HIGH-FREQUENCY TRANSFORMER ISOLATED FUEL-CELL TO UTILITY INTERFACE POWER CONVERTERS

Transcription

1 HIGH-FREQUENCY TRANSFORMER ISOLATED FUEL-CELL TO UTILITY INTERFACE POWER CONVERTERS Ashoka K.S. Bhat Akshay Rathore Department of Electrical & Computer Engineering University of Victoria Victoria, B. C., V8W 3P6 India International Conference on Power Electronics, Chennai, India, December This work is supported by NSERC, Canada. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 1

2 LAYOUT OF PRESENTATION 1. Introduction to FUEL CELL characteristics 2. Classification & Selection of Utility Interfacing scheme for present application 3. Necessity of Soft-switching 4. Comparison & selection of soft-switched DC-DC converter for present application 5. Proposed FULL RANGE ZVS DC-DC converter for present application 6. Work in progress Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 2

3 Increased Energy Demand DISTRIBUTED Power Generation Alternate (Renewable) Energy Sources: Photovoltaic, Wind, Fuel Cell.. Environment friendly & Clean Solar & Wind Power: Subject to weather conditions Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 3

4 Fuel Cell: Continuous power in all seasons as long as continuity of fuel is maintained. Operate silently (no moving parts) Since no combustion of gas, they reduce noise pollution as well as air pollution. Heat from a fuel cell can be used to provide hot water or space heating for a home or for co-generation. Efficient Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 4

5 FUEL CELL: Electrochemical device, converts chemical energy of a fuel directly into electrical energy (DC power), water & heat by the oxidization of hydrogen. The operation is similar to a battery but it requires continuous flow of fuel to keep reactions going on indefinitely. Degradation (primarily corrosion) or malfunctioning of components limits the life of fuel cells [1-2]. Fuel cell voltage is very low, a fraction of volt per cell. To achieve a higher voltage level, fuel cells are connected in series, known as fuel cell stack [1-2]. Can be damaged by reverse current flow. Current feed back into fuel cell must be avoided. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 5

6 - At a given fuel flow rate, fuel cell has an optimum current to supply maximum output power. Therefore, it is usual to operate the fuel cell below that optimum point to keep the reliability high. - Point at the Boundary of Regions R-II & R-III can be regarded as Optimum/Knee point of maximum Power Density. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 6

7 OPERATE AT MAXIMUM POWER DENSITY: OPTIMUM POINT INSTABILITY IN CONTROL, OSCILLATE BETWEEN HIGHER & LOWER CURRENT DENSITIES AROUND THIS OPTIMUM POINT. OPERATE TO THE LEFT OF POWER DENSITY PEAK (R-II REGION) [1] Fuel Cell Handbook, 5 th Edition, EG & G Services Parsons, Inc. Science Applications International Corporation, U. S. Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory, [2] Fuel Cell Handbook, 7 th Edition, EG & G Services Parsons, Inc. Science Applications International Corporation, U. S. Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory, Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 7

8 Effect of Fuel Flow/Pressure For a given electrical load, fuel flow should be adjusted to give proper match. Causes 2 problems: Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 8

9 1. Flow rate can not be adjusted rapidly, internal chemistry must reach equilibrium before cell support increased load. 2. If electrical load increases too rapidly, it could drive the curve over the knee, exceeding maximum power transfer and overheating fuel cell stack with extra losses. For uncontrolled electrical load, an energy buffer (e.g., a battery) is needed to permit instantaneous response to electrical load shifts while the fuel cell stack catches up. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 9

10 FUEL CELL UTILITY INTERFACE - Power transferred from fuel cell stack to grid varies with fuel flow/pressure NEED: High efficiency, compact size and low cost inverter Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 10

11 FUEL CELL INVERTER SPECIFICATIONS Input Voltage (from FC stack)= V Output Power = 5 kw Output/Utility Line Voltage = 240 V AC (RMS) with variation of -10% to +15% Utility/Grid Frequency = 50/60 Hz Switching Frequency = 100 khz THD 5% (no single harmonics 3%) Power factor = nearly unity Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 11

12 MAJOR INVERTER CLASSIFICATION No Transformer Isolation: Boost converter followed by inverter. TRANSFORMER ISOLATION: 1. Line-Frequency (60 OR 50 HZ) Transformer. 2. High-Frequency Transformer. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 12

13 LINE-FREQUENCY TRANSFORMER ISOLATION A. SINGLE-STAGE INVERSION Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 13

14 B. TWO-STAGE CONVERSION I in L in L 0 D b S 1 D 1 C 1 S 3 D 3 C 3 i u D sb Vin S b C sb C d C o Grid Non-isolated boost converter (DC-DC) S 2 D 2 C 2 S 4 D 4 C 4 n t :1 Line frequency Transformer LINE-FREQUENCY ISOLATION TRANSFORMER SIZE: LARGE, HEAVY & COSTLY. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 14

15 NECESSITY FOR HIGH-FREQUENCY POWER CONDITIONING UNIT LOW VOLTAGE DC TO LINE VOLAGE, LINE FREQUENCY AC VOLTAGE. HIGH BOOST RATIO (~ 16): Difficult to Achieve with Non-Isolated Boost Converter. Also Transformer Isolation is Usually Essential for Isolation from Fuel Cell to Utility (For fault, design, safety, regulatory concerns, etc.). HF Transformers Preferred Over Line Frequency Transformers to Reduce Size, Weight & Cost. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 15

16 HF Link Utility Interface Schemes 1. Two-Stage with Single-ended inverter on primary-side (DC-AC-AC: Unfolding type without intermediate DC Link). 2. Two-Stage using Cycloconverter on the Secondary Side [19-20, 33-34, 41] 3. Three-Stage Configuration with Last Stage HF PWM Voltage Source Inverter [21-22, 41]. 4. Three-Stage Configuration with Last Stage HF Current Modulated Inverter [23-30]. 5. Three-Stage Configuration with Last Stage Line Commutated Inverter (~ Square Current Wave Output) [31, 41]. 6. Three-Stage Configuration with Last Stage Line Frequency Unfolding Inverter [32-61]. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 16

17 The above classification is mainly based on the knowledge on PV array to utility interface power converter schemes [33,41, 41a] [33] R. L. Steigerwald and R. E. Tompkins, A Comparison of High- Frequency Link Schemes for Interfacing a DC Source to a Utility Grid, Proceedings IEEE IAS 82, Vol. 17, 1982, pp [41] A. K. S. Bhat and S. B. Dewan, Resonant Inverters for Photo Voltaic Array to Utility Interface, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-24, No. 4, July 1988, pp [41a] A.K.S. Bhat, Resonant Inverters for Photo Voltaic Array to Utility Interface, M.A.Sc. thesis, Dept. of Electrical Engineering, University of Toronto, Toronto, Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 17

18 Scheme-1: Two-stage conversion, Single ended operation on primary side 1. PRIMARY-SIDE CONTROL 2. NO INTERMEDAITE DC LINK 3. CURRENT CONTROL TECHNIQUE SINGLE-ENDED INVERTER: FLYBACK OR FORWARD (DCM PREFERRED) SINGLE-WINDING PRIMARY SECONDARY: TWO WINDING OR SINGLE WINDING. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 18

19 Example-1 S 1 C o L o Utility Input DC Source C in S 2 M 1 SECONDARY-SIDE SWITCHES OPERATE AT LINE FREQUENCY SWITCHES WITH REVERSE CURRENT BLOCKING (THYRTSITORS, MOSFETS/IGBTS WITH SERIES DIODE) Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 19

20 Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 20

21 Example-2: Multi-switch topology (flyback operation) for scheme 1. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 21

22 Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 22

23 FEATUES OF SCHEME-1 TWO-STAGE CONVERSION PRIMARY-SIDE CAN BE SWITCHED AT HF (NO OVERLAP PROBLEM) SIMPLE, LOW COMPONENT COUNT, LOW COST SOLUTION FOR LOW POWER Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 23

24 Lossy resetting and limited duty cycle Risk of transformer saturation. Transformer size will be bigger Input filter inductor size is large Used for low power Difficult to stabilize the feedback circuit in flyback converter Low efficiency Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 24

25 REFERENCES for Scheme 1: [8] M. F. Schlecht, A Line Interfaced Inverter with Active Control of the Output Current Waveform, Proceedings IEEE PESC 1980, pp [9] M. Nagao and K. Harada, Power Flow of Photovoltaic System Using Buck- Boost PWM Power Inverter, Proceedings PEDS 97, Vol. 1, 1997, pp [10] Y. Konishi, S. Chandhaket, K. Ogura and M. Nakaoka, Utility-Interactive High-Frequency Flyback Transformer Linked Solar Power Conditioner for renewable Energy Utilizations, Proceedings IEEE PEDS 01, Vol. 2, October 2001, pp [11] T. Shimizu, K. Wada and N. Nakamura, A Flyback-Type Single Phase Utility Interactive Inverter with Low-Frequency Ripple Current Reduction on the DC Input for an AC Photovoltaic Module System, Proceedings IEEE PESC 02, Vol. 3, 2002, pp [12] S. B. Kjaer and F. Blaabjerg, Design Optimization of a Single Phase Inverter for Photovoltaic Applications, Proceedings IEEE PESC 03, Vol. 3, 2003, pp [13] N. P. Papanikolaou, E. C. Tatakis, A. Critsis and D. Klimis, Simplified High Frequency Converter in Decentralized Grid-Connected PV Systems: A Novel Low-Cost Solution, Proceedings EPE 03, [14] S. Chandhaket, Y. Konishi, K. Ogura, E. Hiraki and M. Nakaoka, A Sinusoidal Pulse Width Modulated Inverter Using Three-Winding High- Frequency Flyback Transformer for PV Power Conditioner, Proceedings IEEE PESC 03, Vol. 3, June 2003, pp [15] S. Chandhaket, K. Ogura and M. Nakaoka, Y. Konishi, High-Frequency Flyback Transformer Linked Utility-Connected Sinewave Soft-Switching Power Conditioner Using a Switched Capacitor Snubber, Proceedings IEEE IPEMC 04, Vol. 3, August 2004, pp [16] S. Chandhaket, Y. Konishi, K. Ogura and M. Nakaoka, Utility AC Interfaced Soft-Switching Sinewave PWM Power Conditioner with Two- Switch Flyback High-Frequency Transformer, IEE Proceedings, Electric Power Applications, Vol. 151, Issue 5, September 2004, pp [17] N. Kasa, T. Iida and L. Chen, Flyback Inverter Controlled By Sensorless Current MPPT for Photovoltaic Power System, IEEE Transactions on Industrial Electronics, Vol. 52, No. 4, August 2005, pp [18] N. Kasa, T. Iida and A. K. S. Bhat, Zero-Voltage Transition Flyback Inverter for Small Scale Photovoltaic Power System, Proceedings IEEE PESC 05, June Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 25

26 Scheme 2: Two stage conversion, line frequency cycloconverter on secondary side) Scheme-2A Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 26

27 Circuit diagram for scheme 2. Operating waveforms for the circuit shown with control shown in Scheme 2A. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 27

28 SCHEME 2B L in L f Fuel Cell + - C in DC-AC HF double-ended inverter HF Transformer AC-AC Line Frequency Cycloconverter C f Grid Control Circuit Reference current (Line frequency) Input Voltage/Current is Sinusoidal Modulated (Amplitude or Pulse-width). No Modulation in Cycloconverter Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 28

29 FEATUES OF SCHEME-2 TWO-STAGE CONVERSION For higher frequency operation, thyristor/mct should be replaced by AC switches (MOSFET/IGBT in series with a diode). It increases component count & losses; therefore advantage of reduction of one stage is eliminated. Cycloconverter switches show commutation overlap when current through the transformer leakage inductance changes direction. It reduces average output voltage & modifies voltage waveform (distortion). At higher frequency, overlap forms large part of HF cycle. COMPONENTS OF BOTH STAGES FOR PEAK POWER Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 29

30 REFERENCES for Scheme 2: [19] H. Fujimao, K. Kuroki, T. Kagotani and H. Kidoguchi, Photovoltaic Inverter with a Novel Cycloconverter for Interconnection to a Utility Line, Proceedings of IEEE IAS 95, Vol. 3, 8-12 October 1995, pp [20] K. C. A. De Souza, M. R. De Castro and F. Antunes, A DC/AC Converter for Single-Phase Grid-Connected Photovoltaic Systems, Proceeding IEEE IECON 02, Vol. 4, 5-8 November 2002, pp [33] R. L. Steigerwald and R. E. Tompkins, A Comparison of High- Frequency Link Schemes for Interfacing a DC Source to a Utility Grid, Proceedings IEEE IAS 82, Vol. 17, 1982, pp [34] R. L. Steigerwald, A. Ferraro and F. G.Turnbull, Application of Power Transistors to Residential and Intermediate Rating Photovoltaic Array Power Conditioners, IEEE International Semiconductor Power Converter Conference Record, IEEE-IAS Record 1982, pp [41] A. K. S. Bhat and S. B. Dewan, Resonant Inverters for Photo Voltaic Array to Utility Interface, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-24, No. 4, July 1988, pp Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 30

31 Scheme-3: Three stage conversion: DC-DC converter followed by PWM VSI No power flow Only reactive power flow Only active power flow Both active & reactive power flow [21] Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 31

32 Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 32

33 FEATUES OF SCHEME-3 FIRST TWO STAGES DESIGNED FOR AVERAGE POWER, SECOND STAGE FOR PEAK POWER CAN BE USED FOR STAND-ALONE OPERATION SECOND HARMONIC PULSATION REFLECTED & ABSORBED BY INTERMEDIATE DC LINK Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 33

34 An extra large inductor is required to control the active power flow between PWM VSI and utility line. A complex control circuit to control the active power flow from fuel cell stack to utility & to feed sinusoidal current at nearly unity power factor i.e. to keep reactive power at minimum is required Interface to utility is complex. Utility line power factor is unstable with load and input voltage variations. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 34

35 REFERENCES for Scheme 3: [21] D. A. Fox, K. C. Shuey and D. L. Stechschulte, Peak Power Tracking Technique for Photovoltaic Arrays, In IEEE Power Electronics Specialists Conference Record, 1979, pp [22] G. K. Andersen, C. Klumpner, S. B. Kjaer and F. Blaabjerg, A New Green Power Inverter for Fuel Cells, Proceedings of IEEE PESC 02, Vol. 2, June 2002, pp [41] A. K. S. Bhat and S. B. Dewan, Resonant Inverters for Photo Voltaic Array to Utility Interface, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-24, No. 4, July 1988, pp Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 35

36 Scheme-4: Three stage conversion: DC-DC converter followed by current controlled inverter HBCC inverter ouput current Utility voltage V u & current i u V u i u Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 36

37 FEATUES OF SCHEME-4 FIRST TWO STAGES DESIGNED FOR AVERAGE POWER, SECOND STAGE FOR PEAK POWER No extra large inductor for power control Current control: not very complex & utility interconnection simpler PF is good & stable, low THD SECOND HARMONIC PULSATION REFLECTED & ABSORBED BY INTERMEDIATE DC LINK SWITCHING LOSSES IN III-STAGE. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 37

38 REFERENCES for Scheme 4 [23] M. Anderson and B. Alvesten, 200 W Low Cost Module Integrated Utility Interactive for Modular Photovoltaic Energy Systems, Proceedings IEEE IECON 95, Vol. 1, 6-10 November 1995, pp [24] A. Lohner, T. Meyer and A. Nagel, A New Panel-Integratable Inverter Concept for Grid-Connected Photovoltaic Systems, Proceedings IEEE ISIE 96, Vol. 2, 1996, pp [25] D. C. Martins, R. Demonti and I. Barbi, Usage of the Solar Energy from the Photovoltaic Panels for the Generation of Electrical Energy, Proceedings of IEEE INTELEC 99, 6-9 June 1999, pp [26] S. Mekhilef, N. A. Rahim and A. M. Omar, A New Solar Energy Conversion Scheme Implemented Using Grid-Tied Single Phase Inverter, Proceedings IEEE TENCON 00, Vol. 3, 2000, pp [27] D. C. Martins and R. Demonti, Interconnection of Photovoltaic Panels Array to a Single-Phase Utility Line from a Static Conversion System, Proceedings of IEEE PESC 00, Vol. 3, June 2000, pp [28] D. C. Martins, R. Demonti and R. Ruther, Analysis of Utility Interactive Photovoltaic Generation System Using a Single Power Static Inverter, Proceedings of IEEE Photovoltaic Specialists Conference, September 2000, pp [29] D. C. Martins and R. Demonti, Photovoltaic Energy Processing for Utility Connected System, Proceedings IECON 01, Vol. 2, 2001, pp [30] D. C. Martins and R. Demonti, Grid Connected PV System Using two Energy Processing Stages, Conference Record 29 th IEEE Photovoltaic Specialists Conference 2002, pp Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 38

39 Scheme-5: Three-stage, HF inverter- Rectifier-Phase-Controlled Inverter Operating with α ~ 180 o I dc = V dcav V R d ABav Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 39

40 FEATUES OF SCHEME-5 UTILITY INTERFACE IS SIMPLE PF is near unity The output current will have high THD. Line filters are necessary to minimize the current harmonics injected into the utility line. Active filter is a complex solution. All 3-Stages Designed for Peak Power Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 40

41 REFERENCES for Scheme 5 [31] A. K. S. Bhat and S. B. Dewan, A Novel Utility Interfaced High-Frequency Link Photovoltaic Power Conditioning System, IEEE Transactions on Industrial Electronics, Vol. 35, No. 1, February 1988, pp [41] A. K. S. Bhat and S. B. Dewan, Resonant Inverters for Photo Voltaic Array to Utility Interface, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-24, No. 4, July 1988, pp Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 41

42 Scheme-6: Three-Stage HF Lnk with Last Stage Line-Frequency Unfolding Inverter HF inverter current output Rectified output at intermediate DC Link Utility line current Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 42

43 FEATUES OF SCHEME-6 UTILITY INTERFACE IS SIMPLE Only I stage to be controlled PF is near unity & Low THD No large inductor (like scheme 3) Size of L f -C f smaller (compared to Schemes 3-5) The components of all three stages are designed for peak power rating. The risk of HF transformer saturation is higher as compared to schemes 3-5. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 43

44 REFERENCES for Scheme 6 [32] L. Bonte and D. Baert, A Low Distortion PWM DC-AC Inverter with Active Current and Voltage Control, Allowing Line-Interfaced and Stand-Alone Photovoltaic Applications, IEEE INTELEC 82, 3-6 October 1982, pp [33] R. L. Steigerwald and R. E. Tompkins, A Comparison of High- Frequency Link Schemes for Interfacing a DC Source to a Utility Grid, Proceedings IEEE IAS 82, Vol. 17, 1982, pp [34] R. L. Steigerwald, A. Ferraro and F. G.Turnbull, Application of Power Transistors to Residential and Intermediate Rating Photovoltaic Array Power Conditioners, IEEE International Semiconductor Power Converter Conference Record, IEEE-IAS Record 1982, pp [35] A. Cocconi, S. Cuk and R. Middlebrook, High-Frequency Isolated 4kW Photovoltaic Inverter for Utility Interface, Proceedings of 7 th PCI Conference, September , pp [36] B. K. Bose, P. M. Szczesny and R. L. Steigerwald, Microcomputer Control of a Residential Photovoltaic power Conditioning System, IEEE Transactions on Industry Applications, Vol. IA-21, No. 5, September/October 1985, pp [37] V. Rajagopalan, K. Al Haddad and J. Ayer, Innovative Utility- Interactive D.C. to A.C. Power Conditioning System, Proceedings of IEEE IECON 85, Vol. 2, November 1985, pp [38] I. J. Pitel, Phase-Modulated Resonant Power Conversion Techniques for High-Frequency Link Inverters, IEEE Transactions on Industry Applications, Vol. IA-22, No. 6, November/December 1986, pp [39] V. Rajagopalan et. al, Analysis and Design of a Dual Series Resonant Converter for Utility Interface, Proceedings of IEEE PESC 1987, June [40] K. S. Rajashekara et. al., Analysis and Design of a Dual Series Resonant Converter for Utility Interface, Proceedings of IEEE IAS 87 annual meeting, October 1987, pp Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 44

45 [41] A. K. S. Bhat and S. B. Dewan, Resonant Inverters for Photo Voltaic Array to Utility Interface, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-24, No. 4, July 1988, pp [42] A. K. S. Bhat and S. B. Dewan, Analysis and Design of a High- Frequency Link DC to Utility Interface Using Square-Wave Output Resonant Inverter, IEEE Transactions on Power Electronics, Vol. 3, No. 3, July 1988, pp [43] A. K. S. Bhat and S. B. Dewan, DC-to-Utility Interface Using Sinewave Resonant Inverter, IEE Proceedings, Vol. 135, Part B, No. 5, September 1988, pp [44] A. Charette, K. A. Haddad, R. Simard and V. Rajagopalan, Variable Frequency and Variable Phase-Shift Control of Dual Series Resonant Converter for Utility Interface, Proceedings IEEE IECON 88, 1988, pp [45] V. Rajagopalan, K. A. Haddad, A. Charette and K. S. Rajashekara, Analysis and Design of a Dual Series Resonant Converter for Utility Interface, IEEE Transactions on Industry Applications, Vol. 26, No. 1, January/February 1990, pp [46] R. Chaffai, K. Al-Haddad and V. Rajagopalan, A 5 kw Utility- Interactive Inverter Operating at High Frequency and Using Zero Current Turn-off COMET Switches, Proceedings of IEEE IAS 90, Vol. 2, 7-12 October 1990, pp [47] U. Herrmann, H. G. Langer and H. Van Der Broeck, Low Cost DC to AC Converter for Photovoltaic Power Conversion in Residential Applications, Proceedings IEEE PESC 93, June 1993, pp [48] S. W. H. de Haan, H. Oldenkamp and E. J. Wildenbeest, Test Results of 130 W AC Module; A Modular Solar AC Power Station, IEEE Proceedings of 1 st World Conference on Photovoltaic Energy Conversion, 5-9 December 1994, pp [49] I. Takahashi, T. Sakurai and I. Andoh, Development of a Simple Photovoltaic System for Interconnection of Utility Power System, Proceedings of IEEE International Conference on Power Electronics, Drives and Energy Systems for Industrial Growth, Vol. 1, 8-11 Jan. 1996, pp Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 45

46 [50] S. Saha and V. P. Sundarsingh, Novel Grid-Connected Photovoltaic Inverter, IEE Proceedings of Generation, Transmission and Distribution, Vol. 143, issue 2, pp , March [51] T. Takebayashi, H. Nakata, M. Eguchi and H. Kodama, New Current Feed Back Control Method for Solar Energy Inverter Using Digital Signal Processor, Proceedings of IEEE Power Conversion Conference, Vol. 2, 3-6 August 1997, pp [52] Soladin 120 Mastervolt, October 2001 Report, Online available: or [53] J. Jung, G. Yu, J. Choi and J. Choi, High-Frequency DC Link Inverter for Grid-Connected Photovoltaic System, Proceedings IEEE International Photovoltaic Specialists Conference, May 2002, pp [54] H. Terai, S. Sumiyoshi, T. Kitaizumi, H. Omori, K. Ogura, S. Chandhaket and M. Nakaoka, Utility-Interactive Solar Power Conditioner Using High Frequency Sine Wave Modulated Inverter for Distributed Small-Scale Power Supply, Proceedings IEEE ISIE 02, Vol. 3, May 2002, pp [55] H. Terai, S. Sumiyoshi, T. Kitaizumi, H. Omori, K. Ogura, H. Iyomori, S. Chandhaket and M. Nakaoka, Utility-Interactive Solar Photovoltaic Power Conditioner with Soft Switching Sine Wave Modulated Inverter for Residential Applications, Proceedings IEEE PESC 02, Vol. 3, June 2002, pp [56] X. Wang and M. Kazerani, A Modular Photo-Voltaic Grid- Connected Inverter Based on Phase-Shifted-Carrier Technique, Proceedings of IEEE IAS 02 annual meeting, Vol. 4, October 2002, pp [57] W. Xualyuan and M. Kazerani, A Novel Maximum Power Point Tracking Method for Photovoltaic Grid-Connected Inverters, Proceedings IEEE IECON 03, Vol. 3, 2-6 November 2003, pp [58] B. M. T. Ho, S. H. Chung and S. Y. R. Hui, An Integrated Inverter with maximum Power Tracking for Grid-Connected PV Systems, Proceedings IEEE APEC 04, Vol. 3, 2004, pp Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 46

47 [59] Q. Li and P. Wolfs, The Analysis of the Power Loss in A Zero- Voltage Switching Two-Inductor Boost Cell Operating Under Different Circuit parameters, Proceedings IEEE APEC 05, Vol. 3, 6-10 March 2005, pp [60] Q. Li and P. Wolfs, A Current Fed Two-Inductor Boost Converter with Lossless Snubbing for Photovoltaic Module Integrated Converter Applications, Proceedings IEEE PESC 05, June [61] B. M. T. Ho and S. H. Chung, An Integrated Inverter with maximum Power Tracking for Grid-Connected PV Systems, IEEE Transactions on Power Electronics, Vol. 20, Issue 4, July 2005, pp [69] V. T. Ranganathan, P. D. Ziogas and V. R. Stefanovic, A DC-AC power conversion technique using twin resonant high frequency links, In conference record, IEEE Industry Applications Society Annual Meeting, Vol. 17, 1982, pp Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 47

48 Selection of Fuel-Cell to Utility Interfacing Scheme Risk of HF transformer saturation Size Efficiency Power Factor and THD Simplicity Fuel cell ripple current Unit cell power Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 48

49 Comparison of HF isolated utility interfacing schemes Parameter Scheme 1 Scheme 2 Scheme 3 Scheme 4 Scheme 5 Scheme 6 No. of power stages Filter circuits Input capacitor Intermediate DC link cap. Large Large Small Small Small Large NA NA Large Large Small Small last stage cap. Small Small Small Small Small Small Extra inductor No No Yes No No No THD low low low low high low Utility line p.f. good good good but unstable good good good *Scheme 5 will become complex, if active filtering is adopted. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 49

50 Comparison of HF isolated utility interfacing schemes (Contd). Parameter Scheme 1 Scheme 2 Scheme 3 Scheme 4 Scheme 5 Scheme 6 Ease of connection to utility line Simple Simple (Voltage Mode complex) complex simple simple simple III stage switching NA NA HF Switched At least one leg HF switched Line frequency switching Line frequency switching Simplicity of control Simple Simple Complex Simple Simple* Simple Size large small small small small small Efficiency low high high high high high Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 50

51 About power decoupling/energy storage electrolytic capacitor for various schemes Scheme Scheme Scheme 3 Scheme 4 Scheme Power decoupling/energy storage electrolytic capacitor Place Input Input Intermediate DC link Intermediate DC link Input Scheme 6 Input Volume Large Large Small Small small Large Life Medium Medium Long Long Long Medium Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 51

52 BASED ON COMPARISON: SELECT SCHEME 4 OR 6 Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 52

53 Fuel Cell Inverter System for Utility Interface Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 53

54 Multi-Cell Concept: DC-DC Converter Fuel cell input + - Cell 1 C d1 + Output - Cell 2 C d2 Cell n C dn (a) 3-Cells of 1.7 kw each (b) 5-cells of 1 kw each (Selected) Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 54

55 L in L o + C C o V in DC-AC in AC-DC V o - n t :1 HF Tr DC-DC converter for fuel cell to utility interface application Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 55

56 HARD-SWITCHED CONVERTERS Switch Voltage & Currents V SW I SW Switch turn-on Switch turn-off Switching Power Loss Turn-on loss Turn-off loss o Limited switching frequency o EMI o Large heat sinks o Lossy snubbers Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 56

57 V A Hard switched Desired Switching Path Switch Closed B I Switching Paths of the Active Switch Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 57

58 Soft switching V switch C I switch + V gate G V switch I switch t E - Fig. 1 Zero voltage switching. I switch V gate V switch t Fig. 2 Zero current switching. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 58

59 ADVANTAGES OF SOFT-SWITCHING It is possible to have a switching at zerocurrent or zero-voltage minimizing switching losses. Lower losses: lossless snubbers or reduced snubber size, reduction in heat sink size. Higher switching frequency: reduced magnetics and filter size. Switching frequency can be high resulting in light, efficient and less expensive converters. Reduction of EMI and lower switch stresses due to soft-switching. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 59

60 Possible HF Transformer Isolated Soft-switched DC-DC Converter Configurations 1. Series resonant converter (SRC) 2. Parallel resonant converter (PRC) 3. Series parallel resonant converter (SPRC) 4. LCL series resonant converter with capacitive output filter 5.LCL series resonant converter with inductive output filter 6.Phase-shifted full bridge PWM converter with inductive output filter 7.Secondary controlled full bridge converter 8.Current-fed two inductor two switch boost converter Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 60

61 SRC & SPRC: Operate with ZVS only for very narrow variations in supply voltage & load for the present application. PRC: Inverter peak current does not decrease much with reduction in the load. First 3 configurations are not considered for further study. Voltage-fed converters (PWM and resonant) use phase-shift control, i.e., phase-shift between gating signals of fixed duty ratio (50%) applied to HF switches of two legs of front-end HF inverter to regulate the output voltage with load and supply voltage variations. Current-fed converter: Duty ratio of the boost switches is modulated. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 61

62 1. LCL Series Resonant Converter with Capacitive Output Filter Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 62

63 Design Equations for LCL SRC with C-Filter Base Values: V B = V inmin, Z B = (L s /C s ) 1/2 and I B = V B /Z B. Converter gain, M = V o /V B, V o = n t V o. Normalized load Current, J = (I o /n t )/I B. Normalized switching frequency F = ω s /ω r = f s /f r, ω s = 2π f s ; ω r = 1/(L s C s ) 1/2. L s M = J V P o 2 B 2 F π f s (A-1) F Po 2 π f s M J VB C s = 2 (A-2) Selecting suitable ratio of L s /L p, value of L p or L p (=L p / n 2 t ) can be calculated. Optimum point: J = 0.427, M = 0.965, F = 1.1, L s /L p = 0.1. L s = 0.35 µh; L p = 3.5 µh; C s = 8.78 µf; C 1 - C 4 = 47 nf, C o = 25 µf; N s /N p = 16.5 Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 63

64 2 switches loose ZVS at high input voltage. Switch peak and RMS current increase with input voltage. Efficiency of the converter decreases with increase in input voltage. High current rating switches are required. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 64

65 2. LCL Series Resonant Converter with Inductive Output Filter Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 65

66 Design Equations for LCL SRC with L-Filter Base quantities: V B = V in,min, Z B = R L and I B = V B /Z B. Output voltage reflected to primary V o = n t V o. Normalized output voltage reflected to primary side: V ' opu ' Vo sin( δ/ 2) = = (B-1) V 2 π 8 B X D D 2 2 Lspu Cspu where D 1 = + 1 ; D 2 = [X Lspu X Cspu ] (B-2) X X Lppu X Lspu = (Q SF )(F), X Cspu = Q SF /F, X Lppu = (F)(Q SF )(L p /L s ) (B-3) Normalized switching frequency, F = ω s /ω r = f s /f r, ω r = 1/(L s C s ) 1/2 ; ω s = 2π f s, δ = inverter output pulse width; full-load Q SF = (L s /C s ) 1/2 /R L ; R L = n 2 t R L. Values at optimum point (from design curves): V o (gain) = pu, F = 1.1, Q SF = 0.5, L s/ L p = 0.075, n t = C s = F/[2πf s (Q SF )(R L )], L s = [Q SF.R L ) 2 (C s ); L p = (n 2 t L p ) = L s /0.075 (B-4) Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 66

67 L s = 0.27 µh, L p = 3.6 µh, C s =11.47 µf, N s /N p = 20, C 1 - C 4 = 80 nf, L o =1.35 mh, C o =1 µf. 2 switches loose ZVS at high input voltage. Duty cycle loss & rectifier diode ringing problems. High voltage rectifier diodes are required. Lossy RCD snubber circuit is required to clamp the rectifier diode voltage Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 67

68 3. Phase-shifted Full Bridge PWM Converter with Inductive Output Filter I in L 0 V in C in S 1 D 1 C 1 S 3 D 3 C 3 a V ab D 2 D 4 b S 2 C 2 S 4 C 4 i Ls L s n t :1 DR 2 HF Tr DR 1 DR 3 DR 4 + I o C o R L V o - V g1 V g2 V g3 V g4 +V in V ab i Ls -V in S 1, S 4 D 1 D 4 S 4 C 1 D 2 C 3 C 2 S 4 C 4 D 2 S 2, S 3 D 2 D 3 Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 68

69 Design Equations for Phase-Shifted PWM Full-Bridge Converter [80-82] Assume peak-to-peak ripple current of I o = 0.3A (10% of FL current) in the minimum input voltage & full load. Taking effect of duty cycle loss & dead gaps, effective duty ratio (assumed) D eff = Then transformer turns ratio: n D V eff in t = (C-1) Vo L s = n t V in 4 I (1 D o f s eff ) (C-2) L o = V ( n in t V ) D 2 I o o f s eff (C-3) Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 69

70 L s = µh, N s /N p = 20, C 1 = C 2 = 65 nf, C 3 = C 4 = 58 nf, L o = 876 µh; C o = 1 µf. 2 switches loose ZVS at high input voltage and at light load at minimum input voltage (Low ZVS range). Duty cycle loss and rectifier diode ringing problems. High voltage rectifier diodes are required. Lossy RCD snubber circuit is required to clamp the rectifier diode voltage. Efficiency decreases at high input voltage Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 70

71 4. Secondary Controlled Full Bridge Converter with Capacitive Output Filter Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 71

72 Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 72

73 Design Equations for Secondary Controlled Full-Bridge Converter [83-84] ZVS condition for primary switches δ 1 π > ( 1- ) (D-1) M 2 ZVS condition for secondary side switches δ π > ( 1-M) (D-2) 2 where M n V t o = ; Vin n = t N N p s Series tank inductance is calculated by [83-84]: L s = n t V o V ω s in δ π P ( π δ) o (D-3) P o = Output power, ω s = angular switching frequency (rad/sec), δ = phase-shift between primary & secondary side voltage across the transformer leakage inductance. All primary and secondary switches will show ZVS if M = 1. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 73

74 L s = 0.5 µh, N s /N p = 16, C o = 50 µf, C 1 - C 4 = 68 nf, C 5 - C 8 = 0.12 nf.. Secondary switches loose ZVS at high input voltage. Switch peak & RMS current increase with input voltage. Efficiency of the converter decreases with increase in input voltage. 2 Active bridges are required. Higher Number of switches. Two driving circuits (gating controls) are required. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 74

75 5. Current-fed Two Inductor Two Switch Converter with Capacitive Output Filter Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 75

76 Design Equations for Current-Fed Isolated DC-DC Converter [90] Boost inductor values [86]: L L = 2 = V I in in D 1 (E-1) Transformer turns ratio is [86]: f s D V in = 1 - (E-2) V o n D = duty ratio of main switches = T on. T on = ON time of main switches, T s = switching time period, V in = input voltage, f s = switching frequency, I in = permissible ripple in input current, n t = secondary to primary turns ratio of HF transformer. L ( 1-D) V in Vo n Vin s = 1 (E-3) n f P V ( 1 ) s 0 o D P o = full load output power. T s Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 76

77 L s = 1.5 µh, L 1 = L 2 = 100 µh, N s /N p = 4, C 1 = C 2 = 10 nf, C a = 2.5 µf, C a1 = C a2 = 0.7 nf, C o = 50 µf. ZVS is lost at reduced load at high input voltage. Switch peak and RMS current decrease with input voltage. Efficiency of the converter increases with increase in input voltage. Transformer VA rating and switch VA rating are higher. Highest ZVS range and highest efficiency. Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 77

78 Advantages of Current-fed Converter Peak as well as RMS currents decreases with increase in input voltage Highest efficiency, efficiency increases with increase in input voltage Highest ZVS range, holds ZVS at high input voltage Free from the problems of duty cycle loss, rectifier diode ringing, diode voltage clamped at output voltage ZCS turn-off of the rectifier diodes Component realization Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 78

79 Comparison of Schemes for V in = 22 V at full load & in brackets are for V in = 41 V at full load Parameters Scheme 1 Scheme 2 Scheme 3 Scheme 4 Scheme 5 I Lsp (A) 74.5 (142) 70.6 (76.9) 57 (64.5) 66 (124) 46 (24.7) I Lsr (A) 54.5 (70.62) 56.6 (56.5) 51.4 (54.16) 59 (66.4) 18.9 (13.9) I Lpp (A) (13) (12.23) I Lpr (A) 8.68 (8) 8.45 (10.4) V Csp (V) 14 (15.25) (11.66) V Csr (V) 9.82 (11.92) 7.62 (7.86) I SWp (A) 74.5 (142.3) 70.6 (76.9) 57 (64.5) 66 (123.7) 69 (37.1) I SWr (A) 38.5 (51.85) 40 (39.32) 36.1 (37.86) 41.8 (47.1) 29.3 (18.1) V SW (V) 22 (41) 22 (41) 22 (41) 22 (41) 110 (110) I DRav (A) 1.43 (1.43) 1.43 (1.43) 1.43 (1.43) (1.43) Irs(A) (2.94) I DRp (A) 4.3 (8.28) 2.96 (3.24) 3. (3.4) 4.1 (7.7) (6.6) V DRp (V) 350 (350) 600 (930) 420 (780) 350 (350) 350 (350) Transformer VA rating 1199 (1695) 1245 (1575) 1130 (1596) 1298 (2722) 1980 (1110) Main switch VA rating 847 (2126) 880 (1612) 794 (1552) 926 (1857) 3223 (1755) Tank VA rating 1354 (2135) 1150 (1230) 256 (284) 1146 (1384) 336 (182) Aux. switch VA rating (565) Aux. Cap. C a VA rating (10.2) n = N s /N p Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 79

80 Schem es HF switch Rectifi er diode or switch Selected components for various mentioned schemes Scheme 1 Scheme 2 Scheme Scheme 4 Scheme 5 3 IRF3007 IXFH/IXFT V ds = 75 V, I d = Same as Same as Same as 60N20 75 A@ 25 o C, I d scheme 1 scheme 1 scheme 1 V ds = 200 V, I d = 56 A and = o C, R dson = R dson = Ω 100 o 100 o C 8ETH06 V = 600 V; V F = 1.8 V I Fav = 8 A; t rr = 40 ns HFA08TB120 S V R =1200 V, I Fav = 8A V F = 3 V, t rr = 40 ns Same as scheme 2 IRFIB7N50L V ds =500V, I d =6.8A and R dson = 100 o C Aux. Switch Same as scheme 1 FQD18N20V2 V dc =200V, I d =6.8 and R dson =0.23 o C Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 80

81 Table 5: Losses and efficiency for various mentioned schemes for V in = 22 V at full load and in brackets are for V in = 41 V at full load. Losses Scheme 1 Scheme 2 Scheme 3 Scheme 4 Scheme 5 Conduction losses in (123.7) (34.1) MOSFETs (W) (215) (114.7) (177.47) Turn-on Loss (W) 0 (ZVS) 0 (ZVS) 0 (ZVS) 0 (ZVS) 0 (ZVS) Turn-off Loss (W) 1.5 (18) 2.6 (3) 2 (2.7) 2.67 (9.37) 1.3 (0.52) Transformer Loss (W) Rectifier Loss(W) 10.3 (10.3) 17.8 (17.8) 17.8 (17.8) 13.9 (17.9) 10.3 Output snubber loss (W) Auxiliary circuit loss (12.1) (W) Total Loss (W) (253.3) (164.5) (155.2) (214.74) (67) Efficiency (%) 87.7 (79.8) 85.6 (85.8) 87.4 (86.5) 85.7 (82.3) 88.9 (93.7) Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 81

82 Table 6: Comparison of various mentioned schemes Parameters Scheme 1 Scheme 2 Scheme 3 Scheme 4 Scheme 5 Switch peak current Increases with increase in supply voltage & decreases with decrease in load Nearly constant with increase in supply voltage but decreases with decrease in load Increases a little with increase in supply voltage & decreases with decrease in load Increases with increase in supply voltage & decreases with decrease in load Decreases with increase in supply voltage & decrease in load Duty cycle loss Parasitic rectifier diode ringing Not present Present Present Not present Not present Not present Present, requires lossy RCD snubber Present, requires lossy RCD snubber Not present Not present Rectifier Low High High Low Low diode rating Efficiency Higher High High High Higher ZVS range Rectifier diode turnoff 100%-load to 10% load at low input, 2 switches loose ZVS at high input 100%-load to 10% load at low input, 2 switches loose ZVS at high input 100%-load to 35% load at low input, 2 switches loose ZVS at high input 100%-load to 10% load for all input for primary switches, secondary switches loose ZVS at high input line 100%-load to 35% load at low input, & 100%- load - 80% load at high input ZCS Hard Hard ZCS ZCS Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 82

83 To overcome the difficulties, a Full Range ZVS Current-fed DC-DC Converter is Proposed and details will be presented in a future paper. NOTE: A comparison of DC-to-DC converters discussed above is the subject matter of an accepted paper: A. Rathore, A.K.S. Bhat and R. Oruganti, A comparison of soft-switched DC-DC converters for fuel cell to utility interface application, to be presented at the Power Conversion Conference, Nagoya, Japan, April, Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 83

84 FUTURE WORK Practical implementation and closed loop control of the DC-DC Converter Design, analysis and implementation of current-controlled soft-switched inverter Synchronization circuit for utility interface Protection circuits for the system TESTING THANK YOU Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 84