Page 75 - Offshore Electrical Engineering Manual

62 CHAPTER 4 Prime Mover Selection Criteria

ALTERNATORS AND EXCITATION SYSTEMS

The control system parameters for the generator excitation system, like those for the
governor, will be required if any computer simulation work is to be carried out.
The following aspects need particular attention in offshore systems:

1. Although, as with large onshore machines, two-pole 50 or 60 Hz designs are
used because of the greater efficiency of energy transfer at the higher speeds,
it should be remembered that this requires the need for complex engineering
analysis and the use of high-grade materials, particularly as the generator mod-
ule could be located on a 600-ft-high steel structure, 100 ft above sea level.
2. To the author’s knowledge, the highest generator rated voltage offshore is 13.8 kV
and alternator manufacturers have no difficulty in producing machines at this rat-
ing. Voltages up to 22 kV could be used, provided suitable switchgear is available.
3. The alternator subtransient reactance (X″d) is a useful regulator of maximum
prospective fault current, and alternator manufacturers are usually prepared,
within certain limits, to vary the design of the windings to enable the system
designers to limit the prospective fault level to a value suitable for the switch-
gear available. For a machine of, for example, 30 MW, the degree of variation
for X″d would be approximately 15%–21%.
4. Although, as discussed earlier, a highly reactive machine may be beneficial to
limit prospective fault currents, this is accompanied by the penalty of poor motor
starting performance because of the increased transient reactance (Xʹd). The wind-
ing reactance must therefore be optimised for the best motor starting performance
allied with prospective fault capabilities within the capacity of the switchgear
installed. This ‘trade-off’ is best accomplished using computer simulation.
5. The conventional configuration of brushless alternator with pilot and main
exciter is commonly used offshore for machines of 500 kW rating and above.
Machines with static (rectifier derived) excitation are acceptable, provided
fault currents can be maintained for at least the full generator fault time rating.
A definite time overcurrent device should be used to shut down the generator
within this time, so that the machine is not left running with a fault still on the
system after the voltage has collapsed.
6. The automatic voltage regulator should incorporate a means of detecting a con-
trol loop disconnection, such as that caused by open-circuit voltage transformer
fuses, in order to avoid excessive voltages being developed on the machine sta-
tor if such a disconnection occurs.

NEUTRAL EARTHING

Typical low-voltage solidly earthed and medium-voltage resistance earthed systems
are shown in Figs. 2.4.3 and 2.4.4. There is little difference between offshore and
onshore practice with regard to generator neutral earthing. However, it is worth
repeating that earth cables and earthing resistors should be adequately rated, both for