Page 243 - Offshore Electrical Engineering Manual

230 CHAPTER 5 Conductor Sizing, Load Flow and Fault Calculation

The voltage V refers to a value between node and earth, and the current I is the
injected nodal current. The total nodal injected power S is obtained from the product
of voltage and current conjugate, as follows:-
[S] = [V][I] *
By taking the current conjugate, the reactive power is given the same sign as
active power for lagging current.
There are three basic types of nodal constraints:

1. fixed complex voltage
2. fixed complex power
3. fixed-voltage modulus with real power.

Type (1) constraint is given to the reference node, usually known as the ‘slack’ or
‘swing’ (in the United States) bus. The type (2) constraint represents a load bus; the
type (3), a generation bus.
The formation of the nodal matrix and methods available for digital iterative
sequences of solution are given in Bergen (1986).

BUSBAR SIZING

Switchboard main busbars must be rated to carry the maximum continuous load
which can flow in any healthy power system operating condition. Transient condi-
tions giving rise to higher currents, such as those due to large motors starting or
downstream faults, may be tolerated momentarily, provided the protection devices
are incorporated which will ensure that the outgoing equipment is removed from the
system before the busbars get overheated.
The continuous current rating must be for the busbars as enclosed in their off-
shore environmental protection, with natural cooling only. This also applies to the
switching and isolating devices in the switchboard.

CABLE SIZING
Cable sizing is generally carried out in accordance with current versions of the
following:
IEC 61892: Mobile and fixed offshore units – Electrical installations:

Part 2: System design
Part 4: Cables
Part 6: Installation
IEC 60287: Electric cables – Calculation of the current rating.
IEC 92-201: Electrical installations in ships
Part 201: System design – General
Part 202: System design – Protection
Part 352: Choice and installation of electrical cables