Control of Parallel Converters for Load Sharing

Prévia do material em texto

Control of Parallel Converters for Load Sharing 
with Seamless Transfer between Grid Connected and 
Islanded Modes 
 
Ritwik Majumder, Student Member, IEEE, Arindam Ghosh, Fellow, IEEE 
Gerard Ledwich, Senior Member, IEEE and Firuz Zare, Senior Member, IEEE 
 
ABSTRACT: This paper describes control methods for proper 
load sharing between parallel converters connected to microgrid 
supplied by distributed generators. The model of the microgrid 
power system is simulated in PSCAD. It is assumed that the mi-
crogrid supplies a load both in grid connected and islanded 
modes. Both passive loads and inertial loads are considered. A 
control strategy is proposed to improve the system performance 
through seamless transfer between islanded and grid connected 
modes. The controller is capable of handling constant impedance, 
as well as motor loads. The smooth transition between the grid 
connected and off grid mode is achieved by changing the control 
mode from voltage control in islanded mode to state feedback 
control in grid connected mode. Its efficacy has been validated 
through simulation for various operating conditions. 
 Index Terms: Distributed power supply, Microgrid, Islanding 
and Resynchronization, Active and Reactive Power sharing. 
 
I. INTRODUCTION 
 
 
HE INTERCONNECTION of distributed generators 
(DGs) to the utility grid through power electronic convert-
ers has raised concern about system stability and smooth trans-
fer between the grid connected and standalone modes. Many 
innovative control techniques have been used for stability of 
the system as well as for proper load sharing. The most com-
mon method is the use of droop characteristics for wireless 
load sharing. Parallel converters have been controlled to de-
liver desired real and reactive power to the system. Local sig-
nals are used as feedback to control the converters, since in a 
real system, the distance between the converters may make an 
inter-communication impractical. The real and reactive power 
sharing can be achieved by controlling two independent quan-
tities – the power angle, and the fundamental voltage magni-
tude [1-4]. 
The system stability during load sharing has been explored 
by many researchers [2, 5-7]. Transient stability of power sys-
tem with high penetration level of power electronics interfaced 
(converter connected) distributed generation is explored in [2]. 
But the study is based on presence of an infinite bus. The im-
portant issue of isolated operation of the network has been 
overlooked in the study. Moreover, smooth transfer between 
the grid connected mode and the standalone or islanded mode 
is crucial in microgrid operation. 
 
R. Majumder, A. Ghosh, G. Ledwich and F. Zare are with the School of 
Engineering Systems, Queensland University of Technology, Brisbane, Qld 
4001, Australia. 
 
 
A transient droop characteristic is used [8] to achieve steady 
state invariant frequency and good current balance. Sometimes 
an additional faster loop is added to program the output im-
pedance. Both inductive and resistive output has been investi-
gated. In the resistive output, the active power is controlled by 
terminal voltage where the reactive power is controlled by the 
source angle. Karimi et al [9] developed a dynamic model and 
a control system for autonomous operation of a stand-alone 
Distributed Resource (DR). A stand-alone DR includes a dis-
tributed resource and a local load which are interfaced elec-
tronically. The DR is represented by a DC voltage source in 
series with a three phase voltage-sourced inverter and an RL 
filter, while the local load is modeled by a parallel RLC net-
work. A state-space dynamic model is developed for the DR 
including the RLC network. Based on the dynamic model of 
the DR, a controller is designed to maintain stability and to 
control voltage and frequency of the stand-alone DR. Control 
of the DG system is important in both grid connected and 
islanded modes and the system stability becomes very crucial 
during the transfer between grid connected and islanded 
modes. A seamless transfer can ensure a smooth operation 
with proper load sharing and quick attainment of steady state. 
The aim of this paper is to set up power electronics inter-
faced microgrid containing distributed generators. Schemes 
for controlling parallel connected converters in islanded and 
grid connected mode are presented. The control techniques for 
a smooth transfer between these two modes are also shown. A 
modular structure of the controller is presented. The structure 
can be modified to meet the control requirement for any other 
distributed system configuration. 
 
II. SYSTEM STRUCTURE 
 
The basic power system model with two DG sources con-
nected to the load at the point of common coupling (PCC) is 
shown in Fig. 1. In this, the system runs in islanded mode 
when the circuit breaker (CB) is open; otherwise it runs in grid 
connected mode. The load can be a constant impedance load 
or a motor load. In Fig. 1, the voltage source VS is the utility 
voltage that is connected to the PCC with a feeder of imped-
ance RS + jXS. The current drawn from the utility is denoted by 
Ig, while Pg and Qg are respectively the real and reactive power 
supplied by the grid. It is assumed that the DGs are constant 
dc voltage sources Vdc1 and Vdc2. The converter output voltages 
are denoted by V1∠δ1 and V2∠δ2 and they are connected to the 
PCC through reactances jX1 and jX2 respectively. P1, P2 and 
Q1, Q2 represent the real and reactive power supplied by the 
DGs. 
T
©2008 IEEE.
 
Fig.1. The system under consideration. 
 
III. PROPOSED CONTROL STRUCTURE 
 
The starting point of the control is the conventional droop 
method, given by 
nQVV
mPs
−=
−=
∗
ωω
 (1) 
where m and n are the droop coefficients, ωs is the synchro-
nous frequency, V is the magnitude of the converter output 
voltage and ω is its frequency, while P and Q respectively 
denote the active and reactive power supplied by the con-
verter. Thus the frequency and the voltage are being controlled 
by the active and reactive power output of the DG sources. 
 
A. Modular Control Structure 
 
The conventional droop controller, given by (1), is modified 
in the proposed control scheme. This is shown in Fig. 2 for 
DG1 only. A similar structure is also used for DG2. In this 
structure, V1∠δ1 and I1 are used for calculating the real power 
(P1) and reactive power (Q1) injected by DG1. These are then 
used in (1) to calculate ωs and V1* as per (1). The quantity ωs 
and the angle of the PCC voltage δPCC are then used to calcu-
late the reference angle δ1*. This is described in the next sub-
section. The reference magnitude V1
* and its angle δ1* are then 
used to generate the instantaneous reference voltages for the 
three phases. These are then compared with the measured val- 
 
Fig.2. The modular control structure, 
ules of the instantaneous phase voltages of V1. The resultant 
error is used in the feedback control to generate the switching 
action (u = ± 1) of VSC-1. The feedback control and converter 
structure are discussed in Section IV. 
 
B. Converter Voltage Angle Calculation 
 
The converter voltage angle control loop is shown in Fig. 3. 
The frequency ω1 is calculated from the droop given in (1). 
This is then compared with the frequency (ωPCC) of the PCC 
voltage. The error is passed through an integrator with a gain 
of KI and is then added with the integral of ωPCC to obtain φ1*. 
The angle φ1* rotates at the synchronous speed ωs while mak-
ing an angle δ1* with the reference. Note that by changing the 
value of KI, we can influence the close loop dynamics without 
affecting the steady state frequency regulation. 
 
Fig. 3. Voltage angle control loop. 
 
IV. CONVERTER STRUCTUREAND CONTROL 
 
The converter structure is shown in Fig. 4. This contains 
three H-bridge converters that are connected to the DG 
sources, denoted by Vdc1. The outputs of the H-bridges are 
connected to three single-phase transformers that are con-
nected in wye for required isolation and voltage boosting [10]. 
The resistance Rf represents the switching and transformer 
copper losses, while the inductance Lf represents the leakage 
reactance of the transformers. The filter capacitor Cf is con-
nected to the output of the transformers to bypass switching 
harmonics. The inductance L1 is added to provide the output 
impedance of the DG source. 
 
Fig. 4. Converter structure. 
 
The advantage of the VSC scheme shown in Fig. 4 is that it 
can readily be used even when the loads (or supply side) con-
tain unbalance and harmonics. In fact, the DG will be able to 
compensate for these to make the microgrid voltage balanced 
and distortion free. To achieve the same performance from a 
three-phase VSC, the dc bus neutral must be clamped. Since a 
DG is often directly coupled to the dc bus, this will not be 
possible. Furthermore, the transformers can boost a lower 
level dc voltage to the microgrid level. 
 
A. Converter Control 
 
The equivalent circuit of one phase of the converter is 
shown in Fig. 5. In this, u⋅Vdc1 represents the converter output 
voltage, where u = ± 1. The main aim of the converter control 
is to generate u. 
 
Fig. 5. Equivalent circuit of one phase of the converter. 
 
From the circuit of Fig. 1, state space description of the sys-
tem can be given as 
PCCc vBuBAxx 21 ++=& (2) 
where uc is the continuous time version of switching function 
u. The discrete-time equivalent of (2) is 
( ) ( ) ( ) ( )kvGkuGkFxkx PCCc 211 ++=+ (3) 
Based on this model and a suitable feedback control law, uc(k) 
is computed. The switching function u is then generated as 
1 then elseif
1 then If
−=−<
+=>
uhu
uhu
c
c (4) 
where h is a small number. Two types of feedback controllers 
are used here. They are discussed below. 
 
B. Output Feedback Voltage Controller 
 
Let the output of the system given in (3) be vcf. From Fig. 2, 
it can be seen that the reference for this voltage is given in 
terms of the magnitude of the rms voltage V1
* and its rotating 
angle φ1*. From this the instantaneous voltage reference v1* for 
the three phases are generated. Neglecting the PCC voltage 
since it is a disturbance input, the input-output relationship of 
the system in (3) can be written as 
( )
( )
( )( )1
1
−
−
=
zN
zM
zu
zv
c
cf (5) 
The control is computed from 
( ) ( )( ) ( ) ( ){ }zvzvzR zSu cfc −= ∗−
−
11
1
z (6) 
Then the closed-loop transfer function of the system is given 
by 
( )
( )
( ) ( )( ) ( ) ( ) ( )1111
11
1
−−−−
−−
∗ += zSzMzRzN
zSzM
zv
zvcf (7) 
The coefficients of the polynomials S and R can be chosen 
based on a pole placement strategy [11]. Once uc is computed, 
the switching function u can be generated from (4). 
 
C. State Feedback Controller 
 
Let the state vector be defined by 
[ ]1iivx cfcfT = (8) 
The reference for vcf is v1
* mentioned in the previous sub-
section. Given V1
* and φ1*, the phasor current through the ca-
pacitor Cf is given by 
( )°+∠= ∗∗∗ 9011 δω VCI fcf (9) 
The reference icf
* is obtained from the instantaneous value of 
Icf
*. 
We shall obtain the reference for i1 through its phasor quan-
tity I1
*. Note from Fig. 1 that if the references are strictly fol-
lowed 
*
1
*
1
*
111 IVjQP ×−∠=− δ (10) 
Let us define I1
* = I1p
* + j I1q
*. We can then write the follow-
ing from (10) 
[ ]
[ ]∗∗∗∗
∗∗
∗
∗
−=
+=
1111
1
1
1111
1
1
cossin
1
sincos
1
δδ
δδ
QP
V
I
QP
V
I
q
p
 (11) 
Therefore the phasor reference is given by 
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛+= ∗
∗
−∗∗∗
p
q
qp
I
I
III
1
11
111 tan (12) 
 
Fig. 6. Source angle extraction from rotating angle. 
 
To calculate the reference from (12), the value of δ1* has to 
be obtained. The voltage angle controller of Fig. 3 generates a 
rotating angle φ1* which is equal to ωst + δ1*. The angle φ1* is 
reset after every 2π. Fig. 6 shows the variation along with the 
reference ωst. From this figure we can write 
∗+== 110 2 δωπω tt ss 
Therefore we have 
( )011 tts −=∗ ωδ (13) 
Once the references for the state vector (8) are obtained, the 
control law is computed from 
( )∗−−= xxKuc (14) 
where K is the gain matrix and x* is the reference vector. The 
gain matrix, in this paper, is obtained through linear quadratic 
regulator (LQR) design. From uc, the switching function is 
generated as in (4). 
 
V. SIMULATION STUDIES 
 
Simulation studies are carried out with different types of 
loads and operating conditions to check the system response 
and controller action. Some of the results are discussed below. 
The system data used are given in Table I. 
 
TABLE I. SYSTEM PARAMETERS 
System Quantities Values 
Systems Frequency (ωs) 100π rad/s 
Source voltage (Vs) 11 kV rms (L-L) 
Feeder impedance (Rs + jXs) 3.025 + j12.095 
DG-1 
DC voltage (Vdc1) 
Transformer rating 
 
VSC losses 
Source inductance (L1) 
Filter Capacitance (Cf) 
Frequency droop coefficient (m) 
Voltage droop coefficient (n) 
 
220 V 
220 V/11 kV, 0.5 MVA, 2.5% 
reactance (Lf) 
1.5 Ω 
0.0578 H 
30 µF 
0.005 rad/s/kW 
0.2045 kV/kVAr 
DG-2 
DC voltage (Vdc1) 
Transformer rating 
 
VSC losses 
Source inductance (L1) 
Filter Capacitance (Cf) 
Frequency droop coefficient (m) 
Voltage droop coefficient (n) 
 
220 V 
220 V/11 kV, 0.5 MVA, 2.5% 
reactance (Lf) 
1.5 Ω 
0.0722 H 
30 µF 
0.00625 rad/s/kW 
0.2727 kV/kVAr 
Passive load The load is varied between 
4.84 + j30.25 Ω and 
102.85 + j157.3 Ω 
Motor load (synchronous) 
Rated rms voltage (L-N) 
Rated rms line current 
Inertia constant 
Iron loss resistance 
 
6 kV 
5 kA 
1 s 
300 pu 
Motor load (induction) 
Rated rms voltage (L-N) 
Rated rms line current 
Rated power 
 
6 kV 
0.11 kA 
50 hp 
 
A. Islanded Mode 
 
In the islanded mode, the VSCs are operated in voltage con-
trol mode through output feedback. In this mode, the grid is 
not available and the total power demand of the load is sup-
plied by the DGs. The frequency is also not fixed and is calcu-
lated from the modified droop to meet the active and reactive 
power requirements. With any load change, the active and 
reactive power requirements change and the VSC reference 
voltage magnitude and angle must change to meet the new 
load requirement. Two types of load are considered here – 
constant impedance type load and motor load. 
Fig. 7 shows the response with impedance load, where the 
values of the load impedances are doubled at 1 s. The load is 
changed back to its nominal value at 1.5 s. It can be seen that 
DG-1 shares more load than DG-2 in accordance with their 
droop characteristics, while the grid does not supply any 
power. 
0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7
-0.5
0
0.5
1
1.5
A
ct
iv
e 
p
ow
er
 (
M
W
)
 
 
0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7
-0.2
0
0.2
0.4
0.6
R
ea
ct
iv
e 
po
w
er
 
(M
V
A
R
)
 
 
0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7
-0.1
0
0.1
Time(s)
C
u
rr
en
t-
p
ha
se
 a
 
(k
A
)
 
 
DG1
DG2
Grid
DG1
DG2
Grid
DG1
DG2
Grid
 
Fig.7 System response with impedance load in islanded mode. 
 
Fig. 8 shows theresults when the inductor motor is con-
nected in parallel with the passive load at 2 s and disconnected 
at 2.75 s. The motor is operated in speed control mode. The 
change in active power supplied by the DGs and output cur-
rent of the inverters show proper load sharing with a quick 
steady state attainment. The zero power and zero current from 
the grid confirm the islanded condition. 
1.8 2 2.2 2.4 2.6 2.8 3 3.2
0
0.5
1
1.5
2
A
ct
iv
e 
p
o
w
er
 (
M
W
)
 
 
1.8 2 2.2 2.4 2.6 2.8 3 3.2
-0.6
-0.4
-0.2
0
0.2
0.4
Time (s)
C
u
rr
en
t-
p
h
as
e 
a 
(k
A
)
 
 
DG1
DG2
Grid
DG1
DG2
Grid
 
Fig .8. System response with motor load in islanded mode. 
 
B. Grid Connected Mode 
 
In the grid connected mode, the steady state system fre-
quency is fixed to the utility frequency. It is assumed that the 
distributed generators supply their rated power at rated fre-
quency. When the load requirement is less than the total rated 
power of the DGs, the excess power flows from DGs go to the 
grid. In case of a motor load, the PCC voltage should not de-
viate much from its nominal value as a sudden large change in 
the motor terminal voltage will result in a large power swing. 
The swing is reduce in those situations where a passive load is 
connected in parallel to the motor. Further to avoid power 
swing, the VSCs must supply the change in the power demand 
as quick as possible. To accomplish this, only a voltage con-
trol may not be sufficient. It is desirable that a current control-
ler is added to the voltage controller to ensure better load shar-
ing and quick response. Therefore the control is changed to a 
state feedback control which uses the feedback of DG output 
voltage, output current and the current through the filter ca-
pacitor. 
The reference voltage magnitude and angle are calculated 
from the droop similar to the islanded mode. However the 
steady state frequency is fixed to the grid frequency. The 
power sharing by the DGs is rating based. Thus the active and 
reactive power requirements from individual DG are calcu-
lated based on their rating. The output current reference is 
calculated from the power and voltage reference. The refer-
ence for the filter capacitor current is calculated from the volt-
age reference. 
Fig. 9 shows the system response during a load change in 
the grid connected mode with impedance. In the grid con-
nected condition, any change in load is picked up by the grid 
as the DGs always provide the rated power. In this case, the 
excess power is sent to the grid. The change in grid current 
with active power demand ensures a stable operation. 
1.2 1.4 1.6 1.8 2 2.2 2.4
-0.5
0
0.5
1
1.5
A
ct
iv
e 
p
o
w
er
 (
M
W
)
 
 
1.2 1.4 1.6 1.8 2 2.2 2.4
-0.2
-0.1
0
0.1
0.2
Time(s)
G
ri
d
 C
u
rr
en
t 
-p
h
as
e 
a 
(k
A
)
 
 
DG1
DG2
Grid
 
Fig. 9. System response with impedance load in grid connected mode. 
 
Fig. 10 shows the results with a passive load when an in-
duction motor gets connected at 2 s and disconnected at 2.75 s. 
It is obvious that the additional power required by the motor is 
coming from the grid as the DGs supply the rated power. 
Fig. 11 shows the results when a synchronous motor is con-
nected in parallel with an impedance load. With the motor and 
impedance operating in the steady state, the motor is discon-
nected at 1.5 s and reconnected at 3 s. It can be seen that the 
powers supplied by the DGs remain constant during both the 
transients and the oscillations in the grid current die out within 
1 s. 
 
1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2
0
1
2
3
A
ct
iv
e 
p
o
w
er
 (
M
W
)
 
 
1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2
-0.4
-0.2
0
0.2
0.4
Time(s)
G
ri
d
 c
u
rr
en
t 
-p
h
as
e-
a 
(k
A
)
DG1
DG2
Grid
 
Fig.10. System response with induction motor load in grid connected mode. 
1 1.5 2 2.5 3 3.5 4 4.5
0
0.5
1
A
ct
iv
e 
p
o
w
er
 (
M
W
)
 
 
1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3
-0.04
-0.02
0
0.02
0.04
G
ri
d
 c
u
rr
en
t
(k
A
)
2.8 3 3.2 3.4 3.6 3.8 4 4.2 4.4
-0.04
-0.02
0
0.02
0.04
Time (s)
G
ri
d
 c
u
rr
en
t
(k
A
)
DG1
DG2
Grid
 
Fig.11. System response with synchronous motor load in grid connected 
mode. 
 
C. Seamless Transfer between Grid Connected and Islanded 
Modes 
 
The results simulated so far show that a better load sharing 
and a quick steady state attainment are achieved with the volt-
age control in the islanded mode, while the state feedback 
ensures better response in the grid connected mode. A seam-
less transfer between these two modes is proposed in this pa-
per by changing state feedback to voltage control and vice 
versa. Ordinarily, in the grid connected mode, the DGs operate 
under the state feedback control. When an islanding is de-
tected, the DGs are switched to the voltage control mode. 
These are switched back to the state feedback control mode 
after resynchronization. The sequence of control from a grid 
connected operation to islanded mode and then again back to 
grid connected is given below. 
1. Detect the islanding by zero current in the breaker con-
necting the grid with the microgrid. 
2. If islanded, switch to voltage control mode. 
3. Calculate the reference voltage required for the individ-
ual DGs to maintain the load power requirement from 
the droop. 
4. Check for the clearing of the fault. 
5. If resynchronization is desired, detect the grid voltage 
and frequency. 
6. Calculate the reference voltages for the DGs to make the 
PCC voltage same as the grid voltage while continuing 
to supply the load requirement and with the proper shar-
ing. 
7. Calculate the change in angle required to match the 
phase for proper synchronization. 
8. Change the voltage reference to the new calculated 
value. 
9. Connect the breaker. 
10. Switch to the state feedback control mode. 
 
Figs. 12 and 13 show the response of the system during 
islanding and resynchronization with the impedance load 
when the islanding occurs at 1.5 s and the resynchronization 
occurs at 2 s. Fig. 12 shows the power sharing and currents, 
while the PCC voltages are shown in Fig. 13 during the island-
ing and resynchronization operation. 
 
Fig. 12, System response during islanding and resynchronization with imped-
ance load. 
 
Fig. 13. PCC voltage during islanding and resynchronization with impedance 
load 
 
Islanding and synchronization with motor load have also 
been also investigated. An impedance load is an infinite sink 
as it can absorb any change in instantaneous real and reactive 
power. However an inertial load such as motor is not capable 
of sinking much power. Thus any change in the terminal volt-
age will result in large oscillation in the real and reactive pow-
ers. So the stability becomes a major issue during islanding 
with inertial load. Since the voltage control is a slow process, 
a re-initialization in the reference value is required to force the 
system to a new steady state quickly. 
It is assumed that an online load flow study is always per-
formed in background with the microgrid load and generation. 
At the instant of islanding, the values obtained from the load 
flow are used to determine the new voltage reference. The new 
reference ensures minimal change in the load voltage after 
islanding and proper sharing of the loads among the DGs. 
These new values are assigned as the new reference for the 
controllers. 
Fig. 14 shows the active power sharing during islanding and 
resynchronization with a motorload. An induction motor is 
used here. The active power input to the motor load is also 
shown in this figure. It can be seen that this power remains 
constant during islanding and resynchronization, validating a 
seamless transfer between the two modes. Phase-a of the DG 
output currents along with the grid currents are shown sepa-
rately during islanding and resynchronization in Fig. 15. It can 
be seen that all the currents reach their steady state values with 
0.2 s, both during islanding and resynchronization. 
0.5 1 1.5 2 2.5 3 3.5
0
0.5
1
1.5
2
2.5
A
ct
iv
e 
p
o
w
er
 (
M
W
)
 
 
0.5 1 1.5 2 2.5 3 3.5
0.5
1
1.5
2
2.5
Time (s)
A
ct
iv
e 
p
o
w
er
 o
f 
M
o
to
r 
L
o
ad
 (
M
W
)
DG1
DG2
Grid
 
Fig. 14. System response during islanding and resynchronization with motor 
load. 
 
Fig. 15. System response during islanding and resynchronization with motor 
load. 
 
V. CONCLUSIONS 
 
In this paper, a modular controller structure with modified 
voltage angle control loop is proposed for better load sharing 
between the parallel connected inverters in a distributed gen-
eration system. The integral control in the voltage angle loop 
helps to influence the close loop dynamics without affecting 
the steady state frequency regulation. By switching the control 
action of the DGs from state feedback in grid connected mode 
to voltage control in islanded mode, a seamless transfer is 
achieved. A step by step control method is proposed for a 
smooth transition during islanding and resynchronization. The 
efficacy of the controller is verified through PSCAD simula-
tions with impedance as well with motor loads. 
 
ACKNOWLEDGEMENT 
 
The authors thank the Australian Research Council (ARC) 
for the financial support for this project through the ARC Dis-
covery Grant DP 0774092. 
 
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Ritwik Majumder (S’07) received his B.E. in Electrical Engineering from 
Bengal Engineering College (Deemed University) in 2001 and his M.Sc. 
(Engg.) degree from Indian Institute of Science in 2004. From July 2004 to 
November 2004, he was with Tata Motor Engineering Research Centre in 
Jamshedpur, India. From November, 2004 to January 2006, he was with Sie-
mens Automotive India and from January 2006 to May 2007, he was with 
ABB Corporate Research Centre, Bangalore, India. Since June 2007, he is a 
Ph.D. scholar in Queensland University of Technology. His interests are in 
Power Systems dynamics, Distributed Generation and Power Electronics 
Applications. 
 
Arindam Ghosh (S’80, M’83, SM’93, F’06) is the Professor of Power Engi-
neering at Queensland University of Technology, Brisbane, Australia. He has 
obtained a Ph.D. in EE from University of Calgary, Canada in 1983. Prior to 
joining the QUT in 2006, he was with the Dept. of Electrical Engineering at 
IIT Kanpur, India, for 21 years. He is a fellow of Indian National Academy of 
Engineering (INAE) and IEEE. His interests are in Control of Power Systems 
and Power Electronic devices. 
Gerard Ledwich (M’73, SM’92) received the Ph.D. in electrical engineering 
from the University of Newcastle, Australia, in 1976. He has been Chair Pro-
fessor in Power Engineering at Queensland University of Technology, Austra-
lia since 2006. Previously he was the Chair in Electrical Asset Management 
from 1998 to 2005 at the same university. He was Head of Electrical Engi-
neering at the University of Newcastle from 1997 to 1998. Previously he was 
associated with the University of Queensland from 1976 to 1994. His interests 
are in the areas of power systems, power electronics, and controls. He is a 
Fellow of I.E.Aust. 
 
Dr Firuz Zare (S’97, M’01, SM’06) was born in Iran in 1967. He holds a 
PhD degree in Electrical Engineering from Queensland University of Tech-
nology in Australia. He has worked as a development engineer and a consult-
ant in industry for several years. He has joined the school of engineering 
systems in QUT in 2006. His research interests are power electronic applica-
tions, pulse-width modulation techniques, renewable energy systems and 
electromagnetic interferences.