soft-switching power converters - ieee · necessity of soft-switching 4. comparison & selection...

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 2006.

This work is supported by NSERC, Canada.

Ashoka Bhat, University of Victoria, Invited Talk, IICPE-2006, Chennai, India page # 1

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


Increased Energy Demand

DISTRIBUTED Power Generation

Alternate (Renewable) Energy Sources: Photovoltaic, Wind, Fuel Cell..

Environment friendly & Clean Solar & Wind Power: Subject to weather conditions


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


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.


- 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.


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, 5th Edition, EG & G Services Parsons, Inc. Science Applications International Corporation, U. S. Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory, 2000.

[2] Fuel Cell Handbook, 7th Edition, EG & G Services Parsons, Inc. Science

Applications International Corporation, U. S. Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory, 2004.


Effect of Fuel Flow/Pressure

• For a given electrical load, fuel flow should be adjusted to give proper match. Causes 2 problems:


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.


• 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


FUEL CELL INVERTER SPECIFICATIONS

• Input Voltage (from FC stack)= 22 – 41 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


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.


LINE-FREQUENCY TRANSFORMER ISOLATION

A. SINGLE-STAGE INVERSION


B. TWO-STAGE CONVERSION

nt:1Line frequency

Transformer

iu

CoVin

S2

D2

C2

Iin

S1

D1

C1 S3

D3

C3

S4

D4

C4

L0Lin

GridSb

Dsb

Csb

Db

Non-isolated boost converter (DC-DC)

Cd

LINE-FREQUENCY ISOLATION

TRANSFORMER SIZE: LARGE, HEAVY & COSTLY.


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.


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].


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. 759-766.

[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. 377-386.

[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, 1982.


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.


Example-1

M1

S1

S2Cin

Co

Lo

Input DCSource

Utility

SECONDARY-SIDE SWITCHES OPERATE AT LINE FREQUENCY SWITCHES WITH REVERSE CURRENT BLOCKING (THYRTSITORS, MOSFETS/IGBTS WITH SERIES DIODE)


Example-2: Multi-switch topology (flyback operation) for scheme 1.


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


• 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


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. 234-241. [9] M. Nagao and K. Harada, “Power Flow of Photovoltaic System Using Buck-

Boost PWM Power Inverter,” Proceedings PEDS’97, Vol. 1, 1997, pp. 144-149.

[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, 22-25 October 2001, pp. 628-632.

[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. 1483-1488.

[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. 1183-1190.

[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, 2003.

[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, 15-19 June 2003, pp. 1197-1201.

[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, 14-16 August 2004, pp. 1242-1247.

[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. 526-533.

[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. 1145-1152.

[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 12-16 2005.


Scheme 2: Two stage conversion, line frequency cycloconverter on secondary side)

Scheme-2A


Circuit diagram for scheme 2.

Operating waveforms for the circuit shown with control shown in Scheme 2A.


SCHEME 2B

DC-AC AC-AC

Lf

Cf Grid

HFTransformer

Lin

Cin

Control Circuit

Reference current

+-

Fuel Cell

(Line frequency)

HFdouble-ended

inverter

Line Frequency Cycloconverter

Input Voltage/Current is Sinusoidal Modulated (Amplitude or Pulse-width).

No Modulation in Cycloconverter


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


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. 2461-2467.

[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. 3268-3273.

[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. 759-766.

[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. 84-96.

[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. 377-386.


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]


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


• 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.


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. 219-227.

[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, 23-27 June 2002, pp. 727-733.



Scheme-4: Three stage conversion: DC-DC converter followed by current controlled

inverter

Utility voltage Vu & current iu

HBCC inverter ouput current

Vu

iu


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.


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. 572-577.

[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. 827-831.

[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. 17.3.

[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. 524-527.

[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, 18-23 June 2000, pp. 1207-1211.

[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, 15-22 September 2000, pp. 1719-1722.

[29] D. C. Martins and R. Demonti, “Photovoltaic Energy Processing for Utility Connected System,” Proceedings IECON’01, Vol. 2, 2001, pp. 1292-1296.

[30] D. C. Martins and R. Demonti, “Grid Connected PV System Using two Energy Processing Stages,” Conference Record 29th IEEE Photovoltaic Specialists Conference 2002, pp. 1649-1652.


Scheme-5: Three-stage, HF inverter-Rectifier-Phase-Controlled Inverter

Operating with α ~ 180o

d

ABavdcavdc R

VVI

−=


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


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. 153-159.



Scheme-6: Three-Stage HF Lnk with Last Stage Line-Frequency Unfolding Inverter

HF invertercurrent output

Rectified output at intermediate

DC Link

Utility line current


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 Lf-Cf 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.


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. 90-95.

[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. 759-766.

[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. 84-96.

[35] A. Cocconi, S. Cuk and R. Middlebrook, “High-Frequency Isolated 4kW Photovoltaic Inverter for Utility Interface,” Proceedings of 7th PCI Conference, September 13-15 1983, pp. 325-345.

[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. 1182-1191.

[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, 18-22 November 1985, pp. 471-476.

[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. 1044-1051.

[39] V. Rajagopalan et. al, “Analysis and Design of a Dual Series Resonant Converter for Utility Interface,” Proceedings of IEEE PESC’1987, 22-27 June 1987.

[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. 711-716.



[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. 355-363.

[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. 193-201.

[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. 563-568.

[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. 80-87.

[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. 1081-1085.

[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. 588-594.

[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 1st World Conference on Photovoltaic Energy Conversion, 5-9 December 1994, pp. 925-928.

[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. 88-93.


[50] S. Saha and V. P. Sundarsingh, “Novel Grid-Connected Photovoltaic Inverter,” IEE Proceedings of Generation, Transmission and Distribution, Vol. 143, issue 2, pp. 219-224, March 1996.

[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. 687-690.

[52] Soladin 120 Mastervolt, October 2001 Report, Online available: www.mastervolt.com or www.mastervoltsolar.com

[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, 19-24 May 2002, pp. 1410-1413.

[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, 26-29 May 2002, pp. 942-947.

[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, 23-27 June 2002, pp. 1501-1505.

[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, 13-18 October 2002, pp. 2520-2525.

[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. 2332-2337.

[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. 1559-1565.


http://www.mastervolt.com/

http://www.mastervoltsolar.com/

[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. 1851-1857.

[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 12-16 2005.

[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. 953-962.

[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. 786-792.


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


Comparison of HF isolated utility interfacing schemes Parameter Scheme 1 Scheme 2 Scheme 3 Scheme 4 Scheme 5 Scheme 6

No. of power stages

2

2

3 3 3 3

Filter circuits 2 2 3 3 3 3

Input capacitor

Large Large Small Small Small Large

Intermediate DC link cap.

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.


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 HFswitched

Line frequency switching

Line frequencyswitching

Simplicity of control

Simple Simple Complex Simple Simple* Simple

Size large small small small small small Efficiency low high high high high high


About power decoupling/energy storage electrolytic capacitor for various schemes

Power decoupling/energy

storage electrolytic capacitor

Scheme 1

Scheme 2

Scheme 3 Scheme 4 Scheme 5

Scheme 6

Place Input Input Intermediate DC link

Intermediate DC link

Input Input

Volume Large Large Small Small small Large

Life Medium Medium Long Long Long Medium


BASED ON COMPARISON: SELECT SCHEME 4 OR 6


Fuel Cell Inverter System for Utility Interface


Multi-Cell Concept: DC-DC Converter

Cell 1

Cell 2

Cell n

+ -Fuel cell input Cd1

Cd2

Cdn

Output+

-

(a) 3-Cells of 1.7 kW each (b) 5-cells of 1 kW each (Selected)


nt:1HF Tr

CoVoVin

Cin+

-

DC-AC

Lin

AC-DC

Lo

DC-DC converter for fuel cell to utility interface application


HARD-SWITCHED CONVERTERS

Switch Voltage & Currents

VSW ISW

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


V

I

Hard switched Desired Switching Path A

B

Switch Closed

Switching Paths of the Active Switch


Soft switching

Vswitch

Vgate

Iswitcht

+

-

Vswitch

Iswitch

G

C

E

Fig. 1 Zero voltage switching.

Iswitch

VgateVswitch

t

Fig. 2 Zero current switching.


ADVANTAGES OF SOFT-SWITCHING It is possible to have a switching at zero-current 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.


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


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.


1. LCL Series Resonant Converter with Capacitive Output Filter


Design Equations for LCL SRC with C-Filter

Base Values: VB = Vinmin, ZB = (Ls/Cs)1/2 and IB = VB/ZB.

Converter gain, M = Vo′/VB, Vo′ = nt Vo.

Normalized load Current, J = (Io/nt)/IB.

Normalized switching frequency F = ωs/ωr = fs/fr,

ωs = 2π fs; ωr = 1/(LsCs)1/2.

⎥⎦

⎤⎢⎣

⎡⋅⋅⎥

⎥⎦

⎤

⎢⎢⎣

⎡ ⋅⋅=

so

Bs fπ

FP

VJML

2

2

(A-1)

⎥⎥⎦

⎤

⎢⎢⎣

⎡

⋅⋅⋅⋅⋅

⋅= 22 Bs

os VJMfπ

PFC (A-2)

Selecting suitable ratio of Ls/Lp′, value of Lp′ or Lp (=Lp′/ nt

2) can

be calculated.

Optimum point:

J = 0.427, M = 0.965, F = 1.1, Ls/Lp′ = 0.1.

Ls = 0.35 µH; Lp′ = 3.5 µH; Cs = 8.78 µF;

C1 - C4 = 47 nF, Co = 25 µF; Ns/Np = 16.5


• 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.


2. LCL Series Resonant Converter with Inductive Output Filter


Design Equations for LCL SRC with L-Filter

Base quantities: VB = Vin,min, ZB = RL′ and IB = VB/ZB.

Output voltage reflected to primary Vo′ = ntVo.

Normalized output voltage reflected to primary side:

( )2

22

1

2sin

DD

δ/VV

VB

'o'

opu+

== (B-1)

where⎥⎥⎦

⎤

⎢⎢⎣

⎡+

−= 1

8

2

1Lppu

CspuLspu

XXXπD ; D2 = [XLspu – XCspu] (B-2)

XLspu = (QSF)(F), XCspu = QSF/F, XLppu = (F)(QSF)(Lp′/Ls) (B-3)

Normalized switching frequency, F = ωs/ωr = fs/fr,

ωr = 1/(LsCs)1/2; ωs = 2π fs, δ = inverter output pulse width;

full-load QSF = (Ls/Cs)1/2/RL′; RL′ = nt2RL.

Values at optimum point (from design curves):

Vo′ (gain) = 0.795 pu, F = 1.1, QSF = 0.5,

Ls/Lp′ = 0.075, nt = 0.05.

Cs = F/[2πfs(QSF)(RL′)],

Ls = [QSF .RL′)2(Cs); Lp′ = (nt2Lp) = Ls/0.075 (B-4)


• Ls = 0.27 µH, L’p = 3.6 µH, Cs=11.47 µF,

Ns/Np = 20, C1- C4 = 80 nF, Lo=1.35 mH, Co=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


3. Phase-shifted Full Bridge PWM Converter with Inductive Output Filter

RLLs

nt:1HF Tr

DR2

Co

Io

Vo

DR1 DR3

DR4

iLs

VinCin

S2

D2

C2

Iin

+

-

S1

D1

C1 S3

D3

C3

S4

D4

C4

L0

a

bVab

Vg1

-Vin

Vg2

Vg3 Vg4

+Vin

D1D4

S1, S4

S4C1C2

D2S4

C3C4D2

D2D3

S2, S3

iLsVab


Design Equations for Phase-Shifted PWM Full-Bridge Converter [80-82]

Assume peak-to-peak ripple current of ∆Io = 0.3A (10% of FL current) in the output @ minimum input voltage & full load. Taking effect of duty cycle loss & dead gaps, effective duty ratio (assumed) Deff = 0.85. Then transformer turns ratio:

o

inefft V

VDn

⋅= (C-1)

so

effints fI

DVnL

⋅⋅

−⋅=

4)1( (C-2)

so

effot

in

o fI

DVnV

L⋅∆⋅

⋅−=

2

)( (C-3)


Ls = 0.154 µH, Ns/Np = 20,

C1 = C2 = 65 nF, C3 = C4 = 58 nF,

Lo = 876 µH; Co = 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


4. Secondary Controlled Full Bridge Converter with Capacitive Output Filter


Design Equations for Secondary Controlled Full-Bridge Converter [83-84]

ZVS condition for primary switches

211 π )M

- (δ ⋅> (D-1)

ZVS condition for secondary side switches

21 π -M) (δ ⋅> (D-2)

where in

ot

VVn

M ⋅

= ; s

pt N

Nn =

Series tank inductance is calculated by [83-84]: ( )os

inots Pπ ω

δπδVVn L

⋅⋅−⋅⋅⋅

= ⋅ (D-3)

Po = Output power, ωs = 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.


Ls = 0.5 µH, Ns/Np = 16,

Co = 50 µF, C1 - C4 = 68 nF, C5 - C8 = 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.


5. Current-fed Two Inductor Two Switch Converter with Capacitive Output Filter


Design Equations for Current-Fed Isolated DC-DC Converter [90]

Boost inductor values [86]:

sin

in

f∆I DV

LL⋅

⋅== 21 (E-1)

Transformer turns ratio is [86]:

o

in

V nV

- D ⋅

= 1 (E-2)

D = duty ratio of main switches =s

on

TT .

Ton = ON time of main switches, Ts = switching time period,

Vin = input voltage, fs = switching frequency,

∆Iin = permissible ripple in input current,

nt = secondary to primary turns ratio of HF transformer.

( )( ) ⎥

⎦

⎤⎢⎣

⎡−

−⋅⋅

⋅⋅⋅⋅

= 11

1

0 DVVn

Pfn V V-D L

o

in

s

oins (E-3)

Po = full load output power.


• Ls = 1.5 µH, L1 = L2 = 100 µH, Ns/Np = 4,

C1 = C2 = 10 nF, Ca = 2.5 µF,

Ca1 = Ca2 = 0.7 nF, Co = 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.


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


Comparison of Schemes for Vin = 22 V at full load & in brackets are for Vin = 41 V at full load

Parameters Scheme 1 Scheme 2 Scheme 3 Scheme 4 Scheme 5 ILsp (A) 74.5 (142) 70.6 (76.9) 57 (64.5) 66 (124) 46 (24.7)ILsr (A) 54.5

(70.62) 56.6 (56.5) 51.4

(54.16) 59 (66.4) 18.9

(13.9) ILpp (A) 14.85 (13) 12.2

(12.23) - - -

ILpr (A) 8.68 (8) 8.45 (10.4) - - - VCsp (V) 14 (15.25) 11.94

(11.66) - - -

VCsr (V) 9.82 (11.92)

7.62 (7.86) - - -

ISWp (A) 74.5 (142.3)

70.6 (76.9) 57 (64.5) 66 (123.7)

69 (37.1)

ISWr (A) 38.5 (51.85)

40 (39.32) 36.1 (37.86)

41.8 (47.1)

29.3 (18.1)

VSW (V) 22 (41) 22 (41) 22 (41) 22 (41) 110 (110)

IDRav (A) 1.43 (1.43) 1.43 (1.43) 1.43 (1.43)

- 1.43 (1.43)

Irs(A) - - - 2.61 (2.94)

-

IDRp (A) 4.3 (8.28) 2.96 (3.24) 3. (3.4) 4.1 (7.7) 11.46 (6.6)

VDRp (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

- - - - 622 (565)

Aux. Cap. Ca VA rating

- - - - 40.8 (10.2)

n = Ns/Np 16.5 20 19 16 4


Selected components for various mentioned schemes Schem

es Scheme 1 Scheme 2 Scheme

3 Scheme 4 Scheme 5

HF

switch

IRF3007 Vds = 75 V, Id = 75 A@ 25oC, Id

= 56 A and Rdson = 0.02Ω @

100oC

Same as scheme 1

Same as scheme 1

Same as scheme 1

IXFH/IXFT 60N20

Vds = 200 V, Id = 60 A @ 25oC, Rdson = 0.052 Ω

@ 100oC Rectifi

er diode

or switch

8ETH06 V = 600 V; VF =

1.8 V IFav = 8 A; trr =

40 ns

HFA08TB120S

VR=1200 V, IFav = 8A

VF = 3 V, trr= 40 ns

Same as scheme 2

IRFIB7N50L Vds=500V,

Id=6.8A and Rdson = 0.51Ω

@ 100oC

Same as scheme 1

Aux. Switch

-

-

-

-

FQD18N20V2 Vdc=200V, Id=6.8 and

Rdson=0.23 Ω @100oC


Table 5: Losses and efficiency for various mentioned schemes for Vin = 22 V at full load and in brackets are for Vin = 41 V at full load.

Losses Scheme 1 Scheme 2 Scheme 3 Scheme 4 Scheme 5 Conduction losses in

MOSFETs (W) 118.6 (215)

128 (123.7) 104.25 (114.7)

139.78 (177.47)

89.3 (34.1)

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) 10 10 10 10 10

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) 0 10 10 0 0

Auxiliary circuit loss (W)

- - - - 14 (12.1)

Total Loss (W) 140.4 (253.3)

168.4 (164.5)

144.05 (155.2)

166.35 (214.74)

124.9 (67)

Efficiency (%) 87.7 (79.8) 85.6 (85.8) 87.4 (86.5) 85.7 (82.3) 88.9 (93.7)


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

Not present Present Present Not present Not present

Parasitic rectifier diode

ringing

Not present Present, requires

lossy RCD snubber

Present, requires

lossy RCD snubber

Not present Not present

Rectifier diode rating

Low High High Low Low

Efficiency Higher High High High Higher

ZVS range

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


100%-load to 35% load at low input, 2 switches


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

Rectifier diode turn-

off

ZCS Hard Hard ZCS ZCS


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, 2007.


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


soft-switching power converters - ieee · necessity of soft-switching 4. comparison & selection...

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