Download International Electrical Engineering Journal (IEEJ) Vol. 6 (2015) No.4, pp. 1884-1890

International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.4, pp. 1884-1890
ISSN 2078-2365
http://www.ieejournal.com/
Synchronized Reactive Power
Compensation in a Microgrid System
Using FOPID Voltage Regulator
T. Nageswara Prasad and S. Farook
Sree Vidyanikethan Engineering College, Tirupathi, Andhra Pradesh, India
[email protected] and [email protected]

Abstract— Distributed generators connected to the
grid, when operating in parallel, the load sharing among
them without affecting the security and reliability is a
crucial issue. In such a large interconnected system it is
necessary to have communication among the DG’s for
effective load sharing which otherwise will influence the
security of the grid. The paper concerns with a strategy
for real and reactive power sharing independently based
on the locally measured signals without having
communication between the DG’s. A Fractional order
Proportional Integral Derivative (FOPID) based control
strategy was proposed to determine the power control
error and to control the parameters of DG’s for effective
load sharing between them. The proposed strategy was
implemented with three DG’s feeding linear loads and
was realized in MATLAB / SIMULINK platform. The
simulation results depict an improved dynamic
performance with fractional order PID controller over
the conventional PI controller.
Index Terms— Distributed generation (DG), Droop control,
Reactive power compensation, Power sharing, Fractional
Order PID controller.
I. INTRODUCTION
The evolution of potential generating technologies and
resources springs up the distributed generation which
fortifies the concept of microgrid. A microgrid in its state
consists of multiple generators connected in decentralized
locations to supply the aggregated loads. The one potential
advantage of having distribution generation is, it relives the
Dr. T. Nageswara Prasad, Professor and Head of Department, Department
of Electrical and Electronics Engineering, Sree Vidyanikethan Engineering
College, Tirupathi, Andhra Pradesh, India. E-mail: [email protected].
Dr. S. Farook, Associate Professor, Department of Electrical and Electronics
Engineering, Sree Vidyanikethan Engineering College, Tirupathi, Andhra
Pradesh, India.. E-mail: [email protected].
stress on the stipulated transmission and distributions
systems and adequates the operational reliability, security
and economy of the electrical system. The sharing of power
in a microgrid by the DG’s particularly during the island
operation is a crucial issue because of the presence of DG’s
having diverse power capabilities and generation
characteristics, which will adversely affect the stability and
security of the grid if appropriate control measures were not
implemented [1]-[15].
The load sharing control strategy of the each DG will
determines the power generator set-points of each unit and
assists in restoring the voltage and frequency of the system
responding to disturbances. The distributed generators
spanning over a wide area must be operated with minimal
communication as their inter-communication is impractical
and the most common method is to use the droop
characteristics [8], [13]. The use of measurable local signals
such as voltage and frequency droop as a feedback signals to
control the converters of DG’s to share the load proportional
to their ratings is a viable approach, which aims to achieve
the effective power sharing in a decentralized manner [18] –
[22]. However, the droop control governed microgrid is
prone to have some power control stability issues, when the
DG feeders are highly resistive. Also the real power sharing
at the steady state is insensitive to feeder impedance while the
reactive power sharing does depend on the feeder impedance
[9]. During the autonomous mode of operation, the errors in
reactive power sharing is investigated and then the reactive
power sharing is realized by modifying the injected real and
reactive power supplied by the DG converters [1] using a
fractional order PID controller. The proposed control
strategy accurately shares the load demand in a microgrid
with different configurations and for diversified loads.
Several control strategies have been proposed for the
load sharing control in a microgrid system with multiple
DG’s and with diversified loads and some proposed PI, Fuzzy
regulator, reduced order regulators etc., [18], [19]. The
1884
Nageswara Prasad and Farook
Synchronized Reactive Power Compensation in a Microgrid System Using FOPID Voltage Regulator
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.4, pp. 1884-1890
ISSN 2078-2365
http://www.ieejournal.com/
present work concerns with the implementation of FOPID
controller to control the voltage magnitude of the DG’s and
to eliminate the steady state reactive power tracking errors.
A fractional order PID controller is the generalized PID
controller whose derivative and integral is fraction rather
than integer, extending this derivative and integral order
from integer to fractional provide more flexibility in design
of the controller thereby controlling the wide range of
dynamics. In fractional order ( PI  D ) besides integral
( K I ), proportional ( K p ) and derivative gain ( K D ) the
controller has additional integral order (  ) and derivative
order (  ) as two more parameters. Thus the use of two extra
operators adds more degree of freedom in design and makes
it possible to further improve the performance [23]-[25]. In
the proposed work presents the reactive power load sharing
compensation method using a FOPID controller on a system
consisting of three DG’s feeding linear loads.
The paper is organized as follows: Section II presents the
concepts of microgrid and conventional droop control
strategy. Section III presents the reactive power sharing
control strategy and its implementation aspects. The
simulation of the proposed microgrid system is presented in
section IV. Finally the results, discussions and conclusions
were presented in section V and VI.
II. MICROGRID SYSTEM DESCRIPTION
A simplified microgrid system with three DG’s feeding
non-linear loads is depicted in Fig.1. As shown in Figure,
each DG unit is interfaced to the microgrid with an inverter
which is provided with the load sharing control (LSC) units
for load sharing between them.
A. Conventional droop control strategy
Most of the microgrids during the autonomous operating
mode use conventional droop strategy which adopts P-f and
Q-V control methods to share appropriate power to DG’s by
maintaining the frequency and voltage.
Fig.2: Droop control block diagram
The role of droop control is to control the real power on
the basis of the frequency and controls the reactive power
based on the voltage control, thus the frequency and voltage
droop control equations can be stated as [10]:
f  f0  K p (P  P0 )
(1)
V  V0  Kq (Q  Q0 )
(2)
Where f and V are the frequency and voltage at new
operating point, P and Q are the real and reactive power at
the new operating point, K p and Kq are the droop constants
for frequency and voltage magnitudes respectively and were
defined as:
K p  ( f o  f ) / Pmax  f / Pmax
K q  (Vo  V ) / Qmax  V / Qmax



(3)
III. REACTIVE POWER SHARING AND ERROR
COMPENSATIONS METHOD
Fig. 1: Simplified microgrid system
During the normal grid connected operation, the real
and reactive power commands are usually assigned by the
centralized controller and the normal droop control method
is used for the power tracking. However during the islanded
mode of operation, the errors due to reactive power sharing is
compensated by introducing a real-reactive power coupling
command [15] to the local load sharing controller such as
integral or FOPID controller to control the voltage
magnitude in a synchronized manner.
The errors during the reactive power sharing is due to
various factors such as feeder impedance, microgrid
configurations etc., which will influence the load sharing
among the DG’s, hence it is necessary to develop an efficient
compensation method will eliminate the reactive power
sharing errors without going into the details of the microgrid
configuration. In the proposed compensation strategy, the
compensation is carried out in two stages with initial power
sharing done by using conventional droop method and later
the power sharing through the synchronized compensation
method [1].
A. Synchronized reactive power compensation
During the initial stage the conventional droop control
governed by the eq. (1) and eq. (2) is used for the initial load
1885
Nageswara Prasad and Farook
Synchronized Reactive Power Compensation in a Microgrid System Using FOPID Voltage Regulator
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.4, pp. 1884-1890
ISSN 2078-2365
http://www.ieejournal.com/
sharing between the DG’s. During this stage the averaged
real power is computed which will be used for reactive power
sharing in later stage of compensation. For computing the
average power, a moving average filter is used to filter out the
power ripples. The measured average power is saved in this
stage for improving the accuracy of reactive power sharing by
synchronized compensation in the second stage. In later
stage, the reactive power sharing error is compensated by
introducing a real-reactive power coupling transient and by
using FOPID control as a voltage regulator. As the
compensation is based on the transient coupling control and
this will be carried out in all DG units synchronously. Very
immediately after the compensation signal is received to the
local load sharing control units, the last computed average
power is used as an input to the compensation scheme.
During the compensation, both real and reactive powers are
used in frequency droop control and the reactive power error
is suppressed by the use of integral term given by [1], [4]:
f  f0  (K p P  KqQ)
B. Multi loop FOPID voltage controller for DG
The conventional multi loop voltage regulators uses PI
controller which has the limitations to react for abrupt
changes in the error, as it is capable of determining the
instantaneous error without considering the rise or fall of the
error, in the proposed work a controller based on fractional
calculus is employed which overcomes the limitations of
conventional PI and PID controllers. Fractional Order
Proportional-Integral-Derivative ( PI  D  ) is a PID
controller whose derivative and integral orders are of
fractional rather than integers. With the inclusion of integral
order (  ) and derivative order(  ) as a fraction, the
controller has additional parameters to tune which allows
more flexibility in design and to improve the wide operating
space of the controller with respect to the load variations
[23]-[25].
(4)
K 
(5)
V  V0  K q Q   c  ( P  Pavg )
 S 
Where Kc is the integral gain of the synchronized reactive
power compensator in each DG unit.
Hence from eq. (4) it is observed that the real and
reactive power control is coupled with frequency droop
control and for any reactive power errors, the unequal offsets
from different DG units will affect the DG output
frequencies, which subsequently will introduce a real power
disturbance. This real power disturbance will cause the
controller to regulate the output voltages of DG’s thus
providing the required reactive power sharing among them.
Once the reactive power is shared properly the real power
flow will follow back to the nominal value with frequency
droop control and the controller of each DG unit will not
contribute the voltage regulation [1].
Fig. 4: FOPID multi loop voltage regulator for DG units
B.1. fractional calculus
The fractional order controllers were originated from the
branch of mathematics called Fractional calculus which
deals with non-integer order derivatives and integrals. The
earliest theoretical contributions in the domain were made by
Euler and Lagrange and were further fortified by Liouville,
Riemann and Holmgren. The results from Riemann and
Liouville were unified and are accepted as the most
admittable definition for fractional integral and derivatives.
For a primitive function f (t ) whose Laplace transform
is F ( S ) , from the fundamentals, the Laplace inverse of nth
order integral operator
1
, n  R  is expressed as:
Sn
 S   t (n)
L1 1
n 1
(6)
n
1
in
Sn
Laplace domain corresponds to convolution product in time
domain, and is expressed as:
x
t n 1
1
n 1
(7)
D  n f (t ) 
* f (t ) 
f (t )  x  t  dt

(n)
(n) 0
The product of the Laplace functions F ( S ) and
Fig. 3 Synchronized reactive power compensation strategy
1886
Nageswara Prasad and Farook
Synchronized Reactive Power Compensation in a Microgrid System Using FOPID Voltage Regulator
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.4, pp. 1884-1890
ISSN 2078-2365
http://www.ieejournal.com/
Similarly the operator S n in Laplace domain gives rise to an
dn
operator n in time domain. From the fundamentals, the
dt
iterating operation of fundamental derivative gives nth
derivative of the function is generalized as:
m

n!
(8)
Dn f ( x)  lim h n   1
f ( x  mh)
h 0
m!  n  m  1
m0
The above equations (7) and (8) correspond to
Riemann–Liouville’s definition for the fractional order
integral and derivatives of order n  R respectively [24].
B.2 Fractional order PID Controller
The Differential equation used to describe the
conventional PID controller is used to describe the fractional
controller with integral and derivative orders as fractional.
The differential equation for fractional controller is:
U c (t )   K p e(t )  K I Dt e(t )  K d Dt e(t ) 
(9)
Applying Laplace transformation results in the
transformed fractional PID controller with continuous
transfer function of the controller given by:
K


Gc ( s)   K p  I  s  K d 
s


 ,   0
(10)
The output of the controller is given by
K


(11)
U c ( s)   K p  I  s  K d  e(s)
s


The error function e( s) is modelled as errors in reactive
power during the compensation.
IV. DIGITAL SIMULATION
The proposed networked microgrid model has been
modelled in MATLAB / SIMULINK platform consisting of
three identical DG having same ratings, feeding three linear
loads connected to the microgrid. The detailed modelled
configuration is depicted in Fig. 5. To validate the
effectiveness of the proposed compensation strategy the
FOPID voltage regulator performance was compared against
the conventional PID controller.
The microgrid parameters used for the simulation is
depicted in appendix. The simulation results were depicted in
Fig. 6 to Fig. 13.
Fig. 5: Networked microgrid system
1887
Nageswara Prasad and Farook
Synchronized Reactive Power Compensation in a Microgrid System Using FOPID Voltage Regulator
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.4, pp. 1884-1890
ISSN 2078-2365
http://www.ieejournal.com/
Fig. 6: Reactive power sharing in Microgrid using PI controller
Fig. 10: Reactive power sharing in Microgrid using FOPID controller
Fig. 7: Real power sharing performance in a Network Microgrid using PI
controller
Fig. 11: Real power sharing performance in a Network Microgrid using FOPID
controller
Fig. 8: DG’s voltage magnitude using PI controller
Fig. 12: DG’s voltage magnitude using FOPID controller
Fig. 9: DG’s current during compensation using PI controller
Fig. 13: DG’s current during compensation using FOPID controller
1888
Nageswara Prasad and Farook
Synchronized Reactive Power Compensation in a Microgrid System Using FOPID Voltage Regulator
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.4, pp. 1884-1890
ISSN 2078-2365
http://www.ieejournal.com/
V. RESULTS AND DISCUSSIONS
The simulation results shown in Fig.6 and Fig. 10 depict
the reactive power outputs of each DG unit using
conventional PI and FOPID voltage regulator. From the
simulation results, it is inferred that the reactive power
initially was shared unequally which is due to unequal
voltage drops in the networked microgrid feeders. These
significant errors were compensated effectively by enabling
the compensation at t=1 Sec and then FOPID controlled
voltage regulator effectively modulates the reactive power
and thus nullifying the reactive power error and allowing
equal sharing of reactive power between the DG’s.
The real power of various DG’s with conventional and
FOPID controller is depicted in Fig. 7 and Fig. 11
respectively. From the simulation results it is inferred that
the real power is shared evenly before compensation and
when the compensation is enabled, the transient real and
reactive power coupling defined by eq. (4) and eq. (5)
respectively initiates the disturbance in the real power. These
disturbances in real power flow however were controlled by
the frequency droop control and FOPID controller effectively
than the conventional PI controller.
The voltage magnitudes of each DG unit is depicted in Fig.
9 and Fig. 12 with conventional PI and FOPID voltage
regulator, the simulation results infers that the voltages have
a deviations which is due to unequal voltage drops in the
feeder which are compensated by the DG units. The
simulation results show a significant improvement in
stabilizing the voltage variations with FOPID controller
compared with conventional controller.
TABLE 1: REAL AND REACTIVE POWER OF DG,S BEFORE AND AFTER
COMPENSATION
Reactive Power ( var)
DG
Units
Before
Compensati
on
( t < 1 Sec )
After
Compensation
( t > 3 Sec )
Before
Compensati
on
After
Compensatio
n
( t < 1 Sec )
( t > 3 Sec )
810
475
1175
1175
DG 2
400
475
1175
1175
DG 3
200
475
1175
1175
TABLE 2: PERFORMANCE OF MICROGRID SYSTEM
DG
Units
PI
FOPID
VI. CONCLUSION
The paper has contrived an effective reactive power
sharing strategy in a microgrid system using FOPID as a
voltage regulator. The proposed load sharing methodology
effectively compensates the errors in reactive power sharing
and effectively shares the load among the various DG’s in a
synchronized manner by modulating the magnitude of DG
voltage by a fractional order PID controller of each DG
during autonomous grid operating mode. The simulation
results evidence an improved performance of the system with
the proposed FOPID controller over the conventional PI
controller.
APPENDIX
TABLE 3: MICROGRID PARAMETERS
Parameter
Filter Inductor
Interfaced
Inverter
Microgrid
parameters
Real Power (W)
DG 1
Real Power peak (W)
The line currents of each DG is shown in Fig. 10 and Fig.
13, infers that there is no significant change in the current
before and during compensation and hence leads to
consistent power sharing among the DG units. The
quantitative performance measures of the microgrid system
with conventional PI and proposed FOPID voltage regulator
is depicted in Table 1 and in Table 2.
Real Power settling time
(Sec)
PI
Droop
coefficients
FOPID
Voltage
regulator
1090
3
2.0
DG2
1250
1220
3
2.1
DG3
1340
1241
3
2.0
L:5mH/R:0.2Ω
40µF
Sampling-switching
frequency
9kHz-4.5kHz
Rated RMS voltage
(Line-Line)
208V (60Hz)
Total Loads
3525W-1425Var
Frequency droop Kp
0.00125 Rad / (Sec . W)
Voltage droop KQ
0.00143 V/Var
Integral dead-band
6W
Integral gain Kc
0.0286 V/(Sec . W)
LPF time constant ґ
0.0159 Sec
DG 1
0.36

0.951 
7.89  S 0.402  8.5924S 
DG 2
DG 3
930
(Lf /Rf )
Filter Capacitor (Cf )
FOPID
DG1
Values
0.42

1.250 
9.45  S 0.621  2.7825S 
0.17

0.811 
10.00  S 0.150  6.9814S 
1889
Nageswara Prasad and Farook
Synchronized Reactive Power Compensation in a Microgrid System Using FOPID Voltage Regulator
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.4, pp. 1884-1890
ISSN 2078-2365
http://www.ieejournal.com/
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Nageswara Prasad and Farook
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