Page 50 - Offshore Electrical Engineering Manual

Generator Capability Diagram 37

TRANSIENT REACTANCE
Unfortunately, increasing the subtransient reactance also increases the transient
reactance and this will adversely affect the power system performance under motor-
starting conditions. Therefore, there needs to be a trade-off between the two reactance
values to ensure starting of the largest motor with the available spinning reserve,
whilst avoiding intolerable fault levels.

POWER FACTOR
The power factor of the system is dependent on the type of load. There is little
filament lighting these days, but normally trace heating, switched heating elements
for heating the accommodation and for hot water will provide some loads at unity
power factor.
Motors, transformers, fluorescent lighting and motors will provide loads at less
than unity power factor. In addition, there will be nonsinusoidal, semiconductor con-
trolled loads such as some forms of vessel propulsions (i.e., thrusters), process heat-
ing and variable-frequency and variable-speed process drives.
The nominal power factor for generator rating on both ships and offshore instal-
lations is taken as 0.8 lagging.

THE BRUSHLESS ALTERNATOR
In this typical offshore machine, slip rings and brushes are eliminated and excita-
tion is provided not by conventional direct current exciter but by a small alternator.
The AC exciter has the unusual arrangement of three-phase output windings on the
rotor and magnetic poles fixed in the casing. The casing pole coils are supplied with
direct current from an automatic voltage regulator (AVR) of the type described in the
previous section. Three-phase current generated in the windings on the exciter rotor
passes through a rectifier assembly on the shaft and then to the main alternator poles.
No slip rings are needed.
The silicon rectifiers fitted in a housing at the end of the shaft are accessible for
replacement and their rotation assists cooling. The six rectifiers give full-wave recti-
fication of the three-phase supply (Fig. 2.2.2).

GENERATOR CAPABILITY DIAGRAM

The physical capability of the AC generator is limited by the following:

1. The real power available from the prime mover – if this is a gas turbine, as is
often the case for the larger generators offshore, the power available will depend
on the fuel in use (diesel or fuel-gas) and the ambient temperature.
2. The field heating limit – this will limit the maximum current in the rotor
field.