Reverse Power Protection: Time Setting

Vide IEEE C 37.102,

From a system standpoint, the primary indication of motoring is the flow of real power into the generator acting as a synchronous motor. The reverse-power relay detects the reverse flow of power (i.e. watts) that would occur should the prime mover lose its input energy. The magnitude of motoring power varies considerably depending on the type of prime mover, as shown in Table.

The sensitivity and setting of the relay are dependent upon the type of prime mover involved because the power required to motor is a function of the load and losses of the idling prime mover.

The reverse-power relay should have sufficient sensitivity so that the motoring power provides 5 to 10 times the minimum pickup power of the relay.

In gas turbines, for example, the large compressor load represents a substantial power requirement from the system, up to 50% of the nameplate rating of the unit; therefore, the sensitivity of the reverse-power relay is not critical. A diesel engine with no cylinders firing represents a load of up to 25% of rating; therefore, again, no particular sensitivity problem exists.

With hydro turbines, when the blades are under the tail-race water level, the percent motoring kilowatts is high. When the blades are above the tail-race level, however, the reverse power is low, between 0.2% to 2.0% of rated, and a sensitive reverse-power relay may be needed.

Steam turbines operating under full vacuum and zero steam input require about 0.5% to 3.0% of rating to motor. This condition may be detected by a sensitive reverse-power relay. If the turbine were operated with its valves only partially closed to, for example, slightly less than the no-load value, the electrical input from the system could be essentially zero, and the reverse-power relay could not detect the condition. Because overheating of the turbine could still occur, some additional means of protection is required.

A directional power relay with either definite- or inverse-time characteristics is frequently used to introduce sufficient time delay necessary to prevent operation during power swings caused by system disturbances or when synchronizing the machine to the system. A time delay of 10 s to 30 s is typical. Either a single-phase or a three-phase relay may be used although a single-phase relay calibrated in three-phase watts is frequently selected.