Basler Electric BE1-51/27C - Manual (Page 66)

60 9137200998 Rev E
(7) Assuming that the 67 relays will not operate for fault 3, the generators will be unnecessarily tripped
if the line relaying fails to operate or if breaker R fails to interrupt the fault. Redundant line relaying
will minimize the need for generator tripping. Use of a breaker-failure timer set to about 0.25
seconds would avoid generator tripping for a failure of breaker R to interrupt.
Neutral Element Backup Settings
(1) The 51N element has no relays to coordinate with because the generators are ungrounded. An
instantaneous element, if used, must be set above the maximum false residual current that can be
developed due to dissimilar current transformer saturation during an external three-phase fault.
Similarly, the time setting of the time overcurrent element must ride through this transient resulting
from current transformer saturation.
(2) Ground current contribution from each transformer is 485 amperes. Normal total current level is:
A 970 485 2
= ×
(3) Set the time element for 0.8-ampere pickup.
(4) Set the time element (51N) at 0.2 s for 970 amperes primary (see B3 curve):
DIAL TIME 08 and multiples 20
0.8 60
970
=
×
(5) Set the instantaneous (50N) pickup at 5 amperes:
7 . 6
0.8
5 =
The maximum ground fault current is:
pickup Relay the times 2 . 3
300
970 =
Tests and Adjustments BE1-51/27C