advanced design and simulation of a hybrid electric

Advanced Design and Simulation of a Hybrid Electric Vehicle

by

Swathi Sidhanthi, B.Tech

A Thesis

In

ELECTRICAL AND COMPUTER ENGINEERING

Submitted to the Graduate Faculty

Of Texas Tech University in Partial Fulfillment of

the Requirements for

the Degree of

MASTERS OF SCIENCES

IN

ELECTRICAL ENGINEERING

Approved

Dr. Stephen Bayne

Committee Chair

Dr. Richard Gale

Ralph Ferguson

Dean of the Graduate School

December, 2010

Texas Tech University, Swathi Sidhanthi, December 2010

ACKNOWLEDGEMENTS

The completion of this thesis is due to the guidance, efforts, assistance, time and

motivation of many people. Firstly I would like to express my gratitude to my advisor

and committee chair Dr. Stephen Bayne, for guiding me throughout the process of my

research. Secondly, I would like to thank Dr. Richard Gale for his valuable input during

this process, and I am also grateful to him for the financial support I have received in the

form of a scholarship for this research. I would also like to thank Dr. Timothy Maxwell

for giving me an opportunity to work in EcoCAR competition.

I would also like to acknowledge the Electrical and Computer Engineering Department

(Professors, graduate students, and the staff) for their major contribution to my success at

Texas Tech University.

Lastly, I wish to acknowledge support from my family and friends throughout my

graduate studies.

ii


CONTENTS

ACKNOWLEDGEMENTS………………………………………….…………………..ii

ABSTRACT…………………………………………………………………………….. v

LIST OF TABLES…………………………………………………………………........vi

LIST OF FIGURES……………………………………………………………………..vii

CHAPTER

1.INTRODUCTION……………………………………………………………………..1

1.1 Powertrain configuration…......................................................................................... 1

1.2. Control Algorithm and Strategy..................................................................................4

1.2.1 Engine Operation, Battery SOC, and Regenerative Braking……………………5

1.2.2 Two-mode transmission…………………………………………………………5

1.2.3 Two EVT Modes………………………………………………………………...6

1.2.4 Four fixed Gear ratios……………………………………………………………6

2.BACKGROUND………………………………………………………………………7

2.1 HEV Technology…………………………………………………………………….10

2.2 Need for HEV………………………………………………………………………..10

2.3 Types of HEV………………………………………………………………………..11

2.3. 1 Series Hybrid…………………………………………………………………...12

2.3.2 Parallel Hybrid………………………………………………………………….14

2.3. 3 Series - Parallel Hybrid/ Power Split / Two – Mode…………………………..14

2.3.3.1 Traction power inverter module…………………………………………16

2.3.3.2 Accessory Power Module………………………………………………..16

2.3.3.3 Air conditioning compressor module……………………………………16

2.3.3.4 Auxiliary fluid pump control module……………………………………16

3.VEHICLE ARCHITECTURE………………………………………………………..17

3.1 EcoCAR architecture components…………………………………………………17

3.2 Hardware connections……………………………………………………………...20

iii


3.3 Power Flow………………………………………………………………………….22

3.4 Charge/ Discharge of the Battery……………………………………………………27

4.MODELING/ SIMULATIONS………………………………………………………30

4.1 Modeling/ Simulations…………………………………………………………...30

4.1.1 Battery Model……………………………………………………………….32

4.1.2 DC - DC Converter Model………………………………………………….36

4.1.3 Inverter/ Rectifier Model……………………………………………………43

4.1.4 Motor Model………………………………………………………………...47

5. SUMMARY AND CONCLUSION…………………………………………………48

REFERENCES………………………………………………………………………….51

iv


ABSTRACT

This thesis illustrates the modeling of power electronics components for a two- mode

hybrid electric vehicle. The model designed is for a Texas Tech University

Advanced Vehicle Engineering Competition EcoCAR: The Next Challenge. Model

of the hybrid electric vehicle is built and simulated in this research. Power electronic

components such as the DC – DC converter, Inverter, Rectifier have been built and the

power management has been discussed during various modes of operation. These models

have been built using PSIM (power simulator) software and the Mathworks tool called

the Simpower systems. Models such as the Battery, Bidirectional Buck Boost have been

built in PSIM whereas Motor model has been done using the Simpower systems. PSIM

tool has been chosen because of the advantage of having most of power electronic

components in built. The motor model built in Simpower systems is then tied into the

PSIM model with the help of a Simcoupler tool. This Simcoupler tool is a bidirectional

tool, hence allowing for the power to flow between the two software tools. The

simulation results have been analyzed during the motor mode of operation as well as

during the regenerative braking. The battery voltage, Buck Boost Converter Voltage,

Inverter output 3 phase current and voltage, motor output Torque and speed are discussed

in this thesis.

v


LIST OF TABLES

1. VTS Table ....................................................................................................................... 3

2. Cell Specification…………………………………………………………………...…27

vi


LIST OF FIGURES

1. Regenerative braking.......................................................................................8

2. Regeneration circuit ........................................................................................9

3. Series Hybrid architecture…………………………………………………..12

4. Parallel Hybrid architecture........................................................................ ..13

5. Power split architecture ...............................................................................15

6. Current and voltage of each components .....................................................18

7. Hardware connections of components .........................................................21

8. Power flow during vehicle powered by battery............................................22

9. Power flow when Battery SOC is low........………………………………..23

10. Power flow during Regenerative braking.................................................. 25

11. Power flow when vehicle powered both by battery and engine ................26

12. Discharge curves for 26650....................................................................... 28

13. Discharge curves for 18650........................................................................29

14. Power Electronic components ................................................................... 31

15. Battery model in PSIM ..............................................................................33

16. Battery simulation during mode 1 ..............................................................34

17. Battery simulation during regenerative braking .........................................35

18. Bidirectional Buck Boost Converter ..........................................................37

19. Voltage mode control .................................................................................39

20. Current mode control..................................................................................39

21. Buck Converter simulation........................................................................ 41

22. Boost Converter simulation .......................................................................42

23. Inverter/Rectifier Model ............................................................................43

24. Inverter 1 phase of 3 phase voltage ...........................................................44

25. Inverter 3 phase output current..................................................................45

26. Rectifier simulation ...................................................................................46

vii


27. Motor Model............................................................................................... 48

28. Motor Speed ............................................................................................... 49

29. Torque output for the motor ........................................................................50

viii


CHAPTER I

INTRODUCTION

1.1 Powertrain configuration

Throughout the first year of the EcoCAR competition, the Texas Tech Advanced Vehicle

Engineering (AVE) Team followed the GM Global Design plan in order to develop the

vehicle with most efficiency. The first step in the design process was to perform stock

vehicle modeling in Microsoft Excel. The second step in this process for the AVE team

was to model several different powertrains using PSAT (powertrain simulation and

analysis toolkit) [1]. The three powertrains that were modeled were the fuel cell E-Drive

system, the front wheel drive Two-Mode hybrid, and a mild hybrid configuration [2].

Once all of the architectures were simulated to show each of their performances based on

gasoline equivalencies, modeling was performed using the GREET software to determine

the pros and cons of the different fuels selected for the vehicle architectures. The Texas

Tech Advanced Vehicle Engineering team initially had three different powertrain designs

that were to be considered. The first of these three was the fuel cell and traction drive

system. The fuel cell was initially the team’s first choice for the powertrain because of

several reasons. Firstly, fuel cells produce energy from fuel at a much higher efficiency

than an engine burning hydrocarbons. Secondly, using a hydrogen fuel cell to produce

power means no use of petroleum products to actually power the vehicle, and the on road

emissions of a fuel cell vehicle are cut to zero [2]. While this architecture has some

1


disadvantages, it did meet the team’s Vehicle Technical Specification standards for

emissions and efficiency which was the primary goal. All of the team’s initial powertrain

simulation was done in PSAT. The range of the vehicle is extremely limited

The second powertrain chosen for testing was the two-mode plug in hybrid. The

components for this architecture are a Two-Mode FWD Transmission, a 1.6L 4 cylinder

engine, a A123 25S2P battery pack. As with the fuel cell, there are several advantages to

this vehicle architecture. Firstly, the vehicle can be driven anywhere from zero to thirty

miles an hour without using the engine at all. Secondly, when the engine is on, the torque

output of the transmission can be controlled by the two electric motors inside the two-

mode transmission [3]. The PSAT simulations for this vehicle were very promising.

The third architecture considered was the mild hybrid. The components chosen for this

architecture are a 2.0L SIDI engine equipped with VGT from the Cobalt SS, BAS+

system for hybrid starts/stops and an 80V Cobasys pack. While this vehicle excelled in

the performance parameters, the vehicle performed less than admirably in the fuel

economy and emissions standards because E10 was the fuel to be used [2]. During the

development process, it was discovered that the Two-Mode transmission is mechanically

limited to 30 miles per hour all electric, so the decision was made to develop a purely

Two-Mode vehicle. The Two-Mode architecture does have some tradeoffs when

compared to the other two vehicles. The Two-Mode does not get near the fuel cell

vehicle’s fuel economy. However, the Two-Mode Vue will be much faster [3] than the

2


fuel cell Vue shown in Table 1. The emissions of the two mode view will be much

cleaner however, because the Mild Hybrid Vue would be burning E10 instead of E85 like

the Two-Mode[3]. Table 1 shows the comparison of the three models.

Table 1: VTS Table

Specification Architecture

EcoCAR FCV 2-Mode Mild

Accel 0–60 10.7 s 8.2 s 7.3 s

Accel 50–70 7.2 s 4.0 s 3.8 s

Passenger

Capacity

5 5 5

Mass 2154 kg

(4748

lb)

2040 kg

(4497

lb)

1720 kg

(3791

lb)

Range > 173

km (108

mi)

> 1050

km

(653 mi)

> 1132

km

(704 mi)

3


The Advanced Vehicle Engineering team at Texas Tech University has selected the two-

mode hybrid power train. It consists of a 1.6 L GM Europe engine converted to run on E-

85, a GM two-mode transmission with two 55 kW electric motors, and a A123 Li ion

battery pack containing four A123 25S2P modules each rated at 82.5V. Ideally the

vehicle will operate on electric power at low speeds(less then 25mpg) and blend power

for higher speeds and towing. The voltage range of the battery pack is 260–330 V.

A model of power electronic components of a Hybrid Electric Vehicle is designed and

dynamic simulation was done and analysis was completed. The research also includes

the study of power electronics components along with the induction motor for complete

analysis and performance of the system. The dynamic simulation of the system is done

using the PSIM (Power simulator) software which is integrated with Simpower systems

software using Simcouple integration tool.

1.2 Control Algorithm and Strategy

The purpose of any hybrid powertrain control system is to optimize the efficiency of the

vehicle it is powering. The main functions for the vehicle control strategy are to:

1) Supervise engine operation to ensure proper operating ranges in pursuit of the best

BSFC possible. This includes turning the engine off/on and deciding when to

operate at constant speed.

2) Maintain the SOC of the battery pack. This includes regenerative braking

schemes (partial/full).

3) Control the operation of the two-mode transmission. Decide operational mode of

transmission (EVT 1, EVT 2, FG 1/2/3/4); This includes motor torque requests.

4


1.2.1 Engine Operation, Battery SOC, and Regenerative Braking

The control of the two-mode hybrid vehicle begins with the turning of the ignition key.

This action powers on the ECU and prompts a check of every device on the CAN bus.

Next the controller enters a loop which can only be broken by a car shutoff or an

emergency stop or battery failure.

In this loop, the battery state of charge (SOC) is checked. If it is below the optimal

minimum the controller sends a message to the engine to check itself and turn on. Then,

the controller reads the speed request from the user to calculate the engine throttle and

battery power demand. To decide these power outputs, the controller compares the

calculated horsepower request to the horsepower available from the battery and engine.

If the battery can provide this power at the requested speed and the SOC is above the

optimal minimum, the vehicle runs off only the battery and motor. In this mode, the

vehicle will have the benefits of a plug-in hybrid, but will be less powerful at higher

speeds. If the battery cannot provide this power, the engine is turned on and throttled to

the optimal speed to meet this demand.

1.2.2 Two mode transmission

The advantage to the two different EVT modes is that the engine can be run at a constant

speed for longer without leaving the range of efficient operation for each of the two

electric motors. The vehicle will change from mode one to mode two when the generator

reaches 0 rpm.

5


1.2.3 Two EVT Modes

With innovations of the two mode transmission, much more efficient powertrain

operation is available for normal driving conditions. The Texas Tech EcoCar team will

utilize all of the different operational modes provided by the two-mode transmission to

achieve the best possible performance from the provided powertrain.

1.2.4 Four fixed Gear ratios

In addition to the two EVT modes, the team will also take full advantage of the four fixed

gear ratios that are native to the GM two-mode transmission. With the first fixed gear

ratio, the powertrain is optimized for acceleration. Unlike a pure EVT, the two mode’s

first and third fixed gear ratios allow engine direct drive.

The second fixed gear in the transmission will be used as the synchronous shift ratio.

This is the native fixed gear ratio in the two mode transmission and will allow a seamless

transition from EVT mode 1 to EVT mode 2 [3]. The third fixed gear ratio will be used to

couple the speed, torque, and power from the engine directly to the transmission output.

This serves as a direct drive gear for the engine. This gear will be utilized for towing and

climbing. The fourth fixed gear ratio will be used as an overdrive gear which will be the

most efficient mode for highway cruising. This is done by replacing electricity for motor

B with hydraulic pressure that is already maintaining the brake of clutch two in order to

maintain the proper torque in motor B.

6


CHAPTER II

BACKGROUND

EcoCAR competition is a three-year advanced vehicle technology engineering

competition run by the United States Department of Energy (DOE), General Motors

(GM), and Argonne National Laboratory [4]. This competition is between 16 universities

across North America, the primary goal is to reduce the pollution caused by vehicle and

to also optimize the power management in the vehicle. This is achieved by minimizing

the vehicle’s fuel consumption by utilizing alternative fuels such as ethanol, biodiesel and

hydrogen. This competition gives teams an opportunity to participate in hands-on

research [4]. EcoCAR challenge focuses on modeling and simulation, as well as

subsystem development and testing of the complete power electronics system. University

teams involve with the design and then integrate the power electronics components into

the vehicle [4].

Hybrid electric vehicle (HEV) is one of the most advanced vehicle designs in recent years

in the field of automobile industry [5]. Hybrid electric vehicle has internal combustion

propulsion system along with an electric propulsion system. With the help of electric

propulsion system, a better performance of the vehicle can be achieved. There is a

growing demand for hybrid electric vehicle concepts across the North America. These

hybrid electric vehicles can be further classified based on the power train architecture

being employed, based on the type of fuel being used and based on the mode of the drive

7


train operation. Mostly, these HEVs use alternate fuels such as ethanol (which can be

created from renewable energy sources), helps in reducing the pollution significantly

which otherwise would have caused by the utilization of petroleum, and usage of these

alternate fuels also helps in reducing petroleum consumption [5].

HEV’s make use of regenerative braking operation. During regenerative braking, the

kinetic energy associated with motor is converted into electric energy, during which

motor acts as a generator, and this energy can then be used to charge the battery. The

energy of the motor otherwise would have been dissipated as heat energy. In this way, the

batteries can be recharged during regenerative mode of operation. Thus this operation

helps in recovering some of the energy lost during braking. Regenerative braking would

not take place if the battery is already charged to its rated voltage [6].

Fig 1: Regenerative Braking

8


Fig 1 shows the induction motor which acts as a motor during motoring mode of

operation, and during this time the current flow is from the battery towards the motor.

Fig 2: Regeneration circuit

When MOSFET M1 is ON, the current flows through the field and motor rotates.

The need for D1 is to stop excess voltage drop during built up across M1. When speed

goes low, M1 is OFF and M2 is ON, during this motor acts as a generator. The current

flow is backwards during this through M2 which is ON. When M2 turns OFF, the current

is maintained by inductance L and this will flow through D2 diode [7]. This current then

flows into the battery, which can be charged using this energy to its rated voltage.

9


2.1 HEV Technology

An electric vehicle usually has a battery pack connected to an electric motor and provides

traction power through the use of a transmission. This battery pack is primarily charged

using a battery charger. The use of this charger is during the maintenance period, when

one wants to charge the battery in the vehicle, when vehicle is not being used.

The Electric Propulsion system has following electrical components [8]

• Power Electronics for power distribution

• Electric Machines for power conversion

• Energy storage element like Battery pack

2.2 Need for HEV

Demand from consumers, necessity to reduce emissions and increase in the price of

petroleum lead automobile industry to focus on more advanced design. One such design

is the Hybrid Electric Vehicle [5]. These vehicles use alternate fuels such as ethanol as

opposed to commercial petroleum; this ethanol fuel relatively causes fewer emissions

compared to petroleum.

10


2.3 Types of HEV

This section discusses about the various architectures of HEV. HEV’s can be broadly

classified into three power train configurations. These three architectures differ in the

energy flow through the power train of the vehicle [5].

2.3. 1 Series Hybrid

These vehicles are also referred as Extended Range Electric Vehicles (EREV) [9]. The

vehicle uses an internal combustion engine (ICE) to turn a generator, this generator in

turn supplies current to an electric motor, and this electric motor rotates the vehicle’s

wheels. A battery is used to store excess charge. With an appropriate balance of its

power electronic components this series hybrid can operate over a substantial distance

with its full range of power without engaging the Internal Combustion Engine (ICE) [10].

11


Fig 3: Series Hybrid Architecture

As seen in fig 3, a series-hybrid omits a mechanical link between the combustion engine

and the vehicle’s wheels; the engine can be run at a constant and efficient rate even as the

vehicle changes speed. The engine will be able to achieve efficiency very closer to the

theoretical limit of 37%, rather than the current average of 20% efficiency [11].

2.3.2 Parallel Hybrid

Unlike series hybrid, where the mechanical link between the engine and vehicles wheel is

omitted, parallel hybrids have an internal combustion Engine (ICE) and electric motor

connected to the mechanical transmission [12]. Most parallel hybrids incorporate an

12


electric motor between the vehicle's engine and transmission. It may also be able to use

its engine to drive one of the vehicle's axles, while its generator used for recharging the

batteries. (This type is called a road-coupled hybrid) [12]. An important advantage of this

architecture is that electric motors are used to power the accessories such as air

conditioning instead of accessories being attached to the combustion engine. This leads to

higher efficiency gains as the accessories can run at a constant speed, regardless of the

speed of combustion engine [12]. Figure 4 below shows the architecture of a parallel

hybrid.

Fig 4: parallel Hybrid Architecture

13


Parallel hybrids can use a smaller battery pack as they rely more on regenerative braking

and the internal combustion engine can also act a generator for supplemental recharging

[13].

2.3. 3 Series - Parallel Hybrid/ Power Split / Two - Mode

These models have the flexibility to operate in one of the two above modes, i.e., series or

parallel mode. Hybrid powertrain models currently used by Ford, Lexus, Nissan, and

Toyota [8], are sometimes referred to as “series-parallel with power-split,” can operate in

both series and parallel mode at the same time [14]. To describe HEV architecture in

which the output power from an electric motor and the ICE are joined, at transmission,

the term POWERSPLIT is used. This POWERSPLIT architecture is further classified as

INPUTSPLIT, OUTPUTSPLIT and COMPOUNDSPLIT. In POWERSPLIT designs, the

electric motor speed typically determines the output ratio. This type of transmission is

commonly referred to as an Electromechanical Variable Transmission (EVT). Figure 5

below shows the power split hybrid architecture.

14


Fig 5: Power Split mode Architecture

Following are the power electronics components used in 2- mode hybrid electric vehicles

[8];

• Transaction power Inverter Module (TPIM)

• Accessory Power Module (APM)

• Air conditioning compressor module (ACCM)

• Auxiliary fluid pump control module (AFPCM)

15


2.3.3.1 Traction power inverter module

This module is a DC- AC inverter. This primarily controls and processes dc power flow

in Battery pack and processes and controls ac power in electric machines [15].

2.3.3.2 Accessory Power Module

In a two mode hybrid, the alternator is eliminated [16]. The Accessory Power Module is a

DC- DC converter, which controls and processes dc power for 12V loads and accessory

battery.

2.3.3.3 Air conditioning compressor module

It is a DC- AC inverter. It controls the electric machines [16].

2.3.3.4 Auxiliary fluid pump control module

This is DC- AC inverter, which controls the auxiliary fluid pump motor [16].

16


CHAPTER III

VEHICLE ARCHITECTURE

3.1 EcoCAR architecture components

The Advanced Vehicle Engineering team at Texas Tech University has chosen the two-

mode hybrid powertrain for EcoCAR competition. It consists of a 1.6 L GM Europe

engine converted to run on E-85, a GM two-mode transmission with two 55 kW electric

motors, and a battery pack containing four A123 25S2P modules. The engine suggested

will be lighter than stock, which will help to balance the added weight of the two-mode

transmission. Ideally the vehicle will operate on electric power at low speeds(less then

25mpg) and blend power for higher speeds and towing. The voltage range of the battery

pack is 250–360 V. The RMS power output of the energy storage system is

approximately 19 kW, where the maximum power output of the motors is 110 kW. The

motors will draw 19 kW (RMS) from the battery at 300 V. Based on information from

GM, the inverter is 380 V DC to 300 V AC.

According to our simulations of the US06 drive cycle, the motors will draw 19 kW

(RMS) from the battery at 300 V. Based on information from GM, the inverter is 300 V

DC to 300 V AC. Assuming the worst case of 35% losses from the motor (from a GM

document), 5% losses from the inverter (typical value), and 5% losses from the converter

(typical value). Using the equation for 3-phase power; P=(√3)(V)(I)(cosθ), a voltage of

300 V, at 19 kW (RMS), and a power factor of 85%, results in a current of 63 A (RMS)

17


and 126 A (Peak) on the motor side of the inverter. Between the converter and inverter,

the expected current is 92 A (RMS) and 184 A (Peak). Between the battery and the

converter, the expected current is 84 A (RMS) and 168 A (Peak). Figure 6 shows the

currents and voltages at each point in the system.

Fig 6: Currents and Voltages of each component

After calculations, it was been decided to use the 3AWG HV wire. This particular wire is

the wire that runs from the battery pack in the back of the vehicle up to the front to the

TPIM. In order to ensure that the wire is rated high enough for the current expected at

this point, the current is calculated based upon the current draw of the motors and then

back calculated to get the current supplied by the battery. From the simulations in the

18


ESS Report Section 2.1 it was shown that. According to our simulations of the US06

drive cycle, the motors will draw 19kW (RMS) from the battery at 300V. Based upon

these values the current drawn by the motors can be calculated using the following

equation,

Io,max=VA 3VLL [21] (3.1 )

Where,

VA=PPF (3.2)

From equation 3.2, using a power factor of .85, the apparent power, VA, is calculated to

be about 22.4kVA. Using this value, and a VLL of 300V, Io,max is calculated to be about

60.8A RMS. Assuming a 5% power loss in the inverter, the power at the output of the

DC-DC converter and input to the inverter, is about 19.95kW. Because the output of the

DC-DC converter is DC the following equation can be used to calculate the current,

I=PV (3.3)

From equation 3.3, using the output voltage of the DC-DC converter of 300V the current

between the DC-DC converter and the inverter is about 66.5A. Using the same approach,

assuming a 10% power loss in the DC-DC converter, the power at the output of the

battery and input to the DC-DC converter is about 21.95kW. Using equation 3 and based

on the nominal output voltage of the new battery pack of 330V, the output current of the

battery to the DC-DC converter is about 66.5A. This is the current that the HV cable from

the battery to the TPIM must be able to handle safely. And based upon the table below,

19


the 3AWG HV cable presently in the vehicle will be able to handle this current. Fuses

will be placed before and after the DC-to-DC converter in order to ensure protection from

over currents. Fuses will also be used for either of the motors in order to prevent short

circuits, which could be very damaging to the system.

3.2 Hardware connections

As mentioned in the architecture section, the motors will draw 19 kW (RMS) from the

battery at 330 V. The high voltage cable in the vehicle has a cross sectional area of 25

mm2, which corresponds to a 3 gauge wire, and allows for a maximum continuous current

of approximately 75 A. As Per the EcoCAR Competition Rules, the high voltage

conductors shall not be routed together with low-voltage (CAN) conductors. All

conductors that are either exposed to the road or near high-temperature components will

also be enclosed in conduit . Each high voltage conductor will be placed in an individual

conduit. The spacing between conduits will be maintained using sturdy and durable

conduit spacers. These spacers will be available from the conduit manufacturer. The

portion of the high voltage conductors near the high temperature components in the

vehicle (near where they connect with the power inverters) will be protected with high-

temperature resistant insulation hose. Figure 7 shows the wiring connections of ESS to

the system.

20


Fig 7: Hardware connections of the components

3.3 Power Flow

The following section illustrates and describes the circumstances for the varying types of

power transfer. Figure 8 illustrates the power flow in the case when the vehicle is being

powered solely by the battery. For this to be occurring, a few conditions must be met.

First, the state of charge (SOC) of the battery must be within certain thresholds. The

specific thresholds selected are between 30-80%. These safety thresholds are necessary to

prevent damage to the battery or other system components. The SOC will be monitored

using signals from the battery control module (BCM) in the CAN system. This case of

vehicle being powered solely by battery also can only occur whenever the vehicle is in

motion and not braking. The battery is will discharge through the Boost converter, where

21


battery voltage is stepped up from 330V to 380V to be compatible with the inverter, and

then to the two-mode transaxle in order to propel the car into motion. The engine plays

no part in vehicle propulsion, during this mode of operation.

Fig 8: Power flow during vehicle powered by battery

22


As the arrow indicates in figure 8, the energy flow is from the battery to the motor. The

TPIM in this model consists of Inverter and bidirectional Converter. A second case is

shown in Figure 9. This shows the action taken when the state of charge (SOC) of the

battery becomes too low – that is, if the SOC falls below a lower threshold of 30%. In

this case, the engine is used to power the vehicle. In addition to turning the wheels, the

engine is also used to turn the motors in the 2-mode transaxle. The power generated from

either of the two motors is then transmitted through the DC-DC converter in order to

charge the battery. Once the battery has been charged enough such that it is once again

above a certain threshold, it will once again be enabled to power the vehicle.

Fig 9: power flow when Battery SOC is low

23


Power flow in the previous section will occur only when the SOC of the battery is below

the threshold limits of 30-80%. One feature of this vehicle to promote power efficiency

is regenerative braking, as shown in Figure 10. When the vehicle is decelerating via its

brakes, one or both of the motors is engaged to assist in slowing the vehicle. The

electricity generated is then used to charge the battery. The only circumstance in which

the regenerative braking function will not be utilized is if the battery SOC is already

above an upper threshold of 80%. This is to prevent overcharging, which could result in

damages to the battery, and in turn other system components. In short, if the battery state

of charge is above an upper threshold of 80%, the control system will disable the

regenerative braking feature. The vehicle will still brake normally, but no energy will

flow from the wheels to the battery.

This power flow occurs only during the regenerative braking

24


Fig 10: Power flow during regenerative braking

.

Another important power transfer scenario occurs when both the battery and the engine

are used to power the vehicle. This could occur in a number of situations including when

the car is traveling at speeds above 30 mph, or in cases when high amounts of power are

needed such as towing or traveling uphill. As illustrated below in Figure 11, the battery is

being discharged to power the motors. At the same time, the engine is adding additional

25


power. Of course, when the battery falls below a lower SOC threshold, it will no longer

assist in powering the vehicle until it is charged to a high SOC level.

Fig 11: Power flow when vehicle powered both by battery and engine

26


3.4 Charge/ Discharge of the Battery

The battery uses nanophosphate cells. A123 systems cells are designed to deliver high

power in pulse and continuous applications. Nanophosphate is a positive electrode

material of remarkable rate capability, critical to high power systems. With their low

impedance and thermally conductive design, A123Systems cells can be continuously

discharged to 100% depth of discharge at 35C rate, a marked improvement over other

rechargeable battery alternatives [17]. Table 2 below shows the cell specifications.

Table 2: cell specifications [18]

27


A123Systems cells are designed to deliver high power in pulse and continuous

applications. Figures 12 and 13 show the discharge curves in 26650 and 18650 cells, here

the voltage remains virtually flat during the discharges, and the capacity doesn’t change

significantly, no matter how fast the discharge is [17].

Fig 12: discharge curves for 26650 [18]

28


Fig 13: discharge curves for 18650 [17]

In order to charge or recharge cells, the charger should apply a constant current (CC)

charge, followed by a constant voltage (CV) charge. The charge current for a battery pack

of cells is determined by multiplying the number of parallel elements in the pack by the

recommended charge current for a single cell [18]. The end of charge voltage for a pack

of cells is determined by multiplying the number of series elements in the pack by the

recommended charge voltage of a single cell.

29


CHAPTER IV

MODELING/SIMULATION

4.1 Modeling/ Simulations

For maximum performance of a hybrid vehicle, battery management is critical. For good

battery management the power electronics systems such as the charger and discharger,

and the inverter must be well designed. The areas that are investigated in this research are

the power electronic components which are the 1) Battery 2) bidirectional Buck – Boost

Converter 3) bidirectional Inverter/ rectifier 4) Induction Motor. The power flow in the

first case is when the vehicle is being powered solely by the battery. For this to occur, a

few conditions must be met. First, the state of charge (SOC) of the battery must be within

certain thresholds, which is between 30-80%. The battery is being discharged through

the DC-DC converter to the two-mode transaxle in order to propel the car into motion. A

second case is the regenerative braking operation. Regenerative braking refers to the

process of capturing energy that is available for use but is wasted during braking or coast-

down of a traditional vehicle. In a non-hybrid vehicle during braking, the momentum of

the vehicle is converted into heat by the braking system. The kinetic energy that is stored

in a vehicle while moving is essentially wasted as heat during braking. In a hybrid design

some of this wasted vehicle energy can be converted into electrical energy allowing the

electric motor to operate as a generator to generate electricity and charge the hybrid

batteries and provide a level of vehicle braking. When the vehicle is decelerating via its

30


brakes, one or both of the motors is engaged to assist in slowing the vehicle. The power

generated is then transmitted through the DC-DC converter in order to charge the battery.

Once the battery has been charged enough such that it is once again above a certain

threshold, it will once again be enabled to power the vehicle. Figure 14 shows the high

level power electronic components modeling using PSIM software.

Fig 14: Power Electronics components

In the above model, S1 block represents a Battery rated at 330V, S2 block represents a

bidirectional DC – DC converter capable of steeping up voltage and stepping down the

voltage based on modes of power transfrer, S3 block represents a bidirectional Inverter/

31


Rectifier Model. The terminal blocks in S2 and S3 seen as S are the Simcouple

bidirectional link connectors, Simcouple links of Simcoupler tool is used to connect the

elements of PSIM blocks to elements of Simpower system. In the above model, the PWM

generator block from Simpower systems is connected to the Inverter switches with the

help of this link. Also, the load in this case a 3 phase motor developed in Simpower

systems is connected to Inverter with the help of this simcouple link.

4.1.1Battery Model

The A123 battery pack is rated at a nominal voltage of 330V and 22.3kWh. Battery pack

consists of 4 25S2P modules. The C-rate of battery which is defined as the amount of

current applied to battery during the charging process is 2C. The battery model developed

in this thesis is based on literature [19]. The battery model used from literature was

adjusted for this research. The state of charge (SOC) of the battery ranges from 30 – 80%.

Fig 15 shows the battery model. The Battery is the main energy storage system (ESS) in a

hybrid electric vehicle.

Non- hybrid vehicles contain a 12V dc battery and a generator that supplies about 14V dc

to keep the 12V battery charged. A 12V engine cranking motor is utilized to crank the

ICE. All the electrical devices on a non-hybrid vehicle operate from this 12/14 volt dc

system. Hybrid electric vehicles also contain a 12v dc battery system. In addition, a

higher voltage dc battery system is required to supply the energy needs to the power

32


inverter and motor/generator. The capacity and voltage of high voltage dc battery systems

vary with the hybrid systems requirements. This additional dc battery system in a hybrid

vehicle is 36volts. The dc batteries are connected in series and operate at one large

battery to obtain the required system dc source voltage.

Fig 15: Battery Model in PSIM [14]

33


In the above model, a capacitor and a current-controlled current source are used to model

SOC of the battery [19]. The RC network is used to achieve transient response. SOC is

function of open-circuit voltage; this can be achieved using a voltage-controlled voltage

source. By initializing voltage across Capacitor to 1 V or 0 V, the battery is set to either

it’s fully charged (i.e., SOC is 100%) or fully discharged (i.e., SOC is 0%) state. The

Capacitor is charged or discharged with the usage of a Current-controlled current source

[18].

Figure 16 through 17 show the simulation of battery. The simulation shows the

discharging and charging of the battery.

Fig 16: Battery simulation during mode 1

34


Fig 17: Battery Simulation during regenerative braking

Initially the State of charge (SOC) of the battery is 100%. Battery is used to power the

motor. When the battery falls to lower threshold voltage of 260V, during the regenerative

braking the motor behaves as a generator and generator charges the battery. The capacity

of the battery is 35Ah and current is 70A max, the charge rate is defined as the ratio of

current to capacity, and the charge rate for this model is 2C.

From the simulations it can be seen that, the battery is sued to power the motor initially

when SOC is 100% and when the battery is fully charged, during this the battery

discharges through the boost converter, inverter then runs the motor and gives a 3 phase

output. This discharging can be seen till 0.5ms, thereafter as the battery voltage falls to

260V, during the regenerative braking, the kinetic energy associated with the inertia of

35


the motor is used to charge the battery, and this mode of operation can be seen after

0.5ms where the battery voltage is charged to 330V, which is its rated nominal voltage.

4.1.2 DC - DC Converter Model

As the Battery voltage varies from 250 to 360 V and the inverter requires an input voltage

of 380 V, a DC to DC converter is required. A switch mode converter is used to meet this

requirement. Switch mode converter like a DC- DC converter have vital advantages

compared to that of a linear regulator, as in a switch mode converter the transistor

operates in either cut off or saturation mode as opposed to linear region operation of

linear regulator. Input energy in DC – DC converter is stored temporarily using storage

elements like inductors or capacitors. This energy can then be released to the output at a

different voltage level. The switches hence would be either completely ON or completely

OFF and hence relatively less losses and significantly higher efficiency than linear

regulators [7]. By adjusting the duty cycle of the switch (that is, the ratio of on/off time

duration), the amount of power transferred can be controlled.

A DC – DC converter can be defined as an electronic device capable of transferring a dc

voltage from one level to another. In addition to voltage conversion, it is also useful in

regulating the output voltage. Conventional linear regulator can only output lower

voltages from the input. They cannot boost the input voltage level. In order for the battery

to be compatible with the inverter, a bidirectional DC-DC converter is necessary. The

battery is rated at 330 V nominal. Because the inverter is rated at 380 V; a boost

converter will be used, which steps up the battery voltage from 330 V to 380 V. Figure

36

http://en.wikipedia.org/wiki/Duty_cycle


18 shows the model of a bidirectional buck boost converter which is modeled using PSIM

(power simulator) tool.

Fig 18: Bidirectional Buck Boost Converter

The converter is composed of energy storage elements like an inductor L, a capacitor C,

and two controllable IGBT switches for switching. These IGBT’s have parallel Diodes

37


across them. A fixed-frequency of 20KHZ PWM is applied on either IGBT to transfer to

and fro. Buck operation transfers energy from the generator to the battery pack in turn

charging the battery, this is achieved by triggering upper IGBT. By triggering bottom

IGBT, boost operation can be obtained, during this energy is transferred from the Battery

to the Motor. In the model of a bidirectional buck boost converter, the input voltage is

300V and output voltage is 380V, using the equation of buck boost converter, the duty

cycle when voltage level is at 330V is equal to 0.868, based on the equation 4.2.

DC – DC converter can be operated using the following two techniques

Voltage mode control

Current mode control

In the voltage mode control, a reference voltage is compared with the output voltage is

used to generated the control voltage, i.e., amplified error signal between the reference

value and actual output voltage serves the control voltage [8]. The PWM then compares

this control voltage with repetitive saw tooth waveform of fixed frequency to generate the

duty ratio of the switch. With this control, the switch duty ratio will adjust the inductor

voltage and thereby inductor current to make output voltage equal to the desired

reference value. Figure 19 shows the voltage mode control.

38


Fig 19: Voltage Mode Control [7]

In contrast, in a current mode control, there is an additional inner loop; in this the control

voltage directly controls the output inductor current and thus the output voltage [7].

Current mode control has fast response in controlling the output inductor current

compared to that of voltage mode control. Figure 20 below shows the current mode

control.

Fig 20: Current Mode Control [7]

39


Due to the advantages of current mode control over voltage mode control Current mode

control operation is employed in the DC – DC converter model.

Design of the DC - DC Converter

The dc-dc converter steps down 380 V up to 330 V during regenerative mode of

operation and perform the reverse during motor mode. The buck converter is operated in

continuous mode for this model.

The switching frequency is assumed to be 20 KHz.

Switching time is reciprocal of switching frequency.

Ts = 1/Fs

(4.1)

Substituting the value of Fs in the above equation, we get switching time as 50µs.

The duty cycle is given by

D = Vin/ Vout [21] (4.2)

In this model, duty cycle is given as the ratio of battery voltage to the inverter voltage, by

substituting the known values in equation 4.2; we get duty cycle equal to 0.868, when the

battery voltage is at 330V.

The boundary current between continuous and discontinuous mode is given by the

equation 4.3 below

ILB = (DTs/2L)* (Vout – VIN) [21] (4.3)

40


Inductance can be calculated using the equation 4.3 above

Output voltage ripple is given by equation 4.4 below

(4.4)

Having known the value of Inductance, the capacitance can then be calculated using the

equation 4.4. L is calculated equal to 1.5mH. Cut off frequency is chosen to be less than

the switching frequency, by substituting the values in above equation 4.4, C is calculated

to be equal to 1900µF.

From equation 4.2, we get a duty cycle of 0.868. In order to prevent subharmonics due to

duty cycle being greater than 50%, slope compensation technique is utilized to the control

voltage [8]. This slope compensation would provide stability to the system. During the

regenerative breaking, as the voltage across the battery increases, the duty ratio

decreases.

In this model, slope compensation is added to control voltage, which then is compared

with the inductor current, this establishes the duty ration for the switch, which in turn

equals the output voltage to the desired value.

The RS flip-flop is used for timing. Fixed frequency clock sets the flip-flop; this flip flop

provides the necessary pulse which is utilized to turn on the IGBT and to energize the

inductor. In this way, by controlling the IGBT, we can control the mode of operation. A

41


voltage sensor element is used to sense the voltage of the battery and then this voltage is

compared with a reference voltage of 260V, which is the lower limit of battery voltage, if

during boost mode of operation, the battery voltage falls below this threshold vale,

bottom IGBT is turned OFF and top IGBT is triggered for buck conversion, during which

battery is charged. Thus during the regenerative mode of operation, Current mode control

is employed, this is to charge the battery at a constant current. And, during the boost

mode of operation, voltage control is used. Figures 21 and 22 show the simulation results

of Buck and Boost converters.

Fig 21: Buck Converter simulation

42


Fig 22: Boost Converter simulation

4.1.3 Inverter/ Rectifier Model

Inverter/rectifier is a bidirectional block which is used to convert the DC voltage coming

out from the converter which is 380V to an AC voltage of 300V (line to line) to run the

motor and vice versa when the power flow is from generator to the battery during the

regenerative braking. In this design the motor/generator supplies the electricity necessary

to charge the dc battery pack. The motor/generator produces three phase ac voltage and 3

phase ac current. This ac voltage is converted to dc voltage to charge the high voltage

43


batteries using rectifier. The bidirectional inverter/ rectifier model inverts the ac and dc

voltage to and fro between the battery and the motor/generator.

Fig 23 shows the 3- phase inverter/ rectifier model. Pulsed width modulation waveforms

are generated from PWM generator block and are fed to the switches in the inverter

block.

Fig 23: Inverter / Rectifier Model

In the inverter/ rectifier model, there are three legs, one for each phase. The output

depends on the switch status; this output is independent of output load current as one of

the two switches is always on in a leg [19]. The objective here is to control three phase

output magnitude and the frequency. Figure 24 shows the sample 1 phase output of a 3

phase voltage, the output voltage is 300 V line to line and figure 25 shows the 3 phase

44


current. Figure 26 shows the rectifier simulation during the regenerative mode of

operation.

Fig 24: Inverter 1 pahse of 3 phase voltage

45


Fig 25: Inverter 3 phase output current

Fig 26: Rectifier Simulation

46


4.1.4 Motor Model

The motor used is a 55 KW motor, this model is developed using Mathworks tutorial

provided for EcoCAR competition students. The complete system was modeled in

Mathworls tool Simpower system. The output from the PSIM was integrated into

Simpower using Simcoupler tool. From Fig 27, we can see that battery which is initially

rated at 330V is used to drive the motor via the dc – dc converter, by boosting its voltage

to 380V to drive the inverter. Now, the inverter converts dc voltage to an ac voltage and

drives the motor. The inverter is used to drive the motor, it can be depicted that when

battery is driving the motor, it is producing a positive torque. When battery has depleted

to its lower threshold voltage of 260V, motor starts acting as a generator and is used to

charge the battery via the converter as a result producing a negative torque. This charging

of the battery . Torque output shows steps as there are few simulation points.

Regenerative braking refers to the process of capturing energy that is available for use. In

this motor in regenerative condition, the AC power from the motor flows backward

through the inverter bridge to the battery.

47


Fig 27: Motor Model [20]

This model consists of the blocks such as the PWM generator block, which is used to

generate PWM signal for the inverter switches at a desired carrier frequency which

establishes the switching frequency of the switches which is set as 20Khz. and amplitude

modulation of 0.8. This PWM signals are fed into the inverter switches using the

bidirectional Simcouple link of the Simcoupler tool. Figure 28 shows the motor speed.

48


Here, we can see that after 8s , the motor speed is decreasing, this is during the

regenerative braking, when motor acts as generator, and the kinetic energy is utilized to

charge the battery via the buck converter.

Fig 28: Motor speed

During the regenerative mode of operation, the motor torque goes negative, this is

because the rotor speed falling below the synchronous speed and slip becomes negative,

49


hence the torque which is proportional to the slip speed also becomes negative during the

regenerative mode of operation, and this can be seen in figure 29.

Fig 29: Torque output for the motor

50


CHAPTER V

SUMMARY/CONCLUSION

An accurate model has been designed for the Hybrid electric vehicle using PSIM

software and the Simpower systems software. The simulated results show the operation

of motor both as motor and a generator. The power flow can be seen in both the

directions. The concept of regenerative braking is illustrated with the help of simulated

results this model helps in utilization of energy during regenerative braking to charge the

battery.

From the simulation result it can be inferred that, when the battery is fully at 330V, the

battery is used to power the motor, during this case fig 16 shows that battery discharges

through the boost DC -DC converter, here voltage mode control is used and the output of

converter is stepped up to 380V and then the inverter is used to drive the 3 phase motor.

Figures 23, 24 show the output of a 3 phase inverter. It can be seen that the output voltage

is 300V line to line and the 3 phase current is 78A. In the output of inverter voltage,

sample of 2 phases are shown. Also, from the simulation results of figure 25, 26, we can

see that during the motoring mode of operation, the speed in ramping and the torque

being positive. During the regenerative breaking, when motor behaves as a generator,

from figure 16 we can see that the battery is being charged through the buck converter

using a current mode control. Figure 19 shows the slope compensation technique

51


employed to adjust the output voltage. From figure 25, 26 it can be seen that during the

regenerative mode of operation, the speed is declining to zero and also the associated

torque going negative.

This complete model consists only of power electronics components used in hybrid

electric vehicle. Each individual component is run for a longer time, due to the limitation

of the computer, the complete integrated system could not be run for a long time. Hence,

the future work will consists of 1) considering the mechanical components like engine 2)

running the complete system for longer time.

52


REFERENCES

[1] PSAT (Powertrain System Analysis Toolkit)

http://www.transportation.anl.gov/modeling_simulation/PSAT/index.html

[2] Grewe, Conlon and Holmes (2007). “Defining the General Motors 2-Mode Hybrid

Transmission, SAE International, 2007 World Congress, Michigan

[3] Jerome Meisel (2009). “An analytical Foundation for the Two-Mode

Hybrid- Electric Powertrain with a Comparison to the Single-Mode Toyota

Prius THS-II Powertrain” SAE International 2009-01-132

[4] EcoCAR Challenge. (2009). "About EcoCAR".

http://www.ecocarchallenge.org/about_ecocar.html

[5] Maples, Moore, Patterson and Schaper ( 2000). “Alternative Fuels for U.S.

Transportation” Transportation Research Board Business Office, Washington, DC

[6] Li Yu-shan, Zeng Qing-liang, Wang Cheng-long, Wang Liang “Research on

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Public Bus “ , International Conference on Intelligent Computation Technology

and Automation

[7] Ehsani, Mehrdad, Yimin Gao, Sebastien E. Gay, Ali Emadi. (2004). “Parallel

Hybrid Electric Drive-train Design ". In Modern electric, hybrid electric, and fuel

cell vehicles: fundamentals, theory, and design, p.117-136. Florida: CRC Press.

[8] Prakash H Desai, Mohammad Anwar, Sean Gleason, Shawn Hawkins “Power

Electronics for GM 2- Mode Hybrid Electric Vehicle”, SAE International

[9] Ehsani, Mehrdad, Gao, Gay, Emadi. (2004). " Hybrid Electric Vehicles". In

Modern electric, hybrid electric, and fuel cell vehicles : fundamentals, theory,

and design, p.117-136. Florida: CRC Press.

[10] Ehsani, Mehrdad, Gao, Gay, Emadi. (2004). "Series Hybrid Electric Drive-train

Design". In Modern electric, hybrid electric, and fuel cell vehicles:

fundamentals, theory, and design, p.239-258. Florida: CRC Press

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http://www.ecocarchallenge.org/about_ecocar.html


[11] Goro Tamai, Mary Ann Jeffers, Chihsiung Lo, Cindy Thurston, Steven Tarnowsky

and Stephen Poulos. (2005) “Development of the Hybrid System for the Saturn

VUE Hybrid” .

[12] Ehsani, Mehrdad, Yimin Gao, Sebastien E. Gay, Ali Emadi. (2004). “Parallel

Hybrid Electric Drive-train Design ". In Modern electric, hybrid electric, and fuel

cell vehicles: fundamentals, theory, and design, p.117-136. Florida: CRC Press.

[13] Husted (2003). “A Comparative Study of the Production Applications of

Hybrid Electric Powertrains”SAE International, Future Transportation

Technology Conference, California

[14] Brendan Conlon (2005). “Comparative Analysis of Single and Combined Hybrid

Electrically Variable Transmission Operating Modes” SAE International SAE

World Congress, Michigan

[[15] Holmes Alan G, Schmidt Michael R (2002), “Hybrid Electric Powertrain Including

a Two- Mode Electrically Variable Transmission”, SAE Technical Paper

[16] B Conlon (2005), “Comparative Analysis of Single and Combined Hybrid

Electrically variable transmission operating modes”, SAE Technical Paper

[17] A123 Datasheet

http://www.a123systems.com/

[18] Min Chen and Gabriel A. Rinc´on-Mora (2006),” Accurate Electrical Battery

Model Capable of Predicting Runtime and I–V Performance,” IEEE

TRANSACTIONS ON ENERGY CONVERSION, VOL. 21, NO. 2.

[19] PSAT (Powertrain System Analysis Toolkit)


[20] Mathworks Tutorial

http://www.mathworks.com/

[21] Mohan Ned, Undeland Tore, Robbins Williams “Power Electronics: Converters,

Applications, and design

54

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advanced design and simulation of a hybrid electric

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