326 HANDBOOK OF ELECTRICAL ENGINEERING
In view of the need to continue operation it is possible to tolerate some relaxation in the pro-
tective relays that are provided for emergency generators, especially low voltage offshore machines.
The following relays in sub-section 12.2.1 may therefore be deleted for low voltage generators:-
- Negative phase sequence (46).
- Differential stator current (87).
- Field winding earth fault (58).
In addition the current setting of the overcurrent relay (51) may need to be higher than for
a ‘normal service’ generator. The voltage-restraining element of a 51 V relay may not prove to be
particularly beneficial for low voltage emergency generators.
12.4 Feeder Transformer Protection
Power systems for offshore platforms and onshore plants, which have their own generators seldom
have feeder transformers with ratings greater than about 10 MVA. These transformers usually feed
radially to the consumer switchboards. Several levels of operating voltages are used in these situations,
for example:
a) Generators and main distribution HV switchboards.
Voltages: 13, 800 , 11 , 000 , 10 , 000 , 6600 ,6000 volts,
occasionally 4160 and 3000 volts.b) Secondary distribution HV and LV switchboards.
Voltages: 6600, 6000 , 4160 , 3000 , 600 , 440 , 400 ,380 volts.c) Sub-circuits
Voltages: 254, 240 , 230 , 220 , 120 ,110 volts.
The power system frequency is either 50 or 60 Hz.
The construction of transformers will be either liquid insulated in a steel tank or cast resin
in a safety enclosure. Either type can be used for outdoor and indoor services, although additional
weatherproofing will be needed for cast resin units.
For transformers having ratings up to 10 MVA the following protection schemes would nor-
mally be provided:
Typically schemes for different transformer configurations that contain most of the following
relays are shown in Figures 12.9, 12.10 and 12.11.
- Overcurrent.
- High-set or instantaneous current.
- Primary earth fault current.
- Secondary earth fault current.
- Differential current.
- Winding and core high temperature.
- Buchholz oil tank surge protection.