Parformance - based Comparision of UPQC

Compensating Signal Generation Algorithms under

Disstorted Supply and Non linear Load Conditions

S. S. Wamane, J. R. Baviskar and S. R. Wagh Shantanu Kumar

Electrical Engineering Department Electrical Engineering Department
VJTI University of Western Australia
Mumbai, India Perth, Australia
sudhirwamane@gmail.com Shantanu.210903@gmail.com

Abstract The recent growth in the use of power electronic with these problems, the shunt active power filters (SAPF)
converters which are considered as nonlinear loads have were employed in combination with passive filters [1]-[2], but
problems of drawing non sinusoidal current and reactive power this method does not reduce the voltage harmonics at the PCC.
from source which in turn pollutes the power quality. This paper In other words, even though harmonic currents are
presents the power quality problems and methods for its
successfully compensated by SAPF, the voltage at PCC will
correction. Two control strategies (p q instantaneous power
theory and d q synchronous reference frame theory) have been not be a sinusoidal. In order to deal with current and voltage
used to extract reference currents for active power filters (APF), harmonic problems simultaneously, the most sophisticated
to solve power quality problems, have been evaluated and their mitigating device i.e. unified power quality conditioner
performances are compared under distorted supply and non- (UPQC) has been developed [2]-[3]. To ensure both the load
linear load conditions. These theories are used to implement the voltage and the supply current sinusoidal, the UPQC has a
control algorithm of a Unified Power Quality Conditioner combination of series and shunt active power filters sharing a
(UPQC) which compensates the harmonic current, harmonic common dc link. The two active power filters (APFs) have
voltage and load unbalance. This paper implements the three different functions; where the series APF is operated as a
phase shunt and series active power filters to compensate current
controlled voltage source to suppress and isolate voltage
and voltage harmonics. The evaluation of UPQC performance is
validated using MATLAB/Simulink showing the comparisons for harmonics, meanwhile shunt APF acts as a controlled current
above two algorithms, and is proved as the efficient way to source to compensate the current harmonics [3].
address power quality issues. This paper presents a comprehensive analysis of UPQC
using two (p - q and d - q theory) algorithms for derivation of
Keywords Active filters, harmonic compensation, reference signals. These reference signals are then compared
instantaneous power and synchronous reference frame theory, with sensed three phase input signals in a hysteresis controller
power quality, total harmonic distortion (THD), UPQC. for generation of switching signals. The basic configuration of
the UPQC is presented in section II. In section III, the control
I. INTRODUCTION algorithms of series and shunt active filters are presented. A
In a modern power system due to the wide use of nonlinear comparative analysis of simulation results is presented in
loads such as adjustable speed drives, electric arc welders, and section IV. Finally, section V concludes the results.
furnaces it has become necessary to establish criteria for
II. POWER CIRCUIT CONFIGURATION OF UPQC
limiting power quality problems. These problems cause
reduction in system efficiency, poor power factor,
maloperation of electronic equipments and reduction in
equipment mean life time. The nonlinear load injects the
harmonic current into the networks and consequently distorts
the voltage waveform at the point of common coupling (PCC).
This distorted voltage waveform affects other loads connected
at PCC. To avoid this problem and to protect the loads from
distortion, the harmonic components of the voltage and
current must be compensated. Conventionally, passive LC
filters were used to mitigate harmonic currents and to improve
power factor [1]. However, passive filters have many
disadvantages such as fixed compensation, large size, Fig. 1 Basic UPQC configuration
resonance problems and bulk passive components. To cope

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c IEEE 38
 Fig. 1 shows a basic system configuration of general UPQC synchronous reference frame variables (d,q,0) whose direct (d)
consisting of two voltage source inverters: one acts as a series and quadrature (q) axes rotate in space at the synchronous
APF and the other as shunt APF, which are connected back to speed given by e = 2fs , where fs is supply frequency. If is
back through dc link capacitor. The series APF which is the transformation angle, then the voltage transformation from
connected between the source and PCC using three single a-b-c to d-q-0 frame is calculated as:
phase series transformers has the capability of compensating
the voltage harmonics, voltage flicker and improving voltage





regulation [4]. A small rated capacity capacitor filter is = 



(1)

connected across the secondary of each series transformers to
eliminate the high switching ripple content in the series APF
injected voltage [5]. The shunt APF has capability of
suppressing the current harmonics, compensating reactive

power, negative sequence current and regulation of the dc link
=  (2)
voltage between both APFs [4]. The shunt APF is connected 

through a small rated capacity inductive filter in order to
eliminate the high switching ripple content in the shunt APF
injected current. The implemented control algorithm mainly
consists of the generation of three phase reference voltages at
load terminals and the reference source currents. Control
strategies to generate the reference signals of the voltage and
current of UPQC have been developed to mitigate the
harmonic voltage and currents [2]-[7]. The most common are
the instantaneous power (p - q) theory [6], modified p - q
theory [7], and synchronous reference frame (d - q) theory [8].

III. CONTROL STRATEGY TO GENERATE REFERENCE

SIGNALS
The control strategy of UPQC can be looked upon as a three
separate steps: series inverter control, shunt inverter control
and DC link voltage control. The important part of UPQC
control is the development of reference signals in terms of
voltage and current signals. The control methods used to
extract reference signals are based on frequency and time
domain techniques. The compensation in frequency domain is
based on Fourier analysis of voltage and current signals to
Fig. 2 Series APF reference voltage signal generation block diagram
extract reference signals whereas compensation in time
domain is based on instantaneous derivation of reference
signals in terms of voltage and current signals from distorted The and voltages are sent through low pass filter (LPF)
voltage and current signals, which consist of simple algebraic for filtering the harmonic components of the supply voltage,
calculations and transformations. The proposed control which allows only the fundamental frequency components.
strategy aims to generate reference signals for both shunt and The LPF is fifth order low pass butterworth filter used for
series APF of the UPQC. The synchronous reference frame (d eliminating the higher order harmonics. In Fig. 2 the phase
- q) theory is used for series APF where as instantaneous locked loop (PLL) is used to achieve synchronization with the
power (p - q) theory is used for shunt APF to generate supply as the supply voltage is unbalanced and/or distorted
reference signals which then compared with sensed three [9]. After inverse transformation in a-b-c coordinates the
phase input signals in hysteresis controller to generate computed voltages from (2) are given to the hysteresis
switching signals. controller along with the sensed three phase PCC voltages
VLabc. The output of the hysteresis controller is switching
A. Reference Voltage Signal Generation for Series APF
signals, used to control the six switches of VSI of the series
In order to make the voltages at the PCC (VLa,VLb ,VLc) APF. The hysteresis controller generates the switching signals
perfectly balanced and sinusoidal, the series APF is controlled such that the voltage at the PCC becomes the desired
such that it injects voltages which cancel out the distortions sinusoidal reference voltage [9]. Therefore, the injected
and/or unbalance present in the supply voltages (VSa,VSb,VSc). voltage across the series transformer by series APF through
The control strategy for the series APF is shown in Fig. 2 the ripple lter cancels out the harmonics and unbalance
where the SRF theory is used. present in the supply voltage.
The SRF theory is based on the transformation of the
stationary reference frame three phase variables (a,b,c) to

2013 IEEE 8th Conference on Industrial Electronics and Applications (ICIEA) 39

B. Reference Current Signal Generation for Shunt APF hysteresis controller to generate the required switching signals
for the shunt APF switches [9].
The control algorithms for the shunt APF consists of the
generation of three phase reference currents. The control
algorithm is based on the p-q theory as shown in Fig. 3. This
=
(5)
theory consists of an algebraic transformation (Clarke
transformation) of three phase voltages and currents from a-b-   ;   (6)
c coordinate to --0 coordinate given by



= (7)





=

(3)












= (8)







= (4)





C. DC link voltage control
The DC voltage regulator shown in Fig. 4 is used to generate a
control signal to keep the DC voltage constant and self-
regulated which may vary because of abnormal operation and
transients. The dc link voltage, VDC is sensed at a regular
interval and is compared with reference signal VDC*. The error
signal is processed in a PI controller. This signal forces the
shunt active filter to draw additional active current from the
network to compensate losses in the power circuit of UPQC.
The parameters of PI Controller [G(s) = Kp+ KI /s] plays an
important role in DC voltage control system response.

Fig. 3 Shunt APF reference current signal generation block diagram

The source side instantaneous real and imaginary power Fig. 4 DC link voltage control system
components can be calculated by using (4) and (5). The
instantaneous real and imaginary powers include both
oscillating and average components given by (6). Average Too much increase in proportional gain (Kp) leads to
components of p and q consist of positive sequence instability in DC voltage control system and reduction in Kp
components ( and ) of source current. The oscillating decreases the responding speed of control system. Integral
components ( and ) of p and q include harmonic and gain (KI) of controller corrects the steady state error of the DC
negative sequence components of source currents [10]. In voltage control system. If this gain value is selected large, the
order to reduce neutral current (for 3-phase , 4-wire system), resulted error in steady state is corrected faster and increase in
is calculated by using average and oscillating components
its value ends in overshoot in system response.
of imaginary power and oscillating component of the real
power; as given in (7) if both harmonic and reactive power
IV. SIMULATION RESULTS
compensation is required. The , and are the reference
currents of shunt APF in --0 coordinates. These currents are The model of UPQC under proposed control algorithm is
transformed into three phase a-b-c coordinates as given by (8). developed in MATLAB/Simulink software as shown in Fig. 5,
The reference currents are calculated in order to compensate where the performance of the UPQC is evaluated in terms of
harmonic and reactive currents in the load. These reference current and voltage harmonic mitigation under distorted load
source current signals are then compared with sensed three- current and source voltage conditions. A 3-phase diode bridge
phase source currents, and the errors are processed by rectifier with RL load acts as nonlinear load which is
connected to AC mains to demonstrate the effectiveness of the

40 2013 IEEE 8th Conference on Industrial Electronics and Applications (ICIEA)

UPQC with the proposed method. The UPQC circuit TABLE II. THD LEVELS OF CURRENT AND VOLTAGE WAVEFORMS AT
PCC
parameters used in MATLAB/Simulink are given in Table I.
THD Without With UPQC
TABLE I. SYSTEM PARAMETERS (%) UPQC
Parameters Value p q theory d q theory
Fundamental voltage
Vsabc 440 V Current 28.89 1.53 1.53
Supply (rms, ph-ph)
Frequency fs 50 Hz
Voltage 14.99 1.81 1.81
Diode rectifier load
Ldc 10mH
inductance
Load
Diode rectifier load
RL 10
resistance
Reference voltage VDC* 600 V
DC-link
Capacitance Cdc 2200F
Shunt Filter inductance Lsh 1mH
APF Switching frequency fsw 5 kHz
Filter inductance Lf 0.4mH
Series
Filter capacitance Cf 25F
APF
Switching frequency fsw 5 kHz (a)

In the simulation studies, the results are specied before

and after the operation of the UPQC system. The voltage and
current harmonic compensation capability of the proposed
UPQC control method is shown in Table II as total harmonic
distortion (THD) levels.

(b)

Series APF Controller Shunt APF Controller

Fig. 5 MATLAB based simulink model of UPQC system

The response of the UPQC in voltage, and current harmonic

mitigation is presented in Fig. 6.

Fig. 6 Performance of UPQC using proposed algorithms for

harmonic compensation

2013 IEEE 8th Conference on Industrial Electronics and Applications (ICIEA) 41

To make the load voltage distortion free, series APF operate to current unbalance. Finally, the UPQC is proposed as a general
compensate the voltage harmonics by injecting the voltage of solution for the compensation of all the disturbances due to
variable magnitude and phase. Fig. 6 (a) and (b) shows that voltages and currents.
the series APF corrects the source voltage appropriately. From
Table II it can be seen that before compensation, the THD REFERENCES
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42 2013 IEEE 8th Conference on Industrial Electronics and Applications (ICIEA)