Page 242 - Offshore Electrical Engineering Manual

Sizing of Conductors 229

An alternative is to use definite time relays. However, on LV systems, it is less
feasible to use definite time relays because, if generator and main switchboard clear-
ance times are to be kept reasonably short, there are likely to be discrimination prob-
lems with fuses and MCBs lower down in the system.
As cables are more exposed to mechanical damage than switchboard busbars, it is
advisable to protect cables which interconnect switchboards by some form of simple
unit protection, rather than IDMT relays with intertripping. This has the advantage
of faster operation and may also relieve any discrimination problems associated with
the unrestricted method of protection.

SIZING OF CONDUCTORS
LOAD FLOW
When the electrical distribution system has been configured for optimum conve-
nience, safety and reliability, the various busbars and cables should be sized for the
maximum continuous load in each system operating condition.
The first task is to ensure that the system 24-h load profile and the load schedule
are as up to date as possible and that diversity factors and operating modes have been
agreed by all parties and ‘frozen’. If the system is simple, with few parallel paths,
load flows may be manually calculated.
In either steady-state or transient conditions, the power system can be represented
by a physical model, such as produced in a network analyser, or by a mathematical
model using a digital computer. With the proliferation of desktop computers, the use
of network analysers, even on small systems, is now rare.
On larger installations, with many parallel paths, computer load flow pro-
grams should be used in any case. Such programs are now available for use on
desktop microcomputers at prices starting from a few hundred pounds. Load flow
calculations by nodal analysis have become firmly established. Such methods
involve

1. the solution of a set of linear simultaneous equations which describe the system
configuration,
2. the application of restraints at each node to enable the required complex power
and voltage conditions to be maintained.

The advantages in using nodal voltage analysis are that the number of equations
is smaller than that with the alternative mesh current analysis method, and the system
may be described in terms of its node numbers and the impedances of the intercon-
necting branches. In nodal analysis, the node voltages V are related to the nodal
injected currents I by the system admittance matrix Y.
In the matrix form,

[I] = [Y][V]