more significant changes in neutral-to-earth voltage occur at and

near node at which the faulted secondary circuit is attached. The

106

farther the distance along the line away from the fault node, the less
significant the neutral-to-earth voltages change. A similar result was
also reported by Kehrle (1984) in her simulation.

The change in level of neutral-to-earth voltage due to a secondary
ground fault is greater at the end of the line than at the middle of the
line as can been seen from Figures 36 through 39. Near the substation
the amount of change was less, but whether a particular location would
experience a net increase or decrease in neutral-to-earth voltage was
difficult to predict. At the end of the line as was the case at the middle
of the line, an out-of-phase secondary ground fault caused an increase
in neutral-to-earth voltage for all customers. For the in-phase
secondary ground fault, customers experienced a reduction of
neutral-to-earth voltage unless the amount of reduction was great
enough to result in a phase angle change in the neutral-to-earth voltage.
In this latter situation, it is possible for the net rms value of the voltage
to actually become higher. '

It is important to note that a secondary ground fault can be
additive or subtractive to the primary line load produced
neutral-to-earth voltage. The elimination of a secondary ground fault at
one location resulting in a lowering of neutral-to-earth voltage at that
location may indeed cause a significant increase in the neutral-to-earth
voltage at other locations. When a secondary ground fault is
discovered, it is recommended that voltage measurements be taken

along the distribution line in the area after the elimination of the

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ground fault attached to node 57 (in—phase)

108

secondary ground fault to determine if any customers have experienced
an increase in neutral-to-earth voltage.

The out-of-phase and in-phase secondary ground fault condition
at the end of the distribution line is illustrated in Figures 40 and 41.
Figure 40 shows the end of the distribution line with the simulated
secondary electrical system attached. The net grounding electrode
resistance to earth of the three grounding electrodes (25, 17, and 10
ohm) is 5 ohms which is the same as used in all simulations at node 57.
The out-of-phase fault condition with respect to the primary line is
defined by observing the phase relationship of the phase angle of the
fault current of the secondary ground fault circuit with respect to the
primary current, in Figure 40 compared with Figure 23. It is important
to note that the fault current and the normal line load current flowing
through the grounding electrodes is in-phase thus resulting in a net
increase in grounding electrode current at the end of the line. Figure 38
shoWs the increase in neutral-to-earth voltage at the end of the line as a
result of the increase in grounding electrode current.

Figure 41 shows the end of the distribution line with simulated
secondary network attached at node 57. In this case the secondary
ground fault is in-phase with respect to the primary line. Note that the
phase relationship of the secondary ground fault current of Figure 41 is
in the same phase of the primary ground fault current of Figure 23.
Note also in Figure 41 that the normal line load produced grounding
electrode current is out-of-phase with the secondary ground fault

current flowing on the grounding electrodes. There will be a net

109

reduction of the grounding electrode current at the end of the line when
there is an in-phase secondary ground fault. This can be seen by the
reduction of grounding electrode current in the in-phase secondary
ground fault condition as shown in Figure 42. The secondary ground
faults of 2.5, 5.0, and 10 amperes were located at the last farm
transformer on the line which was at nodes 428 -- 57. The
neutral-to-earth voltage will be reduced as shown in Figure 39.

Figure 42 shows the reduction of the grounding electrode current
of the grounding electrodes near the end of the line when there is a
secondary in-phase ground fault at the end of the line. Note that at
node 57 grounding electrode, a 10 A secondary ground fault causes
enough current flow on the grounding electrode to result in a change in
phase of the current. This accounts for the change in phase of the
voltage near the end of the line in Figure 39.

Compared with the base case, from Figure 43, it can be seen that
the primary neutral currents did change to a large extent. The neutral
current change trend was very similar to that of the neutral-to-earth
voltage of the corresponding case (compared with Figure 39). The most
significant neutral currents increase (in reverse direction of that in the
portion of the line near the substation along the neutral line mainly due
to secondary fault current returning from earth) occurred near the end
node 57 where the fault secondary network was attached and maximum
neutral-to-earth voltage change occurred. At node 57, the neutral
current was increased significantly as the fault current increased. In the

base case, when the fault current was zero, the value of the neutral

110

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secondary in-phase ground fault attached to node 57

114

current was 0.42 A When the fault current was increased to 2.5 A, the
neutral current also increased by 1.95 A inversely, to -1.53 A. When the
fault current was increased to 5.0 A, the neutral current increased by
another 1.87 A inversely, to -3.40 A. When the fault current was
increased to 10 A, the neutral current increased by one more 3.49 A
inversely, reaching -6.89 A, 17.3 times of the base case value 0.42 A.

The previous analysis of secondary ground faults assumes a 0
degree or a 180 degree phase deference between the current flowing in
the neutral, grounding electrodes and earth as a result of normal line
loading and secondary ground fault current. This will only occur if the
power factor of the primary distribution line and of the secondary
ground fault circuit are unity. This analysis shows the maximum
changes possible as a result of a secondary ground fault. If the primary
neutral grounding electrode current and the secondary fault current
flowing on the grounding electrode are at some phase angle difference
other than 0 degrees or 180 degrees, the resultant current flowing in the
grounding electrode will be at some magnitude less than the maximum
shown here and more than the minimum values. It would, therefore, be
expected that less extreme results would occur for a secondary ground

fault on an actual operating single-phase distribution line.
5.2 Operation of the AC Model

So far, only the results and analyses for the DC model have been

presented. A question, of course, should be asked: How effective is the

115

DC model? Figures 44 through 48 show some comparisons between
the DC based model and its relevant AC based model.

Figure 44 shows the neutral-to-earth voltage magnitude profiles of
the two AC models (with and without tranSmission line inductance)
compared with the 7,200 DC base model profile. To make the DC
model profile more comparable with that of the AC model, the minus
signs of the voltage values near the substation in the DC profile were
taken away in Figure 44. Although the curve position of the AC model
was slightly higher than that of DC model, the shapes of these two
profiles were identical. The profile of the AC model with the line
inductance effect did not experience the "zero voltage value” point like
the DC model did, the reason being that the neutral return currents
from the earth could not cancel each other out completely when the AC
model transmission line series inductance effect was taken into account.
This implies that the DC model can indicate some changing trends of
the neutral—to—earth voltage magnitude. 4

Figure 45 and Figure 46 show the comparison between the AC
model neutral-to-earth voltage phase angle profiles with and without
the transmission line inductance effect. Note that the profile with the
line inductance effect was smoother than that without line inductance
effect.

Figure 47 and Figure 48 show the comparison between voltage
regulation along the ungrounded distribution lines of the DC model and

the AC model with the transmission line inductance effect. There is

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121

little difference between these two regulation curves. The regulation for
the DC model was 0.9% and for the AC model was 1.2%.

Figures 49 through 52 show the transformer bank operating
conditions of the AC network model with the distribution line
inductance effect. Figure 49 and Figure 50 show the magnitude of the
primary current and secondary current of each transformer. For each
residential transformer, primary current was 1.2 A and secondary
current was almost 30 ( 29.6 -- 29.9 ) A. For each farm transformer, the
primary current was 2.1 A; and secondary current was almost 60 (59.2 --
59.7) A. The ratio of the primary current magnitude to that of
secondary was not strictly 1 / 30 because the iron core excitation currents
were neglected.

Figure 51 and Figure 52 show the primary current and secondary
current phase angles of each transformer. It was assumed that the
voltage source phase angle was zero degrees. For each residential
transformer, the primary current phase angle was within the range -27.5
-- 27.3 degrees; and the secondary current phase angle was close to zero
(-0.3 -- 0.1) degrees. For each farm transformer, the primary current
phase angle was within -15.3 -- 15.1 degrees; and the secondary current
phase angle was also close to zero ( -O.4 -- 0.2 ) degree. Both the
residential and farm transformer secondary current phase angles were
close to zero degrees due to the pure resistance loads assumed.

Figures 53 through 55 present the neutral-to-earth voltage
simulation curves resulted from the two different connection styles of

primary and secondary neutrals of the distribution system. In one case

Figure 49.

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126

the primary and secondary neutrals were bonded together, and in the
other case the neutrals were separated.

Figure 53 shows the the primary neutral-to-earth voltage profiles
with primary and secondary neutrals bonded and separated in normal
no fault condition. It can be seen that the voltage profiles of the two
different neutral connection styles were identical.

Figure 54 shows the the primary neutral-to-earth voltage profiles
with primary and secondary neutrals bonded and separated in 10 A
secondary in-phase fault attached to node 33. It can be seen that the
voltage profile of the separated neutral case was different from the
voltage profile of the neutral bonded connection style. From node 11
(near the first farm transformer) toward the end of the line, the
neutral-to-earth voltage decreased significantly. At node 18 (near the
second farm transformer) the voltage with bonded neutrals was 1.9 V,
40% of the 4.7 V of the separated neutral connection case. At node 32
(near the fourth farm transformer in the middle of the line) the voltage
with bonded neutrals was 3.4 V, 42% of the 8.0/8.1 V of the separated
neutral connection case. At node 57 (near the seventh farm
transformer at the end of the line) the voltage with bonded neutrals was
6.7 V, 70% of the 9.5/9.6 V of the separated neutral case. From the
substation to node 10 the neutral-to-earth voltage was slightly increased
by less than 1.1 V for the bonded neutral case as compared with the
separated neutral case.

Figure 55 shows the primary neutral-to-earth voltage profiles with

primary and secondary neutrals bonded and separated in 10 A

VOLTAGE (v)

Figure 53.

 

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bonded and separated in no fault condition (AC model with

line inductance effect)

Figure 54.

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Profile of the neutral-to-earth voltage along the primary distribution
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line from substation to 57th node with primary and secondary neutrals
bonded and separated in 10 A secondary out-of-phase fault attached
to node 33 (AC model with line inductance effect)

130

secondary out-Of-phase fault attached to node 33. It also can be seen
that the voltage profile of the separated neutral connection and the
voltage profile of the neutral bonded connection were significantly
different. The voltage change trend of the latter was similar to the
secondary ground fault DC simulation in the same operational
condition mentioned in 5.1.9 in Figure 34. From node 9 (near the first
farm transformer) toward the end of the line, the neutral-to-earth
voltage increased significantly. The most significant voltage increase
occurred at node 33 where the fault was attached. At this node (near
the fourth farm transformer in the middle of the line) the voltage with
bonded neutral was 14.8 V, 179% of the 8.3 V of the separated neutral
connection case. At node 18 (near the second farm transformer) the
voltage with bonded neutral was 7.2 V, 153% of the 4.7 V of that with
neutral separated. At node 57 (near the seventh farm transformer at
the end of the line) the voltage with the bonded neutral was 12.1 V,
127% of 9.6 V of that with neutral separated. From the substation to
node 8, the neutral-to-earth voltage was slightly decreased by less than
1.0 V compared with the case with primary and secondary neutral
separated.

Prothero et al. (1988) concluded from their field measurements
that the common practice of solidly bonding primary and secondary
neutrals was consistent with the goal of minimizing primary
neutral-to-earth voltage on rural feeders. Figure 54 was consistent with
this conclusion when the secondary side had a large current in-phase

with primary side current. On the other hand, from Figure 55,

131

theoretical analysis indicated when the secondary side had abnormally
high fault current out-of-phase with the primary current, the local
primary neutral-tO-earth voltage could increase.

From Figure 53 through Figure 55 it also can be seen that
removing the bond or separating the ground connection between the
primary and secondary neutrals can prevent secondary faults from
affecting the original primary neutral-tO-earth voltage distribution

system.

VI. CONCLUSIONS

A computer simulation was developed and used to study the
neutral-to-earth voltage profile along a single-phase, radial distribution
line with different normal and abnormal operating conditions. The

conclusions drawn from this study are:

1. The DC circuit model developed in this research is sufficiently
valid to predict the changing trends of the neutral-to-earth voltage
profile along a single-phase primary distribution line.

2. Neutral-tO-earth voltage caused by a high resistance segment in
the neutral conductor of a primary distribution line will be
greatest in the local area of the high resistance segment and will
decrease in magnitude as the distance increaSes away from the
high resistance segment, and the magnitude of this
neutral-to-earth voltage is also dependent upon the level and
location of loads with respect to the abnormal resistance in the
neutral.

3. A high resistance connection between the transformer primary
neutral terminal to down ground and the primary neutral
conductor, in the middle Of the distribution line, will result in an

elevated level of neutral-to-earth voltage when load is drawn at

132

133

the transformer. No significant neutral-tO-earth voltage changes
occur elsewhere along the distribution line.

A phase-to—neutral fault on a primary distribution line is the same
as placing a large load at that location on the line, and will result
in a local increase in the neutral-tO-earth voltage.

When a primary phase-to-earth fault occurs anywhere along the
line, the neutral-to-earth voltages increase in line segments near
the substation and decrease for line segments from the middle to
the end of the line with the most significant changes near the
substation.

A high substation resistance-to-earth for a single-phase radial
distribution line will result in an increase in neutral-to-earth
voltage along the line near the substation when load current is
carried on the primary neutral.

Lowering the resistance-to-earth of the single-phase distribution
line may not result in a significant lowering of the neutral-to-earth
voltage of the line, and usually the lowering is most significant in
the local area where the resistance-to-earth was reduced.
Increasing the primary distribution line operating voltage resulted
in a decrease in primary line current and thus the neutral-to-earth
voltage along the line. The reduction Of neutral-to-earth voltage
was less significant when the voltage increased above 7.2 kV than

when the voltage was increase from a lower value up to 7 .2 kV.

10.

134

A secondary ground fault may cause a significant increase or
decrease in the neutral-to-earth voltage along the distribution line
in the local area of the ground fault depending upon whether the
secondary ground fault current is in-phase or out-Of-phase with
the primary current. The change in neutral-to-earth voltage is
location dependent along the distribution line.

An AC distribution line model operated with line inductance and
an equivalent transformer circuit replacing the load resistor in the
DC model produced a neutral-to-earth voltage profile which was

higher in magnitude but similar in shape to the DC profile.

REFERENCES

REFERENCES

Althouse, J .R. and T.C. Surbrook, 1990. Voltage Gradient Modifications near
Equipotential Planes. IEEE Paper No. 90CH2823-3-A3, Institute of
Electrical and Electronic Engineers.

Aneshansley, D.J., R.C. Gorewit, D.C. Ludingdon and Z. Xin, 1987. Effects
of Neutral-to-Earth Voltage on Behavior, Production and Water Intake in

Dairy Cattle. American Society of Agricultural Engineers, Paper No. 87-3034,
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Appleman, RD. and R]. Gustafson. Source of Stray Voltage and Effect on
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Borcherding, J.R., 1979. A Shocking Story with a Happy Ending. Successful
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Britten, A.M., 1980. Insulate Your Cows from Stray Voltage. Dairy Herd
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Currence, H.D., B.J. Steevens, D.F. Winter, W.K. Dick and GE Krause,
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Gustafson, R.J., Z. Sun and TD. Brennan, 1988. Dairy Cow sensitivity to

Short Duration Electrical Current. American Society of Agricultural
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136

Gustafson, R.J., T.M. Brennan and RD. Appleman, 1985. Behavioral Studies
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Gustafson, R.J, 1985. Understanding and Dealing with Stray Voltage in
Livestock Facilities. IEEE Paper Catalog No. 85CH2126—1—C2, Institute of
Electrical and Electronic Engineers.

Gustafson, R.J., H.A. Cloud, 1982. Circuit Analysis of Stray Voltage Sources
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The Institute of Electrical and Electronic Engineers, 1986. IEEE Guide for
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Kehrle, A.M., 1984. Neutral-to-Earth Voltage Analysis of A Single-Phase
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Norell, R.J., R.J. Gustafson and RD. Appleman, 1982. Behavioral Studies of
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Pearce, J .A., J.D. Bourland, W. Neilsen, L.A. Geddes, and M. Voelz, 1982.
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Volume 5, pp 52-58.

Phillips, D.S.M, 1969. Production Losses from Milking Plant Voltage. New
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Phillips, D.S.M. and R.D.J. Parkinson, 1963. The Effects of Small Voltages
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Prothero, J.N., B.W. Lukecart and CM. DeNardo, 1988. Primary
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Rusch, R.J. and M.L. Good, 1990. Wyes and Wye Nots of Three-Phase
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