Phase-comparison relaying provides only primary protection; backup protection must be provided by supplementary relaying equipment.
Fig. shows schematically the principal elements of the equipment at both ends of a two-terminal transmission line, using a carrier-current-pilot.
As in a-c wire-pilot relaying, the transmission-line current transformers feed a network that transforms the CT output currents into a single-phase sinusoidal output voltage.
This voltage is applied to a carrier current transmitter and to a comparator. The output of a carrier-current receiver is also applied to the comparer.
The comparer controls the operation of an auxiliary relay for tripping the transmission-line circuit breaker.
These elements provide means for transmitting and receiving carrier-current signals for comparing at each end the relative phase relations of the transmission-line currents at both ends of the line.
T = carrier-current transmitter; R = carrier-current receiver.
Let us examine the relations between the network output voltages at both ends of the line and also the carrier-current signals that are transmitted during external and internal fault conditions.
These relations are shown in Fig. It will be observed that for an external fault at D, the network output voltages at stations A and B (waves a and c) are 180° out of phase; this is because of the current-transformer connections at the two stations are reversed.
Since an a-c voltage is used to control the transmitter, carrier current is transmitted only during the half cycles of the voltage wave when the polarity is positive.
The carrier-current signals transmitted from A and B (waves b and d) are displaced in time so that there is always a carrier-current signal being sent from one end or the other.
However, for the internal fault at C, owing to the reversal of the network output voltage at station B caused by the reversal of the power-line currents there, the carrier-current signals (waves b and f) are concurrent, and there is no signal from either station every other half cycle.
Phase-comparison relaying acts to block tripping at both terminals whenever the carrier current signals are displaced in time so that there is little or no time interval when a signal is not being transmitted from one end or the other.
When the carrier-current signals are approximately concurrent, tripping will occur wherever there is sufficient short-circuit current flowing.
This is illustrated in Fig. where the network output voltages are superimposed, and the related tripping and blocking tendencies are shown.
As indicated in Figs. the equipment at one station transmits a blocking carrier-current signal during the one-half cycle, and then stops transmitting and tries to trip during the next half-cycle; if carrier current is not received from the other end of the line during this half cycle, the equipment operates to trip its breaker.
But, if carrier current is received from the other end of the line during the interval when the local carrier-current transmitter is idle, tripping does not occur.