AN INTEGRATED OF CONTROL AND PROTECTION SCHEME FOR AC MICROGRIDS

By

Saad Atitullah AlZahrani

A THESIS

Submitted to

Michigan State University

in partial fulfillment of the requirements

for the degree of

Electrical Engineering – Master of Science

2021

AN INTEGRATED OF CONTROL AND PROTECTION SCHEME FOR AC MICROGRIDS

ABSTRACT

By

Saad Atitullah Alzahrani

Numerous advances have occurred in the area of microgrids (MGs) in the last two decades.

Protection is one of the most significant challenges facing the deployment of MGs. With the uti-

lization of renewable energy resources (RES) to reduce emissions and costs, short circuit levels

have drooped in comparison to those produced by conventional generating sources. Therefore,

traditional protection schemes that apply to distribution system are no longer effective in protect-

ing the microgrid against fault currents, either in grid–connected or islanded mode. In microgrid

framework, distributed generation (DG) based RES require an interface of power electronic con-

verters to regulate their output voltage, current, and frequency as well as to share the generated

power properly. Mitigation the impacts of fault currents in microgrid system is an important aspect

of restricting the output current of converters from exceeding their rated value, preventing power

discontinuity, enhancing reliability of protection system and improving the stability of the network.

For microgrid control and protection challenges, several approaches have been proposed in the

last decade. However, the need for efficient and reliable protection schemes of islanded microgrid

system still exists. This research thesis develops and synthesizes an adaptive integration of control

and protection framework for MGs. The proposed strategy is based on detecting the faults and

limiting the fault currents for short periods until the protection devices make a proper decision.

A single state observer has been developed to detect the faults that occur within protection

zones. Moreover, fault current limiter (FCL) devices have been utilized to achieve rapid switching

with instant reduction of fault current contribution. The adaptive integrated protection proposed in

this research is achievable using either centralized or decentralized control in MGs. The proposed

framework has been applied to islanded microgrid configuration and is demonstrated to be an

effective means to protect the system and maintain the voltage and frequency within acceptable

range with the capability of power continuity during both transient and persistent faults.

Dedicated To

My parents, my wife and my lovely sons

iii

ACKNOWLEDGMENTS

First and foremost, I would like to convey my sincere gratitude to my advisor, Prof. Joydeep Mitra.

This research could not have been done without his continuous advice and assistance. Prof. Mi-

tra patient guidance, constructive critiques, expansive knowledge, and superior research approach

have been invaluable during this research process. I also would like to thank my committee mem-

bers, Dr. Bingsen Wang and Dr.Shanelle Foster for their helpful classes, valuable feedback, and

being members of my committee.

I sincerely acknowledge the funding sources that helped me to complete my M.S. research. I

was funded by King Khalid University in Saudi Arabia, and the Saudi Arabian Cultural Mission to

the USA.

I would also like to thank my colleagues in the ERiSe lab for their help and valuable friend-

ship, Dr.Saleh Almasabi, Mr.Atri Bera, and Mr.Khalil Sinjari. I would like to express my special

appreciation to my closest friend Mr. Ibrahim Allafi for his support, collaboration, and suggestions.

Last but not least, I express my sincere thanks and gratitude to my parents, my brothers, my sis-

ters, my lovely sons, and my wife for their constant support, understanding, and patience through-

out the duration of my studies.

iv

TABLE OF CONTENTS

.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1
2
2

LIST OF TABLES .

.

LIST OF FIGURES .

.

.

.

.

Chapter 1 Introduction .

.

.

.

.

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.

.

.

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.

.

1.1 Objective and Motivation .
1.2 Organization of the Thesis

.

.

.

.

.

. .

2.3

Introduction .

Frequency ride-through (FRT)

2.4 Control System in Microgrids

. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.1
2.2 Renewable Energy Sources in Microgrids

Chapter 2 Microgrid System: Operation, Control and Protection Background . . . . .
.
. . . . . . . . . . . . . . . . . . . . . .
2.2.1
Photovoltaic (PV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2 Wind Turbine (WT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3 Energy Storage System (ESS)
. . . . . . . . . . . . . . . . . . . . . . . .
IEEE 1547 Standards for Interconnection of RES . . . . . . . . . . . . . . . . . .
2.3.1 Voltage disturbance ride-through (VRT) . . . . . . . . . . . . . . . . . . .
2.3.2
. . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . .

4
4
5
5
6
6
7
8
8
9
2.4.1
Power Converter Control in Microgrids . . . . . . . . . . . . . . . . . . . 10
2.4.2 Hierarchical Control Methods of Microgrid System . . . . . . . . . . . . . 14
Primary Control .
2.4.3
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4.4
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Secondary Control
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.4.5 Tertiary Control .
2.5 Protection System in Microgrids . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.5.1 Voltage Based Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.5.2 Adaptive Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.5.3 Differential Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.5.4 Distance Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.5.5 Over-current Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.5.6 Advanced Protection Techniques
. . . . . . . . . . . . . . . . . . . . . . 21

Chapter 3 Effectiveness of Solid State Fault Current Limiter on Operation and Pro-

.

.

.

.
.

. .

Introduction .

tection of Microgrids
.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.1
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.2 Development of FCLs Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.3 Principle of Fault Current limiters FCLs . . . . . . . . . . . . . . . . . . . . . . . 24
3.4 Basic Control Method of FCLs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.5
Impact of FCLs in Islanded Microgrid Protection . . . . . . . . . . . . . . . . . . 27
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.6 Case Study and Results .
. 30
3.7 Conclusion .
.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

. .

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.

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v

. .

.

.

.

Introduction . .

Chapter 4 Control and protection Integration Scheme Using State Observer and FCLs

4.2.1 Voltage and Frequency Regulation . . . . . . . . . . . . . . . . . . . . .
4.2.2

35
4.1
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.2 Consideration of Control and Protection in Microgrids . . . . . . . . . . . . . . . . 36
. 36
Fault Currents Contribution of Inverters . . . . . . . . . . . . . . . . . . . 38
4.3 Adaptive Control and Protection Scheme . . . . . . . . . . . . . . . . . . . . . . 39
. 39
. 43
4.4 Distributed Generation Control in Isolated AC Microgrids . . . . . . . . . . . . . . 45
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.5 Simulation of Test system .
4.6 Results and Discussion .
.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.6.1 Case Study 1: Low Impedance Transient and Persistent Fault . . . . . . . . 50
4.6.2 Case Study 2: High Impedance Transient and Persistent Fault . . . . . . . . 54
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

State Observer Model . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Proposed Framework . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.7 Conclusion . .

4.3.1
4.3.2

. .

.

.

.

Chapter 5 Conclusion and Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

BIBLIOGRAPHY .

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. 58

vi

LIST OF TABLES

Table 2.1: LVRT capability requirements

. . . . . . . . . . . . . . . . . . . . . . . . .

9

Table 2.2: Frequency Ride Through . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Table 3.1: Test System Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Table 4.1: IEEE 1547 Voltage and Frequency Standard [1]

. . . . . . . . . . . . . . . . 38

Table 4.2: Test System Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

vii

LIST OF FIGURES

Figure 2.1: Basic Configuration of Microgrid . . . . . . . . . . . . . . . . . . . . . .

Figure 2.2: Configuration of PV System Model

. . . . . . . . . . . . . . . . . . . . .

Figure 2.3: Configuration of Wind Generation Model

. . . . . . . . . . . . . . . . .

.

Figure 2.4: Configuration of Energy storage Model (Battery)

. . . . . . . . . . . . . .

5

6

6

7

Figure 2.5: Basic control structure in a three-phase grid-forming power converter.

. . . 11

Figure 2.6: Basic control structure in a three-phase grid-feeding power converter.

. .

. 12

Figure 2.7: Basic control structure of three-phase grid-supporting power converters(operating
as a current source) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 2.8: Basic control structure of three-phase grid-supporting power converters(operating
as a voltage source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 2.9: Hierarchical control structure of the AC microgrid . . . . . . . . . . . . . 15

Figure 3.1: Structure of FCL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. 25

Figure 3.2: FCL under normal status and fault status . . . . . . . . . . . . . . . . . . . 25

Figure 3.3: Simple Power system Network . . . . . . . . . . . . . . . . . . . . . . .

. 26

Figure 3.4: Simple Power system Network with FCL . . . . . . . . . . . . . . . . .

. 27

Figure 3.5: Basic Control of FCLs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Figure 3.6: Single DG Connected to FCL Based Islanded Microgrid . . . . . . . . . . 28

Figure 3.7: Representation of single DG Islanded Microhrid with FCL . . . . . . . . . 29

Figure 3.8: System Under Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Figure 3.9: System under study in case of using conventional switch (No FCL)

. . .

. 32

Figure 3.10: Fault Current in zone protection (No FCL) . . . . . . . . . . . . . . . . . . 32

Figure 3.11: System under study in case of using FCL . . . . . . . . . . . . . . . . .

. 33

Figure 3.12: Three phase current in zone protection . . . . . . . . . . . . . . . . . . .

. 33

viii

Figure 3.13: Load voltage during the fault with FCL . . . . . . . . . . . . . . . . . . . 34

Figure 4.1: Single protection zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Figure 4.2: State Observer Model

. . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Figure 4.3: Proposed protection Framework . . . . . . . . . . . . . . . . . . . . . . . 44

Figure 4.4:

proposed control and protection scheme . . . . . . . . . . . . . . . . . . . 45

Figure 4.5:

basic structure of power electronics interfaced with DGs

. . . . . . . . . . 46

Figure 4.6:

Inverter voltage control mode

. . . . . . . . . . . . . . . . . . . . . . . . 47

Figure 4.7:

Inverter current control mode

. . . . . . . . . . . . . . . . . . . . . . . . 47

Figure 4.8: Protection algorithm using DG control mode

. . . . . . . . . . . . . . . . 48

Figure 4.9: Microgrid Under Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Figure 4.10: Control logic of proposed scheme . . . . . . . . . . . . . . . . . . . . . . 50

Figure 4.11: Phase output current in case of low impedance fault

. . . . . . . . . . . . 51

Figure 4.12: Output current in case of FCLs failure . . . . . . . . . . . . . . . . . . . . 51

Figure 4.13: Voltage at bus 1 in normal case (No fault) . . . . . . . . . . . . . . . . . . 52

Figure 4.14: Voltage at bus 2 in normal case (No fault) . . . . . . . . . . . . . . . . . . 52

Figure 4.15: Voltage at bus 1 in faulted case . . . . . . . . . . . . . . . . . . . . . . . . 53

Figure 4.16: Voltage at bus 2 in faulted case . . . . . . . . . . . . . . . . . . . . . . . . 53

Figure 4.17: Load output current in case of low impedance fault

. . . . . . . . . . . . . 54

Figure 4.18: Residual and phase A currents in case of high impedance fault

. . . . . . . 55

ix

Chapter 1

Introduction

Microgrids (MGs) have been shown to be a reliable and efficient solution to provide energy to

remote communities. The configuration of microgrids is generally referred to an area of an electric

distribution network that embeds a palpable number of distributed energy resources (DERs), in

addition to local loads. Microgrids are managed and controlled by an intelligence and would be

capable of operating in two operation modes: isolation from the utility grid (islanded mode), as

well as in connection with the main grid (grid-connected mode). In some cases, microgrids could

be forced to switch to the autonomous operation mode, for example, due to the occurrence of

a fault in the host grid, or it could deliberately disconnect itself from the utility, for instance, if

economics warrant the islanded mode of operation [2].

Regardless of isolated or islanded mode, microgrids have been shown to be a reliable and effi-

cient solution to provide energy to remote communities or those with no access to electricity [3].

However, frequency and voltage fluctuation are major challenges in such remote microgrids due to

the intermittent nature of renewable resources like wind and solar PV. In comparison to the large

interconnected systems, an isolated microgrid has a smaller size but could have abnormal condi-

tions like faults and disturbances which lead to power interruptions, loss of stability, and blackouts.

Due to these cases, a smart control and protection play an important role in maintaining safe op-

eration of the microgrid system. Control of power electronics, interfacing DERs, and switching

1

devices (e.g Circuit breaker) in the network should be coordinated to achieve voltage and frequency

requirements to keep those variables in acceptable range according to IEEE-1547 standard.

1.1 Objective and Motivation

Microgrid configuration required a significant change in the control and protection methodologies

compared with conventional distribution systems. In the last few decades, there has been significant

research on the topic of microgrid design, control, and protection. However, many strategies and

methods have focused on designing power electronic interfaces which enable connection of DERs

to the distribution system, while some others, constitute advances in traditional protection schemes

that rely on detecting the faults and isolating or disconnecting the equipment. In this research, we

propose an adaptive control and protection framework that performs detection based not only on

the fault type and location but also its influence on the power continuity of the system. Single state

observer and FCLs have been integrated to detect and limit the fault currents providing achievement

of adaptive protection and continuity of power serving to the loads.

1.2 Organization of the Thesis

The content of this thesis is divided into five chapters, which are summarized in the following:

Chapter 2 presents the foundations and principles of operation, control, and protection back-

ground of microgrids. In this chapter, a comprehensive background of microgrid control level, as

well as protection devices have been briefly illustrated.

Chapter 3 focuses on the effectiveness of utilizing FCL devices in protraction systems of mi-

crogrids. The location of FCLs plays a crucial role to limit the contribution of fault current based

DGs and increases the reliability and stability of the microgrid system.

In Chapter 4, an adaptive integrated control and protection scheme based on a single state

observer and FCLs have been proposed and implemented in various cases. The implementation

results carry out the feasibility and effectiveness of proposed methods in order to protect the mi-

2

crogrid against transient and persistent faults. Additionally, the integrated framework maintains

the voltage and frequency within acceptable limits.

Chapter 5 provides concluding remarks and avenues for future research.

3

Chapter 2

Microgrid System: Operation, Control and

Protection Background

2.1

Introduction

Renewable energy sources (RES) are being increasingly used to reduce greenhouse gas emissions,

as well as to improve efficiency and reliability of microgrids [4]. Microgrid framework has local

loads and a variety of RESs such as photovoltaic (PV) arrays, wind turbines (WT), micro-turbines

(MT) and energy storage such as battery, fuel cell (FS), and super capacitor (SC).

The RES could be integrated off-grid to provide power to local loads, known as islanded mode

of operation. Further, they would be connected to the grid through the point of common cou-

pling (PPC), which is known as grid-connected operation. However, under any mode of operation,

microgrid-based RES has to be designed with proper control and protection to achieve the require-

ment of normal operation condition in voltage and frequency boundaries according to IEEE-1547

standards.

In this chapter, a brief background of microgrid operation, control, and protection has been

demonstrated in the following subsections. The basic microgrid configuration system has been

illustrated in Figure. 2.9 [5].

4

Figure 2.1: Basic Configuration of Microgrid

2.2 Renewable Energy Sources in Microgrids

Renewable sources of energy are becoming the most important sources for supplying electrical

energy straight to the customer without traditional distribution systems, especially for communities

living in far areas where grid extension is difficult.

RES are the most sustainable remedies for producing energy and heat. The main advantages of

renewable energy generation are the instant availability, less dependence on fossils fuels, low-cost

variation, and no transportation cost with higher economic efficiency. In addition, the capability to

operate under the different modes of grid-connected and islanded is convenient [6, 7].

2.2.1 Photovoltaic (PV)

In a microgrid, PV arrays are connected through power electronic interfaces to regulate their output

power and voltage by inverting the DC voltage to AC voltage. The DC-DC chopper converts

fixed DC voltage/power to variable DC voltage or power with maximum power point tracking and

constant efficiency. Figure 2.2. shows the general configuration of PV connected to AC bus.

5

MVLVDCACACACACACACDCACACDCDCACLCLCLCMCCHPBatteryFlywheelFuel CellMCMCPVWindMCLCLCMCMGCCMGCCTransformerACACMCSwitchesFigure 2.2: Configuration of PV System Model

2.2.2 Wind Turbine (WT)

The wind turbine system is one of the most known and used source of microgrids. It produces

electrical energy from wind speed after converting it from mechanical energy by the generator [8].

Permanent magnet synchronous generator (PMSG) is the most popular machine connected to the

wind system due its reliability, efficiency, low cost and small size.

A wind generation system connected to power electronics interfaces to support local loads

through a DC-AC rectifier and DC-AC inverter is shown in Figure.2.3.

Figure 2.3: Configuration of Wind Generation Model

2.2.3 Energy Storage System (ESS)

Energy storage devices play a significant role in the transition to efficient and reliable energy

sources for power sustainability [9]. In microgrids, due to the variability of DER, energy storage

6

Inverter ControlInverter ControlMPPTMPPTAC BusvPVInverterDCDCDCDCACDCACDCChopperiPVRest of the SystemInverter ControlInverter ControlAC BusInverterACDCACDCRectifierDCACDCACRest of the Systemsystems must be able to provide the amount of power required to balance the system following

disturbances and load changes.

Many types of energy storage devices(ESD) have been integrated into microgrid applications

such as flywheels and batteries. They are modeled as constant DC voltage sources using power

electronic converters to be interfaced with the electrical network (AC/DC/AC) converters for fly-

wheels and DC/AC inverters for batteries [5]. Figure 2.4 shows the basic configuration of battery

system which is connected to AC bus through (DC/AC) inverter.

Figure 2.4: Configuration of Energy storage Model (Battery)

2.3

IEEE 1547 Standards for Interconnection of RES

Microgrids are mainly covered by the distribution network and RES standards. In low or medium

voltage, high penetration of RES distribution systems reduces the dependence and control capabil-

ity of a centralized conventional generator, which has significantly different operating characteris-

tics than power grids.

In the bulk power system, microgrids are treated as negative loads. However, voltage and

frequency of system are regulated by the synchronous generators [10, 11]. IEEE published IEEE

Std. 1547-2018 (revision of IEEE Std 1547-2003) for DERs interconnection and interoperability

with existing power systems. IEEE Std.1547-2018 defines two main categories of RES, A and B

based on performance requirements, such as voltage and frequency regulation, and another three

sub-categories of RES I, II and III based on disturbance ride through (low-voltage ride-through)

7

soC CalculationsoC CalculationAC BusInverterACDCACDCRest of the SystemBattery Management Controlibat(LVRT). Hence, microgrids can be considered as a group of RES, the requirements stipulated in

the IEEE Std. 1547-2018 also applicable to microgrids [1, 12, 13].

2.3.1 Voltage disturbance ride-through (VRT)

The voltage disturbance ride–through requirements are not applicable if the frequency is outside

the specified ride–through range. The RES should have the capability to provide specified voltage

disturbance ride–through while the RES is within the capability limits. Therefore, the voltage dis-

turbance ride–through thresholds and the clearing times must be specified over a limit of allowable

settings according the data given in Table 2.1 [12].

The RES current exchange discontinuation with energy system is not more than the maximum

particular time and with no delay. That does not necessarily imply RES disconnection, isolation or

a trip but may include momentary cessation or trip.However, if the voltage disturbance is existing

within the mandatory operation region, the RES functionalities must meet three conditions: 1)

synchronize with the power system, 2) continue to exchange current with the power system and 3)

neither trip nor cease to energize. In the permissive operation region, the RES would maintain the

synchronization with the power system and it will not trip. Moreover, when the voltage disturbance

is within the continuous operation region of any duration, it will not cause the RES to cease to

energize or trip from the power system. Therefore, it will keep continuously delivering the power

[12].

2.3.2 Frequency ride-through (FRT)

In case of disturbance, the frequency ride–through requirements such as under frequency thresh-

olds, over frequency thresholds, and critical clearing time during abnormal conditions for category

I, II, and III RES are illustrated in Table.2.2. [12]. It is considered that the nominal system fre-

quency is 60 Hz.

Basically, the power system operator specifies the values of maximum clearing times and the

over frequency/under frequency thresholds within a range of allowable settings.

In both cases

8

Table 2.1: LVRT capability requirements

Voltage Range

Operating mode

ride through time

ride through time

(pu)

V >1.20

Cease to energize
1.75<V ≤ 1.20 Permissive operation
1.15<V ≤ 1.75 Permissive operation
1.10<V ≤ 1.15 Permissive operation
0.70<V ≤ 0.88 Mandatory operation
0.50<V ≤ 0.70 Permissive operation
Cease to energize

V <0.50

(s)
Min

-
0.2
0.5
1.0
4s/pu
0.16

-

(s)
Max
0.16

-
-
-
-
-

0.16

of under frequency trip functions (under frequency-1 and under frequency-2), under frequency-2

is utilized as the limiting requirement. Similarly, for the two over frequency trip functions (over

frequency-1 and over frequency-2), over frequency-2 is utilized as the limiting requirement. There-

fore, there are various distinct operating regions in the frequency tripping requirements.

• f >62.0

Ride-through requirements.

• 61.2< f ≤ 61.8

• 58.8 ≤ f ≤ 61.2

• 57.0 ≤ f ≤ 58.8

Mandatory operation.

Continuous operation.

Mandatory operation.

• f <57.0

No ride-through requirements.

2.4 Control System in Microgrids

The frameworks of microgrids operate under different conditions in two modes of operation. Con-

trol systems of microgrids should be designed to manage microgrids under both modes of opera-

tion. In on-grid mode, the control system monitors and controls the voltage, power, and frequency

9

Table 2.2: Frequency Ride Through

Trip

Over frequency-1
Over frequency-2
Under frequency-1
Under frequency-2

f

(Hz)

62
61.2
85.5
56.5

Clearing Time Range settings

(s)

0.16
300
300
0.16

(V)

Voltage
61.8 to 66
61 to 66
50 to 59
50 to 66

(s)
Time

0.16 to 1000.0
180 to 1000.0
180 to 1000.0
0.16 to 1000.0

for utility grid and microgrids. While in off-grid, the control system monitors and controls the

voltage, power, and frequency of the microgrid only. Further, control systems should be designed

to protect the microgrids against abnormal conditions such as faults and disturbances.

Most of RES are controlled using converter control system. Microgrids contain converter-based

RES that behave differently than conventional generators. Therefore, RES based converters should

be modeled depending on the operation mode of the microgrid system. In grid connected mode,

it should be modeled as a constant power source, and a voltage regulator in the case of islanded

mode.

2.4.1

Power Converter Control in Microgrids

Depending on the mode of operation, converter operation can be classified into three categories,

grid-forming, grid-feeding, and grid-supporting [14, 15]. In this section, a brief description of the

operation for all types will be demonstrated.

• Grid-Forming Power Inverters

The grid-forming power converters are controlled in closed-loop to work as ideal AC voltage

sources with reference set-points of voltage amplitude and frequency. As voltage sources,

they present a low output impedance. Therefore, they need an accurate synchronization

method to operate in parallel with other grid forms. In a microgrid, the AC voltage sup-

plied by the grid–forming power converter to be utilized as a reference for the rest of the

10

grid-feeding power converters. Figure.2.5 shows the construction of a controller for a grid–

forming power converter. The inputs to the control system are the voltage and the frequency

of the source to be formed by the power converter at the point of common coupling (PCC).

The external loop controls the grid voltage to match its reference value, while the internal

control loop regulates the current supplied by the converter. Therefore, the controlled cur-

rent flowing through the inductor charges the capacitor to maintain the output voltage close

to the reference provided to the voltage control loop. Generally, in industrial applications,

these power converters are fed by stable dc voltage sources driven by batteries, fuel cells or

another primary source.

Figure 2.5: Basic control structure in a three-phase grid-forming power converter.

• Grid-Feeding Power Inverters

Grid-feeding power converters are controlled as current sources, which presents a high paral-

lel output impedance. They are suitable to operate in parallel with other grid-feeding power

converters in grid-connected mode. generally, most of the power converters belonging to

DG systems operate in grid-feeding mode, as in PV or wind turbine systems [16]. The

grid-feeding power converters can be participated in the control of the microgrid AC voltage

amplitude and frequency by adjusting, at a higher level control layer, the references of active

11

DGioVoltage ControlVoltage ControlCurrent ControlCurrent ControlPWMLoadPCCLfCfvovovreffrefvabcirefioInverterPLLØ and reactive powers to be delivered. For stand-alone microgrid, grid–feeding power convert-

ers cannot operate in this mode if there is no grid-forming, grid-supporting power converter,

or a local generator to set a reference of the voltage amplitude and frequency for the system.

A typical control structure for an AC grid-feeding power converter is shown in Figure.2.6.

Maximum power point tracking (MPPT) controllers or power plant controllers are usually

implemented to regulate the operation of the grid feeding converters to set reference values

for voltage and frequency.

Figure 2.6: Basic control structure in a three-phase grid-feeding power converter.

• Grid-Supporting Power Inverters

A grid-supporting power converter is controlled as a voltage source with a link with series

impedance, or alternatively, as a current source with a parallel impedance. In either mode,

the main objective of grid-supporting is to participate in the regulation of the AC grid voltage

amplitude and frequency by controlling the active and reactive power delivered to the grid.

Two main types of power converters can be found within the grid-supporting group:

The power converter is controlled as a current source, as in Figure.2.7. The main objective of

this type is not only to supply the load connected to the microgrid, but it should additionally

12

DGioPower ControlPower ControlCurrent ControlCurrent ControlPWMLoadPCCLfCfvovoPrefQrefvabcirefioInverterGridbe adjusted to contribute to regulate the voltage amplitude and frequency of both the AC grid

and the microgrid.

Figure 2.7: Basic control structure of three-phase grid-supporting power converters(operating as a

current source)

2) The power converter is controlled emulating the behavior of an AC voltage source, as

shown in Figure.3.4. It is connected to the grid through a link impedance, like in the sim-

plified scheme of a synchronous generator. In such a control scheme, the active and reactive

power delivered by the power converter is a function of the AC grid voltage, the AC voltage

of the emulated voltage source, and the link impedance. This kind of converter can partici-

pate in regulating the amplitude and the frequency of the grid voltage in both grid-connected

and island modes, with no need for connecting any grid-forming converter in the microgrid.

13

DGioPower ControlPower ControlCurrent ControlCurrent ControlPWMLoadPCCLfCfvovoErefQrefvabcirefioInverterDroop ControlDroop ControlvoPrefwrefFigure 2.8: Basic control structure of three-phase grid-supporting power converters(operating as a

voltage source

2.4.2 Hierarchical Control Methods of Microgrid System

The AC microgrid is the most commonly used microgrid configuration as the existing power net-

works are operating at AC, while the DC microgrid is getting the attention due to advantages such

as increasing the utilize of DC-based loads and sources, no synchronization.

Different control strategies are adopted for optimal energy management of microgrids. The

most important control objectives are stability, power balance, synchronization, and protection. To

fulfill these objectives advanced control architecture is required. Mainly, hierarchical control archi-

tecture is employed in AC microgrids. This control architecture has three level control schemes, 1)

primary control, 2) secondary control, and 3) tertiary control.A hierarchical control structure level

of the microgrid is illustrated in Figure.2.9 and detail descriptions are presented in the subsequent

subsections.

14

DGioVoltage ControlVoltage ControlCurrent ControlCurrent ControlPWMLoadPCCLfCfvovoErefQrefvabcirefioInverterDroop ControlDroop ControlvoPrefwrefigigvoFigure 2.9: Hierarchical control structure of the AC microgrid

2.4.3 Primary Control

Primary controller controls voltage and current of the DERs and energy storage system ESS in-

terfacing converters. To maintain stable voltage and frequency, optimal power management and

power-sharing of multiple DERs and ESS are essential. Existing literature in Subsection 2.4.1

proposed two converter control modes for the primary control: grid forming and grid following

mode. In primary control level, the inverter control strategy should be selected based on the mode

of operation of the microgrids [17, 18].

In grid following mode, the utility grid maintains stable voltage and frequency at the microgrid

while the DER and the ESS are operating in current-controlled voltage source inverter (CCVSI) for

maximum power generation and charging mode respectively. In grid forming mode, the microgrid

15

Utility GridTransformerPCCDERDCACDERDCACDCACVSCVSCVSCFeederAC BusLoadESSLoadTertiary ControlSecondary ControlPrimary ControlMGCCData CommunicatiomControl signalControl signalis isolated from the utility grid, and multiple DER and ESS would maintain stable voltage and

frequency of both the AC and the DC buses. Thus, all DERs operate in voltage control voltage

source inverter (VCVSI) and the ESS operate in discharging mode. Depending on the number of

DERs participating in voltage control, this control method is further subdivided into two categories:

• A single DER interfacing converter operates in grid forming mode and is maintaining stable

voltage and frequency, while other DERs operate in CCVSI.

• More than one DER interfacing converters operate in grid forming mode and are maintaining

stable voltage and frequency. Proper synchronization is required for the DERs interfacing

converters, which are operating in grid forming mode.

2.4.4 Secondary Control

The secondary control method is employed to compensate the DC bus voltage deviation in the

voltage and frequency deviations in the AC microgrid. This controller also performs black-start

and synchronization after mode switching.

The secondary control is mainly divided into two categories: 1) centralized control and 2)

decentralized control. In the centralized control method, a global controller known as the microgrid

central controller (MGCC) performs power management in the microgrid. However, to maintain

the exact power balance, MGCC acquires the active and the reactive power information from DER,

ESS and critical loads. Moreover, it has a functionality of security and energy market operation

management. A communication infrastructure is required in this control scheme [19].

In the decentralized control method, there is no central controller, and the MGCC control

function is implemented into the local controllers. Therefore, DER and ESS are responsible for

power management. As there is no global controller however in case of a fault, the microgrid can

maintain a safe operation by isolating the faulty part from the rest of the microgrid system [20].

16

2.4.5 Tertiary Control

This control approach is applied in the grid forming mode. Therefore, this method controls real

and reactive power flows between microgrid and the utility grid to regulate voltage and frequency.

In addition, tertiary control method can also be implemented either in centralized or in distributed

manner.

In centralized management, active and reactive power are measured at the PCC, while set–point

active and reactive power are calculated based on the microgrid power requirements and energy

market operations. In this way, power quality, efficiency and economic operation are ensured.

In contrast to the distributed control, the tertiary control level is implemented in the main grid

rather than at the microgrid MGCC. However, a microgrid architecture with the tertiary control ap-

proach is proposed with distributed control. In this tertiary control approach, the consensus/gossip

algorithm helps to gather global information, and the optimization algorithm is used to find the

local optimal decision which compensates power quality issues [21].

The tertiary controller generates a compensation signal for the RES local controller to improve

power quality at the local bus. This control approaches have two communication links, one is ded-

icated for consensus/gossip-based tertiary control and the second link is dedicated to the primary

controller [17, 22].

2.5 Protection System in Microgrids

The basic framework of microgrid system has been designed to operate in two modes, utility–grid

connected and stand-alone modes. Microgrid would be with When there are no disturbances in the

utility network, microgrids should be integrated with the utility grid. Otherwise, microgrids work

in islanded mode. The switching between two modes guarantees the power continuity to customers

without any interruptions. however, there will be a challenges for protection system to be effective

in both operation modes.

17

2.5.1 Voltage Based Protection

Voltage–based protection methods involve voltage measurements to protect microgrids against var-

ious disturbances. The basic function of this approach is to monitor distributed generation (DG)

output voltages and convert their values into DC quantities using the d-q reference frame. The

voltage–based protection methods can protect microgrids against faults at any location in the net-

work. A communication channel is utilized in the scheme to distinguish which faults are in or out

of the protection zone. Although distances within a microgrid are mostly not long, communication

links could use Ethernet, pilot wires or optical fibers [23].

Upon an occurrence of fault, the faulty zone would be identified and will be isolated if the

fault voltage exceeds the threshold that corresponds to the fault type. The communication links are

working to transmit the measured voltage between the protection relays to compare these values

with the set– reference in the relays. Therefore, the faulted zone in the network would be identified

properly [24].

2.5.2 Adaptive Protection

Adaptive protection approaches enable different microgrid configurations to be protected against

more types of faults. In this scheme, relay settings for different setups are stored in a database.

Therefore, when the configuration gets changed, the relays will be modified in the database with

the updated settings [23, 25].

A MGCC is connected to directional over-current relays at each bus by using a communica-

tion system. In this approach, the analysis would be implemented offline by making action and

event tables for circuit breaker statuses. So, the settings are then applied to many other microgrid

frameworks. During online operating, the MGCC monitors the state of operation of the microgrid

and use the information from the action and event tables to configure the relays. Measured current

values will be compared with the settings of the relays to detect the occurrence of a fault. The

direction of fault current is tested in contrast to the present interlock direction to find fault location.

The characteristics of the adaptive protection scheme are adaptable to various configurations of mi-

18

crogrids. Hence, they provide protection against all types of faults and the communication system

increases the speed of operation. In contrast, this scheme is not appropriate for the large network

of microgrid because of the excessive memory required to store all of the off-line data [25, 26].

2.5.3 Differential Protection

The basic concept of differential zone protection is based on comparing the currents entering and

leaving the protected zone. A number of relays and sensors utilize for each protection zone. Cur-

rent sensors are positioned on the secondary side of transformers for every load and relays located

at the source location of distributed generation. As soon as the difference between entering and

leaving current exceeds a predetermined value, the protection system will operate. Therefore, the

fault is detected and the relays will send a trip signal to the circuit breakers at the faulty zone [27].

Although differential protection schemes can act as fast as 5ms and successfully overcome the

challenge of changing fault current level, the need for protective devices at each line ends may

increase the cost. Hence, a genetic algorithm (GA) is usually applied to find the optimal placement

for sensors, relays, and circuit breakers to minimize the total cost [28].

This type of scheme needs a wired communication infrastructure for exchanging the data of

current measurements. Moreover, it requires synchronized measurements and unbalanced load

transient periods still pose a serious problem [29].

2.5.4 Distance Protection

Distance protection uses an idea of admittance or impedance measurements to effectively identify

and detect faults. The faults can be detected with inverse–time characteristics relay based on mea-

sured admittance of the line segment. In a microgrid system, this scheme of protection has the

capability to detect the faults in both modes of operation: grid–connected or islanded mode. De-

spite the utilizing of inverse–time characteristics relay to each single protection zone, the distance

protection approach is able to isolate the external faults that happen on either side of protected

zone. Therefore, it can operate in the case of forwarding or reverse faults [30, 31].

19

The main advantage of distance protection is that it is not affected by variation of current

levels in case of switching between the operation modes, as it mainly depends on the measured

admittance or impedance. Moreover, it can be coordinated with other relays to work effectively in

the microgrid network. However, distance protection has vital problems. In distribution system,

the small value of impedance is challenging to detect the fault. In terms of performance, loss of

accuracy is another drawback with fundamental extraction due to harmonics, current transients,

decaying DC magnitude, and time constant [32].

2.5.5 Over-current Protection

Over-current protection protects microgrid system against excessive currents which are caused

by short circuits, ground faults, and overload. Although coordination of over current protection

ensures that the microgrid system can be protected securely during both modes of operation, device

selectivity should be considered for suitable operation [33].

There is also an instantaneous over current protection approach that is based on two routines

that can perform instant protection for local line and remote buses regardless of their locations

in the network [34]. Over current protection schemes may benefit from utilizing a communica-

tion assisted protection selectivity strategy that has different levels which are applied with voltage

restrained directional over current protection [35].

In a low voltage microgrid system, There is another protection strategy that using micropro-

cessor based over current relays that is not requiring communication links and sufficient for both

modes of operation.

The main drawback in these types of protection schemes are usually related to the need for a

communication channel. Hence, In the case of a failure in the communication system, the entire

protection system may be endangered [28]. Moreover, Over current protection, is not capable to

trip the fault in a reasonably short time due the different levels of fault currents.

20

2.5.6 Advanced Protection Techniques

In distribution system and microgrids, the fault current levels are considerably different between

the grid–connected and islanded operation, usually when an inverter-interfaced distributed genera-

tion. Therefore, design a reliable protection scheme that operates in either mode of operation poses

a real challenge. In this scenario, there are several opportunities to apply different methods and

approaches that manage the fault current level when the microgrid switches between both modes of

operation. Using a new switching technology by installed external devices is a promising solution

to detect and limit fault currents at any operating conditions. These devices can either decrease the

contribution of fault level or isolated the faulted area [36, 37].

Some strategies have been applied to reduce the combined contribution of many distributed

generations that change the fault current level enough to go beyond the limitations of various

system equipment. These methods guarantee a sufficient level of coordination regardless of the

feeding effect of distributed generations.

21

Chapter 3

Effectiveness of Solid State Fault Current

Limiter on Operation and Protection of

Microgrids

3.1

Introduction

In general, fault currents have a major effect on the stable operation of power system. It may causes

an equipment damage, voltage dip or high drop to the frequency. In microgrids, fault currents

increase due to fault contribution from DGs, utility grid contributions at the same time decrease.

Once microgrid is islanded, the short circuit level may drop to low values that may not pick up by

over current relay. Thus, conventional protection devices utilized in conventional power system

are not longer appropriate and reliable for modern microgrid protection. In addition, most of DGs

interfaced with power inverters which have different characteristics and behaviors. At any case

of disturbance, contribution currents of inverter should not exceed 1.5 to 2 of their rated currents.

Otherwise, isolated of DGs must be placed to protect the inverter against stress or damaging. The

islanded microgrids usually are connected to critical load. So, maintaining power continuity of

DGs during the fault poses a major challenge.

22

Many challenges have been faced to limit the significant impact of the fault current. The most

challenges originate from the fact that microgrids differ in their topology, power electronics con-

verter control, generation types, feeder sizes, power flows direction and fault interruption devices

types and locations.

For developing microgrid operation and protection, many research has been concentrated on

this issue and several possible solutions have been suggested. In [38], a comparative survey of

emerging technologies of fault current limiter devices of power distribution system has been stud-

ied. This survey summarizes the principles and operational of a variety of fault current limiters and

explores their key features and advantages. Therefore, modern power switching technologies uses

static fault current limiters, which provide a feasible and viable solution to reduce and solve the

issues caused by high fault currents in the system [39]. However, optimized location, cost and co-

ordination of these devices with protection relays causes a major challenge. Existing technologies

of fault current limiting are very promising for protection of transmission and distribution systems.

• Limit the first peak of a fault current.

• Low impedance when the protected circuit operates normally.

• Extending the service life of system equipment.

• Fail-safe, high reliability and long lifetime. Avoiding updates to the transmission and distri-

bution facilities.

• Result in considerable savings for the utilities.

• Compact and light weight.

In this chapter, the impact of using fast- acting switches of fault current limiting in islanded micro-

grid have been demonstrated. FCLs have connected next to inverter based DGs in order to avoid

interruption of DGs and maintain the continuity of service by reducing the contribution of fault

current fed by inverters. The performance of using FCLs instead of conventional mechanical cir-

cuit breakers has been tested on small system, and several scenarios have been implemented. The

23

results of FCLs performance under different fault condition are comparing with result of using a

conventional circuit breakers.

3.2 Development of FCLs Devices

The development of FCLs devices is introduced here.

It proves that the FCLs utilization can

effectively present electric system performance. Therefor, the outcome of this chapter can be

generally demonstrated application of FCLs in microgrid protection system.

FCLs have being developed over the past few decades, based on different physical principle

sand technologies. The art statuses of FCL equipment are presented in [40].Classifying by the

main principles, FCLs devices mainly consist of superconducting fault current limiters (SCFCLs),

magnetic fault current limiters and solid-state fault current limiters (SSFCLs) [41].

Additionally, there are also some other kinds of FCLs based on power electronics switches

which are applied to direct current (DC) systems. Alternatively, FCL can be classified into in-

ductive FCLs and resistive FCLs by their impedances. Unlike reactors or high-impedance trans-

formers, FCLs limit contribution fault currents during fault conditions without adding redundant

impedances to the circuit during normal operations.

3.3 Principle of Fault Current limiters FCLs

A FCL is a variable impedance device that is connected in series with a circuit to limit the over

current under fault condition. It has low impedance (ideally negligible impedance) under normal

operating condition, and high impedance under fault condition. The basic representation of FCL is

shown in Figure.3.1. The structure of FCLs come in many different topologies depending on sys-

tem requirement. The operational principle of FCLs is that: during a normal condition, controlled

solid-state switches stay on either “Close” status or “Open” status Figure.3.2. In contrast, When

a fault occurs, the switches would be triggered to change their statuses if the fault current exceed

the threshold of set- point value in controller of FCLs. Therefore, the current limiting impedances

24

are shown up in circuits in order to limit the level of contribution fault currents depending on the

applied control scheme. The ZnO varistor is used to limit the over-voltage across FCL devices.

Figure 3.1: Structure of FCL

Figure 3.2: FCL under normal status and fault status

Figure 3.3. illustrates a simple power system network, consisting of a supply voltage, Vs and

impedance Zs and a load impedance Zload.

In normal operating condition, the circuit current io, is given by the equation 3.1:

io =

Vs

Zs + Zload

25

(3.1)

ABVaristorImpedance  semiconductor switchABVaristorImpedance  semiconductor switchABZfcl=0BNormal StatusZfcl=0AFault StatusZfcl=0BNormal StatusZfcl=0AFault StatusFigure 3.3: Simple Power system Network

If a fault occurs and causes load to be shorted out, the circuit current is given by 3.2:

io =

Vs
Zs

(3.2)

Since the Zs is internal impedance, it is much lower than the load impedance. So, the current during
faults are significantly large compared to normal current. Though, circuit breaker will interrupt this

fault current, it does this immediately, taking about 2-3 cycles to act. Within this span of time, a

heavy inrush of current can damage the components between the supply and load. Therefore,

proper precautions must be taken to ensure the safety of the components and protect them under

fault conditions. The role of a fault current limiter is to prevent damage faster than 2-3 cycles of a

fault current rising.

In the next figure 3.4, the same power circuit with additional FCLs device with impedance Z f cl
which works as a fault current limiter, Z f cl should automatically increase on the occurrence of the
fault and deactivate when the fault get cleared. Ideally Z f cl would equal to zero in the normal
(non-faulty condition) state and equal to Zload (in- faulty condition) when a fault occurs. Even if
the Zs = Z f cl during the presence of a fault, the fault current would be half that without the FCLs

26

VsZsZloadCBioShort Circuitin the network.

Figure 3.4: Simple Power system Network with FCL

3.4 Basic Control Method of FCLs

In application of microgrid system, one important feature of FCLs is to limit excessive contribution

of fault currents to prevent a sudden interruption of DGS and keep the continuity of power pro-

duction. The basic operation of FCLs depends on the comparison between the maximum absolute-

value of the three phase currents and the current threshold. Upon the occurrence of fault, trigger

signal will generate to activate FCLs. The basic logic control of FCL is shown in Figure. 3.5.

3.5

Impact of FCLs in Islanded Microgrid Protection

In order to properly demonstrate the effectiveness of FCLs in microgrid fed by DGs. FCLs have

been connected in series next to power converters immediately to avoid an extreme contribution

of fault currents Figure. 3.6. In conventional distribution system, maximum threshold of fault

currents is determined by the relay setting of the protected system. However in microgrid based

27

ZsZloadVsC.BioShort CircuitZfclFigure 3.5: Basic Control of FCLs

DGs, output current of inverter must not exceed 1.5 to 2 of its rated value as illustrated in expression

3.3.

Max Output Iinv<{1.5 , 2} Irated

(3.3)

The representation of islanded microgrid in normal and faulted condition is shown in Figure 3.7,

Figure 3.6: Single DG Connected to FCL Based Islanded Microgrid

where the Zs, Z f cl, Zload, Zl (In fault case: Zla and Zlb) and Zg represent virtual impedance
of source, FCL impedance, loads impedances, line impedance, and ground impedance at the fault

location, respectively, where the Z f cl is active ted only during faults. The impedances Zs, Z f cl,

28

MaxComparatorTriggerMeasuredIthrIaIbIcIcDCACBus 1Load 2DCACFeederDERFCLLoad 1Bus 2VSCB1B2Protection ZoneZl can be written as:



Zs =| Zs | (cid:54) θs
Z f cl =| Z f cl | (cid:54) θ f cl
Zl =| Zl | (cid:54) θl

where θs , θ f cl and θl are the phase angles of the corresponding impedances.

Figure 3.7: Representation of single DG Islanded Microhrid with FCL

In the normal condition, the output current io of DG can be calculated as:

| Io |=

Vo

Z f cl + Zload1 (cid:107) (Zl + Zload2)

< Ithr

In this case the Z f cl = 0 is not active. Hence, the output current become:

| Io |=

Vo

Zload1 (cid:107) (Zl + Zload2)

< Ithr

(3.4)

(3.5)

(3.6)

In the other hand, the output current in case of fault depends on the fault type and location in the

network. Figure 3.6, demonstrates the ground fault location that placed on the feeder section. In

29

Bus 1ZfclBus 2ZsZload1Zload2ZgZlaZlbVsifioi1i2DER+-vothis scenario, the output current will extremely increase and the zone protection system will send
a trigger signal to activate the FCL ( Z f cl (cid:54)= 0). Therefore, the output current in faulted case
calculated as:

| Io |=| I f |=

Vo

Z f cl + kZl + Zg

> Ithr

(3.7)

Where the output here is equal to peak of fault current. kZl is the line impedance, where k represent
ratio of impedance value which depends on the fault location.

In order to identify the Z f cl value and avoid the interruption of DG and instability issue, the
impedance of FCLs should be design properly. As mentioned previously in 3.3, the output inverter

currents should be not exceeding their boundary. Hence, depending on the output current, Z f cl
has two states:

0,

if Io<{1.5 to 2} Irated

(3.8)

Z f cl =

1, otherwise

In this configuration system, instantaneous value of output current should be monitored in order to

control the trigger signal of FCL. So, In case of fault the FCL will be activated at the maximum

value and calculated with expression:

| Z f cl |=

Vo(re f )

Imax(rated)

(3.9)

Where, Vo(re f ) is the output nominal voltage in normal condition and Imax(rated) is the maxi-
mum output current of inverter.

3.6 Case Study and Results

The case study of islanded microgrids with a single DG has been implemented to verify the effec-

tiveness of FCLs to the protection system. Figure 3.8 shows a single line diagram of a small three

phase microgrid system. The studied system has a distributed PV generator, two buses connected

with a distribution feeder, and two local loads at each bus. The PV system operates in maximum

30

power point tracking (MPPT) and is connected to a power inverter that controls in a voltage source

mode. A FCL has been connected in a series with the inverter to keep the output current of the

inverter within the acceptable limits. It has been controlled to switch in two states depending on

the system condition.

Figure 3.8: System Under Study

The protection system relies on centralized protection control (CPC) which is employed to

send and receive data through the communication channel.Current sensor provide an instant data

of output current while the two relays protection are communicated with the CPC to exchange the

data of protection zone and send a trip signal to the circuit breakers. The system parameters are

illustrated in Table 3.1. A single line to ground fault has been applied in the feeder segment and

the simulation results have shown the performance of FCL in a loading and faulted case.

31

DCACBus 1Load 2DCACFeederPVFCLBus 2VSCB1B2Protection ZoneCentralized Protection Control (CPC)FioZlCommunication ChannelR1R2Current SensorLoad 1Table 3.1: Test System Parameters

Frequency

Based Voltage
Line impedance

Load1
Load2

60 Hz
380Vl−l

0.01+j0.12 Ω

10 kW
15 kW

Figure 3.9: System under study in case of using conventional switch (No FCL)

Figure 3.10: Fault Current in zone protection (No FCL)

32

DCACBus 1Load 2DCACFeederDGSWLoad 1Bus 2VSCB1B2Protection ZoneioilifFigure 3.11: System under study in case of using FCL

Figure 3.12: Three phase current in zone protection

3.7 Conclusion

An Integration of an external FCLs device to protect the system in islanded microgrid was pre-

sented in this chapter. Since the FCLs were connected in a series next to the power inverter, the

fault current contribution would not exceed the max output current at any events. The centralized

protection control (CPC) utilized a communication channel to exchange the data with current sen-

sor, relay protection, and FCL. An implementation of a single DG islanded microgrid system was

33

DCACBus 1Load 2DCACFeederDGFCLLoad 1Bus 2VSCB1B2Protection ZoneioilifFigure 3.13: Load voltage during the fault with FCL

tested and the simulations are carried out to validate the effectiveness and feasibility of the FCL to

the protection system.

34

Chapter 4

Control and protection Integration Scheme

Using State Observer and FCLs

4.1

Introduction

The increasing trend in integrating intermittent distributed energy resources (DERs) into AC micro-

grids presents significant operational challenges in stability and protection. In islanded microgrid-

based DGs, the conventional protection system may not detect all faults due to the small level of

contribution fault currents. In addition, power electronics interface DGs have a different character-

istic and behavior under disturbance event. Upon occurrence of fault, the power converters cannot

exceed their maximum values of output currents. However, the existing protection schemes may

not differentiate between the load and fault currents. Another issue which causes disturbance cases

is that voltage and frequency deviations should be regulated to the specified nominal values in or-

der to guarantee safe and stable operation. For the above reasons, a reliable protection system is

needed that would adapt to the different level of faults.

This chapter presents an integrated framework of protection and control to protect the islanded

microgrid against any level of faults as well as to maintain the voltage and frequency within accept-

able limits. This approach is based on detecting and mitigating the contribution fault currents until

35

the protection devices take appropriate action. In the proposed approach, a single state observer

has been used as a fault detector since it would not be affected by changes in network topology. To

overcome the limitation of the power converter, a controlled FCLs have been utilized to avoid the

interruption of DGs. To achieve the integration protection framework, the system has been divided

into multizone protection.Therefore, the single state observer will detect the faults at any zone and

will be coordinated with FCLs to limit the fault currents. Each FCL will be operated in two states

and triggered according to the maximum rated currents of each inverter.

The proposed control and protection scheme provides a good performance during the transient

and persistent fault. It will achieve the power continuity without load interruption. The practical

benefit of FCLs comes at the rapid responding of any required action and their coordination with

protection devices in the network.

This framework has been applied to 4 bus islanded microgrid configuration and demonstrated

effective results in different cases of faults. The proposed approach can maintain the voltage and

frequency within acceptable range with the capability of self-healing during the transient or per-

sistent fault. Furthermore, the results of simulation case studies on MATLAB/Simulink of an AC

Microgrid are analyzed and compared with available results in the literature.

The reminder of the chapter is organized as follows: consideration of control and protection in

microgrids are briefly summarized in Section 4.2. The proposed adaptive control and protection

scheme has been briefly discussed and described in Section 4.4. Section 4.5 demonstrates the sim-

ulation model of the test system and different scenarios of faults provided with a brief discussion.

Section 4.6 provides some concluding remarks.

4.2 Consideration of Control and Protection in Microgrids

4.2.1 Voltage and Frequency Regulation

Voltage and frequency have an essential influence of power system operation. Although, under

normal operation condition, their values must remain within acceptable limits around the nominal

36

values.

In conventional power systems, synchronous power generators set the nominal values of volt-

age amplitude and frequency with proper control and equipment to respond to any variations. By

adjusting the generator field current, the output reactive power of the generator is controlled, which

helps to control the terminal voltage of the generators. In addition, automatic voltage regulator

(AVR) equipment is usually installed at power plants to maintain the voltage levels at the generator

terminals. Frequency control is done to maintain an adequate balance between the consumed and

generated active powers [21, 42].

In applications of microgrids, a cluster of distributed generation systems utilize a green re-

source, such as a photovoltaic system, wind energy conversion system, or energy storage system

(ESS). All these technologies may have variable output power that has to be controlled properly.

Power electronic converter interfaces regulate the output power, voltage and frequency. Since the

microgrid operates in two modes of operation, namely, grid connected mode (GC) or islanded (IS),

different strategies and schemes have been developed to meet the requirement of normal operation

conditions.

Despite the nature of renewable energy resources, voltage and frequency should be restricted

within acceptable limits either in the event of a load change or any unexpected event. Generally, in

GC mode, the main grid governs the operating voltage and frequency of an overall system, while

in IS mode of operation, the microgrid maintains the voltage and frequency to a nominal reference

value and shares the active and reactive powers among all the connected distribution generation

units.

In microgrid configuration, a three-layer control comprises of primary, secondary, and tertiary

control levels. The primary control structure is locally implemented by the droop control method at

each DG unit to stabilize the voltage and frequency disturbances which occur owing to islanding.

Droop control technique emulates the behavior of a synchronous power generator, which drops its

operating frequency with increase in load demand. While the primary control stabilizes the voltage

and frequency of the microgrid, it leads to voltage and frequency offsets. Thus, the secondary

37

control scheme is needed to restore the voltage and frequency to their nominal values. Lastly,

tertiary control optimizes the operating cost and power flow of the microgrid system [43], [33].

IEEE 1547 standard in [1] laid out the voltage and frequency rang values and specified the trip

time as illustrated in Table 4.1.

Table 4.1: IEEE 1547 Voltage and Frequency Standard [1]

Voltage Range

(% of based voltage)

V <50

50 ≤ V <88
110 ≤ V <120

V ≥ 120

Frequency Range

(Hz)

f <59.3 or f >60.5

Trip Time

(s)
0.16
2.00
1.00
0.16

Trip Time

(s)
0.16

4.2.2 Fault Currents Contribution of Inverters

At any disturbance event, distributed generations in microgrid have different characteristics and

behaviors compared to with synchronous generators used in the traditional system. Absence of

inertia in DGs poses a major challenge in control and protection on the framework of the mi-

crogrid. However, many solutions have been proposed in the literature to mitigate the impact of

fault currents. Most of the suggested schemes are developed to improve the control response of

power converters. Therefore, external FCL devices have been used in this research to mitigate fault

currents without updating control of power converters.

38

4.3 Adaptive Control and Protection Scheme

4.3.1 State Observer Model

Fault detection is an important function of the protection system because all systems are subject to

fault events with unexpectedly time. An observer-based fault detection technique has been utilized

in this research. The basic function of the observer is to reconstruct the outputs of the system from

the local measurements with help of the observer and to use the estimation error as a residual value

to detect the faults [44].

A single observer-based fault detection technique is implemented to a multi-zone microgrid.

The observer has been designed based on the instantaneous measurement of voltages and currents

of the protection zones. Therefore, in case of fault, only the faulted zone will draw a residual output

on the observer. Some local measurements of inputs and outputs are needed to design the observer

model. Hence, the microgrid system has been divided into multiple zones. DGs and loads have

been considered out of the zone. However, loads can be inside the zone, but their values should

be taken into account when designing the observer model since the load value, in this case, would

appear in the residual of observer [45].

Figure. 4.1. represents the protection zone of a single feeder in the microgrid system. R and

L are the resistance and inductance of the line feeder between two buses.The shunt impedance

is neglected due to the short length in the microgrid system. The two-end voltage termination

measurements Vi and Vj are defined as input states while the measured line current represents as
an output state.

39

Figure 4.1: Single protection zone

• State Space Representation of Single Protection Zone

In normal operation, the state space model represents as follow:

˙x = Ax + Bu

y = cx

Hence, a circuit of protection zone describe in 4.2

−vi + vR + vL + v j = 0
dt + vi − v j = 0
Ri + L
i− (Vi −Vj)
−R
L

di
dt =

di

L

(4.1)

(4.2)

If x = i; thus, ˙x = di
state space model of 4.1 will be:

dt and the output y = i = x, and input u = vi− v j. Therefore, parameters of

40

DCACBus 1Load 2DCACFeederDG 1FCLLoad 1Bus 2VSCB1B2Protection ZoneDG 2DCACACDCFCLVSCDG 2DCACACDCFCLVSCRLViVj+-+-RLViVj+-+-A =

−R
L
1
B =
L
C = 1

(4.3)

• State Observer Representation

In faulted condition, the observer model is developed as follow:
Let ˆx be the state estimate, and (y− ˆy) is the output error; then, the observer with additive
part as an error is

˙ˆx = A ˆx + Bu + ∆ ˙ˆx

∆ ˙ˆx = k(y− ˆy), where k is the gain, and ˆy = C ˆx. Then, the output error will be

e = y− ˆy
= y−C ˆx
= C(x− ˆx)

The state observer would be

˙ˆx = (A− kC) ˆx + Bu + ky

(4.4)

(4.5)

(4.6)

Figure. 4.2. shows the model of state observer based microgrid protection zone. It can

be noticed that the e residual can be obtained from the difference between estimated and

measured outputs.

Let the state error be ˜x = x - ˆx, then ˙˜x = ˙x - ˙ˆx.

If the real system and observer model

parameters are identical and by using the state equation in (4.2) and state observer equation

in (4.6). Therefore,

˙˜x = (A− kC) ˜x

41

(4.7)

In this model, the error vanishes asymptotically. Using the pole placement method, value
of k should be chosen a such that real part of every [λ (A− kC)] is negative, where λ is an
eigenvalue. Therefore,

−R
L

− K
(A− kC) =
λ I − (A− kC) = 0
λ = (A− kC)

(e) = −CA−1L fl0

Hence, the residual value will be

Where fl0 act on the output error e according to the observer dynamic [SI − (A− kC)]−1.

Figure 4.2: State Observer Model

42

ACBK1/s=Ax+Buy= CInputU(vi, vj)   +.^ x^ x.^ xDx.x.Dx.n+++y^ y^ ^ x^ xError (e)Observer Structurex.x.=Ax+Buy= CxActual SystemOutput y(io)4.3.2 Proposed Framework

In power system applications, microgrid has to control and operate in two different modes: grid-

connected and stand-alone mode. Microgrid in normal case of operation is connecting to operate

in parallel with the grid. However, in such cases of unplanned events such as faults in medium

voltage (MV) networks or by emergency maintenance requirement, microgrid should be isolated

to operate in standalone mode [5].

For all different operating conditions, microgrid should be equipped with a reliable protection

system to protect the equipment against an extreme fault currents and reduce the risk of instabil-

ity in the system. In islanded mode, the protection system becomes more challenging since the

microgrid networks are mostly supplied from distributed generation-based inverters which are re-

stricted with a limitation of output currents. Therefore, in case of faults, the level of fault currents

is small, and the existing conventional protection systems are no longer a proper solution to choose

the small values of fault currents. Additionally, it may not differentiate between the load or fault

currents.

To overcome previous challenges of islanded microgrid protection, adaptive integration of con-

trol and protection framework has been proposed in this research. The framework is based on uti-

lizing a single state observer that operates as a fault detector for multi zone protection. The state

observer has used a local measurement of both end terminal voltages and line currents of each zone

to find an instantaneous value of residual currents (Ie). In case of faults at any zone, the residual
currents appear in the difference between the measured line currents and estimated values-based

observer.

In order to achieve the adaptation feature and to reduce the load shedding and source inter-

ruptions, fault current limiter (FCL) devices, which are demonstrated in Chapter 3, have been

integrated with the state observer. FCL has been connected in series with each inverter-based DG

and has two states of operation. The FCLs are controlled based on the residual values that are

instantaneously received from the observer. Upon the occurrence of fault with 2 cycle time delay,

all FCLs will be triggered to work in state 1 by increasing their impedances to the maximum value.

43

On the other hand, when the fault gets cleared, FCLs will be deactivated to work in state 0 with

minimum value of impedances. The values of impedances in FCL devices are specified according

to the maximum rated currents of each inverter.

Figure. 4.3. shows the structure of proposed protection scheme and the adaptive of control and

protection algorithm has been illustrated in Figure. 4.4.

| Z f cl |=

if Io<[1.5 to 2]Irated

State 0 =| Z f cl |min

State 1 =| Z f cl |max, otherwise

| Z f cl |=

Vo

Iomax

(4.8)

(4.9)

where, Vo is the output inverter nominal voltage in normal condition and Iomax is the maximum
output current of inverter.

Figure 4.3: Proposed protection Framework

44

ACBK1/sAnalog/DigitalAnalog/DigitalTo Protection and Control AlgorithmMeasuredCurrents ioInput Voltagesvi, vj +-  +-.^ x^ x.^ xDx.x.Dx.n+++y^ y^ ^ x^ xError (e)Observer StructureFigure 4.4: proposed control and protection scheme

4.4 Distributed Generation Control in Isolated AC Microgrids

In the framework of the microgrid operation, AC microgrid is mainly integrated by two types

of voltage source inverters (VSI) which represent DGs control in voltage source mode (VSM) or

45

StartIe>IthrEndYes Residual current Ie of observer(n protection zone) NoTrigger FCLs|Zfcl|max = Vo /Iinv.rTime Delay     0                 TIe<IthrNoPersistent faultTrip CB of zoneClose tie lineTime Delay = 0YesTransient faultDeactivate FCLs|Zfcl|min = 0current source mode (CSM). In grid-connected mode, all connected sources would be operated in

current source mode, following the grid reference. However, in stand-alone operation, at least one

source has to be operated in voltage source mode to set the nominal reference for other sources.

This scenario of control strategy can be achieved with different control schemes such as master-

slave and central control. The basic structure of power electronics interfaced with DGs is shown

in Figure. 4.5. L and C are respectively the inductance and capacitance of the output filter.

Figure 4.5: basic structure of power electronics interfaced with DGs

• Voltage Source Mode (Grid Forming)

In this stage, the power inverter is providing a voltage source support to the microgrid and a

set reference voltage for other inverters that operate in current mode of operation. A Multi

loop control strategy with an inner current control loop and an outer voltage control loop is

adopted for the voltage source shown in Figure. 4.6. In this study, a fixed value of voltage

has been considered for simplicity.

• Current Source Mode (Grid Feeding)

In this stage, power inverters in current control mode regulate their output current and supply

specific real and reactive power depending on the system loads. The voltage level in this

source is set up by the inverter which is set as a master in voltage source mode. Figure.4.7

shows the block diagram of current control inverter.

46

LLLCCCvavbvcFigure 4.6: Inverter voltage control mode

Figure 4.7: Inverter current control mode

4.5 Simulation of Test system

A multi-bus islanded microgrid configuration system is considered to verify the proposed frame-

work scheme. As shown in Figure.4.9. A 4 bus radial islanded microgrid system has been con-

nected with 3 feeder segments and supplied with three paralleled DERs connected at bus 1, bus 2,

and bus 3. Each DER is comprised of RES sources, a voltage source inverter, and a FCL device to

limit extreme fault currents. The configuration has a local load at each bus and a tie line (normally

open) between bus 2 and bus 3. The tie line provides an alternative path of power in case of per-

sistent fault at any feeders. DER 1 is operating in a voltage source model to set a reference voltage

for DER2 and DER 3 which are operating with constant output currents. The system parameters

47

KpKi1/s1/Ls+-++irefirefiLiL-Control LoopPlantvrefvrefvovoLs1/Cs-kd-++++PowerControlKpKi1/sCs1/LsPrefPrefQrefQref+-++++ioiovrefvrefirefirefvovo+--+vovoControl LoopPlantFigure 4.8: Protection algorithm using DG control mode

48

StartTrip CBsDG Control ModeTrigger FCLs|Zfcl|max = Vref /Iinv.rTime DelayIe > 0VCMCCMCompare Measured Values with settingsYesYesNovD0=vD0=Di0=i0=Di0=&Go  to StartNoare illustrated in Table. 4.2.

The islanded microgrid network is divided into protection zones.The local loads can be inside

the zone, and their values should be considered when designing the observer since the load values

in this case, would appear in the residual of observer. For simplicity, the loads have been considered

out of zone and the proposed method applied to distribution feeders. Therefore, different cases of

faults have been applied to verify the effectiveness of the proposed framework.

Figure 4.9: Microgrid Under Study

Table 4.2: Test System Parameters

Frequency

DC voltage of PVs

Based kVA

Based Voltage
Line impedance

C
L

Load 1
Load 2
Load 3
Load 4

60 Hz
777
100

380V˙L-L
0.01+j0.12

6 uF
3mH
10 KW
10 KW
15 KW
20 KW

49

DCACBus 1Load 2DCACFeederDG1FCLLoad 1Bus 2VSCB12Bus 4FeederBus 3Feeder Protection ZonesLoad 4DCACACDCFCLVSCB21B34B43Tie line(Normally Open)B14B41F1F2F3Load 3DCACACDCFCLVSCDG2DG34.6 Results and Discussion

The following cases have considered one protection zone to test the validity and feasibility of

the proposed solution. The control logic of proposed scheme is shown in 4.10. However, the

implementation of this approach will provide the same performance at any feeder segment. As

shown in the test system, three phase fault (F1) is applied in the feeder segment between bus 1 and

bus 2 and satisfactory performance has been found as shown below.

Figure 4.10: Control logic of proposed scheme

4.6.1 Case Study 1: Low Impedance Transient and Persistent Fault

A three-phase low impedance fault is applied at t= 0.1 as a transient event. However, the low

impedance fault draws a significant high fault current in the first cycle as shown in Figure. 4.11 but

the residual current has been picked with the observer and disappeared immediately since all FCLs

are triggered to their maximum value of impedances and deactivated with one cycle thereafter.

In this case, the two end circuit breakers of faulted zone and a tie line would not change their

50

Bus iFCLBus jB1B2ioCurrent SensorState ObserverVoltage Sensorvivj+-vivjilIetd≥ IthrirateOR  nIevrefInverterVCMn(vi-vj)Trip TripFCLvrefirateTriggerTriggerBus nTie switchTripBus iFCLBus jB1B2ioCurrent SensorState ObserverVoltage Sensorvivj+-vivjilIetd≥ IthrirateOR  nIevrefInverterVCMn(vi-vj)Trip TripFCLvrefirateTriggerTriggerBus nTie switchTripnormal position due the nature of applied faults. Figure 4.13, 4.14 and 4.15, 4.16 shows the

voltages at bus 1 and bus 2 in pre-fault and during the faults, respectively. The voltage of the

system has slightly dropped which is not required to isolate any DGs according to IEEE standard

in Table. 4.1.

Figure 4.11: Phase output current in case of low impedance fault

Figure 4.12: Output current in case of FCLs failure

51

Figure 4.13: Voltage at bus 1 in normal case (No fault)

Figure 4.14: Voltage at bus 2 in normal case (No fault)

The similar scenario and fault location in previous implementation is applied here but the fault

event applied as a persistent fault. However, the faulted feeder has entered the blocking state with

opening the two end breakers and close the tie line according the control and protection algorithm

process. In this case, alternative path between bus 2 and bus 3 guarantees an adequate value of

52

Figure 4.15: Voltage at bus 1 in faulted case

Figure 4.16: Voltage at bus 2 in faulted case

power to serve all loads in the system. Figure 4.12 shows the extreme high fault current when

FCLs are deactivated during the fault.

53

Figure 4.17: Load output current in case of low impedance fault

4.6.2 Case Study 2: High Impedance Transient and Persistent Fault

The similar scenario and fault location in previous cases is applied here but the fault current applied

with high impedance transient fault. As the inverters output currents draw depending on the load

values, and the loads in this research assume to be fixed, the total outputs currents of all DGs would

not passed the maximum of load currents. Therefore, the residual current would be appear in the

state observer and impedance fault would not cause any miss coordination. Figure 4.18 shows the

residual current and load currents in high impedance case.

The simulation results show the effectiveness of integration framework in this case to be feasi-

ble at any events on feeder segments. In addition, the observer in this framework will be capable

to identify the residual under the load variations since the loads have been considered outside the

protected zone and the protection and control coordination have been achieved under any fault

conditions.

54

Figure 4.18: Residual and phase A currents in case of high impedance fault

4.7 Conclusion

This work proposes an integration consensus based on utilizing a single state observer integrated

with FCLs as a protection framework. This approach addresses the dual objectives of detecting

and limiting the first peak of contribution fault currents at different cases The islanded microgrid

system has divided into multi zones to achieve the selectivity of the faulted zone. A single state

observer is working as a fault detector, and FCLs, on the other hand, have been implemented as

sectional-based GDs to mitigate the fault currents and provide rapid switching during the transient

and persistent faults. Simulation results have shown the framework performance in a loading case

and fault case. Despite the protection purpose in this study, the proposed combination framework

can be utilized to improve the reliability, stability, and power quality of AC microgrids. For future

work, the integration scheme presented in this chapter can be expanded and utilized in different

centralized or decentralized control methods with different renewable resources.

55

Chapter 5

Conclusion and Future Work

In the first part of this thesis, a brief background of existing control and protection in microgrids is

presented in Chapter 2. Traditional control and protection schemes utilized in distribution system

are no longer appropriate for microgrid configuration for several reasons: the distributed genera-

tions in microgrids are interfaced with power converters that have limited output currents. Addi-

tionally, under fault conditions, the low contribution fault current may not be picked up in islanded

mode of operation. Therefore, the FCL device has integrated with centralized protection control

in Chapter 3. It was connected in series next to the power electronics inverter and controlled to

limit the contribution of fault currents. The effectiveness of FCL location was feasible to prevent

interruption of DGs during the faults and support the power continuity to local loads.

Adaptive integration of control and protection framework is developed using a single state

observer as a fault detector for multi-protection zones with integration of FCLs to limit the con-

tribution of fault currents. The state observer representation is developed in Chapter 4. The state

output estimated currents of observer were compared with the measured currents to find the resid-

ual currents in each zone. In Addition, the FCLs that are connected to DGs were triggered with two

states depending on the residual currents in the faulted zone. The proposed framework guarantees

the power continuity to all local loads in transient and persistent faults by utilizing an alternative

path with a controlled tie line. The use of optimal FCLs placement in the proposed integration

56

framework makes cost effective and maintains the voltage and frequency within acceptable limits

according to IEEE 1547 standard.

In future work, the integration scheme presented in this thesis can be expanded to utilize in

different centralized or decentralized control methods with different other renewable resources.

Moreover, the control of FCLs would be developed to integrate with the power converters and to

adapt to the variability of DGs under different conditions.

57

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