Page 1 of 11

European Journal of Business &

Social Sciences

Available at https://ejbss.org/

ISSN: 2235-767X

Volume 07 Issue 04

April 2019

Available online: https://ejbss.org/ P a g e | 1900

A Comprehensive Study of the Impact Analysis of Facts Devices on Relay

Protection and Its Mitigation

RAMESH .S

PG Student

PRIST Deemed to be University, Thanjavur

Abstract- The basic operation principle of distance

relay is based on the fact that the line impedance is

fairly constant with respect to the line length.

However, the implementation of FACTS controllers

in power system transmission for enhancing the

power system controllability and stability have

introduced new power system issues in the field of

power system protection that must be considered and

analyzed. Some of the concerns include the rapid

changes in line impedance and the transients

introduced by the fault occurrence and the associated

control action of the FACTS Controllers. The

presence of the FACTS devices in the faulted loop

introduces changes to the line parameters seen by the

distance relay. The effect of FACTS device would

affect both the steady state and transient

characteristics of the apparent impedance seen by

distance relays due to the fast response time of

FACTS Controllers with respect to that of the

protective devices.

Keywords: Power system protection, TCSC, Zones,

Distance Relay.

I. INTRODUCTION

The measured impedance at the relaying point is the

basis of the distance protection operation. There are

several factors affecting the measured impedance at

the relaying point. Some of these factors are related to

the power system parameters prior to the fault

instance, which can be categorized into two groups

[1-3]. First group is the structural conditions,

represented by the short circuit levels at the

transmission line ends, whereas the second group is

the operational conditions, represented by the line

load angle and the voltage magnitude ratio at the line

ends. In addition to the power system parameters, the

fault resistance, in the single-phase to ground faults,

could greatly influence the measured impedance, in

such a way that for zero fault resistance, the power system parameters

do not affect the measured impedance. In other words, power system

parameters affect the measured parameters becomes more severe [4].

In today’s power system, there are some difficulties for constructing

new transmission lines, because of the limited resources and

environmental restrictions. This leads to interdiction of double circuit

lines and compensated lines by FACTS

Page 2 of 11

European Journal of Business &

Social Sciences

Available at https://ejbss.org/

ISSN: 2235-767X

Volume 07 Issue 04

April 2019

Available online: https://ejbss.org/ P a g e | 1901

devices in to the power systems. In

the case of the double-circuit lines,

because of the low distance of the

conductors of two circuits, two

circuits affect each other mutually

[5]. On the other hand, recently

FACTS devices are introduced to

the power systems to increase the

transmitting capacity of the lines

and provide the optimum utilization

of the system capability. It is well

documented in the literature that the

introduction of FACTS devices in

power systems has a great influence

on their dynamics [6-7]. As power

system dynamics changes, many

sub-systems are affected, including

the protective systems. Therefore, it

is essential to study effects of

FACTS devices on the protective

systems, especially the distance

protection, which is the main

protective device at EHV and HV

levels. Unlike power system

parameters, the controlling

parameters of FACTS devices could

affect the measured impedance even

in the absence of the fault resistance

[8-10]. In the presence of FACTS

devices, the conventional distance

characteristic such as Mho and

Quadrilateral are greatly subjected

to mal-operation in the form of over- reaching or under-reaching the fault

point. Therefore, the conventional

characteristics might not provide the

protective functions satisfactorily in

the presence of FACTS devices [11-

12]. It is well-known that FACTS

series devices such as Thyristor

Control Series Capacitor (TCSC) are

used to improve the power transfer

capability of long transmission lines.

These series connected FACTS

devices inject a series voltage with

the line and change the line

impedance, cause the impedance

seen by distance relay to be lower or

higher than the actual line

impedance and the distance relay

either over reaches or under reaches.

Figure.1. Transmission line

with midpoint line compensation.

Page 3 of 11

European Journal of Business &

Social Sciences

Available at https://ejbss.org/

ISSN: 2235-767X

Volume 07 Issue 04

April 2019

Available online: https://ejbss.org/ P a g e | 1902

II. PRACTICAL TCSC MODEL AND MODES

OF OPERATION IN STEADY STATE

A. TCSC Model

Fig.2 shows a TCSC module with different protective

elements. Basically, it comprises a series capacitor

(C), in parallel with a Thyristor Controlled Reactor

(TCR) (Ls). A metal oxide varistor (MOV),

essentially a nonlinear resistor, is connected across

the series capacitor to prevent the occurrence of high

capacitor over voltages. Not only does the MOV limit

the voltage across the capacitor, but it allows the

capacitor to remain in the circuit even during fault

conditions and helps improve the transient stability. A

circuit breaker is also installed across the TCSC

module to bypass it if a severe fault or equipment

malfunction occurs. A current limiting inductor, Ld, is

incorporated in the circuit to restrict both the

magnitude and the frequency of the capacitor current

during the capacitor bypass operation.

Fig. 2. Practical TCSC module.

B. TCSC Modes of Operation in Steady State

In normal operating conditions, there are four modes

of operation used in Steady State are as follows:

• Blocking mode

• Bypass mode

• Capacitive Boost mode

• Inductive Boost mode.

Blocking mode: When the thyristor valve is not

triggered and the thyristors are kept in non- conducting state, the TCSC is operating in blocking

mode. In this mode, the TCSC performs like a fixed

series capacitor.

Bypass mode: If the thyristor valve is triggered

continuously it will remains conducting all the time

and the TCSC behave like a parallel connection of the

series capacitor bank with the inductor in the thyristor

valve. In this mode, the resulting voltage in the steady

state across the TCSC is inductive and the valve

current is somewhat bigger than the line current due

to the current generation in the capacitor bank. For

practical TCSC’s with ratio between 0.1 to 0.3 ranges, the capacitor

voltage at a given line current is much lower in bypass than in blocking

mode. Therefore, the bypass mode is utilized as a means to reduce the

capacitor stress during faults.

Capacitive boost mode: If a trigger is supplied to the thyristor having

forward voltage just before the capacitor voltage crosses the zero line a

capacitor discharge current pulse will circulate thorough the