Journal of Engineering Research and Studies E-ISSN0976-7916
JERS/Vol.II/ Issue IV/October-December, 2011/1-8
Research Article
TO LIMIT THE FAULT CURRENT USING UNIFIED POWER
QUALITY CONDITIONER Mr. Marshall Arockia Dass .P*, Dr. A. Peer Fathima**
Address for Correspondence *Assistant Professor, **Professor & HOD,
Department of Electrical and Electronics Engineering, Easwari Engineering College, Ramapuram, Chennai
ABSTRACT
This paper deals with unified power quality conditioners (UPQC’s), which aim at the integration of series-active and shunt-active
filters. The main purpose of a UPQC is to compensate for voltage flicker/imbalance, reactive power, negative sequence 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. This paper discusses the control strategy of the UPQC, with a focus on the flow
of instantaneous active and reactive powers inside the UPQC. The present work discusses the compensation principle and
different control strategies (PI, FUZZY) of the UPQC in detail. The control strategies are modeled using MATLAB/SIMULINK.
The performance is also observed under influence of utility side disturbances such as sag, swell, flicker and spikes. The
simulation results are listed in comparison of different control strategies and for the verification of results.
KEYWORDS Active filters, power conditioners, power quality, voltage flicker, voltage imbalance.
I. INTRODUCTION
The applications of power semiconductor devices are
being widely used in various areas, such as large
thyristor power converters, rectifiers, and arc
furnaces. Complications related to the use of the non-
linear loads in these systems are major issues for both
power providers and users alike. Consequently,
utility power system reliability and power quality has
moved to the forefront. As customers increasingly
use process and computer equipment, which are
highly sensitive to power system interruptions,
utilities are being, forced to serve these loads with
transmission and distribution systems that are at our
exceeding capacity.
The quality of the power is effected by many factors
like harmonic contamination, due to the increment of
non-linear loads, such as large thyristor power
converters, rectifiers, voltage and current flickering
due to arc in arc furnaces, sag and swell due to the
switching (on and off) of the loads etc. These
problems are partially solved with the help of LC
passive filters. However, this kind of filter cannot
solve random variations in the load current waveform
and voltage waveform. Active filters can resolve this
problem, however the cost of active filters is high,
and they are difficult to implement in large scale.
Additionally, they also present lower efficiency than
shunt passive filters [1]. One of the many solutions is
the use of a combined system of shunt and series
active filters like Unified Power Quality Conditioner
which aims at achieving low cost and highly effective
control. Voltage sag is one of the prime factors due to
which particularly production industries suffer huge
loss. This is evident from many power quality survey
reports [2]. Most of these voltage sensitive critical
loads are non-linear in nature due to application of
fast acting semiconductor switches and their specific
control strategy whose presence in a system pose
some major concerns as they affect the distribution
utility in some highly undesirable ways.
The aim of the paper is to design different control
strategies for Unified Power Quality Conditioner
(UPQC), which is one of the major custom power
solutions capable of mitigating the effect of supply
voltage sag, swell, flicker and spikes at the load end or at
the Point of Common Coupling (PCC). It also prevents
load current harmonics from entering the utility and
corrects the input power factor of the load. The control
strategies used here are based on PI Controller and Fuzzy
Controller. The relative performance of the two controls
is also studied.
II.UPQC WITH INTELLIGENT CONTROLLERS
Principle of UPQC: UPQC is one of the custom power
devices used at the electrical power distribution systems
to improve the power quality of distribution system
customers. UPQC could be used to cancel current
harmonics, to compensate reactive power, to eliminate
voltage harmonics, to improve voltage regulation, to
correct voltage and current imbalances, to correct voltage
sag or swell and to avoid voltage interruptions. A UPQC
consists of both shunt and series compensators. A shunt
compensator is used to cancel the disturbances in current
whereas series compensator is used to cancel
disturbances in voltage. Shunt compensator could be
connected to the left or right of the series compensator.
Ideally, shunt compensator injects current to achieve
purely balanced sinusoidal source currents in phase with
the supply voltages at rated magnitude and frequency.
On the other hand series compensation is used to inject
voltage to maintain terminal voltage at rated magnitude
and frequency.
Control circuit of UPQC: A three-phase system has
been selected to study the performance of the UPQC
system. (Fig. 1) shows the schematic diagram of the
UPQC system. Voltage source inverters are used for
shunt and series compensation. One may note that both
Journal of Engineering Research and Studies E-ISSN0976-7916
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voltage source inverters are supplied from a common
dc link capacitor. One of the voltage source inverters
is connected in parallel with the a.c. system while the
other one is connected in series with the a.c. system
through injection transformers. The inverter
connected in parallel, together with its control circuit
forms the shunt compensation circuit. On the other
hand the inverter connected in series with appropriate
control circuit forms the series compensation circuit.
For the successful operation of the UPQC, the dc
capacitor voltage should be at least 150 % of
maximum line-line supply voltage. To regulate the
capacitor voltage constant, either a PI controller or a
fuzzy controller could be used. Thus the control
structure of UPQC has been divided into shunt
compensator and series compensator control circuits.
III. Shunt compensation control circuit: A current
controlled VSI connected in parallel to the source
through booster inductors functions as the shunt
compensator. (Fig. 2) shows the block diagram of the
control circuit of the shunt compensator[11]. The
control circuit consists of a voltage control loop and
two current control loops. The ideal requirements of
the shunt compensator circuit are (i) to maintain the
capacitor voltage at a constant steady value and (ii) to
maintain the source current purely sinusoidal in
nature and in phase with the supply voltage.
a) Voltage control loop: The voltage control loop is
used to determine the amplitude of the reference
source current. Under steady state conditions the d.c.
load removes energy from capacitor at constant
average rate and the capacitor voltage can be
maintained constant only if the incoming power from
the a.c. side is equal to the output power demand.
Thus the variation in capacitor voltage is a measure
of the amplitude of the reference current. The voltage
control loop senses the voltage across the capacitor,
increases the current drawn from the supply if the
capacitor voltage tends to decrease from the reference
value and decreases the current drawn from the ac
side when the capacitor voltage tends to increase.
Thus the voltage control loop monitors the output
voltage and determines the amplitude of the source
current. The amplitude of the reference source current
could be evaluated as follows:
where Gc represents a PI or fuzzy controller.
b) Current control loops: Ideally, shunt
compensation is used to achieve a purely sinusoidal
current wave form in phase with the supply voltage.
The voltage control loop cannot ask the shunt
compensation control circuit to draw the required
power at an arbitrary current wave shape. It is the
current control loop that makes sure that the supply
current has the desired shape and amplitude as
commanded by the voltage control loop.
Estimation of reference current templates: A phase
locked loop (PLL) could be used to generate a unit
amplitude wave form in phase with the supply voltage.
The reference source current is obtained by multiplying
the amplitude determined by the voltage control loop
with the unit amplitude waveform generated by the PLL.
The difference between the instantaneous source and
load currents gives the reference compensation currents.
Hysteresis current controller: A hysteresis current
controller [HCC] is used to generate switching pulses
required for the VSI. In a hysteresis controller the
reference compensation current is compared with the
actual current that is being injected by the compensation
circuit. A positive pulse is produced if the actual current
tends to decrease below the lower hysteresis limit, while
a negative pulse is produced if the current exceeds the
upper hysteresis limit. Thus in a hysteresis current
controller the actual compensation current is forced to
stay within a particular hysteresis band.
IV. SERIES COMPENSATION CONTROL
CIRCUIT
Conventional Techniques: A DVR based on a pulse-
width modulated (PWM) VSI, which is capable of
generating or absorbing real and reactive power
independently at its ac output terminals is used as the
series compensator. The series compensator injects three
single-phase ac voltages in series and in synchronism
with the upstream voltages in the distribution system.
The power circuit could be divided into two main parts:
three-phase VSI, three single-phase injection
transformers. The three single-phase transformers are
connected to the distribution system with a star/open
winding. The star/open winding allows the injection of
positive, negative and zero sequence voltages. The
windings on the inverter side are connected in delta
through inductors to provide high voltage. The block
diagram of a series compensator is shown in (Fig. 3).
Fig 1: Schematic diagram of a UPQC
Journal of Engineering Research and Studies E-ISSN0976-7916
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Fig 2: Shunt compensator control circuit
Using Park’s transformation actual
Fig3.Series compensator control circuit
V. SIMULATION RESULTS:
WITHOUT UPQC
AT PREFAULT CONDITIONS
SENDING END V,I
SENDING END VI RMS & PQ
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RECEIVING END V,I RECEIVING END VI RMS & PQ
DURING THREE PHASE FAULT
SENDING END V,I
SENDING END VI RMS & PQ
RECEIVING END V,I RECEIVING END VI RMS & PQ
UPQC WITH PI CONTROLLER:
SENDING END VOLTAGE AND CURRENT
AT PREFAULT CONDITION:
SENDING END VI RMS & PQ
RECEIVING END V,I RECEIVING END VI RMS & PQ
DURING THREE PHASE FAULT TO GROUND
FAULT
SENDING END V,I
SENDING END VI RMS & PQ
Journal of Engineering Research and Studies E-ISSN0976-7916
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RECEIVING END V,I RECEIVING END VI RMS & PQ
UPQC WITH FUZZY CONTROLLER:
SENDING END VOLTAGE AND CURRENT
AT PREFAULT CONDITION:
SENDING END VI RMS & PQ
RECEIVING END VOLTAGE AND CURRENT:
RECEIVING END VI RMS & PQ: DURING THREE-PHASE TO GROUND FAULT
SENDING END VOLTAGE & CURRENT
SENDING END VI RMS AND PQ RECEIVING END VI
RECEIVING END VI RMS
Journal of Engineering Research and Studies E-ISSN0976-7916
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OUTPUT OF POWER SYSTEM WITHOUT UPQC:
AT PREFAULT CONDITION:
SENDING END RECEIVING END
Voltage 400 V 389 V
Current 15.56 A 15.58 A
VRMS 400 V 389 V
IRMS 15.58 A 15.58 A
Real power P 9.314 KW 9.08 KW
Reactive power Q 205 VAR 90.85 VAR
DURING THREE PHASE TO GROUND FAULT:
SENDING END
Voltage 164 V
Current 252 A
VRMS 164 V
IRMS 252 A
Real power P 60 KW
Reactive power Q 44KVAR
UPQC PERFORMANCE WITH PI CONTROLLER:
AT PRE FAULT CONDITIONS:
SENDING END RECEIVING END
Voltage 400 V 350 A
Current 14.3 A 14 A
VRMS 400 V 350V
IRMS 16.5 A 14 A
Real power P 7730 W 7300 W
Reactive power Q 3400 VAR 73 VAR
DURING THREE PHASE TO GROUND FAULT:
SENDING END
Voltage 338 V
Current 53 A
VRMS 338 V
IRMS 53 A
Real power P 12.5 KW
Reactive power Q 26.6 KVAR
UPQC PERFORMANCE WITH FUZZY CONTROLLER:
SENDING END RECEIVING END
Voltage 400 V 392.7 V
Current 15.78 A 15.4 A
VRMS 400 V 392.7 V
IRMS 15.7 A 15.4 A
Real power P 9200 W 9073 KW
Reactive power Q 1746VAR 54.4 VAR
DURING THREE PHASE TO GROUND FAULT:
SENDING END
Voltage 164 V
Current 252 A
VRMS 164 V
IRMS 150 A
Real power P 60 KW
Reactive power Q 44 KVAR
Design Parameters of UPQC System:
Supply Voltage Vs 282.8 V
Load Displacement factor Cos θ 0.866
Line resistance Rs 0.632 Ω
Line inductance Ls 4 mH
DC capacitor C 5 mF
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HARDWARE MODEL DSPIC
GATE PULSE FROM
MICROCONTROLLER:
AMPLIFIED PULSES AT THE
OUTPUT OF GATE DRIVES
WITHOUT UPQC
SENDING END VOLTAGE
RECEIVING END VOLTAGE
WITH UPQC:
SENDING END VOLTAGE
RECEIVING END VOLTAGE
RESULTS:
Table 7: Hardware implementation of single phase UPQC
Sending End Voltage Receiving End Voltage
Without UPQC 48.7 V 46.1 V
With UPQC 48.7 V 48.3 V
VI. CONCLUSION
An AI based UPQC system has been modeled using
MATLAB toolbox. The novelty of this paper lies in
the application of PI and FUZZY controller to
generate switching signals for the series and shunt
compensator of the UPQC system. The performance
of the system for applications such as voltage
interruption, voltage sag or swells, voltage and
Journal of Engineering Research and Studies E-ISSN0976-7916
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current imbalance, harmonic elimination and reactive
power compensation has been successfully examined
and analyzed. The implementation of FUZZY is
better than PI controller for the same output
performance.
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