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International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No. 9, pp. 1994-2002
ISSN 2078-2365
http://www.ieejournal.com/
A Switched Capacitor
Compensator-Fuzzy Logic Controlled
Scheme for Damping Load Excursions in
PV-Powered DC Schemes
Adel M Sharaf*, Maged N. F. Nashed**, Mona Eskander***
*
Life Senior Member IEEE, Sharaf Energy Systems, Inc., Fredericton, NB-Canada
**, ***
Electronics Research Institute, Cairo, Egypt
*
[email protected], **[email protected],***[email protected]
Abstract— Load excursions in PV powered DC interface
Systems can cause loss of energy utilization and voltage
transients as well as inrush current conditions, which leads to
inefficient PV energy utilization and possible damage to
sensitive loads. Large variation in load demand and /or Source
PV-voltage leads to both inefficient operation and load voltage
excursion. In this paper a low cost switched filter compensator
scheme developed by the first Author as a member of a number
of LC –switched Compensation family of modulated PWM
Switched Capacitors is utilized to maintain quasi steady state
stabilization of common bus load voltage and keep the load
voltage constant irrespective of supply voltage or load
variations. The Switched Capacitor compensating circuit
(SCCS) is composed of two capacitors, rectifier-bridge and
IGBT switches. A Fuzzy logic based controller is designed for
PWM-switching the MOSFET/IGBTs to adjust the Common
bus/ load voltage during source variations and load excursions.
The dynamic performance of the unified system employing the
new SCCS- compensator is validated for different load
conditions, as well as faults such as open circuit and short circuit
conditions at the supply side and the compensator circuit
connection. Dynamic simulation results validated the excellent
compensation of load voltage at all conditions.
Index Terms— PV, Buck-Boost converter,
Compensator, Fuzzy Logic PWM-Controller
SCCS-
I. INTRODUCTION
Electric power distribution/utilization systems suffer from
both voltage and power quality problems. Power quality
problem is any power problem manifested in voltage, current,
or frequency deviation that results in failure or mal-operation
of customer equipment. Power Quality-PQ issues can be
classified into a number of sub-categories: sub, super, and
inter-harmonics; voltage swells, sags, fluctuations, flicker,
and transients; voltage magnitude and frequency deviation;
voltage imbalance (Three phases system); hot grounding
loops and ground potential rise (GPR) safety and fire hazards;
as well as monitoring and measurement problems. It is also
known that massive increase in the use of nonlinear loads,
such as static power converters and arc furnaces, causes high
disturbancesin distribution and utilization systems. All
PQ-these issues result in poor/inefficient utilization and
reduction in power factor, feeder capacity-overloading and
noise interference to adjacent communication systems [1].
Since efficient operation and loss reduction is a priority for
energy conservation and efficient electric energy utilization is
required, solutions to power quality deterioration are tackled.
Examples of devices applied to overcome these problems and
enhance power quality are power filter used with new FACTS
based devices such as; active power filters, STATCOM,
MPF, and switched capacitor compensators are [2-4]. The
active power filters (APF) can also be used for power factor
correction and loss reduction [5, 6].
In this paper a low cost design based on SCCS family of
devices developed by the First Author is validated through
digital simulation using Matlab/Simulink Software
Environment. The SCCS-Compensator may be applied to
2-wire loads, which could be a symmetrical-type or
asymmetrical arc type, a SMPS, or adjustable speed drive.
The developed circuit is tested for a variable parallel RC load
aiming at continuous constant voltage supply for the load,
irrespective of any change in the source voltage or in the load
demand. A fuzzy logic controller (FLC) is designed to adjust
switching the IGBTs according to source or load variations, to
achieve constant un-perturbed load voltage, [7]. Simulations
are done for different load and source conditions, giving the
voltage and current responses at the compensator circuit input
and across the load. Results proved the reliability of the
developed circuit for all tested situations, i.e. open and short
circuited load, open and short circuited source, and variable
load demand.
II. DC CIRCUIT DESCRIPTION
The PV-Powered supply system is modified to include the
SCCS-Compensation device which the developed circuit is
connected is shown in Fig. (1). It is consists of a PV module
1994
Adel M Sharaf et. al.,
A Switched Capacitor Compensator-Fuzzy Logic Controlled Scheme for Damping Load Excursions in PV-Powered DC Schemes
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No. 9, pp. 1994-2002
ISSN 2078-2365
http://www.ieejournal.com/
Node A
Developed Circuit
Control Part
Fig. 1 Test PV-Powered DC Systems with the PWM-Fuzzy Logic SCCS-Compensator.
connected to a buck-boost converter acting as voltage
regulator for the PV source. The SCC-Device is shown in Fig
(1) within the dashed box, consists of two IGBT switches,
three capacitors, and a diode rectifier bridge. If the first IGBT
is switched on, the second IGBT is switched off. Controlling
the on-off switching is done via a fuzzy logic controller. The
first input to the Fuzzy controller is the difference between the
load voltage and the reference voltage (error voltage). The
second input is the change in error. The whole circuit is
simulated by Matlab/Simulink Software Environment.
Fig. 2, PV- Cell Equivalent Circuit.
a. PV Array Analysis
b. Buck-Boost DC-DC Converter
Fig. 2 shows a simplified equivalent circuit model of a PV
module used in the study, which consists of a current source in
parallel with a diode and in series with a resistor., The
behavior of a PV array with Ns × Np modules, whose current
and power characteristics are shown in Fig. (3), may be
described by the following equation [8]:
The ideal converter circuit, shown in Fig. 4, is assumed to
simulate the Buck-Boost converter. The state variables for the
shown Buck-Boost converter are chosen as the inductor
current, IL  X1, and the capacitor voltage, Vc  X2.
State-space-averaged equations in matrix form are, [9]:
(1  D) 

 .  
0
 D
X
(2)
L   X1     U 
 .1  

1
L


(
1

D
)
1
X
 X  
  2   0 

 2
C
RC 

V  IARs
I A  N P I sc  N P I o {exp[ A
]  1}
nN s Vt
Where IA = output current of PV array [A]
ISC = short circuit current of PV module [A]
I0 = diode saturation current [A]
VA = terminal voltage of PV array [V]
RS = series resistance [Ω]
n = ideal constant of diode (1~2)
VT = thermal potential of PV module [V]
Ns = No. of series modules
Np = No. of parallel modules.
(1)
c. Fuzzy Logic controller
The structure of the controller in Fuzzy Logic Toolbox
window is presented in Fig. 5, [10, 11]. The membership of
input and output of control are shown in Fig. 6.Table 1 shows
the Assignment Matrix/Table used for the fuzzy rules of
FLC-controller.
1994
Adel M Sharaf et. al.,
A Switched Capacitor Compensator-Fuzzy Logic Controlled Scheme for Damping Load Excursions in PV-Powered DC Schemes
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No. 9, pp. 1994-2002
ISSN 2078-2365
http://www.ieejournal.com/
10
1
9
0
-300
7
6
Degree of membership
Current (Amp)
ln
ze
lp
hp
0.5
8
5
4
3
2
1
0
hn
0
20
40
60
80
100
120
Voltage (Volt)
140
160
180
200
1
-200
hn
-100
0
input1
ln
100
ze
200
lp
300
hp
0.5
0
-0.1
1
hn
mn
-0.8
-0.6
-0.05
0
input2
0.05
ln
ze
lp
0.1
mp
0.15
hp
0.5
1400
0
-1
1200
-0.4
-0.2
0
0.2
output1
0.4
0.6
0.8
1
-3
x 10
Power (Watt)
1000
Fig. 6, Linguistic rules for PWM—Duty Cycle of IGBT/MOSFET
Switches, input1 is the error of voltage output, input2 is the change of
voltage error, and output1 is the change of duty cycle switching.
800
600
400
200
0
0
20
40
60
80
100
120
Voltage (Volt)
140
160
180
200
Fig.3, Relation between voltage, current and power of the PV array at
different insolation.
Error
/ error
hn
ln
ze
lp
hp
Table I: Rules of FLC
hn
ln
ze
lp
hp
hn
hn
mn
ln
ze
ze
lp
mp
hp
hp
hn
ln
ln
ze
lp
mn
ze
ze
lp
mp
ln
ze
lp
lp
hp
Where: hn = negative high, mn = Medium negative, ln=
negative low, lp = positive low, mn = Medium
positive, hp = positive high, and ze =zero
III. DIGITAL SIMULATION AND VALIDATION RESULTS
Fig. 4, Circuit model for the ideal Buck-Boost converter.
Fig. 5, Structure of FLC in Fuzzy Logic Toolbox window.
The PV-powered DC Source Load Test -system employing
the proposed new SCCS-Modulated/switched Capacitor
circuit is modeled and simulated with Matlab/Simulink,
monitoring the voltage and current profiles at the input of the
compensator circuit and at the load branch.
Figures (7) and (8) show the load voltage and current when
the load is increased by 25% at t=0.5 sec. It is noticed that the
load voltage is not affected while the load current increases
momentarily(0.1sec) then is adjusted, by the fuzzy controlled
compensator, to the new load current value. Figure (9a) gives
the current at the input of the compensator circuit "node A"
showing the charging characteristics of the compensating
capacitors, while Fig. (9b) zooms the capacitor charge at the
point of load increase.
Figures (10) and (11) show the load voltage and current
when the load is decreased by 25% at t=0.5 sec. It is noticed
that the load voltage and current are slightly affected, and
regain steady values in very short interval. Figure (12) shows
1996
Adel M Sharaf et. al.,
A Switched Capacitor Compensator-Fuzzy Logic Controlled Scheme for Damping Load Excursions in PV-Powered DC Schemes
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No. 9, pp. 1994-2002
ISSN 2078-2365
http://www.ieejournal.com/
200
150
100
Current Circuit (Amp)
the drop in capacitor voltage at “node A” at the point of load
decrease.
Figure (13) shows the drop taking place in the load voltage
due to an open circuit in the load for 0.1 second. The load
voltage dropped from 1100V to 950 V, and then gains its
constant value after 0.15 seconds only. The load current is
shown in Fig.(14) is decreased but did not reach zero. The
load current reached its steady state value in less than 0.05
seconds. Figure (15) shows the instantaneous drop in the
capacitor voltage at “node A” due to the load open circuit. It
took another 0.1 seconds to return to its normal charge. The
current at the compensating circuit point "node A" is shown in
Fig. (16).
50
0
-50
-100
-150
-200
1200
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec.)
0.7
0.8
0.9
1
(a)
1000
800
100
600
Current Circuit (Amp)
Load Voltage (Volt)
150
400
200
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec.)
0.7
0.8
0.9
1
Fig. 7, the load voltage at increase the load 25%at time t=0.5 sec.
200
50
0
-50
-100
-150
0.485
0.49
0.495
0.5
0.505
Time (sec.)
0.51
0.515
(b)
Fig. 9, Current at the input of the compensator circuit "node A" with zoom in
(b).
180
Load Current (Amp.)
160
1200
140
120
1000
100
80
800
60
600
40
20
400
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
200
Fig. 8, the load current at increase the load 25%at time t=0.5 sec.
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Fig. 10, the load voltage at decrease the load 25%at time t=0.5 sec.
1997
Adel M Sharaf et. al.,
A Switched Capacitor Compensator-Fuzzy Logic Controlled Scheme for Damping Load Excursions in PV-Powered DC Schemes
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No. 9, pp. 1994-2002
ISSN 2078-2365
http://www.ieejournal.com/
250
220
200
180
200
Load Current (Amp.)
Load Current (Amp.)
160
140
120
100
80
150
100
60
50
40
20
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
Fig. 11, the load current at decrease the load 25%at time t=0.5 sec.
Fig. 14, the load current at open circuit from t=0.5 to 0.6 sec.
1800
1600
1600
1400
1400
1200
Circuit Voltage (volt)
Circuit Voltage (Volt)
0
1200
1000
800
1000
800
600
600
400
400
200
200
0
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
0
0.1
0.2
0.3
0.4
1
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
Fig. 15, capacitor voltage at “node A” at open circuit from t=0.5 to 0.6
sec.
Fig. 12, capacitor voltage at “node A” at load decrease.
1200
300
250
1000
Circuit Current (Amp)
Load Voltage (volt)
200
800
600
400
150
100
50
0
200
-50
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
Fig. 13, the load voltage at Open circuit from t=0.5 to 0.6 sec
-100
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
Fig. 16, capacitor current at “node A” at open circuit from t=0.5 to 0.6
sec.
1998
Adel M Sharaf et. al.,
A Switched Capacitor Compensator-Fuzzy Logic Controlled Scheme for Damping Load Excursions in PV-Powered DC Schemes
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No. 9, pp. 1994-2002
ISSN 2078-2365
http://www.ieejournal.com/
2000
1800
1600
1400
Circuit Voltage (Volt)
Figure (17) shows the drop taking place in the load voltage
due to a short circuited load for 10milli-second. The load
voltage dropped from 1100V to 950 V, but with a steeper
change than in the case of open circuited case, then gains its
constant value after 0.15 seconds only. The load current,
shown in Fig. (18), increased instantaneously, but restored its
steady state as soon as the short circuit is removed. The load
current reached its steady state value in less than 0.05
seconds. Figure (19) shows the instantaneous drop in the
capacitor voltage at the compensating circuit point "A" due to
this short circuit. It took another 0.05 seconds to return to its
normal charge. The current at the compensating circuit point
"A", gives similar response to the voltage as shown in Fig.
(20).
1000
800
600
400
200
1200
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
Fig. 19, Capacitor voltage at Short Circuit at t=0.5 to 0.51 (10 m sec) at
node A.
1000
300
800
Load Voltage (Volt)
1200
250
600
Current Circuit (Amp.)
200
400
200
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
150
100
50
0
-50
-100
Fig. 17, the load voltage at Short Circuit at t=0.5 to 0.51 (10 m sec) at
node A.
-150
-200
220
200
160
140
Load Current
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
Fig. 20, Capacitor current at Short Circuit at t=0.5 to 0.51 (10 m sec) at
node A.
180
120
100
80
60
40
20
0
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
Fig. 18, the load current at Short Circuit at t=0.5 to 0.51 (10 m sec) at
node A.
Figure (21) shows the drop taking place in the load voltage
due to a short circuit in the supply voltage for 20 milli-second.
The load voltage dropped from 1100V to 750 V, then restores
its constant value after 30 milli-seconds. This response time is
reasonable when compared with the time constant of the RC
load.
Figure (22) shows the drop taking place in the load voltage
due to an open circuit at the output of buck-boost converter
for 100 milli-second. The load voltage dropped from 1100V
to 900 V, then restores its constant value after 20
milli-seconds. This response time is nearly the same when
compared with the response time for an open circuit in the
load branch for the same interval at “node A”, Shown in Figs
13, to 16. Figure (23) demonstrates the corresponding change
in the load current, while, Fig. (24) shows the corresponding
compensator circuit voltage. Both figures reveal the fast
1999
Adel M Sharaf et. al.,
A Switched Capacitor Compensator-Fuzzy Logic Controlled Scheme for Damping Load Excursions in PV-Powered DC Schemes
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No. 9, pp. 1994-2002
ISSN 2078-2365
http://www.ieejournal.com/
recovery of voltages and currents, proving the effective
operation of the proposed circuit.
1600
1400
1200
1200
Circuit Voltage (volt)
1000
800
600
400
600
200
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Fig. 21, the load voltage at Short Circuit at t=0.6 to 0.62 (20 m sec) at
node A.
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
Fig. 24, capacitor voltage at open circuit from t=0.5 to 0.6 sec at supply
circuit.
Figure (25) shows the load voltage when a short circuit
occurred across the supply, while Fig. (26) depicts the
corresponding load current. Figure (27) shows the voltage at
the “node A”. It is worth noticing that the voltages and current
are not affected by the supply short circuit. This contradicts
with the case of short circuit at “node A”, as in Figs. 17 to 20,
where the short circuit at the input of compensating circuit
caused sharp changes in load voltage and current. These
results reveal that the compensating circuit acts as a
continuous supply for the load demand, in spite of the open
circuited source.
1200
1000
Load Voltage (Volt)
800
400
200
0
1000
800
600
400
200
1200
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
1000
Load Voltage (Volt)
Fig. 22, the load voltage at Open circuit from t=0.5 to 0.6 sec at Supply
circuit.
Load Current (Amp.)
200
150
800
600
400
200
100
0
50
0
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
Fig. 25, the load voltage at Short Circuit from t=0.5 to 0.51 (10 m sec) at
supply circuit.
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
Fig. 23, the load current at Open circuit from t=0.5 to 0.6 sec at Supply
circuit.
2000
Adel M Sharaf et. al.,
A Switched Capacitor Compensator-Fuzzy Logic Controlled Scheme for Damping Load Excursions in PV-Powered DC Schemes
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No. 9, pp. 1994-2002
ISSN 2078-2365
http://www.ieejournal.com/
Digital Simulation results for fault cases of open circuit,
short circuit, load increase and load decrease proved the
stabilizing effectiveness of the SCCS-Device using the PWM
-FLC-Controller, Digital simulation dynamic performance
validated the SCCS-Device as a new tool that can be utilized
in both Ac and DC hybrid renewable energy systems for
effective voltage stabilization, efficient energy utilization as
well as power quality/filtering applications supply.
Load Current (Amp)
200
150
100
50
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
Fig. 26, the load current at Short Circuit from t=0.5 to 0.51 (10 m sec) at
supply circuit.
1600
APPENDIX:
Photovoltaic: Module Type M 75S
1400
Circuit Voltage (Volt)
1200
1000
800
600
400
200
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
Fig. 27, capacitor voltage at Short Circuit from t=0.5 to 0.51 (10 m sec) at
supply circuit.
VI. CONCLUSIONS AND EXTENSIONS
A switched/Modulated SCCS-Compensation Scheme was
validated for effective voltage stabilization and power quality
enhancements with reduced inrush and voltage changes and
excursions on the DC common load bus. The SCCS
-Compensation Device with the two compensating PWM
-Switched capacitors is a simple scheme designed to stabilize
the common bus voltage, reduce voltage transients , inrush
current conditions and improve the power quality of PV
-powered dc utilization circuits feeding RC variable-type
loads. The power quality is improved by neutralizing the
effects of voltage fluctuations/ sags due to source variations in
SX, TX insolation/temperatures as well as load variations.
The fuzzy controller is designed to control the PWM-Duty
Cycle Switching Patterns of the SCCS-compensating Device
to ensure a regulated stabilized output load voltage near unity
with less excursions, variations and inrush current conditions
irrespective of variations in load or in the input source
voltage.
Electrical parameters
Maximum Power Pmax
Rated current IMPP
Rated voltage VMPP
Short circuit current Isc
Open circuit voltage Voc,,
Thermal Parameters
75 W
4.4 A
17.0 V
4.8 A
22 V
Normal Cell Operating Temperature
Temp coefficient. of the short-circuit current
Temp coefficient. of the open-circuit voltage
45±2 oC
+4*10-4/K
-3.4*10-3/K
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