Siemens siprotec SJ62, User Manual

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2 Functions
279
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
On the basis of the indicated data, the pickup values are calculated considering P
prim
= 3.81 MW (10% of 38.1 MW) on the primary level to
on the secondary level. The dropout ratio is set to 0.9. This yields a secondary dropout
threshold of P
sec, dropout
= 15.6 W. If the pickup threshold is reduced to a value near the
lower setting limit of 0.5 W, the dropout ratio should equally be reduced to approxi-
mately 0.7.
Delay for Pickup,
Dropout and Trip The reverse-power protection does not require short tripping times as protection from
undesired power feedback. In the present example, it is useful to delay pickup and
dropout by about 0.5 s and the trip by approx. 1 s. Delaying the pickup will minimize
the number of fault logs which are opened when the reverse power oscillates around
the threshold.
When using the reverse-power protection to disconnect the switchgear quickly from
the utility grid if faults occur, it is useful to select a larger pickup value (e.g. 50% of
nominal power) and shorter time delays.