Page 221 - Offshore Electrical Engineering Manual

208 CHAPTER 1 Alternator Faults and Protection Devices

OVERLOAD PROTECTION
The larger offshore alternators will not have overload protection as such, but
resistance temperature devices (RTDs) are buried in the stator windings and will
give alarm and trip signals at temperatures where overloading or other abnor-
mal conditions such as excitation faults have occurred. The temperature protec-
tion will avoid unnecessary shortening of insulation life. With small emergency
generators, the associated circuit breaker should be provided with a thermal
element.

OVERCURRENT PROTECTION

To protect the generator from downstream faults, overcurrent relays of either the
inverse definite minimum time (IDMT) or definite time type are required. The
protection relay must provide sufficient time for downstream protection relays
to operate to clear the fault, but this time must be within the time rating of the
alternator. Larger generators are usually provided with permanent magnet pilot
exciters to maintain a fault current of around three times rated current contin-
uously after the transient short-circuit current has decayed away. This can be
maintained usually for an absolute maximum of 10 s before the generator over-
heats. However, it should not be necessary to approach even 5 s in order to oper-
ate downstream protection, and the damage due to arcing faults if such a large
amount of fault energy is let through can be extensive. It is therefore advisable
to keep fault times to a minimum by good relay coordination. With small self-
excited generators, it is important to specify a ‘fault current maintenance kit’
consisting of a voltage sensitive relay and compounding current transformers
(CTs) designed to maintain excitation current when the output voltage collapses
because of a fault. If reasonable coordination cannot be obtained using normal
IDMT or definite time relays, voltage controlled or voltage restraint type relay
can be used to take advantage of close-up fault voltage collapse. An example of
this is given at the end of this chapter.

PHASE AND INTERTURN FAULTS
Stator and interturn faults are relatively uncommon but are more likely to occur at
the ends of the windings or in the terminal box. Insulation failures between phases
may be sensed by differential protection. Both types of fault will cause heating and
possibly a fire, which should be detected by suitable smoke detection in the acoustic
hood, if not by the RTDs or the differential protection.

WINDING PROTECTION
Restricted earth fault (if the individual phases are not accessible at the neutral end)
or phase and earth fault circulating current type protection is usually provided on