Underpower Protection; Underpower Protection Implementation - GE MiCOM P40 Agile Technical Manual

P14D
3

UNDERPOWER PROTECTION

Although the Underpower protection is directional and can be configured as forward or reverse, the most common
application is for Low Forward Power protection.
When a machine is generating and the circuit breaker connecting the generator to the system is tripped, the
electrical load on the generator is cut off. This could lead to overspeeding of the generator if the mechanical input
power is not reduced quickly. Large turbo-alternators, with low-inertia rotor designs, do not have a high over
speed tolerance. Trapped steam in a turbine, downstream of a valve that has just closed, can rapidly lead to over
speed. To reduce the risk of over speed damage, it may be desirable to interlock tripping of the circuit breaker and
the mechanical input with a low forward power check. This ensures that the generator circuit breaker is opened
only after the mechanical input to the prime mover has been removed, and the output power has reduced enough
such that overspeeding is unlikely. This delay in tripping the circuit breaker may be acceptable for non-urgent
protection trips (e.g. stator earth fault protection for a high impedance earthed generator). For urgent trips
however (e.g. stator current differential protection), this Low Forward Power interlock should not be used.
3.1

UNDERPOWER PROTECTION IMPLEMENTATION

Underpower Protection is implemented in the POWER PROTECTION column of the relevant settings group, under
the sub-heading UNDERPOWER.
The UNDERPOWER Protection element provides 2 stages of directional underpower for both active and reactive
power. The directional element can be configured as forward or reverse and can activate single-phase or three-
phase trips.
The elements use three-phase power and single phase power measurements as the energising quantity. A start
condition occurs when two consecutive measurements fall below the setting threshold. A trip condition occurs if
the start condition is present for the set trip time. This can be inhibited by the VTS slow block and pole dead logic if
desired.
The Start and Trip timer resets if the power exceeds the drop-off level or if an inhibit condition occurs. The reset
mechanism is similar to the overcurrent functionality for a pecking fault condition, where the percentage of
elapsed time for the operate timer is memorised for a set reset time delay. If the Start condition returns before the
reset timer has timed out, the operate time initialises from the memorised travel value. Otherwise the memorised
value is reset to zero after the reset time times out.
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Chapter 12 - Power Protection Functions
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