Page 244 - Offshore Electrical Engineering Manual

Worked Example: Fault Calculation 231

For motor cables, the basis of the calculation is
(V d × 1000)
L =
(1.732 × I × (RcosΦ+ XsinΦ)
where L, cable length in metres; V , permitted steady-state volt drop in volts; I, motor
d
full-load current; R, cable resistance in ohms per kilometre; X, cable reactance in
ohms per kilometre and cosΦ, motor power factor at full load.
The permissible maximum steady-state volt drop is normally 2.5%, whilst the
permissible volt drop during starting is 10%.
A typical computer spreadsheet generated motor cable sizing chart is shown in
Fig. 2.8.1.
Derating factors for cables which pass through insulation and for bunching, appli-
cation of protective devices, etc. need to be considered in accordance with the current
edition of the IEE Wiring Regulations.

WORKED EXAMPLE: 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 the GE Alstom Grid Solutions Network Protection and Automation Guide
(NPAG) and the Electricity Council’s Power System Protection (IET).
When a short-circuit occurs in a distribution switchboard, the resulting fault
current can be large enough to damage both the switchboard and associated cables
owing to thermal and electromagnetic effects. The thermal effects will be propor-
tional 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 switch-
boards 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 difficult to avoid the problem of long clearance times for resis-
tive faults. However, with smaller generators, clearance times of several seconds
may be required because of the relatively low prospective fault currents available
(see earlier section on busbar protection). 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 recommended, as it allows the
collapse of generator output current within milliseconds of the onset of a fault.
With such small generators, even subtransient fault currents are small, and it is
unlikely that downstream protection relays set to operate for ‘normal’ generation
will have operated before the output collapse. It is usual to provide a fault current