Research Article TO LIMIT THE FAULT CURRENT USING UNIFIED POWER … II/JERS VOL II... ·...

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



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



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



RECEIVING END V,I RECEIVING END VI RMS & PQ

DURING THREE PHASE FAULT

SENDING END V,I



UPQC WITH PI CONTROLLER:

SENDING END VOLTAGE AND CURRENT

AT PREFAULT CONDITION:



DURING THREE PHASE FAULT TO GROUND

FAULT

SENDING END V,I





UPQC WITH FUZZY CONTROLLER:

SENDING END VOLTAGE AND CURRENT



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



OUTPUT OF POWER SYSTEM WITHOUT UPQC:


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:


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


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:


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


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



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



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.

REFERENCES

1. Hirofumi Akagi, Trends in Active Power Line

Conditioners, IEEE Tran. Power Electronics, vol.

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3. E. Destobbeleer and L.Protin, On the Detection

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Research Article TO LIMIT THE FAULT CURRENT USING UNIFIED POWER … II/JERS VOL II... ·...

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