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JSL 1 A High-Efficiency Low-Cost DC-DC Converter for SOFC Performance and Control of V6 Converter Presented by Dr. Jih-Sheng (Jason) Lai Virginia Polytechnic Institute and State University Future Energy Electronics Center May 11 – May 13, 2004 SECA Core Technology Program Review Meeting Boston, Massachusetts
Transcript
Page 1: High-Efficiency Low-Cost DC-DC Converter for SOFC Library/Events/2004/seca/Virginia... · A High-Efficiency Low-Cost DC-DC Converter for SOFC ... (50V/div) 20ms/div With voltage ...

JSL1

A High-Efficiency Low-Cost DC-DC Converter for SOFC

Performance and Control of V6 Converter

Presented byDr. Jih-Sheng (Jason) Lai

Virginia Polytechnic Institute and State UniversityFuture Energy Electronics Center

May 11 – May 13, 2004

SECA Core Technology Program Review Meeting

Boston, Massachusetts

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JSL2

Acknowledgement

• Technical support and encouragement of Don Collins of DOE NETL are greatly appreciated.

• Also quoted by Don Collins that a 1% increase in efficiency is worth $75/kWe given a $6.50/mbtu gas cost for a SOFC power plant of the size about 150kW – A motivation to high-efficiency power converter design

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JSL3

Outline

1. Fuel Cell Power Plant and DC/DC Converter Topology Options

2. Features of V6 Converter 3. Calorimeter Setup and Test 4. Testing with PEM Fuel Cells5. Conclusion

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JSL4

1. Block Diagram of the SOFC Power Plant

• Fuel cell output or converter input is low-voltage DC with a wide-range variation

• Plant output is high-voltage ac• Multiple-stage power conversions including

isolation are needed

DC-ACLF-HF

AC-ACLV-HV

AC-DCHV-HV

DC-ACHV-HV

DC-AC Inverter

+Filter

SOFCAC-DC

Rectifier+

FilterVin

+

+

BridgeConverter

HFXformer

SECA DC/DC converter22 to 50 V

400 V 120/240V

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JSL5

DC/DC Converter Topology Options

• Single-phase – Half-bridge converter• Two-phase – Full-bridge converter• Three-phase – Three-phase bridge

converter• Six-phase – The proposed V6 converter

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JSL6

Single-Phase Half-Bridge Converter

1 : nHF-AC

Half-bridge converter

Xformer

Rectifier+LC filter

Act

ive

Load

20V>250A

Sol

id-O

xide

Fue

l Cel

l

400V5 kW

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JSL7

Two-Phase Full-Bridge Converter

1 : n

HF ACXformer

Rectifier+LC filter

Act

ive

Load

Sol

id-O

xide

Fue

l Cel

l

Full-bridge converter

20V>250A

400V5 kW

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JSL8

Full-Bridge Converter with Paralleled Devices to Achieve the Desired Efficiency

Load6x 6x

• With 6 devices in parallel, the two-leg converter can barely achieve 97% efficiency

• Problems are additional losses in parasitic components, voltage clamp, interconnects, filter inductor, transformer, diodes, etc.

Voltage clampSol

id-O

xide

Fue

l Cel

l

20V250A

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JSL9

A Three-Phase Bridge Converter

vin

L

Cf

+

Co vo

+

1 : nS1 S5

S4 S2

a

c

HF ACXformer Rectifier+LC filter

D1

D4

D3

D6

iA ia

iL

Act

ive

Load

Fuel

Cel

l or

othe

r vo

ltage

sour

ce

3-phase bridge inverter

S3

S6

b

D5

D2

A

B

Cn

–iC

• Hard switching • With 4 devices in parallel per switch• Efficiency ≈ 95%

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JSL10

Circuit Diagram of the Proposed V6 Converter

Sol

id-O

xide

Fue

l Cel

l

Act

ive

Load

Rectifier+LC filterSix-phase bridge converter

HF ACXformer

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JSL11

Major Issues Associated with the DC/DC Converter

• Cost• Efficiency• Reliability• Ripple current• Transient response along with auxiliary

energy storage requirement• Communication with fuel cell controller• Electromagnetic interference (EMI) emission

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JSL12

2. Key Features of the V6 Converter• Double output voltage reduce turns ratio and

associated leakage inductance• No overshoot and ringing on primary side device

voltage• DC link inductor current ripple elimination cost and

size reduction on inductor• Secondary voltage overshoot reduction cost and size

reduction with elimination of voltage clamping• Significant EMI reduction cost reduction on EMI filter• Soft switching over a wide load range• High efficiency ~97% • Low device temperature High reliability

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JSL13

Photograph of the V6 Converter

Transformers

Rectifiers

Output Filter

To Load

DC Input

Input Bulk Cap

Phase-shift Controller

Transformers

Power MOSFET

HF Cap

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JSL14

Voltage Conversion Ratio

0 60 120 180

α(º)

n⋅Vdc

2n⋅Vdc

<Vo>

DC/DCMode

dcoo VnV ⋅⋅>=<60α

°<<° 600 .1 α

°<<° 12060 .2 α

dco VnV ⋅⋅>=< 2°<<° 180120 .3 α

0

2n⋅Vdc

6swT

oswT

360⋅α

Vd

6swT

oswT

360⋅α

Vd2n⋅Vdc

2n⋅Vdc

6swT

Vd

0

3Phase xfmr voltage doubling effect

dcoo VnV ⋅⋅>=<60α

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JSL15

Comparison between Full-Bridge and V6 Converters

Full Bridge Converter V6 Converter

iL

iLvd

vd

• Secondary inductor current is ripple-less; and in principle, no dc link inductor is needed

• Secondary voltage swing is eliminated with <40% voltage overshoot as compared to 250%

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JSL16

Significant DC link Inductor Size Reduction

With V6 converter, an effective 10x reduction in DC link filter inductor in terms of cost, size and weight

Single Phase500W60Hz

Single Phase5 kW50kHz

Three Phase5 kW50kHz

Single Phase500W60Hz

Single Phase5 kW50kHz

Three Phase5 kW50kHz

Single Phase500W60Hz

Single Phase5 kW50kHz

Three Phase5 kW50kHz

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JSL17

Input and Output Voltages and Currents at 1kW Output Condition

Vin

VoVoIin

Vin

Iin

IoIo

(a) Full bridge converter (b) V6 Converter

Significant improvement with V6 converterLess EMIBetter efficiency (97% versus 87% after calibration)

(97%)(87%)

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JSL18

Efficiency Measurement Results

80%82%84%86%88%90%92%94%96%98%

100%

0 1000 2000 3000 4000Output power (W)

Experimental data and trend line

• Measurement error: within 1%• Heat sink temperature rise:

<20°C at 2kW with natural convection

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JSL19

Where are the Losses?

• Switch conduction • Diode conduction • Transformer • Output rectifier • Output filter inductor and capacitor• Input capacitor• Parasitics

– Copper traces– Interconnects

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JSL20

3. Calorimetry for Accurate Loss Measurement

• A 50-liter calorimeter• Calibration of the 50-liter calorimeter • A 160-liter calorimeter• Calibration of the 160-liter calorimeter

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JSL21

Basic Calorimeter Principle

( )

2

where

: heat flow (W)

: thermal conductivity of the barrier 0.029 W/m K for styrofoam: surface area (m ): temperature ( C): thickness of barrier

hot coldk A T TQt d

Qtkk

ATd

⋅ ⋅ −=

o

OutsideAmbient Air

InternalAmbient Air ≅

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JSL22

Calorimeter Setup Diagram

Heatsink

Power

Resist

ors

Fan

Fan

Fan EndMiddle

Lid End

Bottom

To DataLogger

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JSL23

A 50-liter Calorimeter

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JSL24

Calibration Results of the 50-Liter Calorimeter

y = 0.7659x + 3.7749

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90Power Loss (W)

Tem

pera

ture

(deg

C)

Temperature RiseLinear (Temperature Rise)

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JSL25

A 160-Liter Calorimeter Setup

Calibration with resistor bankfans

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JSL26

Test the 160-Liter Calorimeter at 120-W Condition

0102030405060708090

0 100 200 300 400 500 600Time (min)

Tem

pera

ture

(deg

C)

Average probe temperature

Temperature rise

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JSL27

Temperature Rise Versus Power Loss

y = 0.4086x + 2.9771

1520253035404550556065

30 40 50 60 70 80 90 100 110 120 130Power Loss (W)

Tem

pera

ture

(deg

C)

TempRise

Linear (TempRise)

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JSL28

4. Test DC/DC Converter with a PEM Fuel Cell

• Fuel cell polarization curve • Dynamic fuel cell voltage and current output• Dynamic fuel cell model in electrical circuit• Model verification • Comparison with slow time constant power

supply test

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JSL29

Polarization Curves with Different Compressor Speeds

I_Ifc

0A 10A 20A 30A 40A 50A 60AV(fc)

20V

25V

30V

35V

40V

45V

2

3

1

1. Fuel cell runs at parasitic load condition Compressor is running at low speed

2. Fuel cell is fully loaded Compressor is not immediately

responding to load step voltage dips

3. Compressor speeds up, fuel cell voltage picks up

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JSL30

Fuel Cell Dynamic Characteristics

0102030405060

0 2 4 6 8 10 12t (sec)

0

500

1000

1500

2000

0 2 4 6 8 10 12t (sec)

vFC(V)

iFC(A)

pFC(W)Step load: 1.47kWParasitic power: 70W

voltage undershoot (2.5V) due to compressor delay

150W dip

27.2V

300W power overshoot

43V

1st time constant

2nd time constant

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JSL31

Fuel Cell Dynamic Circuit Model

1st time constant

+−

+ −

X

+

+−

2nd time constant

+

• High load current high voltage drop• Low output voltage low voltage drop

Multiplying ratio

parasitic load

Transient load

VocFuel cell

High current branch

Low current branch

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JSL32

Verification of Fuel Cell Model with Resistive Load Transient

Fuel Cell Output Voltage (10V/div)

10µs/div

Fuel Cell Output Current (5A/div)

Fuel Cell Output Power (200W/div)

0s 50us 100us0

10

20

30

40

501

0W

0.2KW

0.4KW

0.6KW

0.8KW

1.0KW2

>>

Vfc

PfcIfc

(b)Simulation

results

(a) Experimental

results

Vfc

PfcIfc

<1µs time response on Vfc

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JSL33

Test Load Transient with a Slow Power Supply

Voltage maintains constant after transient

Voltage dip

Load current

inductor current

Voltage returns to original level

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JSL34

Fuel Cell Voltage Dynamic with V6 Converter Load Transient

Input Voltage from Fuel Cell (5V/div)

100ms/div

SimulatedExperimental

Significantly slower time constant (≈50ms) due to 30 mF V6-converter input capacitor and a long cable

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JSL35

V6-Converter Output under Load Step

Load Current (2A/div)

Input Current (20A/div)Output Voltage (50V/div)

20ms/div

With voltage control loop bandwidth designed at 20 Hz, settling time is about 40ms under load step

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JSL36

V6-Converter Output under Load Dump

Load Current (2A/Div)

Output Voltage (50V/Div)

10ms/Div

Input Current (20A/Div)

With voltage control loop bandwidth design at 20 Hz, settling time is about 40ms under load dump

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JSL37

Findings of Fuel Cell Modeling and Converter Test Results

• Fuel cell stack shows very fast dynamic, nearly instantly without time constant

• Perception of slow fuel cell time constant is related to ancillary system not fuel cell stack

• Output voltage dynamic is dominated by the converter interface capacitor and cable inductor

• Output current dynamic is dominated by the load

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JSL38

Summary

• Static and dynamic performances of a new V6 converter has been presented

• Two calorimeters were built and calibrated• PEM fuel cell has been tested under

dynamic conditions for controller design purpose

• A circuit model has been verified for fuel cell dynamic study

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JSL39

Future Work

• Test V6 converter with Calorimeter• Incorporate DC/AC inverter for current

ripple evaluation• Work with PNNL for SOFC dynamic

modeling in electrical circuits and test V6 converter with SOFC

• Develop energy balancing and control strategies with SOFC

• Test EMI performance at EPRI-PEAC


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