Page 270 - Offshore Electrical Engineering Manual

Earth Fault Relay Setting 257

(160 A) must operate before relay G. Coordination with relay T is not considered
necessary, since the accommodation switchboard would be supplied through cir-
cuit F. Relay Y does not need to coordinate with relay G, since feeder Y is only
used in mode 3 operation (emergency generator only). However, the setting of
relay G must also allow for back feeding of the utilities switchboard via the emer-
gency switchboard and therefore coordination with fuse W, the largest utilities
switchboard fuse.
The maximum fault current through feeder G is 56,000 A (from fault calcula-
tions). The operating time t of fuse Cl at 56 kA is 0.01 s, from the fuse characteristic.
The setting of relay G for coordination with fuse Cl is three times the fuse rating, i.e.,
3 × 160 A = 480 A, but the maximum load is 1000 A.
For back feeding of the utilities switchboard, relay G must coordinate with fuse
W, which gives 3 × 355 A = 1065 A. From the load flow results, the maximum current
flow through feeder G in the direction of the utilities switchboard is 1032 A at 0.8
power factor.
The starting current of the largest motor is 261 + j1281. Therefore the maximum
load is:
(825.6 + j784.8) − (174.4 + j130.8) + (261 + j1281) = 912.2 + j1935
= 2139 A at 0.43 power factor
Therefore use the nearest setting of 2000 A. This is also the rating of the trans-
former which feeds the emergency switchboard.
From the SC calculations, the maximum SC current at fuse W is 44.3 kA.
From the fuse characteristics, the fuse operating time t is 0.01 s.
Therefore relay G operating time is:
t + 0.4t + 0.15 = 0.164s
The fault current as a multiple of the relay plug setting is 44,300/2000 = 22.
Therefore, from the relay characteristic the operating time of relay G at a TMS of
1 is 0.3 s. The required TMS is 0.164/0.3 = 0.54 ≈ 0.6 (nearest upward setting; using a
setting of 0.5 would reduce the grading margin).

COORDINATION
The same methods may be adopted to obtain settings for the other overcurrent relays.
The resulting coordination chart is shown in Fig. 4.7.3.

EARTH FAULT RELAY SETTING
Fig. 4.7.4 shows the earth fault discrimination flow chart for this system.