Page 250 - Offshore Electrical Engineering Manual

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CHAPTER

Calculation of Load 6

Flow, Prospective Fault

Currents and Transient

Disturbances

FAULT CALCULATION
The following calculations and information are not exhaustive but are intended to
give the reader sufficient knowledge to enable switchgear of adequate load and fault
current rating to be specified. The subject may be studied in more detail by reading
the relevant documents listed in Appendix 1. The nomenclature used is generally as
given in ‘Power System Protection’ (IET).
When a short circuit occurs in a distribution switchboard, the resulting fault cur-
rent can be large enough to damage both the switchboard and associated cables due
to thermal and electromagnetic effects. The thermal effects will be proportional to
the duration of the fault current to a large extent and this time will depend on the
characteristics of the nearest upstream automatic protective device which should
operate to clear the fault.
Arcing faults due to water or dirt ingress are most unlikely in the switchboards of
land-based installations, but from experience, they need to be catered for offshore.
For switchboards operating with generators of 10 MW or more, it is usually not dif-
ficult to avoid the problem of long clearance times for resistive faults. However, with
the smaller generators clearance times of several seconds may be required because of
the relatively low prospective fault currents available. (See PART 4 Chapter 4) With
small emergency generators, pilot exciters are not normally provided and the supply
for the main exciter is derived from the generator output. This arrangement is not rec-
ommended, as it allows the collapse of generator output current within milliseconds
of the onset of a fault. With such small generators even sub-transient fault currents
are small, and it is unlikely that downstream protection relays set to operate for ‘nor-
mal’ generation will have operated before the output collapse. It is usual to provide a
fault current maintenance unit as shown in Fig. 4.6.2. This device is basically a ‘com-
pounding’ circuit which feeds a current proportional to output current back to the
exciter field. When the output current reaches a threshold value well above normal
load current, a relay operates, switching in the compounding circuit. Thus a high out-
put current is maintained by this feedback arrangement until definite time overcurrent
protection, set to prevent the generator thermal rating being exceeded, operates.
A worked example for use in setting Main Generator Protection is given in
PART 4 Chapter 5.

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