2 OVERPOWER PROTECTION
With Overpower, we should consider two distinct conditions: Forward Overpower and Reverse Overpower.
A forw
ard overpower condition occurs when the system load becomes excessive. A generator is rated to supply a
certain amount of power and if it attempts to supply power to the system greater than its rated capacity, it could
be damaged. Therefore overpower protection in the forward direction can be used as an overload indication. It can
also be used as back-up protection for failure of governor and control equipment. Generally the Overpower
protection element would be set above the maximum power rating of the machine.
A reverse overpower condition occurs if the generator prime mover fails. When this happens, the power system
may supply power to the generator, causing it to motor. This reversal of power flow due to loss of prime mover can
be very damaging and it is important to be able to detect this with a Reverse Overpower element.
2.1 OVERPOWER PROTECTION IMPLEMENTATION
Overpower Protection is implemented in the PO
WER PROTECTION column of the relevant settings group, under the
sub-heading OVERPOWER.
The Overpower Protection element provides 2 stages of directional overpower 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 quantities. A Start
condition occurs when two consecutive measurements exceed the setting threshold. A trip condition occurs if the
Start condition is present for the set time delay. This can be inhibited by the VTS Slow Block and Pole Dead logic if
desired.
The Start and Trip timer resets if the power falls below 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.
Chapter 12 - Power Protection Functions P14x
260 P14xEd1-TM-EN-1
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