Page 269 - Offshore Electrical Engineering Manual

256 CHAPTER 7 Protection and Discrimination

The largest fuse on the accommodation switchboard is 400 A (fuse U). The
relay must discriminate with fuse U and permit the maximum load current of 547 A
to flow.
For discrimination with the fuse, the primary current setting should be approxi-
mately 3 times the fuse rating, i.e., 3 × 400 = 1200 A. Therefore the nearest relay set-
ting is 0.75 × 1500 = 1125 A (0.75 PSM).
The maximum fault level for fuse grading is 56.009 kA (from computer science
study). Therefore from the manufacturer’s fuse characteristics, the fuse operating
time t at 56 kA is 0.01 s. From the GEC Guide, the minimum relay operating time is
given by:
t + 0.4t + 0.15 = 0.164s

If the fault current from computer studies is 51.44 kA, then the fault current as a
multiple of the plug setting is 51,440/1125 = 46. The relay operating time at 46 times
the plug setting with a time multiplier setting (TMS) of 1 is 0.255 s, from the relay
characteristic. Therefore the required TMS is 0.164/0.255 = 0.64 ≈ 0.7.

RELAY T: RANGE 10%–200%, CT RATIO 1500/1
As the maximum loading of the accommodation switchboard is the same, the exer-
cise is similar to that for relay F but using the fault currents that the computer has
calculated for feeder T.
With the given fault current of 44,280 A, the equivalent relay PSM is
44,280/1125 = 39.4. Therefore from the relay characteristic, using a PSM
of 39.4 and a TMS of 1, the operating time is 0.255 s. The required TMS is
0.164/0.255 = 0.64 ≈ 0.7.

RELAY G: RANGE 10%–200%, CT RATIO 2000/1

In mode 1, the utilities switchboard would be feeding the emergency switchboard,
as the emergency generator would not be running. From the load flow data for the
emergency switchboard:
Load = 455 kW + jl93 kVA r ≈ 649 A at 0.92 power factor
Largest motor load ≈ 84 A at 0.8 power factor
Motor starting current ≈ 504 A at 0.2 power factor
Therefore the maximum load during the starting of this motor is:
Load − motor FLC + motor start

(597 + j253) − (67.2 + j50.4) + (100.8 + j494) = 939.6Aat0.67 power factor ≈ 1000 A
Relay G must permit the maximum load without tripping and must also coor-
dinate with the largest outgoing fuse on the emergency switchboard, i.e., fuse Cl