446
Journal of Scientific & Industrial Research
J SCI IND RES VOL 72 JULY 2013
Vol. 72, July 2013, pp. 446-453
Photovoltaic Based Three-Phase Three-Wire DSTATCOM to improve
Power Quality
V. Kamatchi Kannan1 and N. Rengarajan2
1
Department of Electrical and Electronics Engineering, K.S.R College of Engineering, K.S.R,Tiruchengode, Tamilnadu, India.
2
K.S.R. College of Engineering, K.S.R. Kalvi Nagar, Tiruchengode,Tamilnadu, India
Received 20 July 2012; revised 28 December 2012; accepted 20 March 2013
Three-phase three-wire Distribution Static Compensator (DSTATCOM) with Photovoltaic (PV) array or battery operated
DC-DC boost converter is proposed in this paper. The proposed DSTATCOM consists of a three-leg Voltage Source Converter
(VSC) with a DC bus capacitor and it provides continuous reactive power compensation, source harmonic reduction and load
compensation throughout the day. With the help of PV array or battery, which is connected to the dc link of VSC via the DC-DC
boost converter is used to maintain the desired voltage to the dc bus capacitor for continuous compensation to the load. The
Icos F controlling algorithm is proposed for three-phase three-wire DSTATCOM. In this algorithm, the fuzzy logic controller is
compared with the conventional PI (Proportional Integral) controller at DC bus to regulate the DC link capacitor voltage. The
fuzzy controller is used to maintain the DC link voltage to the reference value. The switching of VSC will occur by Hysteresis
based Pulse Width Modulation (PWM) current controller. The simulations are carried out by using MATLAB/simulink software
to demonstrate the effectiveness of the proposed scheme.
Keywords: Distribution Static Compensator, Photovoltaic Array,Boost Converter, Voltage Source Converter, Icos F Controlling
Algorithm, Fuzzy Logic Controller
Introduction
Electric utilities and end users of electric power are
becoming increasingly concerned about meeting the
growing energy demand. Power quality issues are gaining
significant attention due to the increase in the number of
sensitive loads. The electrical devices like electric motors,
transformer s, generators, computer, printer,
communication equipment or a house hold appliances.
All of these devices and others react adversely to power
quality issues, depending on the severity of problems.
The power quality problems are mostly related to the
source of supply and types of load1. One of the most
severe problems of power quality is harmonic distortion
and its consequences. Hence, the extensive use of power
electronics based equipment and nonlinear loads are
creating a growing concern for harmonic distortion in
the AC power system. According to the IEEE standard,
harmonics in the power system should be limited by two
different methods; one is the limit of harmonic current
that a user can inject into the utility system at the Point
of Common Coupling (PCC) and the other is the limit of
harmonic voltage that the utility can supply to any
*Author for correspondence
E-mail: kannan.ped@gmail.com
customer at the PCC2-3. Hence, the current harmonics
are often treated as a local problem at least for one feeder
in the distribution network. The impedance of the
distribution network dampens the harmonic propagation.
Therefore, harmonic filtering should be performed nearby
to the source of the current harmonics for the best results.
If this is done, other equipment will be unaffected by the
harmonic producing load.
Many researchers have focused on renewable
energy source based power quality improvement in the
power distribution system 4-5. Custom Power Devices
(CPD) has been adopted for the purpose of improving
power quality and reliability. Custom power devices like
DSTATCOM (Shunt active power filter), DVR
(Series active power filter)and UPQC (Combination of
series and shunt active power filter) are used for
compensating the power quality problems in the current,
voltage and both current and voltage respectively 6-7.
Among these new devices, special attention has been
given to the equipment based on the voltage source
converter technology. A representative example of such
devices is the Distribution Static Compensator
(DSTATCOM) is extensively used to compensate
reactive power compensation, source current harmonic
reduction and load compensation at distribution level8-9.
KANNAN & RENGARAJAN: PHOTOVOLTAIC BASED THREE PHASE DSTATCOM
447
Fig. 1—Circuit diagram of proposed DSTATCOM
The DSTATCOM controller continuously monitors the
load voltages and currents and determines the amount
of compensation required by the AC system for a variety
of disturbances. The different topologies of DSTATCOM
are reported in the literature such as a 4-leg VSC (Voltage
Source Converter), three single phases VSC and 3-leg
VSC with split capacitor DSTATCOM 10-11 etc. The
proposed DSTATCOM consists of three-leg VSC with
a dc bus capacitor. For controlling the DSTATCOM and
to generate the reference currents, there are number of
controllers reported in the literature survey such as
instantaneous reactive power theory, adaptive neural
network, synchronous frame theory and power balance
theory12-18. All the above mentioned algorithms have slow
response. Here, the Icos F controlling algorithm is
proposed for generating the reference currents 19-20 .
Since, after tracking the reference currents with the help
of controller and by comparing it with source currents,
the switching of VSC will occur with the help of a
hysteresis based current controller and hence cancel out
the disturbances caused by the nonlinear loads.
The main aim of this paper is to maintain the dc link
voltage of the three-leg VSC to provide continuous
compensation. The photovoltaic (PV) array is used to
drive the boost converter to step-up the voltage and
maintain the dc link voltage. When continuous
compensation is required, the PV array is connected to
the boost converter in the day time and during the night
time battery acts as a dc source for the boost converter.
When excess power is available or compensation is not
required the PV array charges the battery. The boost
converter presented in this paper utilizes a pulse width
modulation technique. By using this technique the boost
converter draws constant power from the source. This
paper does not discuss the maximum power point
algorithm. Generally, PI controller is used to control the
dc bus voltage of the VSC. The linear mathematical
model of the PI controller is difficult to obtain and also
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J SCI IND RES VOL 72 JULY 2013
fails to perform satisfactorily under nonlinearities and
load disturbances. So in this paper fuzzy logic controller
is proposed for dc bus voltage control. The proposed
system is simulated under MATLAB environment using
SIMULINK and simpower system tool boxes to
demonstrate the effectiveness of this approach for
further reducing the harmonics and to maintain the
dc bus voltage.
module. The solar irradiance (G) and temperature (T)
were taken as standard test conditions which are 1000
Watt/m2 and 250C respectively.
The proposed DSTATCOM has been divided into
three operating modes. The modes are (i) Day time
excess power mode, (ii) Day time mode (iii) Night time
mode.
Day time excess power mode
Description of the proposed system
Fig. 1 shows the basic circuit diagram of the threephase three-wire system which is used to feed the
nonlinear load continuously. The nature of the nonlinear
load is to cause distortion in the current. After connecting
the nonlinear load, suddenly there will be a distortion in
the distribution system. In order to eliminate these
distortions, the control of DSTATCOM is achieved by
using the Icos F algorithm. The load currents, source
voltages and the dc bus voltage are given as an input to
the IcosΦ controlling algorithm. The controlling algorithm
is used to generate the reference currents. Then the
Hysteresis based PWM current controller compares the
reference and source currents and gives a switching pulse
to the DSTATCOM. The DSTATCOM consists of six
Insulated Gate Bipolar Transistor ’s (IGBT) with
antiparallel diode based three-leg VSC connected in shunt
with the dc bus capacitor. The PV module with the DCDC boost converter is connected with the dc bus
capacitor, which is used to give a desired voltage across
the capacitor for continuous compensation. According
to the gate pulse given, the switching of VSC will occur
which injects a currents at the PCC through the interface
inductor Lr.
PV system model
Photovoltaic (PV) is one of the major power sources,
becoming more affordable and reliable than utilities 21-22.
Photovoltaic is the method of converting solar radiation
into direct current electricity which generates an electric
power by using semiconductors that exhibit the
photovoltaic effect. PV module is a connected assembly
of photovoltaic cells. Hence, it will be connected in
parallel to produce high current and in series to produce
high voltage. It consists of a current source in parallel
with a diode which represents the nonlinear impedance
of the p-n junction and also a small series and a high
parallel intrinsic resistance.To model the PV module in
MATLAB-SIMULINK, the parameters are obtained
from SHANSHAN ULICAUL-175D photovoltaic
In this mode, the output voltage of the PV array
drives the boost converter based DSTATCOM for
compensating the source as well as charges the 35V
battery.
Day time mode
When continuous compensation is required, if the
PV output voltage is equal to the requirement of the boost
converter input, the PV array can directly connect to
the boost converter so as to step-up the voltage and match
the dc link voltage of the three-leg VSC. In this mode,
the battery is not charged.
Night time mode
In this mode, PV output is absent and only the battery
supplies the boost converter for providing compensation
at the night time.
Boost converter to control dc capacitor voltage
Boost converter is also called as step-up converter
in which the output DC voltage is always greater than
its input voltage. The dc boost converter with PV or
battery unit is connected to maintain the dc link voltage
of the proposed DSTATCOM. It consists of two
semiconductor switches and one storage element 23-25.
The operation of the boost converter circuit is as follows:
When the switch is closed, the inductor gets charged by
the PV or battery and stores the energy. The diode blocks
the current flowing, so that the load current remains
constant which is being supplied due to the discharging
of the capacitor. When the switch is open the diode
conducts and the energy stored in the inductor discharges
and charges the capacitor. Therefore, the load current
remains constant throughout the operation23. The boost
converter is used to maintain the constant output voltage
for all the conditions of temperature and variations in
solar irradiance. The 35V of PV or battery voltage is
given as an input to the boost converter to get the output
voltage of 670V. The switching frequency is chosen to
25 KHz. The inductor and capacitor used in the boost
converter is 0.0191 mH and 7000 mF.
449
KANNAN & RENGARAJAN: PHOTOVOLTAIC BASED THREE PHASE DSTATCOM
vabc
sin Φ, cos Φ
vdc
iLa
vdcr
iLb
Amplitude
Amplitude
PLL
iLc
vtr
vt
Computation of in-phase and quadrature component of load current
vte
vdce
Fuzzy
Controller
ILa cos Φa
ILa sin Φa
ILb cos Φb
ILc sin Φc
ILc cos Φc
ismd
+
PI
Controller
ILb sin Φb
+
+
+
+
1/3
ismq
+
1/3
vabc
+ +
Isd
+
X
X
i* sabcd
isa
isb
isc
-
Unit template
generator
uabc
wabc
Isq
i*sabcq
i*sabc
PWM current generator
Gate pulse to DATATCOM
Fig. 2—Block diagram of Icos F controlling algorithm
Control strategy of DSTATCOM
The block diagram of IcosΦ controlling algorithm is
shown in Fig.2. The source currents(isa, isb and isc) the
load currents (iLa, iLb, iLc) the ac terminal voltages (va,vb,vc)
and the dc bus voltage (vdc) are sensed and used to
extract the reference currents 20, 26. The IcosΦ controlling
algorithm is used to generate only the active component
of the load currents i.e. Icos F (where I = amplitude of
fundamental load current and F = displacement angle
of load current). Hence by combining the in-phase and
quadrature component, the reference current can be
generated.
In-Phase Component of Reference Source Currents
The amplitude of active power component of
fundamental load current is extracted at zero crossing
of the unit template in-phase with PCC voltages20. For a
balanced source current, the magnitude of active
component of reference source currents can be given as,
Isd = {
| ILa1| cosΦa1+ | ILb1| cosΦb1+ | ILc1| cosΦc1
}+ Ismd
3
… (1)
Where,
Ismd =output of the dc bus voltage fuzzy logic controller
of the VSC
The error in dc bus voltage of VSC at nth sampling
instant is given as,
vdce(n) = vdcr(n) - vdc(n)
… (2)
Where,
Vdcr (n) =reference dc bus voltage
Vdc (n) = dc link voltage of the VSC
To maintain the dc bus voltage a Fuzzy Logic Control
(FLC) is implemented27. It consists of two input variable
and one output variable. The two inputs to the FLC are
error voltage e and the rate of change of error voltage
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J SCI IND RES VOL 72 JULY 2013
Δe. This error voltage is generated by sensed dc bus
voltage and compared with the reference voltage.The
FLC is used to convert the crisp input variables into
linguistic variables using the following five fuzzy sets,
which are: NB (negative big), NS (negative small), ZE
(zero), PS (positive small) and PB (positive big). The
output variable of the fuzzy controller is Ismd. The rule
table which is shown in Table 1 contains 25 rules. Here
the fuzzy controller is compared with conventional PI
controller and the performances of both the controllers
are investigated.By using the amplitude of the threephase voltage and unit vector in phase with va, vb and vc
the in-phase component of reference source currents
are estimated as,
… (3)
i*sad= Isdua; i*sbd= Isdub; i*scd= Isduc
Quadrature Component of Reference Source Currents
The unit vectors (wa, wb and wc) in quadrature with
(va, vb and vc) can be calculated using the in-phase unit
vectors(ua, ub and uc). The amplitude of reactive power
component of fundamental load current is extracted at
zero crossing of the unit template quadrature phase of
PCC voltages.
Table 1—Fuzzy control rule table
NB
NS
ZE
PS
PB
NB
NB
NB
NB
NS
ZE
NS
NB
NB
NS
ZE
PS
ZE
NB
NS
ZE
PS
PB
PS
NS
ZE
PS
PB
PB
PB
ZE
PS
PB
PB
PB
ìï | I | sinΦa1+ | ILb1| sinΦb1+ | ILc1| sinΦc1üï
I = í- La1
ý + Ismq
sq ï
3
ïþ
î
…(4)
Where, Ismq = output of the ac terminal voltage PI
controller of the VSC
The output of the PI controller for maintaining the
amplitude of ac terminal voltage at the sampling instant
is given as,
Ismq(n) = Ismq (n-1) + Kpa {vte(n)-vte(n-1)} + Kiavte(n)
vte (n) and vte (n-1) = error in amplitude of ac terminal voltage
at nth and (n-1)th sampling instant
Kpa and Kia = proportional and integral gain constants of
the ac terminal voltage
vte(n) and vte(n-1) = voltage errors in nth and (n-1)th instant
The quadrature component of reference source
currents are estimated as,
i*saq = Isqwa; i*sbq = Isqwb; i*scq = Isqwc
...(6)
30
20
10
0
-10
-20
-30
-40
0.04
0.05
0.06
0.07
0.08
…(5)
Where,
40
Source current (A)
De/ e
For balanced source currents, the magnitude of
reactive component of reference source currents can
be given as,
0.09
0.1
0.11
Time (s)
Fig. 3—Source current waveform without compensation
0.12
0.13
0.14
KANNAN & RENGARAJAN: PHOTOVOLTAIC BASED THREE PHASE DSTATCOM
Injected current (A)
40
20
0
-20
-40
0.04
0.06
0.08
Time (s)
0.1
0.12
0.14
Source current (A)
50
0
Source current (A)
-50
0.04
0.06
0.08
0.1
0.12
Time (s)
Fig. 4—Injected current and source current waveform
0.14
40
20
0
-2 0
-4 0
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
35
40
100
80
60
40
20
Source
current (A)
Mag (% of Fundamental)
Time (s)
Fundamental (50 Hz)= 29.67, THD= 23.98 %
0
40
20
0
-2 0
-4 0
0.04
5
0.06
10
0.08
15
20
Harmonic order
(a)
0.1
0.12
25
0.14
30
0.16
0.18
0.2
30
35
40
Mag (% of Fundamental)
Time (s)
Fundamental (50 Hz)= 31.2, THD= 1.43 %
100
80
60
40
20
0
0
5
10
15
20
Harmonic order
(b)
25
Fig. 5—Current harmonics and its THD waveform (a) without compensation (b) with compensation
451
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J SCI IND RES VOL 72 JULY 2013
Table 2—Comparison of THD values of DSTATCOM before and after compensation
After compensation
Phases
Phase A
Phase B
Phase C
Before
compensation
23.98
23.98
23.98
IcosΦ using PI controller
2.28
2.24
2.22
IcosΦ using fuzzy
controller
1.43
1.38
1.39
Fig. 6—DC bus capacitor voltage
Reference Source Currents
The reference source currents can be extracted by
the sum of in-phase and quadrature components of the
reference source current. Then the reference source
currents are compared with the source currents in
hysteresis based PWM current controller for generating
gate signals for IGBT switches of VSC based
DSTATCOM.
Simulation results
The three-phase three-wire distribution system is
assumed to be connected permanently to the bridge
rectifier nonlinear load. Due to the existence of this
nonlinear load, the three-phase distribution system source
current is highly distorted. The analysis of the proposed
boost converter operated DSTATCOM for a three-phase
three-wire system has been done using MATLAB
software using SIMULINK and Power System Blockset
(PSB) toolboxes. The simulation waveforms for fuzzy
controller operated IcosΦ controlling algorithm are
follows. The resulting distorted three phase source
current waveform is shown in Fig. 3 for the simulation
time interval between 0.04 second and 0.14 second. The
obtained THD for all the three phases are same and it is
23.98%. The highly distorted source currents can be made
sinusoidal when the proposed DSTATCOM system
injects three phase current with appropriate amplitude.
The injected current of the proposed DSTATCOM for
all the three phases and the obtained three phase source
current waveform after compensation is shown in
Fig. 4. The phase A source current waveform with its
harmonic spectrum for without compensation and with
compensation is shown in Fig. 5. It is observed from the
waveform that the phase A source current THD after
compensation is reduced from 23.98% to 1.43%. The
obtained THD for phase B source current is reduced
from 23.98% to 1.38%. Similarly, the THD obtained for
phase C is also reduced from 23.98% to 1.39%. The
comparison of THD values of Icos F with PI and with
fuzzy controller is shown in Table 2.The distorted current
caused by the nonlinear load is compensated and the
source current was made sinusoidal. The DC bus
capacitor voltage waveform for IcosΦ with PI and IcosΦ
with fuzzy controller is shown in Fig. 6.The obtained THD
values for all the three phases are less than 5 % which
meets the requirements of the IEEE-519-1992 standards.
The main drawback of the proposed scheme is that, the
solar irradiation level for simulation is considered to be
constant at 1000 w/m2 throughout the day, which is not
true in practice. Hence, the PV output voltage varies
with change in irradiation level which affects the output
voltage of the dc-dc boost converter. Therefore, the
KANNAN & RENGARAJAN: PHOTOVOLTAIC BASED THREE PHASE DSTATCOM
dc-link voltage is not maintained constant which
influences the operation of continuous compensation.
8
9
Conclusion
The simulation of the Photovoltaic (PV) array or
battery operated DC-DC boost converter fed three-leg
VSC based DSTATCOM has been carried out for
reactive power compensation, source harmonic reduction
and load current compensation in the distribution system.
The DSTATCOM was controlled by Icos F algorithm in
which fuzzy controller is employed at dc bus voltage.
The purpose of boost converter is to step up the voltage
so as to match the dc link voltage of the three-leg VSC
based DSTATCOM for continuous compensation. In
IcosΦ algorithm, the PI controller is compared with fuzzy
controller at dc bus voltage, the Icos F with fuzzy
controller has a less distortion in the source current
waveform and hence the THD value has been further
reduced. The THD value is below the permissible limit
of 5% (IEEE-519-1992). The MATLAB software with
its simulink and Power System Block set (PSB) toolboxes
has been used to validate the proposed system.
Appendix
AC line voltage: 415 V, 50Hz
Non-linear load: Three phase bridge rectifier with
R = 20W
AC inductor: 2.5 mH
DC bus capacitance of DSTATCOM, Cdc: 7000 μF
DC bus voltage of DSTATCOM: 670 V
DC voltage PI controller: Kpd = 0.1, Kid = 1
PCC voltage PI controller: Kpq =0.1, Kiq =1
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