Theory Of Operation; Overview; Introduction; Fundamental Principle Of Phase Comparison - GE L60 Instruction Manual

L60 Line Phase Comparison System
Chapter 9: Theory of operation

Theory of operation

This chapter outlines advanced information on operation.

9.1 Overview

9.1.1 Introduction

Phase comparison relaying is a kind of differential relaying that compares the phase angles of currents entering one
terminal of a transmission line with the phase angles of the currents entering all remote terminals of the same line. For the
conditions of a fault within the protected zone (internal fault), the currents entering all the terminals are in phase. For
conditions of a fault outside the zone of protection (external or through fault), or for load flow, the currents entering any
one terminal are 180° out of phase with the currents entering at least one of the remote terminals. The phase comparison
relay scheme makes this phase angle comparison and trips the associated breakers for internal faults. Since the terminals
of a transmission line are normally many miles apart, some sort of communication channel between the terminals is
required to make this comparison.

9.1.2 Fundamental principle of phase comparison

The basic operation of a phase comparison scheme requires that the phase angle of two or more currents be compared
with each other. In the case of transmission line protection, these currents can originate many miles from each other so, as
noted, some form of communication channel is required as part of the scheme.
If a two-terminal line is considered (see figure), the relays located at terminal A can measure the current at that terminal
directly. The phase angle of the current at the remote terminal B must somehow be communicated to terminal A. Since the
current sine wave is positive for a half-cycle and then negative for the next half-cycle, it can be used to key a transmitter
first to a MARK signal for a half cycle and then to a SPACE signal for the next half-cycle for as long as the current is present.
Such a signal transmitted at B and received at A can be compared with the current at A to determine whether the two
quantities are in phase or out of phase with each other. Conversely, the current at terminal B can be compared with the
signal received from terminal A.
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 9-1