Page 224 - Offshore Electrical Engineering Manual

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CHAPTER

Transformer Faults and 2

Protection Devices

TRANSFORMER FAULTS

EARTH FAULTS
The magnitude of any fault current flowing in a transformer will be a function of the
winding arrangement (and hence leakage reactance) and the type of earthing (solid
or impedance). Most offshore distribution transformers, however, are of the delta star
(usually Dy11) type, the only exceptions normally being those used for supplying the
drilling silicon controlled rectifiers. With the usual solidly earthed star point arrange-
ment, the fault current will be highest when the fault is near neutral. There is no zero-
sequence path through such a transformer and therefore coordination is not required
with any earth fault protection on the primary side. The magnitude of any earth fault
on the delta primary winding will be governed by the type of earthing in that part of
the system. If it is fed from the main switchboard, it will probably be limited by the
generator earthing resistors.

PHASE-TO-PHASE FAULTS

Faults between phases on offshore transformers are relatively rare, especially with
the newer sealed or encapsulated types of transformer. Such a fault is likely to pro-
duce a substantial current large enough to be detected quickly by upstream overcur-
rent protection.

CORE AND INTERTURN FAULTS
Although interturn faults are unlikely on offshore distribution transformers because of
the relatively low voltages (LVs), the fire hazard is such that their probability cannot be
ignored. A fault in a few turns will cause a high current to flow in the short-circuited
loop and produce a dangerous local hot spot. A conducting path through core lamina-
tions will also cause severe local heating. It is difficult to detect such a fault in a resin-
encapsulated transformer. Offshore distribution transformers are less likely to suffer
line surges or steep fronted impulse voltages, as cabling is relatively short and lightning
strikes are rare. However, moisture ingress is more likely in the offshore environment,
and mechanical vibration levels are higher, making chafing and cracking of insulation
more likely, so the possibility of interturn faults cannot be ignored. If the transformer
is housed in a tank containing insulating oil, there is always a danger that the tank will
corrode in the salt-laden atmosphere, or will be damaged by crane operations or the

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