Page 265 - Offshore Electrical Engineering Manual

252 CHAPTER 7 Protection and Discrimination

3
3.4 × 10 /2010 = 1.69 times setting. From the relay characteristic, at 1.69 times the
relay setting and a time multiplier of 1.0, the relay operating time is 5 s. Hence, for
a required operating time of 10.5 s, the required time multiplier T is 10.5/5 = 2.1.
m

OVERCURRENT PROTECTION
To avoid repetition, only the coordination of the system operating in mode 1 will be
considered. The relay and fuse coordination formulae have been obtained from the
GEC Measurements Protective Relay Application Guide (PAG third Edition). Note
that older GEC relays have been used in this illustration as resetting existing relays
is a more likely task than setting new. Because of this, it is strongly recommended
that data on old relays is retained. New (digital) relays are also usually easier to set.

DATA REQUIREMENTS
SYSTEM DATA
Before beginning work on such a scheme, ensure that you have at least the following
information at hand:

1. Load flow study results for each operating configuration to be considered;
2. Short circuit study results for each operating configuration to be considered,
including three-phase-faults and phase-to-earth faults;
3. single-line diagram of the system, identifying circuit breakers and their protec-
tive devices, fuses and any other means of fault interruption (see Fig. 4.7.1);
4. Fault decrement curves for any generator sets, and fault levels for any incoming
subsea cable feeders.

BASE VALUES
The following base values will be assumed (Table 4.7.1):
Base MVA = 10 MVA
2
Base impedance at 440 V = 0.44 /10 = 0.0194 ohms
2
Base impedance at 6.6 kV = 6.6 /10 = 4.356 ohms
[ √ ]
Base current at 440 V = 10 MVA/ 3 × 0.44 kv = 13.12 kA

[ √ ]
Base current at 6.6V = 10 MVA/ 3 × 6.6 kV = 0.875 kA

Z . pu for base 1 = Z . pu for base 2 × MVA base1 /MVA base2