Power Electronics & Clean Power Research Laboratory

Power Electronics & Clean Power Research Laboratory

http://enjeti.tamu.edu

http://enjeti.tamu.edu

1 1

Texas A&M University

Texas A&M University

http://www.tamu.edu

http://www.tamu.edu

Development of High Frequency 

Development of High Frequency 

Link Direct DC to AC Converters for 

Link Direct DC to AC Converters for 

Solid Oxide Fuel Cells (SOFC)

Solid Oxide Fuel Cells (SOFC)

Dr. Prasad Enjeti

Dr. Prasad Enjeti

Power Electronics Laboratory

Power Electronics Laboratory

Department of Electrical Engineering

Department of Electrical Engineering

Texas A&M University

Texas A&M University

College Station, TX 

College Station, TX 

-

-

77843

77843

SECA Industrial Partner: Delphi

SECA Industrial Partner: Delphi

-

-

Auto

Auto

Power Electronics & Clean Power Research Laboratory

Power Electronics & Clean Power Research Laboratory

http://enjeti.tamu.edu

http://enjeti.tamu.edu

Texas A&M University

Texas A&M University

http://www.tamu.edu

http://www.tamu.edu

Fuel Cell Power Conditioning Stage: 

Fuel Cell Power Conditioning Stage: 

Block diagram 

Block diagram 

(dedicated loads)

(dedicated loads)

V

dc

Output Pow er

Calculator &

Ref. Generator

FUEL
CELL

STACK

ACCESSORY

LOADS

_

Gate Drive

DC/DC Control

Fuel Cell

Controller

Hydrogen

Input

PWM

I

a

I

b

V

a

V

b

I

dc

DC-DC

DC-AC

120V/240V

60Hz
Load

a

n

b

Output

Filter LC

DC-AC

Battery

DC-AC

Control Block

For Voltage & Current

Power Electronics & Clean Power Research Laboratory

Power Electronics & Clean Power Research Laboratory

http://enjeti.tamu.edu

http://enjeti.tamu.edu

Texas A&M University

Texas A&M University

http://www.tamu.edu

http://www.tamu.edu

Fuel Cell Power Conditioning Stage: 

Fuel Cell Power Conditioning Stage: 

Block diagram 

Block diagram 

(connected to utility)

(connected to utility)

C

Electric Utility

L

s

L

s

L

s

V

dc

Reference Signal

Generator

P*

ref

FUEL
CELL

STACK

ACCESSORY

LOADS

_

Gate Drive

DC/DC Control

Fuel Cell

Control

Hydrogen

Input

abc

dq

I

d

I

q

*

I

d

*

PI

V

q

PI

w L

s

V

d

V*

d

V*

q

I

q

w L

s

abc

dq

SVPWM

PI

I

a

I

b

A

V

a

V

b

I

dc

B

DC-DC

DC-AC

Start-up Pow er

Controller

Transformer

Power Electronics & Clean Power Research Laboratory

Power Electronics & Clean Power Research Laboratory

http://enjeti.tamu.edu

http://enjeti.tamu.edu

Texas A&M University

Texas A&M University

http://www.tamu.edu

http://www.tamu.edu

High Frequency Link Direct DC to AC 

High Frequency Link Direct DC to AC 

Converters for SOFC

Converters for SOFC

This project proposes to design and develop high 

This project proposes to design and develop high 

frequency link direct DC to AC converters to improve 

frequency link direct DC to AC converters to improve 

performance, optimize the size, cost, weight and 

performance, optimize the size, cost, weight and 

volume of the DC to AC converter in SOFC systems

volume of the DC to AC converter in SOFC systems

The proposed topologies employ a high frequency 

The proposed topologies employ a high frequency 

link, direct DC to AC conversion approach. The direct 

link, direct DC to AC conversion approach. The direct 

DC to AC conversion approach operates without an 

DC to AC conversion approach operates without an 

intermediate dc

intermediate dc

-

-

link stage

link stage

The absence of the dc

The absence of the dc

-

-

link, results in the elimination 

link, results in the elimination 

of bulky, aluminum electrolytic capacitors, which 

of bulky, aluminum electrolytic capacitors, which 

could result in lower weight/volume/size and cost of 

could result in lower weight/volume/size and cost of 

the power electronic converter

the power electronic converter

Power Electronics & Clean Power Research Laboratory

Power Electronics & Clean Power Research Laboratory

http://enjeti.tamu.edu

http://enjeti.tamu.edu

Texas A&M University

Texas A&M University

http://www.tamu.edu

http://www.tamu.edu

R&D Objectives & Approach

R&D Objectives & Approach

The primary objective is to realize cost effective fuel 

The primary objective is to realize cost effective fuel 

cell converter, which operates under a wide input 

cell converter, which operates under a wide input 

voltage range, and output load swings with high 

voltage range, and output load swings with high 

efficiency and improved reliability 

efficiency and improved reliability 

Employ state of the art power electronic devices & 

Employ state of the art power electronic devices & 

configure two unique topologies to achieve direct 

configure two unique topologies to achieve direct 

conversion of DC power (24

conversion of DC power (24

-

-

48V) available from a 

48V) available from a 

SOFC to AC power (120/240V, 60Hz) suitable for 

SOFC to AC power (120/240V, 60Hz) suitable for 

utility interface and powering stand alone loads

utility interface and powering stand alone loads

Investigate direct DC to AC conversion

Investigate direct DC to AC conversion

Power Electronics & Clean Power Research Laboratory

Power Electronics & Clean Power Research Laboratory

http://enjeti.tamu.edu

http://enjeti.tamu.edu

Texas A&M University

Texas A&M University

http://www.tamu.edu

http://www.tamu.edu

Voltage

Voltage

-

-

fed High Frequency Link 

fed High Frequency Link 

Direct DC to AC Converters for SOFC

Direct DC to AC Converters for SOFC

S

1

S

2

T

a

T

b

Direct DC-AC Converter with

High Freqency Link Transformer

120 V
60 Hz

V

dc

a

c

b

(n)

L
O
A
D

L
O
A
D

I

dc

t

V

t

+

-

I

a

I

b

Input
Filter

I

Direct DC to AC power conversion of fuel cell voltage (22V) to 

Direct DC to AC power conversion of fuel cell voltage (22V) to 

120/240V AC, 60Hz

120/240V AC, 60Hz

The switches are operated in high frequency (40kHz), zero curren

The switches are operated in high frequency (40kHz), zero curren

t 

t 

switching (ZCS) mode

switching (ZCS) mode

ZCS also guarantees transformer volt

ZCS also guarantees transformer volt

-

-

second balance

second balance

Power Electronics & Clean Power Research Laboratory

Power Electronics & Clean Power Research Laboratory

http://enjeti.tamu.edu

http://enjeti.tamu.edu

Texas A&M University

Texas A&M University

http://www.tamu.edu

http://www.tamu.edu

Current

Current

-

-

fed High Frequency Link 

fed High Frequency Link 

Direct DC to AC Converters for SOFC

Direct DC to AC Converters for SOFC

L

s

C

oA

L

f

C

f

Q

5

Q

6

D

f

Q

1

Q

2

Q

4

Q

3

T1

−

−

v

cf

S

1

S

2

C

oA

v

oA

+

−

v

oB

+

−

v

AB

+

−

+

i

Ls

i

Lf

i

s

1 : n

i

T2

Fuel

Cell

V

DC

+

−

+

v

Current

Current

-

-

fed direct DC to AC power conversion of fuel cell voltage 

fed direct DC to AC power conversion of fuel cell voltage 

(22V) to 120/240V AC, 60Hz

(22V) to 120/240V AC, 60Hz

Consists of full

Consists of full

-

-

bridge inverter Q1

bridge inverter Q1

-

-

Q4, HF transformer, simplified 

Q4, HF transformer, simplified 

AC

AC

-

-

AC converter

AC converter

The switches Q5, Q6 (are optional) provide input ripple current 

The switches Q5, Q6 (are optional) provide input ripple current 

(120Hz) cancellation

(120Hz) cancellation

Power Electronics & Clean Power Research Laboratory

Power Electronics & Clean Power Research Laboratory

http://enjeti.tamu.edu

http://enjeti.tamu.edu

Texas A&M University

Texas A&M University

http://www.tamu.edu

http://www.tamu.edu

Current

Current

-

-

fed High Frequency Link 

fed High Frequency Link 

Direct DC to AC Converters for SOFC

Direct DC to AC Converters for SOFC

L

s

C

oA

L

f

C

f

Q

5

Q

6

D

f

Q

1

Q

2

Q

4

Q

3

T1

−

−

v

cf

S

1

S

2

C

oA

v

oA

+

−

v

oB

+

−

v

AB

+

−

+

i

Ls

i

Lf

i

s

1 : n

i

T2

Fuel

Cell

V

DC

+

−

+

v

Full

Full

-

-

bridge inverter offers lower switch stress, simple voltage 

bridge inverter offers lower switch stress, simple voltage 

clamping and transformer flux balance

clamping and transformer flux balance

The switches Q5, Q6, Lf (optional components) provide active 

The switches Q5, Q6, Lf (optional components) provide active 

filtering function: i.e. cancellation of fuel cell input ripple 

filtering function: i.e. cancellation of fuel cell input ripple 

current

current

The converter has three operating modes

The converter has three operating modes

Power Electronics & Clean Power Research Laboratory

Power Electronics & Clean Power Research Laboratory

http://enjeti.tamu.edu

http://enjeti.tamu.edu

Texas A&M University

Texas A&M University

http://www.tamu.edu

http://www.tamu.edu

Current

Current

-

-

fed Direct DC to AC Converter: 

fed Direct DC to AC Converter: 

Operating Modes

Operating Modes

V

D C

Powering mode: Diagonal 

Powering mode: Diagonal 

switches Q1, Q4 or Q2, Q3 are 

switches Q1, Q4 or Q2, Q3 are 

turned

turned

-

-

on. 

on. 

Primary inductor current is 

Primary inductor current is 

transferred to secondary side

transferred to secondary side

V

D C

Restoring mode: switches are 

Restoring mode: switches are 

turned

turned

-

-

off, the inductor current 

off, the inductor current 

flows via the diode 

flows via the diode 

Df

Df

and 

and 

charges the input capacitor

charges the input capacitor

Capacitor voltage is controlled 

Capacitor voltage is controlled 

by Q5, Q6 and is maintained 

by Q5, Q6 and is maintained 

higher than fuel cell voltage

higher than fuel cell voltage

V

D C

Restoring mode: switches 

Restoring mode: switches 

Q1

Q1

-

-

Q4 are turned

Q4 are turned

-

-

on

on

Primary inductor current 

Primary inductor current 

increases at the rate of 

increases at the rate of 

Vdc

Vdc

/Ls

/Ls

Power Electronics & Clean Power Research Laboratory

Power Electronics & Clean Power Research Laboratory

http://enjeti.tamu.edu

http://enjeti.tamu.edu

Texas A&M University

Texas A&M University

http://www.tamu.edu

http://www.tamu.edu

Current

Current

-

-

fed Direct DC to AC Converter: 

fed Direct DC to AC Converter: 

Operating Modes

Operating Modes

V

D C

V

D C

Input ripple current cancellation 

Input ripple current cancellation 

modes are shown in this figure

modes are shown in this figure

Capacitor voltage is controlled 

Capacitor voltage is controlled 

by Q5, Q6 and is maintained 

by Q5, Q6 and is maintained 

higher than fuel cell voltage

higher than fuel cell voltage

V

D C

V

D C

Power Electronics & Clean Power Research Laboratory

Power Electronics & Clean Power Research Laboratory

http://enjeti.tamu.edu

http://enjeti.tamu.edu

Texas A&M University

Texas A&M University

http://www.tamu.edu

http://www.tamu.edu

Direct DC to AC Converter 

Direct DC to AC Converter 

Control Strategy

Control Strategy

+

−

G

v

(s)

abs

Q

1

, Q

4

Q

2

, Q

3

v

o

*

+

−

v

o

+

−

phase

shift

Logic

C ircuit

S

1

, S

2

i

T 2

S

1

S

2

v

o A

 =  p o s itiv e  h a lf c y c le

v

o B

  =   n e g a tiv e   h a lf  c y c le

Voltage control strategy for the direct DC to AC Converter

Voltage control strategy for the direct DC to AC Converter

A proportional (P) controller is used for 

A proportional (P) controller is used for 

Gv(s

Gv(s

)

)

The bi

The bi

-

-

directional switches S1, S2 are selected based on the input curr

directional switches S1, S2 are selected based on the input curr

ent 

ent 

polarity

polarity

Power Electronics & Clean Power Research Laboratory

Power Electronics & Clean Power Research Laboratory

http://enjeti.tamu.edu

http://enjeti.tamu.edu

Texas A&M University

Texas A&M University

http://www.tamu.edu

http://www.tamu.edu

Direct DC to AC Converter 

Direct DC to AC Converter 

Control Strategy

Control Strategy

+

v

oA

, v

oB

i

oA

, i

oB

P

av

V

dc

avg.

power

cal.

i

s,av

−

i

Ls

i

Lf

*

i

Lf

G

f

(s)

+
−

abs

+

−

+

−

buck mode

boost mode

PWM pulse

reference calculation

The above control block diagram is used to 

The above control block diagram is used to 

actively cancel the low frequency ripple current 

actively cancel the low frequency ripple current 

from the fuel cell input current

from the fuel cell input current

Power Electronics & Clean Power Research Laboratory

Power Electronics & Clean Power Research Laboratory

http://enjeti.tamu.edu

http://enjeti.tamu.edu

Texas A&M University

Texas A&M University

http://www.tamu.edu

http://www.tamu.edu

Direct DC to AC Converter 

Direct DC to AC Converter 

Simulation Results

Simulation Results

Output Voltage 

Output Voltage 

120V/240V, 60Hz

120V/240V, 60Hz

Input inductor 

Input inductor 

current

current

Fuel cell input 

Fuel cell input 

current

current

Power Electronics & Clean Power Research Laboratory

Power Electronics & Clean Power Research Laboratory

http://enjeti.tamu.edu

http://enjeti.tamu.edu

Texas A&M University

Texas A&M University

http://www.tamu.edu

http://www.tamu.edu

Activities for the Next 6

Activities for the Next 6

-

-

12 months

12 months

Task # 1: Design

: A detailed design of the proposed direct DC to 

AC converters will be completed: high frequency transformer 

design, component ratings, protection circuitry, hardware layout

will be completed followed by a comprehensive simulation of the 

FCI systems. The design will be optimized to handle wide input 

voltage range and output load swings. 

Completed

Task # 2: 

Task # 2: 

Evaluation of Fuel Cell Ripple Current & Energy Storage

Evaluation of Fuel Cell Ripple Current & Energy Storage

: 

: 

A trade study will be initiated to study the performance of vari

A trade study will be initiated to study the performance of vari

ous 

ous 

types of input filter designs

types of input filter designs

Task # 3: 

Task # 3: 

Hardware Construction & Testing

Hardware Construction & Testing

: Hardware 

: Hardware 

components will be procured for the development of the 

components will be procured for the development of the 

proposed converters and tested with DSP control in open loop

proposed converters and tested with DSP control in open loop

Task # 4: 

Task # 4: 

Testing & Verification

Testing & Verification

: The proposed converters will 

: The proposed converters will 

undergo linear & nonlinear load testing on a SOFC fuel cell 

undergo linear & nonlinear load testing on a SOFC fuel cell 

simulator. These tests will be conducted in coordination with 

simulator. These tests will be conducted in coordination with 

SECA Industrial partner: Delphi

SECA Industrial partner: Delphi

-

-

Auto

Auto

Power Electronics & Clean Power Research Laboratory

Power Electronics & Clean Power Research Laboratory

http://enjeti.tamu.edu

http://enjeti.tamu.edu

Texas A&M University

Texas A&M University

http://www.tamu.edu

http://www.tamu.edu

SOFC Fuel Cell Simulator Development 

SOFC Fuel Cell Simulator Development 

to assist in testing/validation

to assist in testing/validation

The purpose of the fuel cell simulator is to enable 

The purpose of the fuel cell simulator is to enable 

testing and validation of power conditioning module 

testing and validation of power conditioning module 

performance under various loading conditions

performance under various loading conditions

A programmable DC power supply (0

A programmable DC power supply (0

-

-

55V, 10kW) is 

55V, 10kW) is 

controlled via 

controlled via 

Labview

Labview

to emulate V

to emulate V

-

-

I characteristics 

I characteristics 

of a SOFC

of a SOFC

Labview

Labview

is employed for SOFC V

is employed for SOFC V

-

-

I curve fitting and 

I curve fitting and 

emulation

P o w e r   S u p p l y

 

A u t o m a t i c   C o m p u t e r

C o n t r o l

U s e r   V i s u a l i z a t i o n   o n

C o m p u t e r

V o l t a g e   a n d

C u r r e n t

F e e d b a c k

C o m m a n d e d

V o l t a g e

emulation