Page 220 - Offshore Electrical Engineering Manual

CHAPTER

Alternator Faults and 1

Protection Devices

INTRODUCTION
As discussed in earlier chapters, the isolated location, onerous operating conditions
and harsh environment make it particularly important that provisions are made in
the design in order to protect electrical equipment from faults on the system and to
remove faulty equipment from the system quickly and safely. Alternators must be
protected from prime mover and fuel system faults. Motors must be protected from
faults in driven equipment. In this chapter, common protection relay applications
are discussed Worked examples are provided in PART 4 Chapter 7 for power system
relay configurations likely to be met in offshore installations.

NEUTRAL EARTHING AND EARTH FAULTS

Medium-voltage generators are normally resistance earthed to limit fault currents.
Stator windings should be designed to minimise third harmonic circulating cur-
rents, so that generators may be paralleled without extra weight and space being
taken up with earthing circuit breakers. The short time ratings of any earthing resis-
tors must be borne in mind when calculating earth fault relay settings. A current
sensing relay of either inverse or definite time characteristic may be used for unre-
stricted earth fault protection. Alternatively, an attracted armature type relay may
be used in conjunction with a time delay relay. In either case, the time delay must
be within the time rating of the earthing resistor and the relay must coordinate with
those downstream. The term ‘back-up’ earth fault protection is sometimes used to
describe this scheme, as it supplements the generator differential protection. But
the use of this term is not recommended, as unrestricted earth fault protection will
not detect phase-to-phase faults and therefore the term ‘back-up’ or ‘standby’ leads
to some confusion. The neutral earthing conductor arrangement must be rated for
the prospective earth fault current flows available from the generator. If the genera-
tor is a low-voltage machine with bolted earth connection, this current is likely to
be high enough to affect cable conductor sizes. With directly earthed low-voltage
machines, the high currents are used to operate fuses and miniature circuit break-
ers usually found in low-voltage distribution systems. With larger high-voltage
machines the damage caused to laminations is limited by the insertion of the neu-
tral resistance, and more sensitive forms of protection may be used in the medium-
voltage distribution system.

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