International Electrical Engineering Journal (IEEJ)
Vol. 4 (2013) No. 1, pp. 953-961
ISSN 2078-2365
NEURO-FUZZY BASED POWER
QUALITY IMPROVEMENTS IN A THREE
PHASE FOUR WIRE DISTRIBUTION
SYSTEM USING DSTATCOM
E.Babu1,R.Subramanian2
1
2
1
, Department of Electrical and electronics engg
Department of Electrical and electronics engg
babuesec@gmail.com,2rama.smani@gmail.com
Abstract- This project presents reactive power
compensation in a distribution power system.
DSTATCOM (distribution static compensator) is a
reactive power compensator for power quality
improvement in a three phase four wire distribution
system. A voltage source converter (VSC) based
DSTATCOM with a zig-zag transformer is used for
the compensation of reactive power for voltage
regulation along with load balancing, elimination of
harmonic currents, and also the neutral current
compensation at the point of common coupling. The
neuro-fuzzy based system is used to control scheme of
the VSC. The zig-zag transformer is used for
providing a path to the zero sequence current in a
three-phase four-wire distribution system. The
performance of the proposed DSTATCOM system is
verified through simulations using MATLAB software
with its Simulink.
KeywordsDistribution
static
compensator
(DSTATCOM), voltage-source converter (VSC), zigzag transformer and neutral current compensation.
I. INTRODUCTION
The present day ac power distribution systems are
suffering from severe power quality problems. These power
quality problems include high reactive power burden, harmonics
currents, load unbalance and excessive neutral current etc. The
power quality at the point of common coupling (PCC) with the
utility grid is governed by the various standards and the IEEE519 standard is widely accepted. Some remedies to these power
quality problems are reported in the literature. A group of
controllers together called Custom Power Devices, which
includes the DSTATCOM (distribution static compensator),
DVR (dynamic voltage restorer) and UPQC (unified power
quality conditioner).
The DSTATCOM is a shunt connected device, which
takes the care of the power quality problems in the currents.
Three-phase four-wire distribution systems are used to supply
single-phase low voltage loads such as computer loads, lighting
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NEURO-FUZZY BASED POWER QUALITY IMPROVEMENTS IN A THREE PHASE FOUR WIRE DISTRIBUTION SYSTEM USING DSTATCOM
International Electrical Engineering Journal (IEEJ)
Vol. 4 (2013) No. 1, pp. 953-961
ISSN 2078-2365
ballasts, small rating adjustable speeds drives (ASD) in air
conditioners, fans, refrigerators and other domestic,
commercial appliances etc. The most of the connected loads are
generally nonlinear loads. The harmonics in currents and
unbalance in load increases the neutral line currents. The
excessive neutral current which has both harmonic and
fundamental component of load currents may over load the
neutral conductor of three-phase four-wire distribution system.
A simple three phase four wire system circuit is shown in fig. 1.
There are many topologies reported in the literature for threephase four-wire DSTATCOM such as a three single-phase
VSC, VSC with four leg, three leg VSC with capacitors and
three leg VSC with a zig-zag transformer and three-leg VSC
with neutral terminal at the positive or negative terminal of dc
bus. The application of a zig-zag transformer for reduction of
the neutral current is having an advantage due to passive
compensation, rugged and less complex over the active
compensation techniques.
Fig. 1. Three phase four wire system circuit
DSTATCOM for a three-phase four-wire system. There are
instantaneous reactive power theory (p q theory), synchronous
reference frame theory, power balance theory, etc. A fuzzy based
control of an active filter is proposed in the literature for
harmonic elimination in the supply currents. In this paper, a
Fuzzy logic control of the three-phase four-wire DSTATCOM is
proposed with a new method control for the voltage regulation at
the PCC.
II. PROPOSED DSTATCOM
The three-phase four-wire DSTATCOM is used for
reactive power compensation and harmonic current
compensation along with load balancing and neutral current
compensation. Fig. 2. Shows the power circuit of proposed VSC
based DSTATCOM along with a zig-zag transformer
connected in the three-phase four-wire distribution system. This
Zig-Zag transformer can supply the path for the zero-sequence
current. Fig. 3. Shows the phasor diagram of zig-zag
transformer.
There are many theories available for the generation of
reference source currents for the control of VSC of
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NEURO-FUZZY BASED POWER QUALITY IMPROVEMENTS IN A THREE PHASE FOUR WIRE DISTRIBUTION SYSTEM USING DSTATCOM
International Electrical Engineering Journal (IEEJ)
Vol. 4 (2013) No. 1, pp. 953-961
ISSN 2078-2365
Fig. 2. Schematics of three phase four wire systems with DSTATCOM.
From phasor diagram, it can be found that the voltage across
the transformer’s winding is of the phase voltage of the threephase four-wire distribution power system.
Fig. 3. Zig-Zag transformer phasor diagram.
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NEURO-FUZZY BASED POWER QUALITY IMPROVEMENTS IN A THREE PHASE FOUR WIRE DISTRIBUTION SYSTEM USING DSTATCOM
International Electrical Engineering Journal (IEEJ)
Vol. 4 (2013) No. 1, pp. 953-961
ISSN 2078-2365
The linear and non-linear, balanced and unbalanced
loads are connected at the point of common coupling. The
DSTATCOM consists of a 3 leg pulse width modulated (PWM)
voltage source converter (VSC) using six insulated gate bipolar
transistors (IGBTs), and dc capacitors. The T- connected
transformer connected at the load terminal provides a
circulating path for zero sequence harmonic and fundamental
currents. The DSTATCOM provides harmonics elimination,
neutral current compensation and load balancing along with
power factor correction or line voltage regulation. The
compensator current is used to compensate the reactive power
component of the load current. The DSTATCOM injects a
current Ic, such that the load current, Is and source voltage, Vs.
III.CONTROL OF DSTATCOM
The block diagram of the control scheme is shown in
Fig. 4. The fundamental real and reactive power components of
load current are extracted in each phase, and they are
considered as the reference source currents (iSa, iSb, iSc). The
load currents (iLa, iLb, iLc) and the PCC voltages (vSa, vSb,
vSc) are extracted.
The extraction of the weights corresponding to the
fundamental active components of the load current (wpa, wpb,
wpc) and the weights corresponding to the fundamental reactive
components of the load current (wqa, wqb, wqc). The averages
of the respective weights are Wp and Wq are updated at each
sample time.
For an ac system, the source voltages and load currents
have harmonic components along with fundamental components
where V1 and Vn are the peaks of the fundamental and
harmonic components of the voltage. Similarly, I1 and In are the
peaks of the fundamental and harmonic components of the
current.
The unit template for three phases can be represented as
ua = U sin ῳt
ub = U sin (ῳt-120)
uc = U sin (ῳt-240)
xa = U cos ῳt
xb = U cos (ῳt-120)
xc = U cos (ῳt-240)
Fig. 4. Extraction of fundamental real and reactive current in a three-phase system.
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NEURO-FUZZY BASED POWER QUALITY IMPROVEMENTS IN A THREE PHASE FOUR WIRE DISTRIBUTION SYSTEM USING DSTATCOM
International Electrical Engineering Journal (IEEJ)
Vol. 4 (2013) No. 1, pp. 953-961
ISSN 2078-2365
where U = 1. The fundamental load current can be
decomposed as
I1 = Ip + Iq
where Ip and Iq are the active and reactive power
components of the load current, respectively.
The estimates of the fundamental active power and
fundamental reactive power components of the load current for
a single phase are obtained by estimating the respective weights
corresponding to the fundamental active components of the
load current (wpa, wpb, wpc) and the weights corresponding to
the fundamental reactive components of the load current (wqa,
wqb, wqc).
The average weight corresponding to the active and
reactive components of the load is shown as
wp =(wpa + wpb + wpc)/3
wq =(wqa + wqb + wqc)/3
The reference source currents are obtained as the sum
of active and reactive power currents as
i*Sa = ipa + iqa i*Sb = ipa + iqa i*Sc = ipa + iqa.
The reference source currents in three phases are used
for the control of the three-leg VSC. The sensed and reference
source currents are compared, and the error is used to generate
the gating signals.
IV.FUZZY LOGIC SYSTEMS
Fuzzy logic systems are one of the main developments
and successes of fuzzy sets and fuzzy logic. It is a rule-base
system that implements a nonlinear mapping between its inputs
and outputs. A fuzzy logic system is characterized by four
modules.
 Fuzzifier
 Defuzzifier
 Inference engine
 Rule base
A schematic representation of a FLS is presented in Fig.
5. The operation of a FLS is based on the rules contained in the
rule base.
Fuzzification


Two proportional–integral (PI) controllers are used for
controlling the dc bus voltage and ac terminal voltage. A self
supporting dc bus is realized using a PI controller over the
sensed (vdc) and reference values (v∗dc) of the dc bus voltages
of DSTATCOM. This PI controller estimates the loss
component of the source current (wloss), and hence, this is
added with the wp. The second PI controller is used over the
amplitude of PCC voltage (VS) and reference values (V ∗S ),
and it estimates the reactive component of the DSTATCOM
current (wqr), and hence, this is added with the wq.
Now, the real components of the reference source
currents are computed as
ipa = wp*ua
ipb = wp*ub
ipc = wp*uc.
Similarly, the reactive components of the reference source
currents are obtained as
iqa = wq*xa
iqb = wq*xb
iqc = wq*xc.


Measure the values of input variables
Perform a scale mapping that transforms the range of
values of input variables into corresponding universe of
discourse.
Function of the fuzzification that converts input into
suitable linguistic values.
Fig.5. Structure of fuzzy based system
Knowledge Base
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NEURO-FUZZY BASED POWER QUALITY IMPROVEMENTS IN A THREE PHASE FOUR WIRE DISTRIBUTION SYSTEM USING DSTATCOM
International Electrical Engineering Journal (IEEJ)
Vol. 4 (2013) No. 1, pp. 953-961
ISSN 2078-2365




It consists of the data base and linguistic control rule
base.


V. MATLAB-BASED MODELING OF THE SYSTEM
The database provides a necessary definitions, which
are used to define linguistic control rules and fuzzy
data, manipulation in an, FLC.
The rule base characterizes of the control goals and
control policy of the domain experts by means of set
of linguistic control rules.
Decision Making Logic

It has a capability of simulating human decision
making based on fuzzy concepts and of inferring fuzzy
control actions employing fuzzy implication and the
rules of inference in fuzzy logic.
Defuzzification

The defuzzification which yields a non-fuzzy, control
action from an inferred fuzzy control action.
A scale mapping which converts a range of values of
input variables into corresponding universe of
discourse.
The three leg VSC and the zig-zag transformer based
DSTATCOM for a three-phase four wire system are modeled
and simulated using MATLAB and its Simulink and SimPower
System toolboxes. The DSTATCOM system shown in Fig. 2 is
modeled in MATLAB, and its developed model is shown in
Fig.6. The multi winding transformer model available in the SPS
is used for modeling the zig-zag transformer. The ripple filter
has connected to the VSC of the DSTATCOM for filtering the
ripple in the PCC voltage.
The control algorithm for the DSTATCOM is also
modeled in MATLAB/SIMULINK. The reference source
currents are derived from the sensed PCC voltages (vSa, vSb,
vSc), load currents (iLa, iLb, iLc), and the dc bus voltage of
DSTATCOM (vdc). Fig. 7 shows the Simulink model of control
model of distribution system.
Fig. 6. Simulink model for three phase four wire system with DSTATCOM
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NEURO-FUZZY BASED POWER QUALITY IMPROVEMENTS IN A THREE PHASE FOUR WIRE DISTRIBUTION SYSTEM USING DSTATCOM
International Electrical Engineering Journal (IEEJ)
Vol. 4 (2013) No. 1, pp. 953-961
ISSN 2078-2365
Fig. 7. Simulink model of control model of distribution system
A pulse width modulation (PWM) current controller is
used over the reference and sensed compensator currents to
generate the gating signals for the IGBTs of the VSC of the
DSTATCOM.
transformer. The DC bus voltage of the capacitor (vdc) of the
VSC of DSTATCOM is regulated by the controller, and the dc
voltage has maintained near the reference dc voltage under
varying load disturbances.
VI. SIMULATION RESULTS AND DISCUSSION
The performance of the three phase VSC based
DSTATCOM and the zig-zag transformer for PCC voltage
regulations, along with the neutral current compensation and
load balancing of a three phase four wire linear load, is shown
in Fig. 8. At 0.10 s, a three phase linear load is
changed to two phase load and again to single phase load at
0.15 s. The PCC voltages (vs), balanced source currents (is),
load currents (iLa, iLb, iLc), compensator currents (iC), load
neutral current (iLn), transformer neutral current (iCn) and dc
bus voltage (vdc) are demonstrated under change of load
conditions. It’s observed that the amplitude of PCC voltage
(VS) is regulated to the reference amplitude by the required
reactive power compensation and that the source neutral current
(isn) is maintained at nearly zero because of the zig-zag
The performance of DSTATCOM for PCC voltage regulation
and harmonic eliminations, along with neutral current
compensation, is shown in Fig. 9. At 0.10 s, a three-phase
nonlinear load is changed to two phase load and again to single
phase load at 0.15 s. These loads are applied again. The PCC
voltages (vs), balanced source currents (is), load currents (iLa,
iLb, iLc), compensator currents (iC), load neutral current (iLn),
transformer neutral current (iCn) and dc bus voltage (vdc) are
demonstrated under varying nonlinear loads. The PCC voltage
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NEURO-FUZZY BASED POWER QUALITY IMPROVEMENTS IN A THREE PHASE FOUR WIRE DISTRIBUTION SYSTEM USING DSTATCOM
International Electrical Engineering Journal (IEEJ)
Vol. 4 (2013) No. 1, pp. 953-961
ISSN 2078-2365
Fig. 8. Performance of three-phase three-leg VSC and a zig-zag
transformer for neutral current compensation, load balancing,
and voltage regulations.
is regulated to the reference amplitude value. It’s observed that
the load is drawing a non sinusoidal and unbalanced current,
but the source currents are sinusoidal and balanced due to the
proper compensation currents injected by DSTATCOM.
Fig. 9. Performance of three-phase three-leg VSC and a zig-zag
transformer for neutral current compensation, harmonics
compensation, and voltage regulation.
CONCLUSION
The analysis and simulation of a new three phase four
wire DSTATCOM consisting of a three leg VSC with a zig-zag
transformer controlled with fuzzy logic have been carried out,
and its performance have been demonstrated for neutral current
elimination along with reactive power compensation, harmonic
eliminations, and load balancing. The VSC is controlled using
the fuzzy logic controller. The voltage regulation and power
factor correction modes of operation of the DSTATCOM have
been observed. The dc bus voltage of the DSTATCOM has been
regulated to the reference dc bus voltage under wide varying
loads.
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NEURO-FUZZY BASED POWER QUALITY IMPROVEMENTS IN A THREE PHASE FOUR WIRE DISTRIBUTION SYSTEM USING DSTATCOM
International Electrical Engineering Journal (IEEJ)
Vol. 4 (2013) No. 1, pp. 953-961
ISSN 2078-2365
The zig-zag transformer has been found to be effective
for compensating the zero-sequence fundamental and harmonic
neutral currents. The kilovolt ampere rating of transformers in
the zig-zag configuration is much less compared to that of a
similar star–delta transformer. It has been found that the control
technique is simple to implement, is fast in response, and gives
nearly zero phase shift in the estimated reference currents.
8.
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4, Jul-Aug.. 2011
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