# On-grid and off-grid operation of multi-input single-output DC/DC converter based fuel cell generation system

**ABSTRACT** This paper presents the modeling and simulation of a fuel cell generation unit for off-grid and on-grid operation and configuration. A fuel cell unit consists of a fuel cell array, DC/DC converter and DC/AC inverter. The dynamic model for fuel cell array and its power electronic interfacing are presented; also a multi-input DC/DC converter and its control scheme is proposed and analyzed. This DC/DC converter is capable of interfacing fuel cell arrays to the DC/AC inverter. The suitable control strategy of DC/AC inverter for different operating conditions is demonstrated. The simulation results based on PSCAD/EMTDC software show the capability of fuel cell generation system for dynamic studying in the various operations.

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**ABSTRACT:**In this paper, a set of interfaces are implemented for supplying residential 3- phase loads by a Fuel Cell (FC). The proposed system consists of a DC/DC converter and a high frequency transformer to boost FC voltage. Furthermore, an auxiliary switch is applied to control the power flow from FC to the output load. Also, an inductance is paralleled with the primary terminals of the transformer to contribute in the power transfer procedure. The other part is a three phase full bridge SPWM inverter with an LC filter. A battery pack is connected to the DC bus by a bidirectional switch to aid supplying required voltage for the inverter at load peaks. The main advantage of this topology is the capability of controlling power flow. Reducing the number of switches and diodes at the DC side, which results in reduction of conducting losses, and simple control strategy, are the other advantages. The simulation results are provided by PSCAD/EMTDC software to verify the advantages of the proposed topology. - SourceAvailable from: Sayyed Mahdi Ale Emran
##### Conference Paper: Modeling the combination of UPQC and photovoltaic arrays with Multi-Input Single-Output DC-DC converter

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**ABSTRACT:**In this paper analysis results of a cooperation of the unified power quality conditioner (UPQC) with the photovoltaic system are presented. The proposed system consists of a series inverter, a shunt inverter, and some photovoltaic arrays connected in the dc link through the multi-input single-output (MISO) DC-DC converter. A numerical formulation of the proposed control schemes, which are based on the instantaneous power theory, is presented. The proposed system can improve the power quality at the Point of Common Coupling (PCC) on power distribution systems. The performance of the proposed system was analyzed using simulations with power system computer aided design/ElectroMagnetic transients DC (PSCAD/EMTDC) analysis program.Power Systems Conference and Exposition, 2009. PSCE '09. IEEE/PES; 04/2009 - SourceAvailable from: Sayyed Mahdi Ale Emran[Show abstract] [Hide abstract]

**ABSTRACT:**In this paper analysis results of a cooperation of the unified power quality conditioner with the fuel cell units are presented. A comprehensive control strategy is introduced to extract the compensating signals for the control of the proposed system. The used control strategy can extract the reference currents and voltages of UPQC fast and accurately in the presence of harmonics and/or frequency oscillation. The proposed scheme can compensate voltage sag, voltage interruption and harmonics at the Point of Common Coupling (PCC) on power distribution system. The simulation results prove the efficiency of using the presented method on power quality improvement.

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been proposed to interface fuel cell DG system with the

Iranian Journal of Electrical & Electronic Engineering, Vol. 5, No. 2, Jun. 2009

131

On-grid and Off-grid Operation of Multi-Input Single-Output

DC/DC Converter based Fuel Cell Generation System

R. Noroozian*, M. Abedi**, G. B. Gharehpetian** and S. H. Hosseini***

Abstract: This paper presents the modeling and simulation of a proton exchange

membrane fuel cell (PEMFC) generation system for off-grid and on-grid operation and

configuration. A fuel cell DG system consists of a fuel cell power plant, a DC/DC converter

and a DC/AC inverter. The dynamic model for fuel cell array and its power electronic

interfacing are presented; also a multi-input single output (MISO) DC/DC converter and its

control scheme is proposed and analyzed. This DC/DC converter is capable of interfacing

fuel cell arrays to the DC/AC inverter. Also the mathematical model of the inverter is

obtained by using average technique. Then the novel control strategy of DC/AC inverter for

different operating conditions is demonstrated. The simulation results show the

effectiveness of the suggested control systems under both on-grid and off-grid operation

modes.

Keywords: distributed generation (DG), modeling, PEM fuel cell (PEMFC), operation, and

power electronic interface.

1 Introduction 1

The fuel cells can be used in the cars, buildings,

hospitals, hotels, industrial facilities fast food outlets,

etc. These applications have two off-grid and on-grid

operation modes [1].

The fuel cells generate DC electrical energy from

hydrogen by using a chemical process and their

emissions are water. Therefore, power electronic

circuits are an enabling technology that is necessary to

convert DC electrical power generated by a fuel cell

into usable AC power for passive loads, automotive

applications, and interfaces with electric utilities. The

fuel cell DG system is interfaced with the utility

network via boost DC/DC converters and a three-phase

pulse-width modulation (PWM) DC/AC inverter. In

recent decades, various power electronic circuits have

Iranian Journal of Electrical & Electronic Engineering, 2009.

Paper first received 25 Sep. 2008 and in revised form 27 Dec. 2008.

* R. Noroozian is with the Department of Electrical Engineering,

Faculty of Engineering, University of Zanjan, P.O.Box 45195-313,

Zanjan, Iran.

E-mail: noroozian@aut.ac.ir.

** M. Abedi and G. B. Gharehpetian are with the Department of

Electrical Engineering, Amirkabir University of Technology, Tehran,

Iran.

E-mail: abedi@aut.ac.ir; grptian@aut.ac.ir

*** S. H. Hosseini is with the Department of Electrical Engineering,

Tabriz University, Tabriz, Iran.

E-mail: hosseini@tabrizu.ac.ir.

utility grid [1-4]. The DC voltage generated by a fuel

cell stack varies widely and is low in magnitude.

Therefore, a boost DC/DC converter is necessary to

generate a regulated higher voltage DC for desired

inverter input voltage. The boost DC/DC converter is

responsible for drawing power from the fuel cell, and

therefore should be designed to match fuel cell ripple

current specifications. Conventional DC/DC converters,

such as push-pull, half bridge and full-bridge converters

can be used to boost the low voltage of the fuel cell to

the required level. However, the transformers in these

converters have considerable turns ratios (such as 1:20),

and hence, high leakage inductances, which results in

low energy efficiency and difficulty in control of the

DC/DC converter [5]-[6]. DC/DC boost converters are

usually used to interface the DC output of fuel cell units

with the utility network. The static (V-I) characteristics

of fuel cells show more than a 30% difference in the

output voltage between no load to full-load conditions

[7-8]. This inevitable decrease, which is caused by

internal losses, reduces the utilization factor of the fuel

cells at low loads. To increase the utilization of fuel

cells, this paper, multi-input single-output (MISO)

DC/DC converter for fuel cell arrays, which provides

well-regulated output voltage, has been presented and

analyzed. The advantages of this converter are its

simple configuration, fewer component number, lower

cost and higher efficiency. Another advantage of MISO

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#2Array

Battery

Meter

Iranian Journal of Electrical & Electronic Engineering, Vol. 5, No. 2, Jun. 2009 132

DC/DC converters is that their switching frequency can

be lower than a traditional converter, which means

reduced switching losses and increased efficiency.

Therefore, the MISO DC/DC converter is useful for

combining fuel cell arrays.

The DC/AC voltage source inverter (VSI) has been

widely used to interconnect a fuel cell energy system to

a utility grid under both on-grid and off-grid operations

[1], [4]. The control strategy of the DC/AC inverter

should be able to deliver a preset amount of active and

reactive power to the grid or be able to supplying the

isolated unbalanced AC load by constant balanced AC

voltage magnitude and frequency. Therefore, the

DC/AC inverter controller in on-grid mode controls the

active and reactive power flows to the utility grid.

Therefore, a novel and simple control system based

instantaneous power control strategy has been proposed

and developed for DC/AC inverter [9-11].

The DC/AC inverter controller in off-grid operating

mode regulates the voltage and the frequency of isolated

unbalanced load. Therefore, a novel control system

based on

0qd

−−

rotating frame has been proposed for

DC/AC inverters. In qd

−−

current and voltage components in the

are given rise to 2ω current ripples. The zero

component appears as a disturbance at ω [12-14].

These three sequences are regulated independently by

the Proportional Integral (PI) controllers.

This paper presents the modeling and controlling of

fuel cell generation system under off-grid and on grid

modes. Also, the mathematical model of the DC/AC

inverter is derived by using the average large signal

model. Two separate novel controllers are designed for

these purposes. The proposed system has been modeled

and simulated. The simulation results show the

generated power from the fuel cell DG system can be

controlled.

0 rotating frame, the load

d − channels

q

The results also show that the fuel cell generation

system is capable to supplying the unbalanced AC loads

by constant balanced AC voltage magnitude and

frequency

2 System Configuration

Fig. 1 shows the schematic diagram of the fuel cell

generation system, which has been studied in this paper.

The basic components of this system are the fuel cell

power plant, MISO DC/DC converter and DC/AC

inverter. A validated 500 W PEMFC dynamic model,

reported in [15], is used to model the fuel cell power

plant. The fuel cell power plant consists of n fuel cell

arrays connected in parallel. The MISO DC/DC

converter is used to boost the low voltage of the fuel

cell to make a high voltage DC bus. In this paper, the

DC bus voltage (MISO DC/DC converter output) is

chosen as Vdc=V 750. The controllers of the boost

DC/DC converters are designed to keep the DC bus

voltage within specified limit (

inverter is a three-phase six-switch VSI with neutral

clamped DC capacitors, which interfaces the DC bus

with a V V/400 220 AC power system. An LC filter

connected to the inverter filters the switching frequency

harmonics and generates a high quality sinusoidal AC

waveform suitable for the load. The VSI controller in

on-grid mode controls the active and reactive power

delivered from the fuel cell energy system to the utility

grid. The active and reactive power flows follow within

specified reference values, which can be set by using

power management units. The VSI controller in off-grid

operating mode regulates the unbalanced load voltage of

balanced and sinusoidal with constant amplitude and

frequency. Supercapacitors or battery banks are

connected to the DC bus to provide energy storage

capability under different operating conditions.

%5

±

). The DC/AC

(MISO)output Single

input -Multi

#1Array

Cell Fuel

Cell Fuel

n#Array

Cell Fuel

DC

DC

Control

DC

AC

Banks

or capacitors

-Super

Filter

LC

Voltage

Meter

Current

and

Voltage

Hz 50

kV 0.4

Grid AC

grid -on

grid-off

Load AC

Unbalanced

Signals

Control

Signals

Controller

converter DC/DC

Controller

Inverter DC/AC

signals

Reference

Bus DC

or

+

System Generation Cell Fuel

signals

Actual

plantpower

cell Fuel

signals

Actual

signals

Reference

VSI

Fig. 1. Fuel cell generation system.

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in parallel to compose a 30 kW fuel cell array is

Noroozian, et al: On-grid and Off-grid Operation of Multi-Input Single-Output DC/DC Converter … 133

3 Dynamic Models for Fuel Cell Array

Fig. 2 shows the electrical circuit model that

describes the dynamic behaviour of fuel cell for its

electrical terminals [15]. In Fig. 1, Efc is the equivalent

internal potential and activation voltage drop. Ract, Rconc,

and Rohmic are the equivalent resistances of activation,

concentration, and ohmic voltage drops inside the fuel

cell stack, respectively. These resistors are current (Ifc)

and/or temperature (T ) dependent. Cfc represents the

equivalent capacitance of the system. VC represents the

voltage across of equivalent capacitance. Dfc is the duty

cycle of each boost converter. Vfc and Ifc is the output

voltage and current of each fuel cell array, respectively.

R is the equivalent resistance of load for MISO DC/DC

converter.

In Fig 2, we have:

fc

fc

fca

CC

C

I

CR

V

dt

dV

+−=

(1)

fc

ohmicCfcfc

I .RVEV

−−=

(2)

fc

2

fc

fc

I

n

R)D1 (

V

−

=

(3)

where, Ra=Ract+Rconc, τfc=RaCfc is the time constant of

associated to activation and concentration voltages. The

activation and concentration losses represent a delay in

the fuel cell output voltage. Corresponding to equation

(3), the power supplied by the fuel cell to the load is

depending on the operating point set by the duty cycle

and the number of fuel cell arrays.

In this paper, the fuel cell current is 20 A (rated

operating point) and its output voltage (Vfc,cell) is about

27 V [4], [15]. Therefore, the number (Ns) of fuel cell

stacks we need to connect in a series to get a voltage of

V 108 is

. 4

27

108==

s

N

(4)

The total power rating of the series connection of 4

PEMFC stacks (0.5 kW each) is 2 kW. The number (Np)

of these 2 kW PEMFC units that need to be connected

.15

kW 0.54

kW 30

×

==

×

=

stack

P

s

array

P

p

N

N

(5)

Therefore, each fuel cell array is composed of

15

stacks with the power rating of 30 kW.

4×

4 MISO DC/DC Converter

The output voltage of fuel cells at the series of the

stacks is uncontrolled DC voltage, which fluctuates with

load variations as well as with the changes in the fuel

input. It has to be controlled by DC/DC boost converter.

The MISO DC/DC converter topology used for the

combination of DC output of fuel cell power array is

shown in Fig. 3.

fc

C

fc

I

fc

V

act

R

+

−

conc

R

fc

E

+

−

ohmic

R

n

RDfc

2)1 ( −

+

−

C

V

Fig. 2. Electrical model of PEMFC.

The low voltage inputs of fuel cells, i.e., Vfc1, Vfc2,

Vfc3 and Vfcn have been connected to the DC bus by

series connected boost converters. The single output of

DC/DC converter is fed to the DC/AC inverter, to

produce the AC output for AC grid under both on-grid

and off-grid modes. The power flow and output voltage

of fuel cells have been controlled by controlling the

duty cycles of n IGBT switches, Sfc1, Sfc2, Sfc3 and Sfcn.

The output voltage of MISO DC/DC converter Vdc can

be expressed by the following (6):

nfc

fcn

D

2fc

2fc

D

1fc

1fc

D

dcn2dc1dc

V

−

dc

1

V

−

...

1

V

−

1

V... VVV

+++=

+++=

(6)

where Dfc1, Dfc2, Dfc3 and Dfcn are the duty cycles of

boost converters and Vdc1, Vdc2, Vdc3 and Vdcn are the

output voltages of boost converters. If the converter

duty cycles and their inputs are equal, the output voltage

of MISO DC/DC converter, i.e., Vdc, can be determined

by the following (7):

fc

fc

dc

V

D1

n

V

−

=

(7)

where Dfc is the duty cycle of each boost converter, Vfc

is the low voltage input of each fuel cell array. Vdc is the

output voltage of boost converters.

The output current of MISO DC/DC converter Idc

can be expressed by the following (8):

)D1 (I...

)D1 (

2

I)D 1 (

1

II

n fcfcn

2fc fc1 fc fc dc

−==

−=−=

(8)

where Ifc1, Ifc2, Ifc3 and Ifcn are the input currents of boost

converters, If the converter duty cycles and their inputs

are equal, the output current of MISO DC/DC

converter, i.e., Idc, can be determined by the following

(9):

)D1 (

fc

II

fcdc

−=

(9)

where Ifc is the input current of each fuel cell array.

Applying now Ohm’s law on the resistance R , we

have:

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grid support. Off-grid (or stand-alone) mode allows the

DC/AC inverter to operate completely isolated from AC

grid. There can be a dual mode of operation. In this

mode, DC/AC inverter could be automatically switched

between the two modes.

5.1 Control Strategy for the On-grid Operation

The average large signal model of the DC/AC

inverter in on-grid operating condition is shown in Fig.

4. This converter is represented with three ideal current

sources ifa

amount of the current injected to AC grid from the DC

Iranian Journal of Electrical & Electronic Engineering, Vol. 5, No. 2, Jun. 2009 134

dc dc

RIV =

(10)

Using equations (7), (9) and (10), the equivalent

resistance in the fuel cell from its terminals is given by

n

R)D 1 (

R

2

fc

fc

−

=

(11)

The value of capacitor of each boost converter can

be determined as follows:

dcsfc

fcfc fc

RV

∆

f

V)D1 (nD

C

−

=

(12)

where f is the switching frequency, ∆Vdc is the output

voltage ripple of DC/DC converter. The value for

inductor of each boost converter can be determined by

the following (13):

s

fcfc

f 2

R . D

L =

(13)

In the control system of MISO DC/DC converter,

the output voltage of converter has been compared with

a reference value and the error signal is applied to PI-

controller. The PI controller (Kp+1/Tis) can be designed

using the classic Bode-plot and root-locus method. The

output signal of this controller is the one input of PWM

switching for adjust the duty cycle. Therefore, the

output voltage follows the reference value. In this paper,

as an example a triple input single output (TISO)

DC/DC converter has been designed and studied. The

main components of the dc/dc converter can be

determined by the prescribed technical specifications,

such as the rated and peak voltage and current, input

current ripple, and output voltage ripple, etc., using the

equations (7) to (13). The component values for the 90-

kW TISO DC/DC converter used in this paper are listed

in Table 1.

5 DC/AC Voltage Source Inverter

The DC/AC voltage source inverter exchanges

power between the utility grid and the DC bus and vice

versa. On-grid (or grid-connected) mode allows the

DC/AC inverter to operate parallel to the grid, providing

ref, ifb

ref and ifc

ref. The converter manages the

bus. As it can be seen in Fig. 4, the input signals of the

DC/AC inverter are source phase voltages, vsa, vsb and

vsc, three phase output currents for this converter ifa, ifb

and ifc, the reference of the active power, Pref and the

reference of the reactive power, Qref. Lf is the

inductance of the inverter filter. Rg, and Lg are the

resistance and inductance of the AC grid. This

controller uses the Hysteresis Current Control (HCC)

switching technique. As it can be seen in Fig. 4, we

have:

−=

fc

fb

fa

sc

sb

sa

i

i

i

i

i

i

(14)

Fig. 5 shows the DC/AC inverter control in on-grid

operation. The required power to be injected to AC grid

is set by Pref and Qref reference signals. These signals

can be chosen by customers or remote power

management units [9-11]. However, this control

strategy is called

QP−

control scheme for on-grid

operation.

Table 1. Parameters of TISO DC/DC converter.

L

mH 0.01

C

F 1553µ

3fc 2 fc1 fc

DDD

==

0.6

R (Equivalent load)

Ω

6.25

dc

V

∆

kV 0.015

sf

kH 10

3fc2fc1 fc

III

==

kA 0.3

3 fc2fc1fc

VVV

==

kV 0.108

3dc2 dc1dc

VVV

==

kV 0.25

p

K

3.75

iT

0.01

In this paper, the

designed based on the instantaneous power control

strategy. The

β−α

transformation in Fig. 5 performs

the following equations:

QP−

control strategy has been

=

αβ

β

α

sc

sb

sa

s

s

v

v

v

T

v

v

(15)

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sc

i

Noroozian, et al: On-grid and Off-grid Operation of Multi-Input Single-Output DC/DC Converter … 135

−

−−−

=

αβ

2

3

2

3

0

2

1

2

1

1

3

2

T

The

β−α

component related to the reference

current of each network converter can be expressed by

equation (16).

−

+

=

αβ

βα

βα

β

α

ref

ref

ss

ss

2

s

2

s

ref

s

ref

s

Q

P

vv

vv

vv

1

i

i

(16)

+

C

−

1

fc

S

1

fc

I

1

fc

V

L

1

dc

V

+

−

dc

I

+

+

C

−

2

fc

S

2

fc

I

2

fc

V

L

2

dc

V

+

−

+

C

−

3

fc

S

3

fc

I

3

fc

V

L

3

dc

V

+

−

+

C

−

fcn

S

fcn

I

fcn

V

L

dcn

V

+

−

−

dc

V

R

1

fc

D

2

fc

D

3

fc

D

fcn

D

Fig. 3. Multi-input and single output (MISO) DC/DC

converter.

The

β−α

inverse transformation box, shown in Fig.

5, calculates the three-phase current references to be fed

into the HCC scheme. Thus:

=

β

α

ref

s

ref

s

abc

ref

ref

sb

ref

sa

i

i

Ti

i

(17-a)

−−

−=

2

3

2

1

2

3

2

1

01

3

2

abc

T

(17-b)

−=

ref

sc

ref

sb

ref

sa

ref

fc

ref

fb

ref

fa

i

i

i

i

i

i

(18)

The comparison of the calculated reference currents

and the actual currents generated by the DC/AC inverter

will result in the error signal, which controls the

switches of the inverter.

dc

V

dc

I

fa

v

fb

v

fc

v

fa

i

fb

i

fc

i

f L

sa

v

sb

v

sc

v

sa

i

sb

i

sc i

g

L

g

R

ref

fc

i

ref

fa

i

ref

fb

i

Controller

Inverter DC/AC

sa

v

sb

v

sc

v

fa

i

fb

i

fc

i

Hz 50

kV 0.4

Grid AC

ref

P

ref

Q

+

C

−

1

2

C

Fig. 4. Average large signal model of the DC/ AC inverter in

on-grid mode.

Transform

ref

ai

Inverse

βα−

Control

Current

PWM

ref

iβ

ref

iα

Filter LC

Bus DC

Meter

Current

Meter

Voltage

Grid AC

Reference

Current

βα−

ref

Q

ref

P

ref

ci

ref

bi

fabc

i

sabc

v

Fig. 5. Block diagram of DC/AC inverter controller in on-grid

operation.

5.2 Control Strategy for the off-grid Operation

The average large signal model of the DC/AC

inverter in off-grid operating condition is shown in Fig.

6. This inverter is represented with three voltage

sources, vfa

DC/AC inverter voltages and currents are expressed by

the following equation:

ref, vfb

ref and vfc

ref. The equations describing

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off-grid mode.

The circuit configuration and control scheme for

DC/AC inverter for supplying unbalanced AC load is

depicted in Fig. 7. The DC/AC inverter between the DC

bus and AC load can be controlled by fV− control

strategy, which regulates the voltage and the frequency

of AC load [12-14]. In the fV − controller, it is clear

that:

a. Frequency (ω) can be obtained by Phase Lock

Loop (PLL) using desirable frequency (e.g., 50

Hz).

Iranian Journal of Electrical & Electronic Engineering, Vol. 5, No. 2, Jun. 2009

136

+

+

=

fc

fb

fa

f

f

f

fc

fb

fa

f

f

f

lc

lb

la

fc

fb

fa

i

i

i

dt

d

L00

0L0

00L

i

i

i

R00

0R0

00R

v

v

v

v

v

v

(19)

where, vfa, vfb and vfc are line to neutral three phase

output voltages of the DC/AC inverter. ifa, ifb and ifc are

three phase output currents. vla, vlb and vlc are line to

neutral three phase voltages of AC loads. The voltage

equations in the

0qd

−−

follows:

reference frame are as

ω

ω−

+

+

+

=

0f

fq

fd

f

f

0f

fq

fd

f

f

f

0f

fq

fd

f

f

f

0 l

lq

ld

0f

fq

fd

i

i

i

000

00L

0L0

i

i

i

dt

d

L00

0L0

00L

i

i

i

R00

0R0

00R

v

v

v

v

v

v

(20)

ref

fc

v

fa

v

fb

v

fc

v

fa

i

fb

i

ref

fa

v

ref

fb

v

Controller

Inverter DC/AC

f

fa

i

fb

i

fc

i

lc

v

lb

v

la

v

Load

fc

i

f L

lci

lb i

la i

Load

Load

lb

v

la

v

lc

v

f

R

dc

V

dc

I

+

−

1

C

2

C

Fig. 6. Average large signal model of the DC/ AC inverter in

b. The load phase voltages (vla, vlb and vlc) can be

detected and transformed to the

synchronously rotating reference frame using

following equations:

0qd

−−

=

lc

lb

la

0 dq

0 l

lq

ld

v

v

v

T

v

v

v

(21)

+ω−−ω−ω−

+ω−ωω

=

2

1

2

1

2

1

)120tsin() 120tsin( ) tsin(

) 120tcos()120tcos() tcos(

3

2

T

0dq

oo

oo

Transform

v

0qd

−−

Load AC

Meter

Voltage

Filter LC

Bus DC

lq

v

ld

0 lv

exp

lq

v

exp

lv0

exp

ld

v

Meter

Current

PI

PI

PI

PIPIPI

PIPI PI

0

ldq

v

lq

v

ld

v

0 lv

ω

ref

fa

ref

fb

ref

fc

v

v

v

Transform

0qd

−−

fq

i

fd

i

0

fi

f L

ω

fabc

i

Control

Voltage

PWM

ω

f

PLL

labc

v

fq

i

fd

i

0

fi

−

+

−

+

−

+

ref

lqi

ref

ldi

−

ref

li0

+

−

+

+

−

+

+

+

+

+

−

+

+

ref

fd

v

ref

fq

v

ref

fv0

Transform

Inverse

0qd

−−

ω

f

R

f L

ω

f

R

+

+

+

+

+

Fig. 7. Block diagram of DC/AC inverter controller in

off-grid operation.

The load phase voltage should be kept balanced and

sinusoidal with constant amplitude and frequency.

Therefore the expected load voltage in the

reference frame should have only the following value:

0qd

−−

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Then the available voltages in the a

−

coordinate are compared with the triangular wave

provided by PWM voltage control block. Therefore the

output provides suitable switching pattern of DC/AC

inverter.

6 Simulation Results

In this paper, to evaluate the performance of fuel cell

generation system, on-grid and off-grid operating

conditions have been modeled by PSCAD/EMTDC.

The control strategy of DC/AC inverter with neutral

clamped DC capacitors has been studied in on-grid and

off-grid operation modes.

Noroozian, et al: On-grid and Off-grid Operation of Multi-Input Single-Output DC/DC Converter … 137

=

0

3

2

.4

0

0

v

v

v

exp

0 l

exp

lq

exp

ld

(22)

The inverter controller based on

reference frame consists of an inner current loop and an

outer voltage loop in a three channel arrangement. The

current and voltage loops include independent PI

controllers for the d , q and 0 channels to eliminate

steady state error. The reference load current loops in

the

0qd

−−

coordinate are:

0qd

−−

rotating

−

−

−

=

)vv (PI

)vv (PI

)vv (PI

i

i

i

exp

0 l

0 l

exp

lqlq

exp

ldld

ref

0 l

ref

lq

ref

ld

(23)

The output signals from PI controller can be

expressed by the equation (19).

ω

ω−

−

−

−

+

+=

0

iR

iR

0

iL

iL

)ii ( PI

)i i ( PI

)ii (PI

v

v

v

v

v

v

fqf

fdf

fdf

fqf

0f

ref

0 l

fq

ref

lq

fd

ref

ld

0 l

lq

ld

ref

0f

ref

fq

ref

fd

(24)

The reference output voltages for the DC/AC

inverter are transformed to the

inverse synchronously rotating frame.

cba

−−

by using

=

ref

0f

ref

fq

ref

fd

abc

ref

fc

ref

fb

ref

fa

v

v

v

T

v

v

v

(25)

+ω−+ω

−ω−−ω

ω−ω

=

1)120t sin() 120tcos(

1)120 tsin()120tcos(

1) tsin() tcos(

Tabc

oo

oo

cb

−

6.1 On-Grid Operation

In this case, the reference values of Pref and Qref have

been changed from 80 kW to 40 kW and from 8 kVAr

to 4 kVAr at t=0.4s, and then from 40 kW to 80 kW and

from 4 kVAr to 8 kVAr at t = 0.8 sec., respectively.

Figures 8 to 11 show the simulation results. As shown

in Fig. 8, the active power injected to the AC grid has

been changed from 80 kW to 40 kW at t = 0.4 sec., and

then from 40 kW to 80 kW at t = 0.8 sec. In Fig. 8, the

reactive power has been changed from the AC grid by

the fuel cell unit is changed from 8 kVAr to 4 kVar at t

= 0.4 sec. and then from 4 kVAr to 8 kVar at t = 0.8 sec.

As it can be seen the parameters follow the reference

points and in the steady-state, the fuel cell generation

system delivers the active power to the grid and

consumes the reactive power from the grid, which

matches the reference values of P and Q . The filtered

output voltages and currents of each fuel cell array for

this case are shown in Fig. 9.

As depicted in Fig. 9, the fuel cell current has some

delay because it takes some time for the fuel to be

converted to the hydrogen, which is demanded for the

request power, and the fuel cell voltage and current

depend on each other as voltage-current polarization

curve of the stack. Fig. 10(a) shows the active power

injected to the DC bus. Corresponding to the Fig. 10(a),

active power injected into the DC bus by using fuel cell

power plant increases and reach to reference value,

which match the above active power reference

variation. Fig. 10(b) show the DC bus voltage (the

output of TISO DC/DC converter), which match the

above grid connected condition. Note that the DC bus

voltage comes up to its reference value, 750 V though

the fuel cell output voltage is fluctuated in during the

simulation time. The voltage ripple at the DC bus is

about 1.2%, which is with the acceptable range.

Fig. 11 shows grid-side phase voltages and line

currents of the DC/AC inverter. The line currents

changes with power reference variations, which goes

into the utility grid. Therefore, the inverter can deliver

the generator’s power of fuel cell DG system to the grid

with low harmonic current. This verifies the

effectiveness of the QP−

control strategy.

6.2 Off-Grid Operation

The response of proposed fuel cell generation

system to unbalanced resistive-inductive loading in the

off-grid mode has been studied, too. Therefore, the

fV − control scheme is activated. The unbalanced load

No. 1 has been changed to unbalanced load No. 2 at

t=0.4s and at t=0.8s the load has been again changed to

its initial value, i.e., load No. 1. The load parameters are

given in appendix A. Figures 12 to 15 show the

simulation results. Fig. 12 shows the active and reactive

power consumed by unbalanced loads. As shown in Fig.

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simulation study of a fuel cell generation system for on-

grid and off-grid operation modes. A validated 500 W

PEMFC dynamic model, reported in [1-5], is used to

model the fuel cell array. The dynamic model for fuel

cell array and its power electronic interface have been

presented, too. The multi-input single-output (MISO)

DC/DC converter has been studied. This converter is

capable of interfacing fuel cell power plant to the

DC/AC inverter. Conventional PI voltage controllers are

used for the MISO converter to regulate the DC bus

voltage. The controller designs for different operating

conditions of DC/AC inverter are given using the

average large signal model. The QP−

control scheme

Iranian Journal of Electrical & Electronic Engineering, Vol. 5, No. 2, Jun. 2009 138

12, the fuel cell generation system is supplying the

unbalanced AC loads. The filtered output voltage and

current curves of each fuel cell array for this case are

shown in Fig. 13. As depicted in Fig. 13, the some delay

of fuel cell output voltage and current depend on each

other as voltage-current polarization curve of stack. Fig.

14(a) shows the power injected to the DC bus.

Corresponding to the Fig. 14(a), power injected into the

DC bus by using fuel cell increases and reach to load

demand, which match the above unbalanced load

requirement. Fig. 14(b) shows the DC bus voltage (the

output of TISO DC/DC converter), which match the

above load variations. Note that the DC bus voltage

comes up to its reference value, 750 V though the fuel

cell output voltage is fluctuated in during the simulation

time. The voltage ripple at the DC bus is about 1.2%,

which is with the acceptable range.

Fig. 15 shows phase voltages and line currents at

unbalanced load terminals. The DC/AC inverter

maintains the output at the desired level irrespective of

unbalanced loads applied on the system.

The AC voltage level across the load remains

unchanged with the unbalanced load variation switching

and stay at the reference value Vlq

simulation. However, the balanced voltages are

provided for the unbalanced AC loads while the load

phase currents are not sinusoidal. This is because of

ability of DC/AC inverter to control its output voltage.

The 3-phase line current at load terminals changes with

the load variation switching. The frequency (50 Hz in

our case) is imposed by a phase lock loop (PLL) block.

This verifies the effectiveness of the

strategy for the off-grid mode. To quantify the level of

the voltage unbalance, the percentage of negative

sequence unbalance is expressed in accordance with the

definition of the “degree of unbalance in three phase

system” [12-14]. In this case, the negative sequence

unbalance is lower then 1% which is acceptable. It must

be noticed that international standards admit unbalances

lower than 2% [12-14].

7 Conclusion

This paper presents the modeling, control and

exp in the all time of

fV − control

based instantaneous power control strategy is used on

the inverter to control the active and reactive power

delivered from the fuel cell generation system to the

utility grid. The fV − control scheme based

transformed current-voltage controller is used on the

inverter under unbalanced load conditions. This

controller regulates the load phase voltage in balanced

and sinusoidal with constant amplitude and frequency.

This point indicates the technical and economical

superiority of the proposed system for parallel

connection of fuel cells. The simulation results based on

PSCAD/EMTDC software show the effectiveness of the

suggested control systems in on-grid and off-grid

operation modes. The results also show the fuel cell

system is maintained within specified limit.

Appendix A

Parameters of the unbalanced AC loads used for

simulation: RL load No. 1:

0qd

−−

Ω

°

6.299

12.473

∠

and

Z

7.2717Z

1la

=

,

Ω

°

17.120

∠

5.3364

14.438

∠

Z

1lb

=

Ω

°

1lc

=

RL load No. 2:

Ω

°

4.2069

16.908

∠

and Z

3.2401Z

2lb

=

,

Ω

°

24.167

∠

Ω

°

2.3018

12.943

∠

Z

2la

=

2 lc

=

Reactive Power(Kvar)

Time(Sec)

Active Power(KW)

Fig. 8. P and Q delivered to the grid.

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Noroozian, et al: On-grid and Off-grid Operation of Multi-Input Single-Output DC/DC Converter … 139

Fig. 9. Filtered output voltages and currents of each fuel

cell array.

Power(KW)

Voltage(V)

Time(Sec)

Fig. 10. (a) Power output from the DC bus; and (b) DC bus

voltage.

Voltage(V)

Current(A)

Time(Sec)

Fig. 11. Grid-side phase voltages and line currents of the

inverter.

Time(Sec)

Active Power(KW)

Reactive Power

Fig. 12. Active and reactive power consumed by unbalanced

loads.

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Iranian Journal of Electrical & Electronic Engineering, Vol. 5, No. 2, Jun. 2009 140

Time(Sec)

Voltage(V)

Current(A)

Fig. 13. Filtered output voltages and currents of each fuel cell

array.

Time(Sec)

DC Voltage(V)

DC Power(KW)

Fig. 14. (a) Power injected into the DC bus; and (b) DC bus

voltage.

Time(Sec)

Time(Sec)

Voltage(V)

Current(A)

Fig. 15. Phase voltages and Line currents at unbalanced load

terminals.

References

[1]

Fuel Cell Handbook (Seventh Edition), EG&G

Services, Inc., Science Applications International

Corporation, DOE, Office of Fossil Energy,

National Energy Technology Lab, Nov. 2004.

Jung J., Dai M., and Keyhani A., “Modeling and

Control of a Fuel Cell Based Z-Source

Converter”, Proc. of IEEE Applied Power

Electronics Conference and Exposition, Austin,

TX, pp. 1112-1118, March ,2005.

Kim Y. and Kim S., “An Electrical Modeling and

Fuzzy Logic Control of a Fuel Cell Generation

System”, IEEE Trans. Energy Conv , Vol. 14,

No. 22, pp. 239-244,1999.

Wang C., Nehrir M. H. and Gao H., “Control of

PEM Fuel Cell Distributed Generation Systems”,

IEEE Trans. Energy Conv.,Vol 21, No.2, pp. 586-

595,2006.

Nergaard T. A., Ferrel J. F., Leslie L. G. and Lai

J. S., “Design considerations for a 48 V Fuel Cell

to Split single phase inverter system with ultra

capacitor energy storage”, Proc. of 33rd IEEE

Annual PESC, Cairns, Australia, pp. 2007-2012,

June 2002.

Wang J., Peng F. Z., Anderson, J., Joseph, A.,

and Buffenbarger, R., “Low cost fuel cell inverter

system for residential power generation”, IEEE

[2]

[3]

[4]

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load”, Energy Conversion and Management,

Vol.48, pp. 2119–2128, 2007.

[15] Wang C., Nehrir M. H. and Shaw S. R.,

“Dynamic Models and Model Validation for

PEM Fuel Cells Using Electrical Circuits”, IEEE

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2005.

[16] Short T., “Electric Power Distribution

Handbook”, CRC Press, 2004.

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Trans. on Power Electronic, Vol. 19, No. 5, pp.

1315-1322, 2004.

Ozpineci B., Tolbert L. M., Su G. J., Du Z.,

“Optimum Fuel Cell Utilization with Multilevel

DC-DC Converters”, Proc. of 19th IEEE annu.

Applied Power Electronics Conf. and Exposition,

pp. 1572-1576, 2004.

Andujar J. M., Segura F. and Vasallo M. J., “A

suitable model plant for control of the set fuel

cell-DC/DC converter”, Renewable Energy, ,

Vol.33,No.4, pp. 813-826,2008.

Mahmoodi M., Gharehpetian G. B., Abedi M.,

Noroozian R., “Novel and Simple Control

Strategy for Fuel Cell Converters in DC

Distribution Systems”, Proc. of the First

International Power and Energy Conf., Putrajaya,

Malaysia, pp. 358-362, Nov. 2006.

[10] Mahmoodi M., Gharehpetian G. B., Abedi M.,

Noroozian R., “A Suitable Control Strategy for

Source Converters and a Novel Load- Generation

Voltage Control Scheme for DC Voltage

Determination in DC Distribution Systems”,

Proc. of the First International Power and

Energy Conf., Putrajaya, Malaysia, pp. 363-367,

Nov. 2006.

[11] Mahmoodi M., Gharehpetian G. B., Abedi M.,

Noroozian R., “Control Systems for Independent

Operation of Parallel DG Units in DC

Distribution Systems”, Proc. of the First

International Power and Energy Conf., Putrajaya,

Malaysia, pp. 220-224, Nov. 2006.

[12] Lin B. R. and Lee Y. C., “Three-phase power

quality compensator under the unbalanced

sources and nonlinear loads”, IEEE Trans. Power

Electronics, V0l.51, No.5, pp. 1009–1017, 2004.

[13] Blazic B. and Papic I., “A new mathematical

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[14] Vechiu I., Camblong H., Tapia G., Dakyo B. and

Curea O., “Control of four leg inverter for hybrid

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[7]

[8]

[9]

Reza Noroozian was born in Bonab,

Iran, in 1975. He received the

B.Sc.degree from

University, Tabriz, Iran, in 2000, and

M.Sc. and Ph.D. degrees from the

Amirkabir University of Technology

(AUT), Tehran, Iran, in 2003 and 2008,

respectively, all Electrical Engineering.

the Tabriz

Currently, he is an Assistant professor in the Department of

Electrical Engineering, Faculty of Engineering, Zanjan

University, Zanjan, Iran. His research interests include Power

System, Distributed Generation, Power Electronic and Power

Quality.

Mehrdad Abedi received his B.Sc.,

M. Sc. and Ph.D. from Tehran

University, London University and

Newcastle University in 1970, 1973,

and 1977, respectively. He worked for

G.E.C. (U.K) till 1978. Since then he

joined

University (Tehran, Iran) where he is

now the professor and member of

Center of Excellency on Power System. Prof. Abedi has

published more than 25 books and 160 papers in journals and

conferences. He is distinguished professor in Iran and is prize

winner for two outstanding books. He is also member of

Iranian Academy of Science and member of CIGRE. His main

interest is electrical machines and power systems modeling,

operation and control.

Gevorg B. Gharehpetian was born in

Tehran, in 1962. He received his BS

and MS

engineering in 1987 and 1989 from

Tabriz University, Tabriz, Iran and

Amirkabir University of Technology

(AUT), Tehran, Iran, respectively,

graduating with First Class Honors. In

1989 he

Engineering Department of AUT as a lecturer. He received the

Ph.D. degree in electrical engineering from Tehran University,

Tehran, Iran, in 1996. As a Ph.D. student he has received

scholarship from DAAD (German Academic Exchange

Service) from 1993 to 1996 and he was with High Voltage

Institute of RWTH Aachen, Aachen, Germany. He held the

position of Assistant Professor in AUT from 1997 to 2003,

and has been Associate Professor since 2004. Dr.

Gharehpetian is a Senior Member of Iranian Association of

Electrical and Electronics Engineers (IAEEE), member of

IEEE and member of central board of IAEEE. Since 2004 he

is the Editor-in-Chief of the Journal of IAEEE. The power

engineering group of AUT has been selected as a Center of

Excellency on Power Systems in Iran since 2001. He is a

member of this center and since 2004 the Research Deputy of

this center. Since November 2005 he is the director of the

EE Dept of Amirkabir

degrees in electrical

joined the Electrical

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Iranian Journal of Electrical & Electronic Engineering, Vol. 5, No. 2, Jun. 2009 142

industrial relation office of AUT. He is the author of more

than 222 journal and conference papers. His teaching and

research interest include power system and transformers

transients, FACTS devices and HVDC transmission.

Seyed Hossein Hosseini was born in

Marand, Iran in 1953. He received the

M.S. degree from the Faculty of

Engineering University of Tabriz, Iran

in 1976, the DEA degree from INPL,

France, in 1978 and Ph.D. degree from

INPL, France, in 1981 all in electrical

engineering. In 1982 he joined the

University of Tabriz, Iran, as an assistant professor in the

Dept. of Elec. Eng., from 1990 to 1995 he was associate

professor in the University of Tabriz, since 1995 he has been

professor in the Dept. of Elec. Eng. University of Tabriz.

From Sept. 1990 to Sept. 1991 he was visiting professor in the

University of Queensland, Australia, from Sept. 1996 to Sept.

1997 he was visiting professor in the University of Western

Ontario, Canada. His research interests include Power

Electronic Converters, Matrix Converters, Active & Hybrid

Filters, Application of Power Electronics in Renewable

Energy Systems and Electrified Railway Systems, Reactive

Power Control, Harmonics and Power Quality Compensation

Systems such as SVC, UPQC, FACTS devices.

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