Switching-Cell current source inverter with improved reliability

Thesis for the Degree of Ph.D.

Switching-Cell Current Source Inverter with

Improved Reliability

School of Architectural, Civil, Environmental, and Energy Engineering,

Major in Environmental and Energy Engineering

The Graduate School

Do Duc Tuan

December 2020

The Graduate School

Kyungpook National University

- i -

Abstract

Switching-Cell Current Source Inverter with

Improved Reliability

By

Do Duc Tuan

Current source inverters (CSIs) are extensively used in many industrial

applications. However, they have a reliability issue because CSIs suffer from

the open-circuit problem. Thus, overlap-time should be inserted in the gate

signals for safe commutation. The overlap-time distorts output current

waveforms and increases the total harmonic distortion of output current.

Moreover, it reduces the current source utilization of the inverter.

The open-circuit problem in the CSIs can be mitigated with switching￾cell (SC) structure. SC capacitors with diodes help to reduce dv/dt of power

switches and clamp the over-shoot voltage under open-circuit faults. The new

topologies of the single-phase dual-output (DO) and three-phase four-leg

CSIs are proposed in this thesis.

The single-phase dual-output switching-cell current source inverter (DO￾SC-CSI) is presented in the first part of the thesis. Two modes of common

and different frequency operations are considered and detailed pulse with

- ii -

modulation schemes are also presented. The two outputs of the proposed

inverter are independent both amplitude and frequency. The proposed

inverter can be used in many applications such as two-phase open-end

winding induction motor drive, smart microgrid applications, photovoltaic

grid-connected systems, etc.

In the second part of the dissertation, a novel four-leg SC-CSI,

abbreviated as H8-SC-CSI, is presented. By using SC structure, current paths

for the input current source are always provided. Therefore, the open-circuit

problem is mitigated and can achieve high reliability. In addition, THD in the

output current is reduced because the switch overlap-time is minimized or

removed.

The switching strategies, analysis, design, simulation, and experimental

verification of two novel topologies of the switching-cell current source

inverter are presented in this dissertation.

- iii -

Contents

Chapter 1. Introduction .......................................................................1

1.1 Background .....................................................................................1

1.2 Benefits and Limitation of the Conventional Inverters...................4

1.3 Switching-Cell Structure.................................................................5

1.4 Contribution of This Thesis............................................................6

1.5 Outline of This Thesis.....................................................................8

Chapter 2. Proposed Single-Phase Dual-Output Switching-Cell

Current Source Inverter........................................................................9

2.1 Conventional Single-phase Dual-output CSI..................................9

2.2 Proposed Topology and Operation Principle ..................................9

2.2.1 Proposed Topology ................................................................9

2.2.2 Carrier-Based PWM with Offset Function ..........................10

2.2.3 Switching State Analysis .....................................................14

2.2.4 Overlap-time ........................................................................17

2.2.5 Reliability Analysis..............................................................20

2.2.6 SC Capacitor Design............................................................23

2.2.7 Inductor Design....................................................................23

2.2.8 Common-Mode Voltage ......................................................24

2.3 Semiconductor Loss Analysis.......................................................26

- iv -

2.3.1 Switching loss......................................................................27

2.3.2 Conduction loss....................................................................28

2.4 Simulation and Experimental Results...........................................30

2.4.1 Simulation Results...............................................................30

2.4.2 Experimental Results...........................................................33

2.4.3 Summary ..............................................................................47

Chapter 3. Proposed Switching-Cell Three-Phase Four-Leg Current

Source Inverter.....................................................................................48

3.1 Conventional Three-Phase Four-Leg Current Source Inverter.....48

3.2 Proposed H8-SC-CSI....................................................................48

3.2.1 Proposed Topology ..............................................................48

3.2.2 Operation Principle of the Proposed H8-SC-CSI ................49

3.2.3 Operation Under Fault Conditions.......................................61

3.2.4 SC Capacitor Design............................................................64

3.2.5 Inductor Design....................................................................65

3.2.6 Common-Mode Voltage ......................................................65

3.3 Power Loss Analysis....................................................................66

3.3.1 Conduction Loss ..................................................................67

3.3.2 Switching Loss.....................................................................68

3.4 Simulation and Experimental Results...........................................70

3.4.1 Simulation Results...............................................................71

3.4.2 Experimental Results...........................................................74

- v -

3.5 Performance Comparisons............................................................81

3.6 Summary .......................................................................................82

Chapter 4. Conclusion ........................................................................84

4.1 Summary and contribution............................................................84

4.2 Publication ....................................................................................85

References ...........................................................................................86

- vi -

List of Figures

Fig. 1.1. Conventional CSI...................................................................2

Fig. 1.2. Conventional single-phase dual-output CSIs.........................3

Fig. 1.3. Conventional four-leg CSI (H8-CSI).....................................5

Fig. 1.4. Series connection of two switching-cell ................................6

Fig. 1.5. Current path under fault condition.........................................6

Fig. 2.1. Proposed single-phase dual-output (DO) SC-CSI ...............10

Fig. 2.2. Carrier and reference signal .................................................12

Fig. 2.3. Switching states of the proposed single-phase DO-SC-CSI.16

Fig. 2.4. Gate signals and simplified equivalent circuits of the proposed

single-phase DO-SC-CSI during overlap-time ....................19

Fig. 2.5. Effects of overlap-time on output current............................20

Fig. 2.6. Simplified equivalent circuits of the conventional and

proposed inverter during dead-time ....................................22

Fig. 2.7. DO-SC-CSI for grid-connected PV system .........................25

Fig. 2.8. The power loss of MOSET and diode of the proposed

inverter... ..............................................................................29

Fig. 2.9. Semiconductor loss comparison in the proposed and other

conventional CSIs. ...............................................................29

Fig. 2.10. Prototype photo of the proposed single-phase DO-SC-CSI..31

Fig. 2.11. Waveforms of the proposed inverter in CF mode with

overlap-time of 0.3 s..........................................................35

Fig. 2.12. Waveforms of the proposed inverter in DF mode with

overlap-time of 0.3 s..........................................................36

Fig. 2.13. Switch voltage stresses of the conventional CSIs under fault

condition during 1 µs dead-time ..........................................37

- vii -

Fig. 2.14. Simulation waveform of the proposed DO-SC-CSI under fault

condition. .............................................................................38

Fig. 2.15. Experimental waveforms of the 2HB CSIs with overlap-time

of 0.8 µs. ..............................................................................39

Fig. 2.16. Experimental waveforms of the DO-CSI [17].....................40

Fig. 2.17. Experimental waveforms of the proposed inverter in the CF

mode with overlap-time of 0.3 s. .....................................42

Fig. 2.18. Experimental waveforms of the proposed inverter in the CF

mode with overlap-time of 0.3 s........................................43

Fig. 2.19. Experimental waveforms of the proposed inverter..............44

Fig. 2.20. Efficiency comparison. ........................................................46

Fig. 3.1. Proposed four-leg SC-CSI (H8-SC-CSI). ............................50

Fig. 3.2. High-frequency loops between legs.....................................50

Fig. 3.3. Space vector diagram for the proposed H8-SC-CSI............51

Fig. 3.4. Key waveforms of the proposed inverter.............................54

Fig. 3.5. Simplified equivalent circuits of the operation of the proposed

inverter in sector I under normal condition..........................59

Fig. 3.6. Simplified equivalent circuits of the proposed inverter during

accidental turn-off. ..............................................................63

Fig. 3.7. Loss breakdown of MOSET and diode of the H8-SC-CSI..68

Fig. 3.8. Simulation waveform of the CSIs under normal

conditions…….....................................................................70

Fig. 3.9. Switch voltage stresses under fault condition with 1 s dead￾time. .....................................................................................71

Fig. 3.10. Simulation waveforms of the proposed CSI under fault

condition with 1 s dead-time in each sector.......................72

Fig. 3.11. Prototype of the proposed H8-SC-CSI ................................74

- viii -

Fig. 3.12. Waveform of the H6-CSI in normal condition with 2 s

overlap-time .........................................................................76

Fig. 3.13. Switch voltage stresses of the conventional CSIs under fault

condition during 1 µs dead-time ..........................................76

Fig. 3.14. Waveforms of the proposed CSI under normal condition with

0.2 s overlap-time .............................................................77

Fig. 3.15. Waveforms of the proposed H8-SC-CSI under fault condition

with 1 s dead-time for each sector. ....................................78

Fig. 3.16. Common-mode voltage of the prototype inverters..............79

Fig. 3.17. Efficiency comparison of the prototype inverters................79