Protective Relay Applications Flashcards

What is the primary function of a distance relay (21) in transmission line protection?

Answer: Measure impedance to the fault location

A distance relay measures the impedance from the relay location to the fault, using both voltage and current inputs to determine fault location.

Which relay characteristic is best suited for protecting a transformer against internal turn-to-turn faults?

Answer: Differential relay (87)

The differential relay (87) compares current entering and leaving the transformer; internal faults cause an imbalance that triggers tripping.

In a Mho distance relay, the operating characteristic on an R-X diagram is shaped like a:

Answer: Circle passing through the origin

The Mho relay characteristic is a circle on the R-X diagram that passes through the origin, providing inherent directionality.

What does the term 'reach' mean in the context of a distance relay?

Answer: The impedance setting that defines the protected zone boundary

Reach is the impedance value (in ohms) up to which the relay will operate, defining the boundary of the protection zone.

A pilot protection scheme uses communications between line terminals primarily to:

Answer: Achieve high-speed tripping for faults anywhere on the protected line

Pilot schemes use communications to allow both terminals to trip simultaneously for internal faults, achieving high-speed protection for the full line length.

Which type of bus protection scheme uses current from all feeders connected to the bus and compares them?

Answer: Differential bus protection (87B)

Differential bus protection sums all currents flowing into and out of the bus; any difference indicates an internal fault.

What is the purpose of a check zone in high-impedance bus differential protection?

Answer: To provide a redundant differential element covering all bus sections

The check zone covers all bus sections and must operate simultaneously with a zone element, preventing false trips due to CT errors.