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2012 International Conference on Computing, Electronics and
Electrical Technologies [ICCEET]
Generalized UPQC system with an improved Control Method under
Distorted and
Unbalanced Load Conditions. A.Jeraldine
Viji\Dr.M.Sudhakaran2
Asst.Professor,Research scholar!, MEC,JNTU,Associate
pro-f,PEC
Abstract- Power quality has become an important factor in power
systems, for consumer and household appliances with proliferation
of various electric and electronic equipment and computer systems.
The main causes of a poor power quality are harmonic currents, poor
power factor, supply-voltage variations, etc. A technique of
achieving both active current distortion compensation, power factor
correction and also mitigating the supply-voltage variation at the
load side, is compensated by unique device of UPQC presented in
this paper and this paper presents a modified synchronous-reference
frame (SRF)-based control method to Shunt active filter and
instantaneous PQ (IPQ) theory based control technique for series
active filter to compensate power-quality (PQ) problems through a
three-phase four-wire unified PQ conditioner (UPQC) under
unbalanced and distorted load conditions. The proposed UPQC system
can improve the power quality at the point of common coupling on
power distribution systems under unbalanced and distorted load
conditions. The simulation results based on Matlab/Simulink are
discussed in detail in this paper.
Index Terms-Active power filter (APF), harmonics, modified phase
locked loop (MPLL), power quality (PQ), synchronous reference frame
(SRF), unified power-quality (PQ) conditioner (UPQC).
I INTRODUCTION
The modem power distribution system is becoming highly
vulnerable to the different power quality problems [1-2]. The
extensive use of non-linear loads is further contributing to
increased current and voltage harmonics issues. Furthermore, the
penetration level of small/large-scale renewable energy systems
based on wind energy, solar energy, fuel cell, etc., installed at
distribution as well as transmission levels is increasing
significantly.
A. Jeraldine viji is a Research Scholar in JNTUK, Andra Pradesh.
and currently working Asst.Professor in Mailam Engineering
college,. E-mail: [email protected]. Dr.M.Sudhakaran is
working as a prof in Pondicherry Engg college,E-mail:
[email protected]
978-1-4673-0210-4112/$31.00 2012 IEEE 193
Unified power quality control was widely studied by many
researchers as an eventual method to improve power quality of
electrical distribution system [1-3]. The function of unified power
quality conditioner is to compensate supply voltage
flicker/imbalance, reactive power, negativesequence current, and
harmonics. In other words, the UPQC has the capability of improving
power quality at the point of installation on power distribution
systems or industrial power systems. Therefore, the UPQC is
expected to be one of the most powerful solutions to large capacity
loads sensitive to supply voltage flicker/imbalance [2]. The UPQC
consisting of the combination of a series active power filter (APF)
and shunt APF can also compensate the voltage interruption if it
has some energy storage or battery in the dc link [3]. The shunt
APF is usually connected across the loads to compensate for all
current-related problems such as the reactive power compensation,
power factor improvement, current harmonic, compensation, and load
unbalance compensation (3,4)whereas the series APF is connected in
a series with the line through series transformers. It acts as
controlled voltage source and can compensate all voltage related
problems, such as voltage harmonics, voltage sag, voltage swell,
flicker, etc.
The proposed control technique has been evaluated and tested
under non-ideal mains voltage and unbalanced load conditions using
Matlab/Simulink software. The proposed method is also validated
through experimental study. The following diagram shows the
generalized UPQC system
Voltage
i:>., iC. il.
rv = r.J',, + rL".; .. J '
pce
Fig-I Generalized Diagram ofUPQC system
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2012 International Conference on Computing, Electronics and
Electrical Technologies [ICCEET]
II. UPQC CONTROL ALGORITHM
DVR APF
ContmJCf Centrdla.: $\I'dlddtUion H.aJTnonu:c:'iIr1ion . ..
Rei'ertnc\'ollage gmtr.lioo. GalesigM generation Gate,ignjgenaanon
ReJCti\'i:powercalru3tion'
Fig-2 control Diagram ofUPQC system
The UPQC consists of two voltage source inverters Connected back
to back with each of them sharing a common dc link. Fig-2 shows the
control diagram of Upqc system. One inverter work as a variable
voltage source is called series APF, and the other as a variable
current source in called shunt APF.The main aim of the series APF
is harmonic isolation between load and Supply; it has the
capability of voltage flicker/ imbalance compensation as well as
voltage regulation and harmonic compensation at the
utility-consumer PCC. The shunt APF is used to absorb current
harmonics, compensate for reactive power and negative-sequence
current, and regulate the dc link voltage between both APFs. The
proposed UPQC control algorithm block diagram in Matlab/Simulink
simulation software is shown in Fig. 2.
A. Reference Voltage Signal Generation for Series APF
.. ---1r-.---+i .. ----jf-H--H
===--------------------- ----------------------------
----------------------------
Fig-3 control diagram of Series Active Filter
The series APF control algorithm calculates the reference value
to be injected by the series APF transformers, comparing the
positive-sequence component with the load side line voltages.
In
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equation (1), supply voltages VSabc are transformed to d-q-O
coordinates.
1 1 1
71 - - - r' 1 ,= ; n(t) n(cut - ) n(cut+ ) Vs!l a a v. 4 CQOJt)
CilOJt - ) CilOJtt ;) a
(1)
The voltage in d axes (VSd) given in (2) consists of A verage
and oscillating components of source voltages (VSd and VSd ) . The
average voltage VSd is calculated by using second order LPF (low
pass filter).
(2)
The load side reference voltages V l'!< 12bc are calculated
as given in equation (3). The switching signals are assessed by
comparing reference voltagesV Lbc and the load voltages (VLabc )
via sinusoidal PWM controller.
(3)
The three-phase load reference voltages are compared with load
line voltages and errors are then processed by sinusoidal PWM
controller to generate the required switching signals for series
APF switches.
B. Reference Current Signal Generation for Shunt APF
Fig-4 control diagram of Shunt Active Filter
The above figure shows the control diagram of shunt active
filter. The shunt active filter compensates the current harmonics
and reactive power generated by the nonlinear load. The
instantaneous active power (p-q) theory is used to
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2012 International Conference on Computing, Electronics and
Electrical Technologies [ICCEET]
control of shunt APF in real time. In this theory, the
instantaneous three-phase currents and voltages are transformed to
a-p-O coordinates as shown in equation (4) and (5).
[t l. v, T 1 l.J [ ' 1 t]= - ' 1 ft '\ a "'/, V
"1 1
' a
1 l .:l !!] l
(4)
(5)
The source side instantaneous real and imaginary power
components are calculated by using source currents and
phase-neutral voltages. The instantaneous real and imaginary powers
include both oscillating and average components as shown in (7).
Average components of p and q consist of positive sequence
components (p andq)of source current. The oscillating components (p
and q) of p and q include harmonic and negative sequence components
of source currents [4]. In order to reduce neutral current, p 0 is
calculated by using average and oscillating components of imaginary
power and oscillating component of the real power; as given in (8)
if both harmonic and reactive power compensation is required. i*sa
i *s, i*so are the reference currents of shunt APF in a-p-O
coordinates. These currents are transformed to three-phase system
as shown in (9).
(6)
p=p+ P (7)
(8)
1 1 @ 1. "Il ro] (9) Oi"" ' jC 1. ..p
The reference currents are calculated in order to compensate
neutral, harmonic and reactive currents
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in the load. These reference source current signals are then
compared with sensed three-phase source currents, and the errors
are processed by hysteresis band PWM controller to generate the
required switching signals for the shunt APF Switches [6].
III. SIMULA TOIN RESULTS
Fig
Fig-5 Simulink diagram of Series Active Filter
In this study, a new control algorithm for the UPQC is evaluated
by using simulation results given in Matlab/Simulink software. The
simulated
UPQC system parameters are given in Table 1. In simulation
studies, the results are specified before and after UPQC system are
operated. Before harmonic compensation, the THD of the supply
current is 26.23% and after is 0.5%.Similally voltage harmonic is
24.6% before compensation and after is 0.3%.
"
Fig-6 Overall simulink diagram of proposed UPQC system
Parameters Value Source
Voltage VSabc 380 Vnn, Frequency f 50 Hz
Load 3-Phase ac Line Inductance LLabc 3 mH Shunt APF Ac Line
Inductance LCabo 4 mH Filter Resistor RCabc 60 Filter Capacitor
CCabo 10 "F Switching Frequency fp",m 10 kHz Series APF
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2012 International Conference on Computing, Electronics and
Electrical Technologies [ICCEET]
ac Line Inductance LTabc 2 mH Filter Resistor RTabc 60 Filter
Capacitor CTabc 20 "F Switching Frequency fpwm 15 kHz
The proposed UPQC control algorithm has ability to compensate
both harmonics and reactive power of the load and neutral current
is also eliminated. The proposed control technique has been
evaluated and tested under dynamical and steady-state load
conditions.
Fig-7. Three phase source current before compensation
Fig-7. Three phase source voltage before compensation
Fig-8 Supply voltage after adding upqc system
Fig-9 .Supply current after adding upqc system
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200 ..
Fig-10 Three phase compensating current
100 .
,;! j 0 ,r r r' r'1l r'f
-3lO ...
-8lO 0 0.01 0.112 0.111 O.IM 0.00 O.IE 0.07 0.111 0.111 0.1
Tme(5)
Fig-II. Three phase load current
_(6IlIz)'6!.1IIFO.51\
i ,
j 0 >
100 200 300 400 500 600 700 8110 900 18110 fr!
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2012 International Conference on Computing, Electronics and
Electrical Technologies [ICCEET]
voltages and source voltage, the shunt APF provides three-phase
balanced and rated currents for the loads.
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