Converter Topologies and Energy Management for EV

ABSTRACT

––––––––

The environmental and energy issues have propelled the need of Electric vehicles (EV) and this mode of transportation is progressing expeditiously combating diverse barriers. The major challenges include enhancing the driving range, battery life and power capability. To address the above issues, this thesis explores the performance of EV with Hybrid Energy Storage System (HESS) employing Li-ion battery and Ultracapacitor (UC). A comprehensive modelling of various system components of 3-wheeled Light Electric vehicle (LEV) based on Indian Driving Cycle (IDC) is presented to  aid in the sizing of ESS.

A bi-directional DC/DC converter (BDC) acts as an interface between the energy sources and ensures efficient power flow. Thus the choice of the BDC gains significance and there is a need to arrive at a suitable topology. In this perspective, this work presents a comparative study of conventional buck-boost, switched capacitor Luo and Multilevel Modular Capacitor clamped Converter (MMCC) topologies based on current ripple and transfer efficiency. The results indicate the suitability of  MMCC topology for a 3-wheeled LEV.

The energy management control strategy plays a vital role in regulating the power flow as per the drive cycle load requirement. This necessitates an efficient control design to accommodate real-time load fluctuations by regulating the power flow from the hybrid sources. In this work, a hybrid control strategy incorporating filtering and fuzzy rule based technique is proposed for effective power flow regulation. The  results  obtained with respect to various performance measures  including  battery stress factor, UC state-of-charge (SOC) difference, energy consumption rate,

system efficiency and tracking of speed profile indicate that the proposed control strategy performs satisfactorily.

The voltage imbalance issues in series stacking of  UC  are addressed by increasing the conversion gain of MMCC. The selection of conversion gain is a compromise between the capacitance value of UC,  number of parallel stacks and cost. A detailed guideline to arrive at an optimal voltage gain of MMCCC considering the above factors is presented. The failure mode analysis of MMCC converter is presented to highlight its performance under fault conditions.

An overall comparative study of EV employing Battery Energy Storage System (BESS) and the hybrid energy storage system is presented to elucidate the merits of HESS. The economic feasibility of BESS and HESS  are performed and presented to emphasize the effectiveness of Li-ion battery/UC HESS.

The converter topology and energy management control schemes presented in this thesis will open up fresh avenues in hybridizing energy sources for EV applications.

LIST OF TABLES

TABLE NO. TITLE PAGE NO.

Comparative analysis of  HESS parallel

configurations 26

Technical specification of the Bajaj RE 3-wheeler 28

Specification of IDC 29

Energy demand of EV for different cases 35

Specification of Li-ion battery cell 45

Specification of UC bank 48

Switching pattern of SCC Luo Converter 67

Performance analysis of conventional buck-boost BDC 84

Comparative analysis of BDC topologies 85

System components for experimental verification of

BDC 87

Rule base for high SOC of battery 120

Rule base for medium range of battery SOC 121

Rule base for low SOC of battery 121

Comparative analysis of energy management

control strategies 126

System components for experimental  verification  of control strategy  128

Comparative evaluation of UC stacking requirement

for various modules of MMCC Converter 139

Various fault operation modes 149

Performance analysis of BESS and HESS 163

Assumed referential cost and factors 169

Cost analysis for BESS and HESS 169

FIGURE NO. TITLE PAGE NO.

2.18 Electrical equivalent model of UC 47

Basic structure of isolated type BDC 52

Dual-half bridge isolated BDC 53

Dual-active bridge isolated BDC 54

Bi-directional buck-boost converter 56

Cascaded buck-boost converter 56

Bi-directional Cuk converter 57

Bidirectional SEPIC/Zeta DC-DC converter 57

Coupled inductor based non-isolated converter 58

Interleaved bi-directional DC/DC converter 58

Conventional buck-boost BDC topology 59

Boost mode 60

Buck Mode 62

Current waveform 64

Current and voltage stress in the main switch 65

2-Q switched capacitor Luo converter 67

Boost operating mode 68

Current waveforms of UC and battery 70

Voltage and current stress in the main switch during

boost operation 70

Buck operating mode 71

Current waveforms of discharging battery and

charging UC 73

Voltage and current stress in the main switch

during buck operation 73

Schematic of single module MMCC converter 74

(a) Equivalent circuit for state I in buck mode 76

(b) Equivalent circuit for state II in buck mode 76

FIGURE NO. TITLE PAGE NO.

(a) Voltage and current waveform of discharging

battery 77

(b) Voltage and current waveform of charging UC 77

(a) Equivalent circuit for state I in boost mode 78

(b) Equivalent circuit for state II in boost mode 79

(a) Voltage and current waveform of discharging UC 79

(b) Voltage and current waveform of charging battery 79

(a) Voltage and current stress in main switch

during buck mode 80

(b) Voltage and current stress in main switch during

boost mode 80

Pulse dropping switching pattern 83

Experimental setup of conventional buck-boost 88

(a) Inductor current waveform and the switching

pulse during boost mode 88

3.30 (b) Auxiliary source current and its corresponding  switching signal during buck/boost operation  89

(c) Main source and auxiliary source battery

voltage 89

Experimental setup of MMCC converter 90

(a) Switching signals of SR and SB group switches 90

3.32 (b) MMCC output voltage, auxiliary source voltage

and current 91

3.32 (c) Auxiliary source current, main source voltage and current 91

Block diagram of energy management system 95

Energy management control strategies for EV system 96

FIGURE NO. TITLE PAGE NO.

––––––––

Co-ordination of battery and UC for different drive

cycle operation 100

Power train architecture of a 3-wheeled LEV 102

Driver Model 103

Voltage and current controller design 104

4.8 Block diagram of filtering based strategy 106

4.10 (a) SOC status of Li-ion battery and UC 107

4.10 (b) Battery and UC voltage 107

4.10 (c) Power distribution between battery and UC 108

4.10 (d) Current distribution between battery and UC 108

4.10 (e) Energy provided by battery and UC 108

Flowchart of deterministic rule-based strategy 110

(a) Current distribution of battery and UC 111

4.11 (b) Power distribution of battery and UC 111

4.11 (c) Voltage distribution of battery and UC 112

4.11 (d) SOC status of battery and UC 112

(e) Energy distribution of battery and UC 112

Block diagram of integrated filtering and rule-

based strategy 113

(a) Voltage distribution of battery and UC 114

4.13 (b) SOC status of battery and UC 114

4.13 (c) Energy distributions of battery and UC 114

4.13 (d) Current distribution of battery and UC 115

(e) Power distribution of battery and UC 115

Block diagram of hybrid energy management strategy 116

Block diagram of fuzzy inference system 118

(a) Membership function of input 1 119

FIGURE NO. TITLE PAGE NO.

4.16 (b) Membership functions of input 2 119

4.16 (c) Membership functions of input 3 119

(d) Membership function of output 120

Response surface plot of input-output relation 122

(a) SOC status of UC 124

(b) SOC status of battery 124

(c) Current distribution of battery and UC 124

4.18 (d) Power distribution of battery and UC 124

4.18 (e) Voltage distribution of battery and UC 125

(f) Energy provided by HESS 125

Vehicle speed profile 126

Experimental setup 127

Speed profile plot 129

(a) Main battery and auxiliary battery source voltage 129

4.22 (b) Switching pulses of MMCC converter 130

4.22 (c) Auxiliary source current corresponds to the

hall signal 130

(d) Auxiliary source current corresponds to

boost pulses 131

(a) Auxiliary source charging current and voltage corresponds to buck pulse  131

4.23 (b) Main source discharging current , main and auxiliary source voltage during constant speed

operation 132

4.23 (c) Hall signal and corresponding phase current of

BLDC motor 132

FIGURE NO. TITLE PAGE NO.

Redundancy check (a) Schematic of two module MMCC converter (b) Boost operation using

2- module with CR>RVS (c) Buck operation using

single module with CR

CR variation in MMCCC using redundancy

capability (a) Battery SOC (b) UC SOC 138

5-level, 4-module MMCCC configuration 140

Buck Operation 141

Boost operation 142

Switching signals of SB and SR switches 144

Voltage across the modules of 5-level,

four-module MMCCC 144

Fault analysis of MMCC (a) Before fault

(b) After fault 146

Voltage across the modules of MMCC converter 147

SOC status of battery and UC 147

Conventional buck- boost BDC topology 148

Current through main switch S1 150

Voltage spikes across main switch S1 150

UC current 151

Current through main switch S2 152

Voltage spike across main switch S2 153

DC bus current 153

Energy requirement of a 3-wheeled LEV 156

Li-ion battery requirement 157

Parallel stack requirement of battery 158

Cost and weight of the battery system 158

BESS without regeneration 160

FIGURE NO. TITLE PAGE NO.

BESS with regeneration 162

Range and IDC cycle estimation for BESS and HESS 164

Estimated life cycle and time durability of

battery system 165

Cost break down of various components of electric

power train 166

Classification of total operating cost of ESS 167

Capital cost analysis 170

Operating cost analysis 170

LIST OF SYMBOLS AND ABBREVIATIONS

-  Acceleration Power

Fad - Aerodynamic Drag

-  Amplitude modulation

- Amplitude of square wave

- Amplitude of triangular wave

CCBatt - Annual capital cost of battery

-  Annual replacement cost

RCBatt - Annual replacement cost of battery

ARAI - Automotive Research Association of India

-  Average charge/discharge current

UC(dec) - Average current of UC during deceleration

UC(acc) - Average current of UC during acceleration

̅ - Average input current

MCBatt - Average maintenance cost of battery

̅ - Average output