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Z-SOURCE INVERTER CONTROL FOR TRACTION DRIVE OF

M.S.

FUEL CELL - BATTERY HYBRID VEHICLES

presented by

Kent Daniel Holland

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Z-SOURCE INVERTER CONTROL FOR TRACTION DRIVE OF FUEL CELL —
BATTERY HYBRID VEHICLES
By

Kent Daniel Holland

A THESIS

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

MASTER OF SCIENCE
Department of Electrical and Computer Engineering

2005

ABSTRACT

Z—SOURCE INVERTER CONTROL FOR TRACTION DRIVE OF FUEL CELL —
BATTERY HYBRID VEHICLES

By
Kent Daniel Holland

Abstract— This paper presents a Fuel Cell Vehicle (FCV) control strategy using the
Z-source inverter to control power from the fuel cell, power to the traction motor, and
State of Charge (SOC) of the battery. The recently presented Z-source inverter is well
suited for many applications, including FCVs. The Z-source inverter utilizes an exclusive
Z—source (LC) network to link the main inverter circuit to the fuel cell (or any dc power
source). A new concept is revealed that substitutes a high voltage battery for one (or
both) of the capacitors from the Z-source network. By controlling the shoot-through duty
cycle we can control the battery voltage (thus controlling SOC) therefore there is no need
for a separate dc-dc converter. By controlling the shoot-through duty cycle and
modulation index we can produce any desired output ac voltage (thus there is no need for
a dc-dc boost converter), regulate battery SOC, and control FC output power (or voltage)
simultaneously. The Z-source inverter has the benefit of enhanced reliability due to the
fact that momentary shoot—through can no longer destroy the inverter (i.e. both devices of
a phase leg can be on for a significant period of time).These facts make the Z-source
inverter highly desirable for use in hybrid electric vehicles, as the cost and complexity is
greatly reduced when compared to traditional inverters. Although the Z-source inverter
could be utilized in any HEV, this paper presents the control strategy and demonstrates
these features for use in a FCV. These new concepts will be demonstrated by simulation

and experimental results.

Dedicated to my Mother
Anja Kaarina Turkki

December 26, 1943 — March 28, 2004

iii

ACKNOWLEDGEMENTS

I first wish to thank my Sister Dr. Karina Holland, congratulations for being the
first doctor in our entire family. I also wish to thank my Mother, I know how proud she
was of our academic accomplishments, I wish she could be there to see us graduate again.
My biggest thanks goes to my Father, who motivated me to start and continue my
education, and also to find direction in education. By his teaching me love and respect for
the great outdoors and the environment, it was easy for me to choose to apply my
education towards alternative vehicles, so that I would make a difference, and protect the
environment that we all take for granted.

I also wish to thank my advisor Dr. Peng, and to all that work in his lab at

Michigan State University, thank you for all the help and sharing your knowledge.

iv

TABLE OF CONTENTS

LIST OF TABLES ............................................................................................................ vii
LIST OF FIGURES ..................................................................................................... viiviii
KEYS TO SYMBOLS OR ABBREVIATIONS ............................................................... xi
1. INTRODUCTION .......................................................................................................... 1
1.1 Conventional Vehicles .............................................................................................. 1
1.2 Hybrid Vehicles ........................................................................................................ 4
1.2.1 Series Hybrid Vehicle ........................................................................................ 5
1.2.2 Parallel Hybrid Vehicle ...................................................................................... 8
1.2.3 Series-Parallel Hybrid Vehicle ........................................................................ 10

1.3 Fuel Cell Vehicles ................................................................................................... 10
1.4 Outline of Thesis ..................................................................................................... 12

2. FUEL CELL VEHICLE POWER ELECTRONICS REVIEW .................................... 13
2.1 Voltage Source Inverter .......................................................................................... 13
2.1.1 Voltage Source Inverter Circuit Analysis ........................................................ 14
2.1.2 Voltage Source Inverter Control ...................................................................... 15

2.2 Current Source Inverter .......................................................................................... 20
2.3 Zsource Inverter ...................................................................................................... 22
2.4 Zsource Inverter Control ......................................................................................... 23
2.4.1 Simple Control ................................................................................................. 24
2.4.2 Maximum Boost Control ................................................................................. 27
2.4.3 Maximum Constant Boost Control .................................................................. 31

2.5 Zsource Inverter Circuit Analysis ........................................................................... 36

3. COMPARISON OF TRADITIONAL INVERTERS AND Z-SOURCE INVERTER

FOR FUEL CELL VEHICLES ........................................................................................ 38
3.1 Switching Device Power Comparison .................................................................... 39
3.2 Passive Component Comparison ............................................................................ 41
3.3 Efficiency Comparison ........................................................................................... 42
3.4 CPSR Comparison .................................................................................................. 44
3.5 Comparison of Traditional Inverters and Z-Source_Inverter for Fuel Cell — Battery
Hybrid Vehicles ............................................................................................................ 45
3.6 Comparison Conclusions ........................................................................................ 47

4. REVIEW OF CURRENT FUEL CELL VEHICLE TECHNOLOGY ......................... 49
4.1 FCV Operating Modes ............................................................................................ 49

4.1.1 Mode 1, medium power ................................................................................... 50
4.1.2 Mode 2, high power ......................................................................................... 50
4.1.3 Mode 3, low power .......................................................................................... 51
4.1.4 Mode 4, regenerative braking .......................................................................... 51
4.2 Current FCV Technology ....................................................................................... 53

4.2.1 Texas Tech University Fuel Cell Vehicle ........................................................ 53

4.2.2 Ford Fuel Cell Vehicle ..................................................................................... 54
4.2.3 Jeep Fuel Cell Vehicle ..................................................................................... 54
4.2.4 Toyota Fuel Cell Vehicle ................................................................................. 55

5. SIMULATION .............................................................................................................. 56
5.1 Simulation Setup ..................................................................................................... 56
5.1.1 Fuel Cell Stack Model ..................................................................................... 57
5.1.2 Battery Model .................................................................................................. 60
5.1.3 Control ............................................................................................................. 62

5.2 Simulation ............................................................................................................... 64
5.2.1 Simulation Case 1 ............................................................................................ 65
5.2.2 Simulation Case 2 ............................................................................................ 67
5.2.3 Simulation Case 3 ............................................................................................ 69

6. PROTOTYPE DEVELOPMENT, AND EXPERIMENTAL RESULTS .................... 71
6.1 Experimental Setup ................................................................................................. 71
6.1.1 Fuel Cell Stack ................................................................................................. 72
6.1.2 Battery .............................................................................................................. 77
6.1.3 Zsource Inverter....................... ........................................................................ 78
6.1.4 Output Filter, and Load .................................................................................... 78

6.2 Experimental Results .............................................................................................. 80
6.2.1 Experiment Case 1 ........................................................................................... 80
6.2.2 Experiment Case 2 ........................................................................................... 82

7. CONCLUSIONS ........................................................................................................... 84
REFERENCES ................................................................................................................. 85

vi

LIST OF TABLES

Table 1. Switching device power comparison example [18] ............................................ 41
Table 2. Required passive components [18] ..................................................................... 42
Table 3. Operation Conditions at Different Power [18] ................................................... 44

vii

LIST OF FIGURES

Figure 1. Typical Internal Combustion Engine (ICE) Vehicle .......................................... 2
Figure 2. Typical Electric Vehicle (EV) ............................................................................ 4
Figure 3. Typical Series HEV [5] ...................................................................................... 7
Figure 4. Typical Parallel HEV [5] .................................................................................... 9
Figure 5. Typical Fuel Cell Vehicle [7] ........................................................................... 11
Figure 6. Traditional voltage source inverter .................................................................... 13
Figure 7. Single phase half-bridge inverter ....................................................................... 14
Figure 8. Simple pulse width modulator [12] ................................................................... 16
Figure 9. SPWM generation ............................................................................................ 16
Figure 10. Saturated Pulse Width Modulation .................................................................. 17
Figure 11. Standard sinewave reference signal at 100% modulation index [14] .............. 19

Figure 12. Reference signal modified with third harmonics, equivilant to standard
sinewave reference signal at 100% modulation index [14] .............................................. 19

Figure 13. Reference signal modified with third harmonics at 100% modulation index

[14] .................................................................................................................................... 20
Figure 14. Traditional current source inverter ................................................................. 21
Figure 15. Zsource inverter for a FCV .............................................................................. 22
Figure 16. Z-source network during the shoot-through state ............................................ 23
Figure 17. Traditional PWM control method ........................................................ ' ........... 25
Figure 18. Modified PWM with shoot through zero states [9] ......................................... 26
Figure 19. Voltage gain of the simple boost control [16] ................................................. 27
Figure 20. Maximum boost control strategy [16] ............................................................. 28
Figure 21. Voltage gain of the maximum boost control strategy [16] .............................. 29

viii

Figure 22. Maximum boost control with third harmonic injection strategy [16] ............. 30
Figure 23. Voltage gain of the maximum boost control with third harmonic injection [136]
........................................................................................................................................... 1
Figure 24. Maximum constant boost control strategy [17] ............................................... 32
Figure 25. Voltage gain of the maximum constant boost control strategy [17] .............. 33
Figure 26. Maximum constant boost control with third harmonic injection strategy [17] 34
Figure 27. Voltage gain of the maximum constant boost control with third harmonic
injection [17] ..................................................................................................................... 35
Figure 28. Voltage stress comparison of different control methods [17] ......................... 36
Figure 29. System configuration using conventional PWM inverter [18] ....................... 38
Figure 30. System configuration using dc/dc boost + PWM inverter [18] ..................... 38
Figure 31. System configuration using the Z—source inverter [18] ................................. 39
Figure 32. Typical fuel cell polarization curve [18] ........................................................ 40
Figure 33. Calculated efficiency of inverters [18] ............................................................ 43
Figure 34. Calculated efficiency of inverters plus motor [18] ......................................... 43
Figure 35. System configuration using a conventional inverter ...................................... 46
Figure 36. System configuration using a DC-DC boosted inverter ................................. 46
Figure 37. System configuration using a Z-source inverter ............................................. 47
Figure 38. Medium power operating mode ....................................................................... 49
Figure 39. High power operating mode ............................................................................ 50
Figure 40. Low power operating mode ............................................................................. 51
Figure 41. Regenerative braking operating mode ............................................................. 52
Figure 42. Schematic of simulation model ....................................................................... 56
Figure 43. Fuel cell polarization curve ............................................................................. 57

ix

Figure 44. Comparison of fuel cell polarization curve and model equation .................... 58

Figure 45. Properties of the fuel cell model ...................................................................... 59
Figure 46. Fuel cell model power versus current graph comparison ............................... 60
Figure 47. Properties of the lithium-ion battery model ..................................................... 61
Figure 48. Battery voltage versus SOC ............................................................................ 62
Figure 49. Maximum constant boost control with third harmonic injection [17] ............. 63
Figure 50. Final simulation schematic .............................................................................. 64
Figure 51. Simulation case 1 ............................................................................................ 66
Figure 52. Simulation case 2 ............................................................................................ 68
Figure 53. Simulation case 3 ............................................................................................. 70
Figure 54. Schematic of experimental medel .................................................................. 71
Figure 55. Linearized, scaled down, 5kW “Typical Fuel Cell Polarization Curve“ ........ 72
Figure 56. Experimental fuel cell model .......................................................................... 73
Figure 57. Experimental fuel cell model polarization curve ............................................ 74

Figure 58. Three phase ac power, circuit breaker, isolation transformer, and variable
transformer, were used to control the voltage level in the experiment ............................. 75

Figure 59. Three phase diode bridge rectifier, and 1.20, were connected in series, to

create the fuel cell model in the experiment ..................................................................... 76
Figure 60. Capacitor bank used to replicate a battery in the experiments ........................ 77
Figure 61. lOkW Zsource inverter prototype ................................................................... 78
Figure 62. Output filter and load, used in the experiments ............................................... 79
Figure 63. Experimental results case 1 ............................................................................. 81
Figure 64. Experimental results case 2 ............................................................................. 83

KEYS TO SYMBOLS OR ABBREVIATIONS

Hybrid electric vehicle (HEV)

Internal combustion engine (ICE)

Electric vehicle (EV)

Zero emission vehicle (ZEV)

Hybrid vehicle (HV)

Fuel cell (FC)

Fuel cell vehicle (FCV)

Fuel cell hybrid electric vehicle (FC HEV)
Pulse width modulation (PWM)
Sinusoidal pulse width modulation (SPWM)
Modulation index (M)

Boost factor (B)

Voltage gain (MB)

Phase A voltage reference (Va)

Phase B voltage reference (Vb)

Phase C voltage reference (Vc)

Upper shoot-through envelope (VP)

Lower shoot-through envelope (Vn)

Time during shoot through (To)

Switching cycle interval (T)
Output ac voltage (I306)

Input dc voltage (V0)

xi

Peak dc-link voltage across the inverter bridge (1%)
Buck—boost factor ( B B )

Capacitor voltage (VC)

Switching Device Power (SDP)

Constant power speed ratio (CPSR)
Shoot-through duty cycle (Do)

Battery terminal voltage (Vb)

State of charge (SOC)

Phase A upper switch gate control signal (Sap)
Phase A Lower switch gate control signal (San)
Phase B upper switch gate control signal (pr)
Phase B Lower switch gate control signal (Sbn)
Phase C upper switch gate control signal (Scp)
Phase C Lower switch gate control signal (Sm)
Upper shoot-through envelope reference signal (mef)

Lower shoot-through envelope reference signal (me)

xii

1. INTRODUCTION

Hybrid electric vehicles (HEVs) have recently become attractive for many
reasons. The US. government is interested in becoming less dependant on foreign oil
reserves. Not to mention that these oil reserves are being rapidly depleted. The typical
consumer is interested in achieving greatly increased fuel efficiency, while maintaining
the performance and conveniences offered by conventional gasoline powered vehicles.
Environmentalists, consumers, and government alike are all interested in decreased
tailpipe emissions [1]. These emissions contain greenhouse gases and other pollutants
that are harmful to humans and thought to be the major cause of global warming [2]. Due
the recent increased gas prices, and the need for reduced emissions and increased

efficiency, the timing for HEV development could not be better.

1.1 Conventional Vehicles
The conventional vehicle uses an internal combustion engine (ICE), which

supplies mechanical power to the wheels, as shown in figure 1. One problem associated
with conventional ICE vehicles is that the typical driver demands quick acceleration,
which in turn requires a more powerful, less efficient engine. The problem is that most of
the time only a small part of the power is used, however the larger engine is always
unnecessarily guzzling gas. Larger engines use more gas because they are heavier, the
displacement of the cylinders is larger, and they usually have more cylinders [3]. The
problem with smaller engines is they may not be able to keep up with traffic as well,
especially when traveling uphill. While millions of automobiles are sold every year, it is

clear that some alternative to the conventional vehicle must be developed.

 

ICE

 

 

 

 

 

 

Mechanical

Tra mi i n
”8 55° Power Flow

 

 

 

 

 

GEAR

 

 

 

 

 

Figure 1. Typical Internal Combustion Engine (ICE) Vehicle

One possible alternative is the conventional electric vehicle (EV). The EV uses
batteries to store and supply electric power to the motor, as shown in figure 2. The main
advantage of the EV is that it is a zero emission vehicle (ZEV). Although there is some
pollution created when the electricity is generated at the power plant, this is insignificant
compared to emissions from conventional ICE vehicles. EVs are also very efficient when
compared to conventional ICE vehicles, only about 20% of the chemical energy in
gasoline is converted into work at the wheels of an ICE vehicle, 75% or more of the

energy from a battery reaches the wheels of an EV [4]. EVs also provide strong

performance, due to the fact that electric motors provide nearly peak power even at low
speeds. However the cost, performance, and convenience of an EV all depend on the
battery. There are several types of batteries commonly used in EVs, advanced lead—acid
batteries, nickel-metal hydride, and lithium polymer batteries [4]. Even the best of these
batteries can store only a limited amount of energy, thus the range of EVs is still limited.
Recharging the batteries is also a problem, although home recharging systems are
available, recharging sites away from home are scarce, and the time required to charge
the batteries can be substantial. While the EV is very promising, currently the drawbacks
outweigh the benefits, and much work needs to be done to make the EV a realizable

solution.

 

 

Battery
Mechanical
Power Flow

 

 

 

 

Electric
Controller Power Flow

 

 

 

 

Motor

 

 

 

 

 

Figure 2. Typical Electric Vehicle (EV)

1.2 Hybrid Vehicles

Currently the most feasible alternative to the conventional vehicle is a hybrid
vehicle (HV). A HV combines two or more power sources in attempt to gain greater
overall vehicle efficiency and reduced emissions. There are many types of HVs, and until
recently they have mainly been large vehicles such as busses, trains, and submarines.
Another more familiar example of a HV is a motorized pedal bike, which combines the
power from a small electric or gas motor along with power from the rider legs. In fact any
vehicle that combines two or more power sources that provide energy for propulsion,

directly or indirectly, is a HV.

A hybrid electric vehicle (HEV) combines an EVs energy storage system, along
with an energy source, typically an ICE. The I-IEV can achieve greater overall efficiency
through the use of regenerative braking. During regenerative breaking an electric motor
acts like a generator, using the wheels to apply torque to the motor to generate electrical
power, this torque in turn slows the vehicle down. The energy created during regenerative
braking is stored in the battery until needed. Electrical storage systems commonly include
batteries, ultra capacitors, or combinations of these. Another energy saving benefit of a
HEV is that the power sources can be optimally controlled such that each source operates
efficiently. For example a typical HEV incorporates a smaller more effiCient ICE
compared to a conventional vehicle. When more power is needed than the ICE can
provide, as in acceleration or uphill travel, the battery delivers the extra needed power.
The ICE may be alternative fuel, such as bio-diesel, methanol, natural gas, or alternatives

to the ICE such as solar panels, and fuel cells.

1.2.1 Series Hybrid Vehicle
HVs can have several different configurations, and many types of power sources.

The series HEV typically uses an ICE to mechanically turn a generator which supplies
electric power to the electric traction motor, while excess power is stored in a battery or
ultra-capacitor, as shown in figure 3. Thus the series HEV can be thought as an EV with
an on board battery charger. This configuration allows the ICE to operate at a maximum
point of efficiency. The ICE runs for extended periods of time at maximum efficiency,
and shuts off when the batteries reach full charge. This also allows the engine size to be
minimized to just satisfy the average power requirements. Also because there are less

mechanical connections, simple packaging and modularization can be achieved.

There remain several problems with the series HEV. First the electric motors and
power electronics need to be sized to provide all of the power to the road, thus the
propulsion system tends to be more expensive. The efficiency of a series HEV also
suffers due to the number of energy conversions, in the form of chemical energy to
mechanical energy to electrical energy back to mechanical energy. Another major issue is
battery life, the batteries available today cannot last for the vehicles expected life,
because of the large number of rapid charge, and deep discharge cycles that they must
withstand during daily driving [4]. In the future it is likely that battery technology will
advance rapidly, and the cost and efficiency of propulsion systems will decrease, which

will make the series HEV more realizable.

 

ICE

 

 

 

 

 

 

Battery

 

 

 

Generator

 

 

 

 

 

 

 

 

Figure 3. Typical Series HEV [5]

Controller

 

 

 

 

Electric
Motor

 

 

 

 

 

Gear

 

 

 

Mechanical
Power Flow

Electric
Power Flow

 

1.2.2 Parallel Hybrid Vehicle
Another common HV configuration is the parallel HV. In the parallel HEV both

the ICE, and the electric motor can independently propel the vehicle or assist each other,
as seen in figure 4. Also the ICE can power the vehicle and recharge the battery using the
motor as a generator. The parallel HEV has several main advantages over the
conventional ICE vehicle, and the series HEV. First in the energy conversion efficiency,
the mechanical connection between the ICE and the wheels reduces the amount of power
conversion from the main energy source. Second the engine and electric motor sizes can
be decreased because of the fact that both can supply power to the wheels
simultaneously. This results in increased fuel economy and reduced pollutant emissions.
However the benefits of a smaller ICE and electric motor may be offset by the

requirement of a complex transmission [4].

 

Mechanical

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Power Flow
Battery

Electric

Power Flow
ICE Controller
. . Electric

Transmissmn
Motor

 

 

 

 

 

 

 

   

 

Gear

 

 

 

Figure 4. Typical Parallel HEV [5]

1.2.3 Series-Parallel Hybrid Vehicle
Series I-IEVs and parallel HEVs can be combined in different ways, commonly

referred to as series-parallel HEVs [5]. In the series-parallel type of hybrid, there are
several possible ways to operate the vehicle, series, parallel, or some combination of
both, which utilizes the advantages of both configurations. Depending on the driving
conditions, the most advantageous operating mode could be selected. The main drawback
is that this configuration suffers from a more complicated structure, and higher cost than
either a series or a parallel does.

Although several automobile companies now have HEVs on the market, it has
been proven difficult to sell cars that cost more, but to the average consumer does not
offer significant and obvious advantages. Experts rank hybrid electric vehicles only as a
temporary solution, before the fuel cell cars and the hydrogen infrastructure are generally

available [6].

1.3 Fuel Cell Vehicles

Fuel cells have achieved global attention as an alternative power source for
HEVs. Fuel cell vehicles (FCVs), are being developed by auto manufacturers, and have
generated interest among industry, environmentalists and consumers. A FCV promises
the air quality benefits of a battery-powered electric vehicle, with the driving range and
convenience of a conventional ICE vehicle. The fuel cell is capable of producing energy
from fuels like hydrogen, methanol, or gasoline, without combustion, or conversion to
mechanical energy between. When using hydrogen as a fuel, the only exhaust will be
water in theory. This technology promises to make use of the advantages of the electric
vehicle drive train without being restricted to short range. Figure 5 shows the typical Fuel

cell hybrid electric vehicle (FC HEV), which consists of the fuel cell, controller, battery

10

(optional) and electric motor [7].

FCVs with regenerative braking and energy storage are clearly the best solution
to provide low (possibly zero) emissions, and long range. However technology is not
ready to combine all promising advantages of fuel cells in one affordable system, and the
infrastructure (i.e. refueling stations) for hydrogen needs to be established. The FCV
appears to be the most reasonable solution for the not to distant future, as the cost of a

fuel cell is reduced, and the hydrogen infrastructure is built.

 

Fuel Cell

 

 

 

Mechanical
Power Flow

 

 

 

Electric

Battery $9 Controller P°Wer F'°W

 

 

 

 

 

 

 

Motor

 

 

 

 

 

Figure 5. Typical Fuel Cell Vehicle [7]

11

1.4 Outline of Thesis
The goal of this research is to develop a prototype of a FCV drive train using the

Z—source inverter. In addition it will be shown that the Zsource inverter can be used for
control of the fuel cell power, battery state of charge (SOC), and traction drive. The
major contributions of this work towards this goal are:

Chapter 2: Reviews the current FCV power electronics technologies, and
introduces their control strategies. The Zsource inverter and its control strategies are also
introduced.

Chapter 3: Compares the Zsource inverter to the current FCV power electronics
technologies.

Chapter 4: Provides an introduction to FCV operating modes. Current FCV
technology will also be presented.

Chapter 5: Will demonstrate that the Zsource inverter can indeed provide the
above mentioned goals, by computer simulations.

Chapter 6: Will further demonstrate that the Zsource inverter can achieve these
goals, and that the Zsource inverter can be implemented in a FCV, through the use of

experiments.

12

2. FUEL CELL VEHICLE POWER ELECTRONICS
REVIEW

FCVs employ an inverter to control the dc power from the fuel cell and battery
and convert the dc power into ac power to be delivered to the motor. In this chapter the
power electronics for a FCV will be discussed. Starting with two traditional inverter
topologies, the voltage source and current source inverters, and finally the Zsource

inverter will be introduced.

2.1 Voltage Source Inverter
Traditionally there have been two inverter topologies; voltage source inverters,

and current source inverters. The voltage source inverter as seen in figure 6 consists of a
dc voltage source, a large dc-link capacitor, and a three phase bridge inverter, which
consists of six semiconductor switching devices. The voltage on the dc side is smoothed

by a capacitor, while the current on the dc side is unsmoothed (contains an ac ripple) [8].

DC-Voltage 3-Phase
Source Inverter To AC
* Load

DCC ::C “'—

 

 

 

 

 

 

 

 

Figure 6. Traditional voltage source inverter

13

The voltage source inverter is the most commonly used inverter, however it
suffers from the following problems. The output ac voltage is limited to below the dc
voltage, therefore an additional dc-dc boost (step-up) converter is needed to obtain an
output ac voltage greater than the dc voltage source [10]. This additional dc-dc converter
will increase the cost and complexity of the system, as well as lowering the systems
efficiency. The voltage source inverter’s reliability also suffers from the problem of
shoot through. Obviously, both devices in any phase leg can never be on simultaneously
because the short circuit would cause huge currents to flow and damage the switching

devices.

2.1.1 Voltage Source Inverter Circuit Analysis
To familiarize the reader to the operation of the voltage source inverter, a single

phase half-bridge inverter, as shown in figure 7, will be examined.

 

 

v,/2 + T1 \ +

 

 

Vs/2 : T2 \

 

 

 

Figure 7. Single phase half-bridge inverter

14

When transistor T1 is ON, a voltage Vs/2 will be applied to the load. Similarly, if
T2 is ON -Vs/2 will be applied to the load. Thus the inverter generates the voltages +
Vs/2 or - Vs/2 at the load. The inverter switches can be digitally controlled in such a way
that the voltages generated at the output can represent an ac sinewave.

A three phase inverter, such as the one shown in figure 6, can be constructed from
three single phase half-bridge inverters in parallel [11]. The output ac voltages are
displaced 120 degrees relative to each other, in order to achieve three phase balanced

operation.

2.1.2 Voltage Source Inverter Control
Many pulse width modulation (PWM) control methods have been developed and

used for the traditional 3-phase voltage source inverter. The traditional V-source inverter
has six active vectors (or switching states) when the dc voltage is impressed across the
load and two zero vectors when the load terminals are shorted through either the lower or
upper three devices. These total eight switching states and their combinations have
spawned many PWM control schemes.

Typically conventional PWM methods are applied to control the voltage source
inverter. Conventional PWM can be achieved using a simple pulse width modulator, as
shown in figure 8. The pulse width modulator produces a logic signal, referred to as
sinusoidal PWM (SPWM), that is used to command the inverters power transistors to turn
on (saturated) and off (cutoff), in such a way as to produce a sine wave at the inverters
output. As figure 8 shows a simple pulse width modulator can be implemented using a
comparator. The comparator is used to compare a reference signal, Vrcf, and a carrier

signal, cha. The comparator produces a logic-level output as shown in figure 9, which

15

is logic-high whenever Vref is greater than cha, and logic-low whenever Vref is less
than Vcarrier [12]. This output is used to control the upper switch in a phase leg. To control

the lower switch the signal is inverted.

Vref

{L +
NW

 

 

- ' SPWM

 

 

Vcarrier

Figure 8. Simple pulse width modulator [12]

 

 

 

 

Figure 9. SPWM generation

The amplitude of the inverters output is controlled by the modulation index,
Which is me divided by che, [11]. Obviously if the amplitude of an is controlled, the

modulation index is controlled, and the amplitude of the inverters output is controlled.

16

The modulation index of the waveforms shown in figure 9 is around .8. If the amplitude
of me (sinewave) is decreased, the modulation index is decreased, the PWM will spend
less time at the logic-high state, and the average of the output signal will be decreased. If
the amplitude of me is increased, the modulation index is increased, the PWM will spend
more time at the logic-high state, and the average of the output signal will be increased.
Using this simple pulse width modulator the maximum modulation index is one, or
100%, that is me can never exceed Vcarrier or the linear relationship between me and the

average output signal will be lost [13], as shown in figure 10.

KTTWRAAAA
iyvvvvyvya

I ‘kk‘a. .- a" I

 

 

 

 

 

Figure 10. Saturated Pulse Width Modulation

A method to increase the modulation index over 100% is third harmonic injection.
Basically third harmonics are added to me which flattens the top of the resulting signal,
allowing the amplitude of Vref to be turned up higher than Vania. Which allows the
modulation index to be increased greater than one, thus the PWM will spend more time at
the logic-high state, and the average of the output signal will be increased.

The maximum available voltage on a motor using a standard three-phase power

17

inverter is around 86% of the dc link voltage [14]. Figure 11 shows the standard sinewave
reference signal, which can achieve 100% (of Venn-er) maximum modulation index.
Adding third harmonics to me decreases the overall amplitude of the resulting reference
signal, that is the resulting reference signal never reaches either 0% or 100% (of Vcam-cr),
Figure 12. This is due to the fact that the minimum of the 3rd harmonic corresponds to
the maximum of the fundamental and vice versa. This allows the fundamental signal
amplitude to be increased up to the point were the modified modulating signal reaches the
100% limit of VOW-6,, as shown in Figure 13. By applying an appropriate coefficient to
the third harmonic component, which is 1/6 of the fundamental [15], the fundamental
amplitude can be increased by 15%.

When considering a balanced three phase system, the third harmonic components
are cancelled out, because a l20-degree phase-shift on the fundamental corresponds to a
360-degree shift for the third harmonic, thus there are sinusoidal voltages (and therefore
currents) on the motor windings, and line to line voltage is 15% higher than with pure

sinewave PWM modulation [14].

18

Sinewave Modulation

 

 

 

 

 

 

 

d)
em:
xm. g
E. \ a
5...! \ ia
a x ,3,
§‘” 5
2 O \L—T

 

 

 

Figure 11. Standard sinewave reference signal at 100% modulation index [14]
3rd Harrnoni c Injection

 

 

 

 

 

 

 

 

 

 

8

g5“ fl“ 5...
5‘ $53
2 - $5
.. (”GE
: “._.:l:
.8 EE§
m o'-
520 EELE
8 V

2° HI

 

 

 

Figure 12. Reference signal modified with third harmonics, equivilant to standard
sinewave reference signal at 100% modulation index [14]

19

3rd Harmonic Injection

 

 

 

 

 

 

 

 

CD
Aim- g
:3: 9m
<13"—
5'” “21359;,
E .- DIE:
C d):
.243- EU!“
a, gag
I—LI.
é
0
III

 

 

 

 

Figure 13. Reference signal modified with third harmonics at 100% modulation
index [14]

The reference signals frequency controls the desired output frequency, the carrier
signals frequency is many times higher and determines the inverter switching frequency
(speed at which the inverters power transistors to turn on and off). The reference signal

can be recovered from the inverters output SPWM by low pass filtering.

2.2 Current Source Inverter
The traditional current source inverter is shown in figure 14. The current source

inverter typically consists of a large dc inductor fed by a dc voltage source (together these
are the current source), and a three phase bridge inverter. The current on the dc side is
smoothed by an inductance, while the voltage on the dc side is unsmoothed (contains an

ac ripple) [8].

2O

DC-Current 3-Phase
Source Inverter To AC
Load

000 ~ L *—

 

 

 

 

 

 

 

Figure 14. Traditional current source inverter

The current source inverter suffers from the following problems. The output ac
voltage has to be above the dc input voltage, thus an additional dc-dc buck (step-down)
converter is needed to obtain an output ac voltage less than the dc input voltage. This
additional dc-dc converter will increase the cost and complexity of the system, as well as
lowering the efficiency. The current source inverter’s reliability also suffers from the
problem of an open circuit. That is, at least one upper switch and one lower switch must
always be on [11]. That is because the inductors current cannot change instantaneously,
the inductor current would momentarily continue to flow through the switching devices,
which have high resistance in the off state, obviously this would cause a huge voltage

spike across the switching devices, which would damage or destroy the switching device.

21

2.3 Zsource Inverter
”5} .g} M}
fuel __ V+
.. 1- -<= 339
stack ~-
JG 1K1} 1K3}
vfidUb - -

Figure 15. Zsource inverter for a FCV

L1

 

 

 

 

 

 

 

 

 

 

 

Figure 15 shows the Zsource inverter for a FCV. The Zsource inverter utilizes an
impedance network (Z network) to link the main inverter circuit to the fuel cell (or any dc
power source). This impedance network provides unique features and advantages over
the traditional inverters. The impedance network consists of two capacitors connected in
an X fashion, and two inductors, to save space a split core inductor can be used. Although
the Zsource inverter can be used for many applications, including dc-ac, ac-dc, ac-ac, dc-
dc power conversions, with any ac or dc source and load, this thesis will focus on a
Zsource inverter for a fuel cell vehicle. That is, for dc-ac power conversion with a fuel
cell as the dc power source, and a motor for the ac load.

The Z-source inverter advantageously utilizes the shoot-through states to boost
the dc bus voltage by gating on both the upper and lower switches of a phase leg. Figure
16 shows the schematic of the Z-source network during the shoot-through state.

Obviously the capacitor voltage is boosted during this state. This is because the current

22

ramps up through the inductors, whose voltages are VI. = L 6%! . Thus inductor voltage

increases, due to the increasing current. The capacitors are in parallel with the inductors
during this state, therefore the capacitor voltage is also increased, which boosts the dc
link voltage during the nonshoot—through states. Therefore, the Z-source inverter can
buck (by lowering the modulation index) and boost voltage to a desired output voltage
that is greater than the available dc bus voltage. In addition, the reliability of the inverter
is greatly improved because the shoot-through can no longer destroy the circuit. Thus it

provides a low-cost, reliable, and highly efficient single-stage structure for buck and

 

 

 

 

 

 

 

 

 

 

 

 

boost power conversion.
.14
——t>I——°—’IYD" a
l n I
II
fuel C1
cell —T—
stack :-
ll C2
I II

Figure 16. Z-source network during the shoot-through state

2.4 Zsource Inverter Control
Conventional pulse width modulation (PWM) methods can be applied to control

the Zsource inverter. The difference between the conventional PWM as previously

explained for the voltage source inverter, and the PWM used to control the Zsource, is

23

that the shoot through zero state is introduced into the PWM. The shoot through zero
state is produced when two switches in any phase leg are both gated on, causing a short
circuit across the Z network. This shoot through zero state is forbidden in the traditional
voltage source inverter, because it would damage or destroy the switching device. The
shoot through zero state provides the Zsource inverter the ability to boost (step up) the dc
bus voltage, by boosting the capacitor voltage, as previously explained. This boosting of
the dc bus voltage obviously can be used to increase the output ac voltage. Clearly the
Zsource inverter has the ability to buck (step down) the output ac voltage by lowering the
modulation index. So a great advantage of the Zsource inverter is revealed, the Zsource

inverter is a buck-boost inverter, provided in one simple stage.

2.4.1 Simple Control
Figure 17 shows the traditional PWM control method as described earlier. In this

case the PWM control is three phase, where Va*, Vb*, and Vc* are the sinusoidal
reference signals. The sinusoidal reference signals can be approximated as straight lines
for a visual aid, because the frequency of the triangular carrier signal is much greater than
the sinusoidal reference signals. So when looking at one cycle of the carrier signal, the
sinusoidal reference signals will not change much. As previously mentioned the control
signals are generated by comparing the reference signals and the carrier signal. Sap, the
control signal for the positive (upper) switch in phase leg a, for instance, is logic-high
whenever Va* is greater than the carrier signal, and logic-low whenever it is less. San, the
control signal for the negative (lower) switch in phase leg a, is simply the inversion of

Sap. The control signals for the other phase legs are generated in the same manner.

24

 

0-..-.. .OI.“I.’I.I O..- ”.‘m....... -CCJ4 0—..4 ............ I vb

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

H San

 

 

 

 

 

 

 

 

 

Figure 17. Traditional PWM control method

When the dc voltage is high enough to generate the desired ac voltage, the
traditional PWM of figure 17 is used. When the dc voltage is not enough to generate a
desired output voltage, a modified PWM with shoot through zero states will be used to
boost voltage [9], as shown in figure 18. The shoot through is introduced into the zero
state because this will not change the function of the PWM. As shown in figure 17, the
zero state (Labeled as V000) is when all of the upper (or lower) switches are gated off
simultaneously creating zero voltage across the load. Introducing the shoot through here
will obviously also create zero voltage across the load, therefore the shoot through does
not change the operation of the PWM. As can be seen in figure 18, the shoot through zero
states (highlighted grey) are evenly allocated into each phase. The shoot through is
created by turning on both switches in any phase leg, for example the grey areas where

both control signals Sap, and San are both logic-high.

25

 

__7,a\ .Va

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

San
[I ' Sbn
I I l Sen

Shoot through zero states

Figure 18. Modified PWM with shoot through zero states [9]

For this simple boost control, the obtainable shoot-through duty ratio decreases
with the increase of the modulation index (M). The maximum shoot-through duty ratio of
the simple boost control is limited to (1-M) [16], thus reaching zero at a modulation index
of one. The thick curve in figure 19 shows the maximum obtainable voltage gain, MB
(where B is the boost factor) versus M, which indicates no voltage boost and no voltage
gain at M=1. The area left of the curve is the possible operation region under the simple
control. In order to produce an output voltage that requires a high voltage gain, a small
modulation index has to be used. However, small modulation indices result in greater
voltage stress on the devices. Using this control method, the voltage stress across the
switches is quite high, which will restrict the obtainable voltage gain because of the

limitation of device voltage rating.

26

 

N DJ J> LII

Voltage gain (MB)

I—fi

 

 

 

 

0 02 (5.4 0.6 0.8 1
Modulation index (M)

Figure 19. Voltage gain of the simple boost control [16]

2.4.2 Maximum Boost Control
Maximum boost control [16] is a control method used to produce the maximum

voltage boost (or voltage gain) under a given modulation index. Reducing the voltage
stress under a desired voltage gain is important to the control of Zsource inverter. Figure
20, shows the maximum boost control strategy. It is quite similar to the traditional PWM
control method as described for the voltage source inverter. This control method
maintains the six active states unchanged, and turns all zero states into shoot-through zero
states. Thus maximum shoot through duty cycle and boost factor (B) are obtained for any
given modulation index (M) without distorting the output waveform. As can be seen from

figure 20, the shoot through state is obtained when the triangular carrier wave is either

27

greater than the maximum curve of the references (Va, Vb, Vc), or less than the minimum
of the references. Compared to the simple boost control, the possible operation region of
this control method is much wider, as can be seen in figure 21, and for any given voltage
gain, a higher modulation index can be used, which means lower voltage stress across the

switches.

V»
5
V»

 

 

 

 

 

_

 

 

 

 

rl:L ‘-|
13
E.
,_I
I
I
I

L]
LIL
L

U

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 20. Maximum boost control strategy [16]

28

 

l
I
I
I
I
l
I
1
|
l
l
I
l
I
I
l
l

    

U)

Voltage Gain (MB)
N

————-—p-—-—————+—-——————4——-—»——-4v——~—————

 

 

I
I
I
I
I
I
I
t
1
I
I
I
|
I
I
l
I
1
I
I
I
I
I
I
I
I
4
I
I
I
l
I
I
I
l
J
I
I
I
I
I
|
I
I
I

I
I
I
I
I
I
I
|
_________ I___—_——-———l
|
|
I
I
|
|
|
I
I

. . El .
0 0.2 0.4 0.6 0.8 1
Modulation Index (M)

 

Figure 21. Voltage gain of the maximum boost control strategy [16]

As previously mentioned third harmonic injection can be used in a three-phase
inverter system to increase the modulation index range. This can also be used here to
increase the range of M so as to increase system voltage gain range. The control scheme
is shown in figure 22. The operation principle is identical to the previous case, the only

difference is that the modulation waveform is changed due to the third harmonics. In this
. . . 2 . . .
control, the maximum modulation index M: T can be achieved at 1/6 third harrnomc

injection [15]. That is, the amplitude of the third harmonics injected is 1/6 of the
amplitude of the reference signals (Va, Vb, Vc). The voltage gain is identical to the

previous control method for the same modulation index. The curve of voltage gain versus

29

modulation index is shown in figure 23, from which we can see that the possible

operation region is extended with the increase of modulation index. The third harmonic

injection does not change the voltage stress benefits, except that the range of voltage gain

is extended.

 

 

ha

 

 

 

 

:L

J"

 

 

 

 

 

J

J
"U
_I

 

 

 

'3.
'_I

 

 

.I

_J

_J
L

.—aI

 

 

Ll
LII—"ll
If
Li

I“
WITH-I

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

513
L1
E.

I

Figure 22. Maximum boost control with third harmonic injection strategy [16]

30

 

 

 

 

 

: : I
s ------- «: ——————— I —————— :
: : I
i E I
a 4 """" ’: “““““ 3“ """ I
E : z I
3 ------- ------- ------- I
o . . I
on : : :
E I l
g 2-------z ------- s------l -------
: : I .
: z 'I :
1 ——————— J. ——————— : ——————— J. i
: : I :
E E I E
0 l : ' . r
0 0.2 0.4 0.6 0.8 1

Modulation index

Figure 23. Voltage gain of the maximum boost control with third harmonic injection
[16]

2.4.3 Maximum Constant Boost Control
The maximum constant boost control method [17] is used to achieve maximum

voltage boost/gain while maintaining a constant boost viewed from the Z—source network
and producing no low-frequency ripple associated with the output frequency. This
maximum constant boost control can greatly reduce the L and C requirements of the Z-
network. Figure 24, shows the control scheme of the maximum constant boost control
method, which achieves the maximum voltage gain while always keeping the shoot-
through duty ratio constant. There are five modulation curves in this control method:
three reference signals, Va, Vb, and V., and two shoot-through envelope signals, Vp and V”.

The upper and lower envelope curves are periodical and are three times the output

31

frequency. When the carrier triangle wave is greater than the upper shoot-through
envelope, Vp, or lower than the lower shoot-through envelope, V.., the inverter is turned to
a shoot-through zero state. In between, the inverter switches in the same way as in
traditional carrier-based PWM control. The boost factor is determined by the shoot-
though duty cycle, and must be kept the same in order to maintain a constant boost. The
basic point is to get the maximum B while keeping it constant all the time. The curve of

voltage gain versus modulation index is shown in figure 25. As can be seen in figure 25,

45

the voltage gain approaches infinity when M decreases to —.

 

 

 

 

Vp
Vb
Va \ VC
0
r
, ~
I T’ - V In: a...”

M n

 

 

j
“I

Sap-LII
Sop'I—I'I_I'|
U

 

 

 

LIL
2

Sop
San‘U—T

S... T
SmLILJLI—‘LI‘II—

 

| LIL
LEI:

__I
II
__J

 

 

'I_I
__J"I__fl
'LITI
L_ _

.1—
_I
___I
"LI II

"I
'I
W

LLL

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

:I
EL
=I

Figure 24. Maximum constant boost control strategy [17]

16

 

__——4

_________

14 ------- --------

12 ------- -------

_--——_—— _-—-_______-____q

 

10 ------- -

_________________________

Voltage gain (MB)
on

---1-—-1--r---I--—

    

-__——---—————_—‘

 

 

 

 

0.5 0.6 0.7 0.8 0.9 1
Modulation index (M)

Figure 25. Voltage gain of the maximum constant boost control strategy [17]

The shoot-through duty cycle is constant and can be expressed as

5-1-1/Ell
T 2

where To is the time during shoot through, T is the time while not in the shoot through
state, and M is the modulation index.

The boost factor B and the voltage gain can be calculated:

 

where 17% is the output ac voltage, and V0 is the input dc voltage.

33

This maximum constant boost control can be implemented using third harmonic
injection. A control scheme of the third harmonic injection control method, with 1/6 of
the third harmonic [15], is shown in figure 26. Using the third harmonic injection a
unique feature can be obtained, only two straight lines, Vp and Vn, are needed to control
the shoot through time with 1/6 (16%) of the third harmonic injected. The voltage gain

versus M is shown in figure 27. The voltage gain can be varied from infinity to zero
. . 3 2 . . .
smoothly by rncreasmg M from —E— to 7:;- wrth shoot-through states (solid curve in

figure 27) and then decreasing M to zero without shoot-through states (dotted curve in
figure 27). The same equations from maximum constant boost with out third harmonic

injection apply, that is for shoot through duty cycle, boost factor, and the voltage gain.

,IVn/I‘A /\ /\ I\
V° .. >I

 

 

 

 

 

IX

 

 

 

Sap-I—I-IJ
smLII_fl_ "I_
8.. 1

Smlr u1_fl_fl_
smr If I‘ ‘
Sml_fl_II_J MI! WI‘IJ‘

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

'—-I,_l

Figure 26. Maximum constant boost control with third harmonic injection strategy
[17]

34

16

 

14

 

 

12

 

10

 

 

 

Voltage gain (MB)
CD

 

—_L—_—-——-——

 

 

 

 

 

 

 

 

 

6 I
. I
2 I
_—_ ‘h
0 —-""’Tf_—
0 0.2 0.4 0.6 0.8 1 1 .2

Modulation index (M)

Figure 27. Voltage gain of the maximum constant boost control with third harmonic
injection [17]

The voltage stresses across the devices with the three different control methods
discussed are shown in figure 28. As can be seen from figure 28, the maximum constant
boost control method will cause a slightly higher voltage stress across the devices than
the maximum boost control method, but a much lower voltage stress than the simple
control method. However, since the maximum constant boost control method eliminates
line frequency related ripple, the passive components in the Z-network will be smaller,
which will be advantageous in many applications such as a FCV [17]. Thus the control
method used in this study will be the maximum constant boost control method with third

harmonic injection.

35

 

9 r l

I

 

 

 

 

 

——-+~—— simple control
8 T“ maximum boost
7 .@ max. const. boost

 

 

 

 

 

 

 

Voltage stress N dc
is u.
x? \ _

 

 

 

 

 

 

 

Voltage gain

Figure 28. Voltage stress comparison of different control methods [17]

2.5 Zsource Inverter Circuit Analysis
As described in [9]. The peak dc-link voltage across the inverter bridge, 17,-, is

expressed as
Where V0 is the dc source voltage, and B is the boost factor resulting from the shoot

through zero state. B can be expressed from the following,

36

Given that the inverter bridge is in the shoot-through zero state for an interval of T ,
during a switching cycle with an interval T.
The peak dc-link voltage is the equivalent dc-link voltage of the inverter. On the

other side, the output peak phase voltage, I7

ac , from the inverter can be expressed as,

v. VO

Where M is the modulation index. This equation shows that the output voltage can be

stepped up and down by choosing an appropriate buck—boost factor, B B .
BB =M *B=(O~00).
'Ihe buck—boost factor B B is determined by the modulation index M and boost

factorB. The boost factor B can be controlled by the duty cycle (i.e., interval ratio) of
the shoot-through zero state over the nonshoot-through states of the inverter PWM.

The capacitor voltage can be expressed as,

37

3. COMPARISON OF TRADITIONAL INVERTERS AND
Z-SOURCE INVERTER FOR FUEL CELL VEHICLES

Three different inverter system configurations are to be investigated: the
conventional PWM inverter, the dc/dc boost + PWM inverter, and the Z-source inverter.
Their system configurations for fuel cell applications are shown in Figures 29, 30, and

31, respectively.

 

SZ
I

 

 

 

 

fuel __.__

cell __I- I: C E
stack 1' t
J lg} _I K1} ”<3:

Figure 29. System configuration using conventional PWM inverter [18]

 

 

 

 

 

 

 

 

 

 

 

fuel
cell

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 30. System configuration using dc/dc boost + PWM inverter [18]

38

 

 

 

 

L1
JG JG JG
fuel __ V+
“‘1 l ‘ C 39
Stack T
1
JG JG JG

- emf- a -

14

 

 

 

 

 

 

 

 

Figure 31. System configuration using the Z-source inverter [18]

3.1 Switching Device Power Comparison
In an inverter system, each switching device has to be selected according to the

maximum voltage impressed and the peak and average current going through it. To
quantify the voltage and current stress (or requirement) of an inverter system, Switching
Device Power (SDP) is used. The SDP of a switching device/cell is expressed as the
product of voltage stress and current stress. The total SDP of an inverter system is
defined as the aggregate of SDP of all the switching devices used in the circuit [18].
Total SDP is a measure of the total semiconductor device requirement, thus an important
cost indicator of an inverter system.

A comparison example for a 50kW fuel cell model shown in figure 32 with the

following specifications is conducted in [18]. The results are given in table 1.

39

0 Fuel cell output voltage at maximum power: 250 V;

0 Maximum fuel cell output voltage: 420 V;

0 Maximum power: 50 kW;

0 Motor power factor at maximum power: 0.9;

0 Output voltage of the boost converter: 420 V;

0 Modulation index of conventional PWM inverter and dc/dc boost + PWM inverter
is 1;

0 Modulation index of Z source inverter is 0.9 at maximum output power. (To keep

the voltage stress of the switches lower than 450 V)

 

1.0

  

0.8 - -------------------------------- ..................

 

 

 

0.0 0.5 1.0 1.5
Current Density (A/omz)

Figure 32. Typical fuel cell polarization curve [18]

40

 

 

 

 

Inverter Total Total
Systems Average Peak
SDP SDP
(kVA) (kVA)
PWM 238 747
inverter
PWM plus 225 528
boost dc/dc
Z source 199 605
inverter

 

 

 

 

 

Table 1. Switching device power comparison example [18]

The Z-source inverter’s average SDP is the smallest among the three while the
conventional PWM inverter’s SDPs are the highest in both average and peak values. The

average SDP also indicates thermal requirements and conversion efficiency.

3.2 Passive Component Comparison
From [18], an example of required passive components at input power of 50 kW

is shown in Table 2 based on Current ripple through inductors <10% of average current,
and the same system specification as in the SDP calculations above, at switching

frequency of lOkHz:

41

 

 

 

 

Inverter
S stems v-r
y 5' E E g 5 > 3 2 E; a 9 2°. < 9
R- B A i: a. < "R S *0 5' 0 *1:
c: 1: o 0 g 0 I” 0 a: "" a:
2g mg an; as 3&0 “£0
0 H V A H A :9. A :0.
as. a a“? 89. 3%2 5°92
conventional 0 N/A N/A 1 106 420
PWM
inverter
dc/dc boost 1 510 200 l 124 420
+ PWM
Z - source 2 384 200 2 115 420
inverter

 

 

 

 

 

 

 

 

 

Table 2. Required passive components [18]

3.3 Efficiency Comparison
Efficiency is an important criterion for any power converter. High efficiency can

reduce thermal requirements and cost. A comparison example is conducted in [18], with
the fuel cell and motor parameters shown in Table 3. The fuel cell model is the one
shown in Figure 32, with [W (per unit) =200 A, and Vp,, (per unit)=420 V. The operation
principle of the inverters are: the conventional inverter is always operating at modulation
index of 1, the dc-dc boost PWM inverter boosts the dc voltage to 420 V, and the Z
source inverter outputs the maximum obtainable voltage while keeping the switch voltage
under 450V. The motor loss is simply modeled as the loss at the stator resistor. The
same motor model is used to calculate the motor loss. The selected devices for loss
calculation are: The switches for the main inverters are FUJI 1PM 6MBP300RA060, the
switch for the dc/dc boost converter is FUJI 2MBI 300N-O60.

Only the semiconductor devices loss is considered. The calculation results are

also shown Figures 33, and 34 respectively.

42

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Inverter efficiency
0.974
I
0.972 ~ 4—7— r __-__. .__. ,._-_
+ Conventional PWM
5‘ 0'97 I inverter
c:
-3 0.968 -C--__._.7/_c:————*\ + dc/dc boost + PWM
E inverter .
m 0.966 Z - source inverter
I
0.964 +-
0.962
0
Power (kW)

 

 

 

Figure 33. Calculated efficiency of inverters [l8]

 

System Efliciency

 

 

0.95

I N + Conventional PWM
09 —~— — :——~-«---~-——~R inverter
' "'1
O 85 i \A + dc/dc boost + PWM

 

 

 

Efficrenc
l

 

 

 

 

 

 

 

 

inverter
0-3 ‘ ‘ ““ " ““ " Z - source inverter
0.75 - ~~ ~-— A:
p
0.7 ' i; I J. i
0 10 20 30 40 50
Power (kW)

 

 

 

Figure 34. Calculated efficiency of inverters plus motor [18]

43

From the above comparisons, the Z source inverter provides the highest

 

 

 

 

 

 

efficiency.
50 kW 40kW 30 kW 20 kW 10 kW
Power Rating 56 kVA 47kVA 38 kVA27 kVA 14 kVA
Fuel cell voltage (V) 250 280 305 325 340
Conventional PWM inverter 209.4 158.5 115.9 77.3 39.7
Motor
current
(A) Dc/dc boost +PWM inverter 124.7 105.6 84.2 59.9 32.1
Z — source inverter 129.5 105.3 81.1 56.2 29.6

 

 

 

 

 

 

 

 

Table 3. Operation Conditions at Different‘Power [18]

3.4 CPSR Comparison
Constant power speed ratio (CPSR), the ratio between the top speed of a motor

and its base speed, is limited mainly by available dc voltage of the PWM inverter. The
fuel cell voltage decreases as the current drawn increases, which greatly limits the
motor’s power output and efficiency at high speed. For the conventional PWM inverter
with the fuel cell model described above, the fuel cell voltage is the dc voltage of the
inverter, which drops to 250 V at 200 A. From the 250 V dc, the conventional PWM
inverter can only yield 170 V to the motor. This low motor voltage limits CPSR and
lowers mechanical output power and efficiency. Both the PWM inverter with dc-dc
boost and Z-source inverter can keep the inverter dc voltage constant at or above 420 V,

which in turn increases CPSR by a factor of 1.68 [18]. In other words, the motor voltage

produced by these inverters is 1.68 times that produced by the conventional PWM
inverter, thus the same motor can output 1.68 times the power than when driven by the

conventional PWM inverter.

3.5 Comparison of Traditional Inverters and Z-Source

Inverter for Fuel Cell - Battery Hybrid Vehicles

As previously mentioned the Z-source inverter has two independent control
freedoms. The following analysis is provided to show that the shoot-through duty cycle
Do, can be used to control the battery terminal voltage Vb (thus controlling SOC), and the

Do and the modulation index M can be used to control the output ac voltage to the motor

170 C. From [9], we have

v,,=VC=1""D°vo lie—=1" 1
1—21)0 and, V0/2 1--2D0

 

 

9

where Vb equals capacitor voltage Vc, and V0 is the fuel cell voltage. The above
equations show that we can independently produce any desired output ac voltage (limited
by the voltage ratings), control the battery terminal voltage (i.e. SOC), and control the
fuel cell voltage (or power), by controlling the shoot-through duty cycle, and modulation
index. The FC HEV uses a battery to assist the fuel cell system output the requested
power to the traction motor, capture excess power from the fuel cell, and to absorb
energy from regenerative braking. The FC HEV using the conventional inverter, Figure
35, must use a bi-directional dc-dc converter to control the SOC of the battery, because
the modulation index is the inverters only control freedom. Also, the conventional

inverter is a buck (step-down) inverter, the output ac voltage is limited below the fuel cell

45

voltage. Because of the wide voltage range of the fuel cell, the conventional inverter

imposes high stresses to the switching devices.

JJG J

 

 

 

 

 

Battery
Bi-dir
fuel T + _—
cell -_—’_— ::7 DC, ”‘0 E
stack DC .

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 35. System configuration using a conventional inverter

'Ihe dc-dc boosted inverter, Figure 36, can improve these stresses, at the price of
higher cost and complexity. The dc-dc boost converter is used to boost (step-up) the
voltage from the fuel cell, to a steady dc bus voltage, and the inverters output ac voltage
is controlled by the modulation index. The system configuration using the dc-dc boosted
inverter typically uses a bi-directional dc-dc converter to control the SOC of the battery

[19].

 

 

 

 

r} Battery J
DC/ Bi-dir
f I _
3's:- D. T no
stack - Boost ’ DC .

 

 

 

 

 

 

 

 

J3}

M}

M}

 

 

 

 

 

 

 

 

1M1} JG JG

Figure 36. System configuration using a DC-DC boosted inverter

 

 

 

 

46

By using the Z-source inverter system configuration, Figure 37, we eliminate the
dc-dc converters. This can be achieved because the Z-source inverter has two
independent control freedoms. By controlling the shoot-through duty cycle we can
control the battery SOC, and by controlling the modulation index we can control the
output ac voltage. This advantage of the Z-source inverter will result in a much more cost

efficient, and power efficient hybrid FCV.

 

 

fuel + x

\$
11 _ I
1K3: 1K1} lg}
our -
Ll

 

 

 

 

 

 

 

 

 

 

v v

Figure 37. System configuration using a Z-source inverter

3.6 Comparison Conclusions
The comparison results show that the Z-source inverter can increase inverter

conversion efficiency by 1% over the two existing systems and inverter-motor system
efficiency by 2 to 15% over the conventional PWM inverter. The Z-source also reduces
the total average SDP by 15%, which leads to cost reduction. Moreover, the constant
power speed ratio is greatly (1.68 times) extended over the system driven by the

conventional PWM inverter. Thus, the Z—source inverter system can minimize stresses

47

and size of the motor and increase output power greatly. It has also been shown that no
dc-dc converters are needed to control the battery SOC. Along with these promising
results, the Z-source inverter offers a simplified single stage power conversion topology

and higher reliability because the shoot through can no longer destroy the inverter.

48

4. REVIEW OF CURRENT FUEL CELL VEHICLE
TECHNOLOGY

4.1 FCV Operating Modes
As seen in figure 38, the system configuration consists of the fuel cell stack, the

battery, the controller (inverter), and the traction motor. The main source of the vehicles
power is the fuel cell. The secondary power source is the battery, which also stores
excess energy from the fuel cell, and from regenerative braking. The four utilized

operating modes are outlined in the following [20].

— :3
Electrical Power

 

 

 

 

 

 

 

 

 

 

Mechanical .
connection connectlon Flow
fl Fuel Cell {1
Battery Inverter

 

 

 

 

 

 

 

 

 

 

 

Motor

 

 

Figure 38. Medium power operating mode

49

4.1.1 Mode 1, medium power
The medium power operating mode, as shown in figure 38, normally occurs after

the vehicle accelerates and the vehicle maintains a fairly constant speed, highway travel
is an excellent example of this. While in the medium power mode, the vehicle traction

motor only receives power from the fuel cell.

4.1.2 Mode 2, high power
The high power mode usually occurs during acceleration, or uphill driving. While

in the high power operating mode, both the fuel cell and the secondary battery provide
power to the traction motor, Figure 39. The battery can also speed up the vehicles
response time for a request of acceleration, because the fuel cell typically has a slow

response time.

 

Fuel Cell r1

 

 

~ __._-

 

 

 

 

 

 

 

Battery Inverter

 

 

 

 

 

 

 

 

 

 

 

 

Motor

 

 

 

 

Figure 39. High power operating mode

50

4.1.3 Mode 3, low power
The low power operating mode, Figure 40, typically occurs at low speed, an

example of this would be driving slowly in heavy traffic. Because of the parasitic loads,
such as the air compressor, associated with the fuel cell, the fuel cell system efficiency
decreases when operated under low power [20]. Thus the vehicle will be operated strictly

as a battery powered electric vehicle under low power.

 

Fuel Cell

 

 

 

 

 

 

 

 

 

Battery anerter

 

 

 

 

 

 

 

 

 

 

 

Motor

 

 

 

Figure 40. Low power operating mode

4.1.4 Mode 4, regenerative braking
Regenerative breaking, Figure 41, usually occurs when the vehicle operator

presses the brake pedal, or when the vehicle coasts downhill. During regenerative

breaking the fuel cell produces no power, and the electric motor acts like a generator,

51

using the wheels to apply torque to the motor to generate electrical power, this torque in
turn slows the vehicle down. The energy created during regenerative braking is stored in
the battery until needed. It is important to note that without shoot-through the Z-source
inverter operates in the same way as traditional inverters, providing the capability for

regenerative braking.

 

Fuel Cell

 

 

 

 

 

 

 

 

Battery Inverter

 

 

 

 

 

 

 

 

a ' 9... ‘.r'n-t.
. 4.: _-.. .

 

 

 

Motor

 

 

Figure 41. Regenerative braking operating mode

It is important to mention that in any of the operating modes, if the SOC of the
battery becomes too low, the fuel cell power will be increased to provide power to

recharge the battery [20].

52

4.2 Current FCV Technology
A review was conducted of work performed to date in the area of FCV design.

Several companies and research institutions have performed work in the FCV area, and a

summary of recent work, focusing on the power electronics, is listed in this section.

4.2.1 Texas Tech University Fuel Cell Vehicle
The Texas Tech FCV [21] is propelled by two electric motors, and powered by a

combination of batteries and a fuel cell. A 1997 Chevrolet Lumina is used and expected
to achieve 60 mpg and have zero emissions. The desired requirements are 0 to 60 mph in
under 16 seconds, and a driving range of 325 miles. This acceleration requirement for a
4400 pound vehicle, requires a motor between 100 and 150 kW depending on the
transmission. They selected to use two ac induction motors, because they are much less
expensive than do brushless motors, however the dc brushless motors are lighter and
more efficient. They used 23, 38Ah 12V sealed lead acid batteries, connected in series,
resulting in 276V, a high enough voltage to run the motors. Thus the inverter can be
connected directly across the batteries. The maximum power created by the batteries is
approximately lOOkW. While the maximum power from the fuel cell is 20kW (60V @
333A). A dc-dc converter is connected in parallel to each of the batteries, which steps
down the voltage from the fuel cell to a voltage suitable for charging the batteries. The
dc-dc converters have the following characteristics, input voltage range: 50V to 100V,
output voltage: 12V to 15V (adjustable), input current: 15A (max), output current: 50A

(max).

53

4.2.2 Ford Fuel Cell Vehicle
The Ford P2000 [22] is built on the contour platform, and consists of only one

power source, the fuel cell, actually 3 fuel cells connected in series, producing a
maximum voltage of 385V. The P2000 is the simplest example of a FCV, because no
battery is incorporated. However without a battery the P2000 will not be able to benefit
from regenerative braking, which could lead to a loss of up to 30% in some urban driving
cycles. The P2000 uses a 67kW ac induction motor. No dc-dc boost converter is needed
because the fuel cells produce high enough voltage to run the motor. Therefore the
inverter is directly connected to the fuel cells. The Inverter has the following
characteristics, maximum current: 280A, minimum voltage: 200V, maximum voltage

385V, and nominal voltage of 255V.

4.2.3 Jeep Fuel Cell Vehicle
In October 2000, DaimlerChrysler introduced the Jeep Commander 2 methanol

FCV [23] in Lansing, Michigan. To use methanol as the fuel, the vehicle must have an on
board methanol fuel reforming system to extract hydrogen from the methanol. This
results in a vehicle that is no longer zero emissions, but it is still much cleaner than a
conventional vehicle using an internal combustion engine. The Jeep FCV incorporates
two 38kW fuel cells, Operating at a voltage between 220V to 270V. Also a 28Ah, 324V
(nominal) battery, capable of producing 90kW is used as a secondary power source, and
connected directly to the inverter bus. A dc-dc boost converter is used to boost the fuel
cell voltage to charge the batteries and drive the two 83kW induction motors more

efficiently.

54

4.2.4 Toyota Fuel Cell Vehicle
In the Toyota FCV [20] the 90kW maximum fuel cell and the traction inverter are

connected directly in series, with the 6.5Ah battery connected in parallel to the inverter
bus through a 20kW dc-dc converter. The Toyota FCV uses a permanent magnet ac
induction motor capable of 83kW maximum power. The battery connected in parallel to
the inverter bus through a dc-dc converter is mainly used to provide power assist when
the fuel cell response is delayed, or when the vehicle is driven under high loads. The
battery also absorbs the energy recovered by regenerative braking, and acts as the

vehicles primary power source under low loads.

55

5. SIMULATION

In this chapter simulations will be used to verify the Z-source inverter can be used
to provide the above-mentioned hybrid system operating modes, control of battery SOC,
control of fuel cell voltage (or power), and control of output voltage (or power). The
simulations will be run using a software package called SABER. Saber software [25]
simulates physical effects in different engineering domains (hydraulic, electric,
electronic, mechanical, etc.) as well as signal-flow algorithms and software. Saber
software is used in the automotive, aerospace, power and IC industry to simulate and
analyze systems, sub-systems and components to reduce the need for prototypes. Many
innovations in the automotive sector today are enabled by hybrid technologies where
software controlled electro-mechanical components replace traditional mechanical

components.

5.1 Simulation Setup

The Zsource inverter shown in figure 41 was modeled and simulated using

SABER. To examine this system, first fuel cell and battery models are needed.

 

 

fuel
cell

stack

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 42. Schematic of simulation model

56

5.1.1 Fuel Cell Stack Model
Although there are many complex subsystems and parasitic loads associated with

a fuel cell, when considering the power electronics, we are mainly concerned with the
voltage, and current. It is possible to model a fuel cell stack in a much more fundamental
manner, incorporating electrochemistry, thermal characteristics, and mass transfer, but
such a model is not necessary for the analysis as accomplished in this work. The fuel
cells voltage (and power) is determined by two main factors. First the rate at which
hydrogen flows through the fuel cell establishes the level of the V-I polarization curve
[26]. Second the amount of current drawn by the inverter determines the point on this
curve where the fuel cell will operate. Thus, by controlling the amount of current drawn

by the inverter, the fuel cell voltage and power can be controlled.

 

420

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

1
I

 

g

 

ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo

Voltage (V)
§ E? 'g’

 

 

 

 

 

-Ponouuuhbc Col-OQIIII DCOOIOIIOU 0.0... .0. I lit-n...

0
EB

IUD 200 350
Current Density (Ncmz)

Figure 43. Fuel cell polarization curve

57

 

450 y ‘- ’ i I I
: . 3 . Model
---+ - - Actual points

 

 

 

 

 

 

350

150

 

 

 

100 i
0

Figure 44. Comparison of fuel cell polarization curve and model equation

To create a fuel cell model, an equation is used to approximate the V-l
polarization curve of Figure 43. Figure 44 shows the polarization curve of the 50 kW fuel
cell stack that was used in this study, along with the model equation curve. The equation
given in MATLAB, a common mathematics software program, by basic curve fitting the

points on the "Typical Fuel Cell Polarization Curve“ is

V: 6.4657e-008*I."4 -5.7400e-005*I."3 + 0.0163*I."2 -2.2381*I +410.09‘76

58

Coefficients of model equation

 

 

 

 

 

 

 

 

 

 

 

  

 

 

Fl
., .2]
l l
[a - ‘ .4 ¢ :2 “I" 4 “374,5 i
'WWC mewydclm . .. . fiBJSS-Ie-B ‘
1 curre W W‘ '.,. r» ‘ . ’5. [new value] _

'12.».w'fi". flue-fll-I WW .

r "r ., a... +» III-x m ,—. _
5'.“ ‘ t .. :+_ _ - V ' 1 . .1 ' '

iv . —.

 

Figure 45. Properties of the fuel cell model

To create the fuel cell model in SABER, a current controlled voltage source
(CCVS) was used. The coefficients of this model equation were used in the CCVS as
shown in Figure 45, and the voltage of the CCVS is controlled by its own current.

To check the accuracy of the CCVS fuel cell model compared to the "Typical
Fuel Cell Polarization Curve“ the power versus current graph as seen in Figure 46 is
checked to verify that the curves do not vary greatly. The power versus current graph
does indeed verify the accuracy of the fuel cell model equation. However one point from
the "Typical Fuel Cell Polarization Curve” does not line up accurately, it can be seen that
when the "Typical Fuel Cell Polarization Curve“ is at 50kW the fuel cell model is
actually at around 53kW. All other points from the "Typical Fuel Cell Polarization

Curve“ line up very accurately.

59

x10

6 r 1 r l r I

Model
---+ - - Actual points

 

 

 

    
 

 

 

 

 

i

I i i i
1 00 1 50 200 250 300 350

 

 

Figure 46. Fuel cell model power versus current graph comparison

5.1.2 Battery Model
FCVs employ a battery for storing energy and to handle the load dynamics. The

battery is used to supply supplementary power to the motor, allowing the fuel cell to
operate in an efficient and safe region [27]. Depending on the batteries SOC, it can be
charged, discharged, or held constant. The SOC can be controlled two ways. First, the
battery terminal voltage (i.e. SOC) is controlled by the shoot-through duty cycle. Second,
the fuel cell power can be adjusted. That is, if the fuel cell power is greater than the load
power the SOC increases, or if the fuel cell power is less than the load power the SOC

decreases. Changes in the SOC occur due to load leveling, because the load dynamics can

60

change much faster than the fuel cell can respond. For example as a FCV climbs a hill the
extra power needed to maintain speed comes from the battery, and as the vehicle
descends the battery will absorb the extra power created by the fuel cell. SABER has a
lithium—ion battery model in its MAST parts Library, which is used in this study. The

properties of the lithium-ion battery model are shown in Figure 47.

Amp-hours, internal resistance,
number of cells, nominal
voltage per cell, charged and
discharged specific gravity

gravity as E

 

Figure 47. Properties of the lithium-ion battery model

61

 

 

   

 

 

 

 

 

400-0' E 1 E I E I a a a
300-0‘ """" ‘ """ f “““““ *2 “““ *' “““ ‘2 “““ E‘ “““ 2‘ “““ E“ """
Zzoov ----- 5 ------ i ------ . ------ i ----- ----- EL ----- -----
mo- ------ : ------ a ----- “E ----- I ----- *2 ----- I ----- ----- I -----
0~0'I I I I I I I I I I I
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
SOC

Figure 48. Battery voltage versus SOC

Figure 48 shows the batteries voltage versus SOC, which is similar to an actual
batteries graph [28], verifying the suitability of the battery model. The battery model was
set to 6.5 Ah, the specific gravity and number of cells was set to achieve the nominal

voltage.

5.1.3 Control
As previously mentioned maximum constant boost control with third harmonic

injection [17] is the control method used in this work. Figure 49 shows the control

scheme of maximum constant boost control with third harmonic injection. The final

62

simulation schematic is shown in figure 50. The reference signals Va, Vb, and Vc are each
created with two ac voltage sources in series, the reference signal added to the 1/6
amplitude third harmonics [15]. The triangular carrier signal is created by a periodic
voltage source. The upper and lower shoot through envelope voltages, Vp and VI] are each
created by comparing dc voltage reference signals mef and me along with the carried
signal. The gate control signals San, Sap, Sb“, pr, Sc“, and Scp are created by comparing the
reference signals along with the carrier signal, as in traditional PWM, and using a logic
OR gate to combine that signal along with Vp and Vn. So for example Sap is logic high
when ever the reference signal Va is greater than the carrier signal, OR the envelope
voltage reference mef is less than the canier signal, OR the envelope voltage reference

me is greater than the carrier signal.

AMI A /\

—r ,‘

Vc

v,\

/
I I ><
TIT

SwLHI
Sup] " U ‘
Sap MIL! LI ‘LJ U
smlf LJ LJI'LFI it

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Saar If
an __J mu uflLJ

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 49. Maximum constant boost control with third harmonic injection [17]

63

A

 

1L

 

 

\I
ll

 

 

 

"I

M‘M I
[_L
-

 

 

 

 

 

 

 

 

 

 

 

,.
__.Varei

4
i
O
l

__.Vcrcf

i

—.

—charrier

O..-
l

 

 

Vbret
Vcarrier ’
1: —VP
Vprei
Vnrci
-Vn
Vcarrier '
__.Vpref

__.Vnrei

Figure 50. Final simulation schematic

 

 

 

 

Varci ' S
Vcarrier V"—
_ VP: San
f Vno—
Vbre '
Vcarrier Vno—
’vC ’ _§.bn
Vn—
Vcref __
: —0 VP._ ’ _Sgp
Vcarrier V".—

 

 

5.2 Simulation
To verify the Z-source inverter can be used to provide the above-mentioned

hybrid system operating modes, control of battery SOC, and control of output voltage (or

power), three cases are examined and simulated. In these cases the circuit parameters are

L1=L2=200uH, C1=400uF, C2 has been replaced with a 6.5Ah battery with a nominal

voltage of 330V, switching frequency of 10 kHz, and using maximum constant boost

64

control with third harmonic injection [17].

5.2.1 Simulation Case 1
The fuel cell voltage is kept constant at 300V (P=30kW), and the load power is

varied from 30kW, to 55kW, to 5kW, back to 30kW. As one would expect the battery
SOC should remain constant while the load is at 30kW (Pin=Pout). When the load is
increased to 55kW (Pin<Pout) the battery should supply the additional power requested
by the load, thus the SOC will decrease. When the load is decreased to 5kW (Pin>Pout)
the additional power provided by the fuel cell will charge the battery, increasing the SOC.

These results are verified by simulation, Figure 51, starting from the top, the fuel
cell voltage is constant, and the fuel cell current is fairly constant. Next are the battery
voltage, SOC, load voltage, load current, and load power. Initially the load absorbs
30kW, and the SOC stays constant. The load is then increased to 55kW and the SOC
decreases. Next the load is decreased to 5kW, and the SOC increases. Finally the load is
returned to 30kW and the SOC remains constant.

This simulation shows that we can operate the fuel cell at an efficient operating
point, while the battery handles the load dynamics. This also verifies the Z-source
inverter can be used to provide the medium, and high power operating modes. Whilst the
load is increased to 55kW the battery and fuel cell both provide power to the load,
verifying the high power mode. The medium power mode is verified when the load is
consuming 30kW, the fuel cell provides power to the load, and while the load is at 5kW,

the fuel cell provides power to the load and charges the battery.

65

I Case 1

 

 

 

(V) 3 t(s)
I 500.0 * v_iue|_cell
2 400.0 _—
300.0 —%

 

 

200.0

7 (A) 1 NS)

 

200.0

J i_tuel_cell

 

1‘; 100.0

 

l

(V) :tts)

 

(V)
to
N
O
O

 

V v_batlary

 

31 0.0

(~) :tts)

 

0.88

i
I
m]
i

 

* soc

 

 

(V)
P
°.

 

 

Si 0.0“

 

-200.0

 

 

80000.0 '
5 40000-0
20000.0 '

 

 

 

 

 

 

 

 

0.0 '

 

I I

0.0 5.0 10.0 15.0 20.0 25.0 30.0

I t(s)

Figure 51. Simulation case 1

66

I —|
35.0 40.0

5.2.2 Simulation Case 2
The load power is kept constant at 30kW, and the fuel cell power is varied

between 30kW, 50kW, 20kW. Again the battery SOC should remain constant while the
fuel cell is producing 30kW. The battery will charge when the fuel cell power is
increased to 50kW (Pin>Pout), increasing the SOC. When the fuel cell power is
decreased to 20kW (Pin<Pout), the battery will supply the additional power requested by
the load, decreasing the SOC.

This can be verified in Figure 52. Starting from the bottom, the load power,
current, and voltage are constant, where the power is at approximately 30kW. Next are
the battery SOC, battery voltage, fuel cell current, and fuel cell voltage. Initially the fuel
cell produces 30kW, and the SOC stays constant. Then the fuel cell power is increased to
50kW and the SOC increases. Again the fuel cell produces 30kW, and the SOC stays
constant. Next the fuel cell power is decreased to 20kW, and the SOC decreases. Finally,
the fuel cell again produces 30kW, and the SOC stays constant.

Case 2 shows that we can control the fuel cell power, thus controlling the battery
SOC. Case 2 also verifies the Z-source Inverter can be used to provide the medium, and
high power operating modes. While the fuel cell is at 30kW the vehicle operates in the
medium power mode, and while the fuel cell produces 50kW the vehicle operates in the
medium power mode while charging the battery. When the fuel cell creates 20kW the
vehicle is operating in the high power mode, as both the fuel cell and battery are

providing power to the load.

67

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(V) 1 NS)
4000 7 v_fuel_cell
2 300.0 _l I——‘
7 (A) INS)
300'0 I i_fuel_cell
g 200-0 _J ————
100.0 ‘ L] J
0'0 ‘ (V) :t(s)
330.0 ' 7 v_battery
21 320.0 '
310.0 ' ’ (-) :t(s)
0.74- - 500
r... 0.72 '
0-7 ‘
“-58 ' (V) :tts)
400.0 " L— '-'.-*2\.. “i'xh‘.- '-'-v--.~r..'.- . - .-;;:,n_- :3'11‘1""E"\4 V_Ioad
A 200.0 “I
it 0.0 - . I
'200-0 .1 Farm't- 2r»: "4‘ a ‘ - «Wt-H - ~Pfii‘flzi-i‘ . -1...- .. . vim-.22.... '. .2'»'.I|
-400.0 ‘ 7 (A) :t(s)
i_load
g
200 0 J
' ' (W) 31(8)
40000.0 I p load
A __ e 4 '—
E. 20000.0
0-0 I I T I I I I I i
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
t(s)

Figure 52. Simulation case 2

68

5.2.3 Simulation Case 3
The fuel cell operation is stopped, and the load power is varied from 5kW, to

20kW. As one would expect the battery SOC should decrease. These results are verified
by simulation as seen in Figure 53, starting from the top, the fuel cell power is zero. Next
are the battery voltage, SOC, load voltage, load current, and load power. Initially the load
absorbs 5kW, and the SOC decreases slowly. The load is then increased to 20kW and the
SOC decreases more rapidly. Case 3 verifies that the vehicle can be operated without

using the fuel cell, strictly as an electric vehicle, as in the low power mode.

69

Case 3
I) INS)

P_fuel_cell

 

1.0

 

0.0

 

 

 

'1'0 (V) 2 1(8)

v_batt ery

 

324.0

M

323.0

 

 

 

 

 

H 106)
SOC

 

0.666
0.864

1.: 0.862
0.66
0.656
0.656

400.0
200.0
0.0
-200.0
-400.0

100.0

 

 

 

 

' ' RMnLH—I- Irm-

(V)

 

 

 

(A)

0.0

 

 

-1 00.0
30000.0

g 20000.0
1 0000.0
0.0

 

 

II, .. ......_...,_ _- ___ .. ,v_load

 

 

 

Figure 53. Simulation case 3

70

6. PROTOTYPE DEVELOPMENT, AND EXPERIMENTAL
RESULTS

In this chapter experiments will be used to verify the Z-source inverter can be
used to provide the above-mentioned hybrid system operating modes, control of battery
SOC, control of fuel cell voltage (or power), and control of output voltage (or power).
The experimental setup, as seen in Figure 54, will be discussed, including the fuel cell,
battery, and load models. The experiment procedure is outlined, and the experimental

results will be given.

6.1 Experimental Setup

L1

 

 

 

22:2 +632 26
mill ——+ x \\
“e T v v
stack 1- _o c / \Battery E
t

 

 

 

 

 

 

 

 

 

 

Figure 54. Schematic of experimental model

Figure 54 shows the schematic of the Zsource inverter to be used in a fuel cell
vehicle. Before the experiments could be run, models of the fuel cell, battery, and load

have to be developed.

71

6.1.1 Fuel Cell Stack

 

420

330 ------------------------------

 

I
I
AA-‘AAAA-uA-A ”A-LA-“A-u-AAA-AA-A-Aj-AQ-A‘utfiu‘ _ ..........................
v
I i ‘

Voltage (V)
F

 

 

 

 

 

0 5 10 _ 20 35
Current Density (A/cmz)

Figure 55. Linearized, scaled down, 5kW “Typical Fuel Cell Polarization Curve“

The fuel cell model was based on the "Typical Fuel Cell Polarization Curve“ as
seen previously. The polarization curve was linearized, as shown by the black line in
Figure 55. The polarization curve was also scaled down to 5kW instead of 50kW, this
was achieved by scaling down the current by a factor of ten. The reason the polarization
curve was linearized was so a fuel cell model could be created from a dc voltage source
in series with a resistor. The dc voltage source is set to the voltage at the Y-intercept, and
the resistance is set to the slope of the line.

The problem with this approach is that the resistor required to achieve the desired

72

AX _ (330—250) _Q
AY (20—6) 14

 

slope R = = 5.71452 would consume too much power.

This problem is solved by selecting the resistance to be smaller, so less power is
consumed. The resistance was chosen to be R=1.2£2, because there were three 2kW .40
resistors available for use in our lab. The dc voltage source was randomly selected to be

250V, a lower voltage was desired for safety reasons.

 

+

R 1.20

Vfuel cell

 

DC + 250v

 

Figure 56. Experimental fuel cell model

The final fuel cell model schematic is shown in Figure 56. The model is created
by connecting a 1.2 Q resistor in series with a 250V dc voltage source.

The equation for voltage across the fuel cell produced by the final fuel cell model

73

is,

VM,_C,,, = 250V —1.2o * I

Figure 57, shows the experimental fuel cell model polarization curve that was
produced in SABER using the final fuel cell model. The approximate operating points
used in the experiments are also shown in figure 57.

Experiment Fuel Cell Modelv l Curve

 

260.0
P=.5kW

“5‘

240.0 ”(2 87>M§I P=3kW

\\
(1 2. 768. 22%6)‘ p=5_5kW

-\.-
N

220 .0 2 ”Pk
“a.

(24.99, 220.201.)

‘..

5“-
~-
\

 

 

Voltage

""‘x.

 

‘..
d

 

 

 

W f l j I I I I I
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40
Current

 

Figure 57. Experimental fuel cell model polarization curve

The final fuel cell model was constructed using the following technique. Three
phase ac power was run through a circuit breaker then an isolation transformer, then a

variable transformer was used to control the voltage level, this can be seen in Figure 58

74

The voltage was then converted into dc using a three phase diode bridge rectifier, a 6mF
capacitor is placed across the rectified dc voltage to smooth the ripple created by the

rectifier, finally three .49 resistors, 1.20 total, were connected in series with the dc

voltage, as can be seen in Figure 59.

 

Figure 58. Three phase ac power, circuit breaker, isolation transformer, and
variable transformer, were used to control the voltage level in the experiment

75

 

Figure 59. Three phase diode bridge rectifier, and 1.29, were connected in series, to
create the fuel cell model in the experiment

76

6.1.2 Battery

 

Figure 60. Capacitor bank used to replicate a battery in the experiments

Because there was no suitable high voltage battery available in our lab to be used
in the experiments, a battery model was constructed using the following technique. As
batteries and capacitors are both energy storing devices, a large capacitor bank can be
used to replicate a battery. Figure 60 shows the capacitor bank used to replicate a battery
in the experiments. The battery model consists of 25 6mF capacitors in parallel, in

parallel with 8 SmF capacitors in parallel, for a total capacitance of 190mF.

77

6.1.3 Zsource Inverter
The lOkW Zsource inverter prototype shown in Figure 61 was used in the

experiments. The circuit has the following parameters: Z-source network: L1 = L2 = 1
mH (60 Hz inductor), C1 = C2 = 1,300 [If (C2 is connected in parallel with the capacitor

bank), and IGBTs are used for the switches.

 

Figure 61. lOkW Zsource inverter prototype

6.1.4 Output Filter, and Load
The output filter and load (motor model), are the last pieces of the experimental

setup to be discussed. Figure 62 shows their actual experimental setup. The low pass
output filter is used to filter out the high frequency switching component of the output ac
voltage. The components parameters are; the capacitors are 50uF wye connected, and the
inductors are lmH connected in series, giving a comer frequency of

1
fc = — = 4.47kHz, which clearly will filter out the lOkHz switching frequency.

JE

78

The load resistors were chosen based on the power requirements. Two lkW 109 resistors
were connected in parallel for each phase, giving each phase 2kW power rating and 59,

again the resistors are wye connected.

 

‘ /

Figure 62. Output filter and load, used in the experiments

79

6.2 Experimental Results

6.2.1 Experiment Case 1
In the case 1 experiment the load power is varied from 3kW (M=.547, Do=. 179) to

5.5kW (M=.741, Do=.179) to .5kW (M=.223, Do=.l79) back to 3kW. The fuel cell power
is designed to 3kW constant, however fuel cell power varies because the l90mF capacitor
bank can not store enough energy to handle the load dynamics, like the battery used in
simulation would. Figure 63 shows that the battery can be used to handle the load
dynamics, because the battery voltage varies with changes in the load. That is, when the
load power is high the battery voltage drops because the battery is supplying power to the
load, and when the load power is low the battery voltage increases because the battery

absorbs the extra power from the fuel cell.

80

2005/05/16 16 47= 43 IE1!“ mm!
. , 216/: m
E : : 3 << Main?!“ >> 3 _ 3 3 Battery
' ‘ ' 3 = 5 3 ‘ Voltage
50V/div

' Fuel Cell
Voltage
50V/div

 

 

Fuel Cell
Current
5A/div

 

1 0A/d iv

 

 

u .
Wl__ 7‘11 I '-

 

2005/05/16 16 38 49 IE0 "or-a1
Stopped 1 216/: 500mm-
.M:I?L31=I""I ' 5 f

   
 
 
  
 
 
  
  
  

 

‘ ......... ......... ......... ......... Fuel Cell
5 : ? Current
5A/div

Fuel Cell
Voltage
50V/div

------ ----- ~* Load

 

Load
Current
20A/div

,_ _IMIIII...IIII IIIIUIII LIIIIIII IIIIIIIIIIIIILII

Figure 63. Experimental results case 1

 

81

6.2.2 Experiment Case 2
In the case 2 experiment the load is designed to 3kW constant. The fuel cell

power is varied from 3kW (M=.556, Do=.104) to 5.5kW (M=.487, Do=.l48) to .5kW
(M=.645, Do=0) back to 3kW. Figure 64 shows that under constant load the fuel cell
power can be controlled, thus controlling the battery SOC, allowing the battery to be
charged, discharged or held constant. That is, when the fuel cell power is high the battery
voltage increases because the battery absorbs the excess power created by the fuel cell,
and when the fuel cell power is low the battery voltage decreases because the battery

must supply the excess power requested by the load.

82

EM
2 :(<Harn:10k 3}: I

2005/05/16 11:55:39
1pm I .

..___'._..‘1 LL I 4].;
1. '- Wr

 

 

llornal
arcs/s 5W

 

 

Fuel Cell

- Voltage

50V/d iv

Battery
Voltage
50V/div

 

Fuel Cell
Current
5A/div

Battery
Current

_ - , 5Aldiv

 

 

 

 

 

 

 

Figure 64. Experimental results case 2

83

“I-“ NIH '-

Fuel Cell
Voltage
50V/div

Fuel Cell

'Cwmm

5A/div

Load
Current
20A/div

Load
Voltage
250V/div

7. CONCLUSIONS

It is obvious that some alternative to the internal combustion engine (ICE) must
be put into practice in order to resolve the many environmental and social issues
associated with the ICE. Fuel cell vehicles (FCVs) are clearly the best solution to provide
low (possibly zero) emissions, and long range. The FCV appears to be the most
reasonable solution for the not to distant future, as the cost of a fuel cell is reduced, and
the hydrogen infrastructure is built.

This paper has presented a FCV system power control strategy to control power
from the fuel cell, power to the motor, and State of Charge (SOC) of the battery, using
the Z—source inverter. This is also verified by simulation, and experimental results. The
Z—source inverter is very promising for use in FCVs because of the following unique
features and advantages: 1) Less complex, more cost effective, and efficient, than a dc-dc
boosted inverter, while providing the same function (i.e. buck boost). 2) Greater
reliability, because shoot-through can no longer destroy the inverter. 3) No need for any
dc-dc converters to control the battery SOC, or boost the dc bus voltage, because the Z-
source inverter has two independent control freedoms. In summary, the Z source inverter

is very promising for fuel cell vehicles.

84

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