International Journal of Engineering Trends and Technology (IJETT) – Volume 8 Number 8- Feb 2014
PI-Fuzzy Hysteresis Controller based Unified Power
Quality Conditioner
T.Guna Sekar#1, Dr.R.Anita#2
#1
#2
Assistant Professor, Kongu Engineering College, TamilNadu, India
Professor and Head, Institute of Road and Transport Technology, TamilNadu, India
Abstract— An unified power quality conditioner (UPQC) is a
custom power device. It solves voltage related and currentrelated Power Quality problems in the power distribution
systems. In this paper, a UPQC topology that eliminates the
harmonic components is proposed. The proposed topology
enables UPQC to have a reduced dc-link voltage without
compromising its compensation capability. This proposed
topology also helps to match the dc-link voltage requirement of
the shunt and series active filters of the UPQC. The topology uses
a capacitor in series with the interfacing of the shunt active filter,
and the series active filter. A fuzzy logic controller (FLC) with
fast reference voltage generation to correct and regulate
unbalance voltage in three-phase system is proposed. The
reference voltage is fed to the FLC, which is a robust closed loop
controller. The proposed algorithm and control scheme of active
filter may correct and regulate unbalance in the-system. PI
controller is used to maintain the DC link capacitor voltage. A
simulation study of the proposed topology has been carried out
using MATLAB/Simulink and the results are generated.
Keywords— Fuzzy Logic, Hysteresis Controller, Phase
Locked Loop, Unified Power Quality Conditioner,
Synchronous Reference Frame.
active power filter. However the passive filters are
suffering from the disadvantages such as sensitive
to the variation of frequency, system impedance,
and possibility of series/parallel resonance and
fixed filter frequency. Because of series/parallel
resonance it may cause the damage to inductor and
capacitor of passive power filters. Performance of
passive power filter can be affected by the system
impedance and APF’s are used to resolve passive
filters problems. Basically APF’s are voltage source
or current source converters to provide
compensation of voltages or currents harmonics.
APF’S are shunt active and series active power
filter. The shunt active power filters (APFs) are
used to eliminate current harmonics, load
balancing, power factor correction of three-phase
four wire distribution and the series active filters
are used to eliminate the voltage harmonics.
I.INTRODUCTION
II.SYSTEM DESCRIPTION
The Proliferation of power electronic equipment
led a serious attention about power quality of a
distribution system. These power electronic
converters are from low power domestic
applications to high power adjustable speed drives
(ASDs). This power electronic converter generates
harmonics which includes fundamental, third, fifth
etc and other higher harmonics. These harmonic
current may cause power quality degradation,
transformer overheating, malfunctioning of medical
facilities, destruction of electric power components,
pollute the power system and rotary machine
vibration etc. Many power quality standards are
proposed, such as IEC1000-3-2 and IEEE519-1992.
These harmonics can be suppressed by a passive or
ISSN: 2231-5381
The Fig.1 shows the block diagram of Unified
Power Quality system, it consists of a three-phase
source, which is connected to non-linear loads. The
UPQC is connected before the load to make the
source and the load voltage free from any
distortions. The UPQC, carried out by using two
VSIs, one VSI acts as the shunt APF and the other
as the series APF. The shunt APF is realized using a
three-phase, three-leg VSI, and the series APF is
carried out using a three-phase, three-leg VSI. Both
APFs share a common dc link between them. The
Series Active Power Filter is coupled using
coupling transformer and the shunt active filter is
connected in parallel with the phases. The proposed
control strategy aims to generate reference signals
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International Journal of Engineering Trends and Technology (IJETT) – Volume 8 Number 8- Feb 2014
for both shunt and series APFs of the UPQC. The
series active filter is controlled to eliminate voltage
harmonics and the shunt active filter is controlled to
alleviate current from harmonics and load balancing.
The inverter can be implemented by IGBTs
operating with the fuzzy hysteresis controller for
the filtering function.
Fig 2: SRF Method


i 


 
2
i   
3
 

i o 



1
0
1
2
1
2
3
2
1
2
1 
2  
 ia
 3 
 
2  i b 

1  i c 
2 
(1)
Fig 1: Block Diagram of UPQC
Now, the two phase current quantities iα and iβ of
stationary αβ-axes are transformed into two-phase
There are different control strategies being synchronous (or rotating) frame (d-q-axes) using
used for the calculation of reference currents in equation (2), where cosθ and sinθ represents the
active power filter namely Instantaneous Reactive synchronous unit vectors which can be generated
Power Theory (p-q theory), Unity Power Factor using phase-locked loop system (PLL).
method, One Cycle Control, Fast Fourier Technique
etc. Here, SRF theory is used to extract the threei d 
s in    i  
 cos 
   


phase reference currents and voltages used by the

s
i
n

c
o s    i  
 i q 



(2)
active power filters. Fig 2 shows the block diagram
which explains three-phase SRF-theory, used for
fundamental
component
extraction.
The The d-q currents thus obtained comprises of AC
synchronous reference frame theory is used to and DC parts. The fundamental component of
extract the fundamental component in the supply current is represented by the fixed DC part and the
voltage or current. It is based on the transformation AC part represents the harmonic component. This
of the currents or voltages in synchronously rotating fundamental component can be easily extracted
d-q frame. If θ is the transformation angle, then the using a Low Pass Filter (LPF), as implemented in
current and voltage transformation from α-β to d-q Fig 2. The d-axis current is a combination of active
is defined as in the Fig 2 In this method, the source fundamental current (id dc) and the load harmonic
currents and voltages are first detected and current (ih). The fundamental component of current
transformed into two-phase stationary frame (αβ-0) rotates in synchronism with the rotating frame and
from the three-phase stationary frame (a-b-c), as per thus can be considered as dc. By filtering id, the
current is obtained, which represents the
equation (1)
III. REFERENCE SIGNAL GENERATION FOR
ACTIVE FILTERS
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International Journal of Engineering Trends and Technology (IJETT) – Volume 8 Number 8- Feb 2014
fundamental component of the load current in the amplitude disturbances, harmonic rejection and line
synchronous frame. Thus, the AC component idh unbalance in the case of three-phase systems.
can be obtained by subtracting id dc part from the
total d-axis current (id), which leaves behind the
harmonic component present in the load current. In
the rotating frame the q-axis current (iq) represents
the sum of the fundamental reactive load currents
and part of the load harmonic currents. So the qaxis current can be totally used to calculate the
reference compensation currents. Now inverse
transformation is performed to transform the
currents from two –phase synchronous frame d-q
into two-phase stationary frame α-β as per
equation(3)
i   cos 
 
i    sin 
 sin   i d 
 
cos   i q 
 
Fig 3 :PLLSystem
(3)
The transformation angle is generated by
using the PLL. The output of the PLL is given to
the SRF as the unit vector templates (cos ,sin )
i * 


and the reference signal is generated. Comparing
i
 ca 
 
*
this diagram to the conventional PLL used in
i   
 
T
abc  i 
cb

telecommunications, it can be seen that the PI
 
 
controller is analogous to the low pass filter, the
 *
i o 
i cc 
(4)
integrator is analogous to the voltage controlled
oscillator, and the SRF transformation blocks
Finally the current from two phase stationary frame scheme is analogous to the phase detector. Its
αβ0 is transformed back into three-phase stationary working principle relies on regulating to zero the
frame abc as per equation (4) and the compensation direct component of the rotating frame. This
reference currents ica*, icb* and icc* are obtained component is calculated using the estimated phase
for the shunt active filter and as same voltage angle , closing the loop.
signals are obtained by giving the voltage signal as
Assuming balanced and harmonic free input
the input.
voltages, the expression of the d-axis component
which is feed to the PI controller is:
IV. SRF WITH PHASE LOCKED LOOP
The correct phase angle is very important
(5)
information in grid-connected systems such as UPS,
Or leading to
controlled rectifiers, active filters, dynamic voltage
=-v
(6)
restorers and also in the emerging distributed
generation systems. To estimate the phase angle
open loop and closed loop methods are available. where: Vd is the phase detector output signal;
The closed loop methods are commonly known as
V is the amplitude of the input voltages;
θ is the angle of phase A
Phase-Locked Loops or PLLs. The figures of merit
is the estimated angle.
of a PLL are the steady state phase angle error,
Using the same procedure for the q-axis component Vq, its
speed of response to phase, frequency and voltage
amplitude is:
=
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v
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(7)
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International Journal of Engineering Trends and Technology (IJETT) – Volume 8 Number 8- Feb 2014
Finally, defuzzification is used to convert the
fuzzy outputs into control signals. In designing of a
fuzzy control system, the formulation of its rule set
When approximates θ ,Vd (8) will approximate plays a key role in improvement of the system
zero and the PLL will be locked. In this situation, performance. The mamdani type fuzzy logic
according to (7), Vq will be equal to the input controller is used;the max-min inference method is
voltage amplitude. The kp and ki gains determine applied in this study.
the speed of response and disturbance rejection of
TABLE 1: FUZZY RULE TABLE
the PLL in a direct relation. However, there is a
trade off between noise, harmonic and voltage
unbalance rejection and speed of response. The
higher the gains, the worse the noise, harmonic and
line unbalance rejection.
The generated reference signal is compared
with the actual signal and the error signal is
generated. The error signal is given as the input to
the fuzzy controller.
=v
- v
(8)
V. FUZZY LOGIC CONTROLLER
Fuzzy set theory exhibits immense potential for
effective solving of the uncertainty in the problem.
VI .HYSTERESIS BAND CONTROLLER
It is an outstanding mathematical tool to handle the
uncertainty arising due to vagueness. Fuzzy logic
The hysteresis band current control (HBCC)
control is divided into fuzzification, inference and technique is used for pulse generation in VSIs. The
defuzzification
control method offers good stability, gives a very
fast response, provides good accuracy and has got a
simple operation. It consists of a hysteresis band
surrounding the generated error signal. The error is
obtained by subtracting the actual signal from the
reference signal. The reference signal used here is
obtained by the SRF method. The error signal is
then fed to the fuzzy and then fed to relay with the
desired hysteresis band to obtain the switching
pulses for the inverter.
Fig 4: Fuzzy Inference System
The knowledge base is composed of a data base
and rule base and is designed to obtain good
dynamic response under uncertainty in process
parameters and external disturbances. The data base
consisting of input and output membership
functions, provides information for the appropriate
fuzzification operations, the inference mechanism
and defuzzification. The inference mechanism uses
a collection of linguistic rules to convert the input
conditions into a fuzzifiedOutput.
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Fig 5 :Hysteresis Controller
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International Journal of Engineering Trends and Technology (IJETT) – Volume 8 Number 8- Feb 2014
VII. DC LINK VOLTAGE CONTROL
The dc side voltage of APF should be
controlled and kept at a constant value to maintain
the normal operation of the inverter. Because there
is energy loss due to conduction and switching
power losses associated with the diodes and IGBTs
of the inverter in APF, which tend to reduce the
value of Vdc across capacitor Cdc. A feedback
voltage control circuit needs to be incorporated into
the inverter for this reason. The difference between
the reference value, Vref and the feedback value
(Vdc), an error function first passes a PI regulator
and the output of the PI regulator is subtracted from
the d axis value of the harmonic current
components. The DC capacitor voltage can be
found by using the equation.
V
dc

2 2V
3
LL
(9)
VIII. RESULTS AND DISCUSSION
The proposed system consists of a three single
phase nonlinear load and single three phase
nonlinear load .The unbalanced condition is
achieved by connecting the three different single
phase loads to the phase and the Neutral. Because
of this condition there will be a flow of current in
the neutral conductor consisting of both harmonic
and fundamental component.Due to variation in the
values of single phase loads connected, the
magnitude in each phase will vary and the neutral
current harmonics will be high. The simulation
results without filter shown in Fig 6 it shows that
the voltage contains harmonics and the load current
contains harmonics and it is unbalanced. Due to the
unbalanced nature the neutral current flow is high.
MATLAB SIMULINK model for the proposed
system is shown in Fig7.
The installation of UPQC compensates the
harmonics and unbalance and the neutral current
magnitude gets reduced effectively. The DC link
Capacitor value is maintained constant using the PI
controller and it is shown in the Fig 8
Fig 6 : Waveform of load voltage, load current and neutral current without
UPQC
Fig 7 :MATLAB SIMULINK model of UPQC
Fig 8 : Waveform of load voltage, load current, neutral current and Vdc with
UPQC
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International Journal of Engineering Trends and Technology (IJETT) – Volume 8 Number 8- Feb 2014
TABLE II. COMPARISION OF THD VALUES WITHOUT UPQC AND WITH UPQC
Controller
Without UPQC
With UPQC
Parameters
RMS
Value of
Current
Current
THD (%)
Voltage
THD (%)
RMS Value
of Current
Current
THD (%)
Voltage
THD (%)
Phase A
35.82
34.37
20.22
26.5
4.58
1.98
Phase B
32.93
38.43
22.64
25.8
4.08
2.64
Phase C
18.61
34.09
13.37
24.62
3.52
1.84
IX CONCLUSION
REFERNCES
This paper describes PI Fuzzy-based control strategy
for UPQC, which compensates the Load voltage
and current harmonics under unbalanced load
conditions. The proposed control strategy uses APF,
based on the SRF theory. The simulation results
show that, during unbalanced and nonlinear load
conditions, the proposed control algorithm
eliminates the harmonic distortion and balances the
load current on the distribution system, making the
power supply to the consumer sinusoidal. The value
of THD obtained from the result shows that the
harmonic content are within IEEE standard limit.
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APPENDIX
Supply voltage: 110V, 50Hz.
Three Single phase loads
Load 1: R=15Ω ; C = 1000µf
Load 2: R=25Ω ; C = 1000µf
Load 3: R=50Ω ; C = 1000µf
Single Three phase load:
R=10Ω ; C = 1000µf
DC link voltage: 240V.
DC link capacitance value: 2500µf
Ripple filter parameters: 1.8mH, 0.25Ω.
ISSN: 2231-5381
[10] MyoungLee.G, Dong-Choon Lee, Member, IEEE, and Jul-Ki Seok,
Member IEEE, “Control of Series Active Power Filters Compensating
for Source Voltage Unbalance and
Current Harmonics” IEEE
Transactions On Industrial Electronics, Vol. 51, No. 1, February 2004.
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