Page 225 - Offshore Electrical Engineering Manual

212 CHAPTER 2 Transformer Faults and Protection Devices

like. Offshore transformers are as likely to suffer from abnormal system operation as
their onshore counterparts. The maintenance of system voltage and frequency may be
dependent for long periods on the satisfactory operation of one generator package. The
governor or automatic voltage regulator of this package may be set low or high, or may
be subject to drift within the limits of the voltage and frequency protection. Therefore,
transformers may be subject to variations in voltage and frequency. High voltage and
low frequency may together cause shifting of flux in structural parts of the transformer,
which will heat up and destroy insulation. The transformer must also be protected from
overloads and overcurrents due to downstream faults.

MAGNETISING INRUSH
Magnetising inrush is a normal healthy but transient condition associated with the
establishment of linking flux between the windings. However, if it is forgotten when
setting upstream overcurrent protection relays, it may cause nuisance tripping. Values
quoted by transformer manufacturers are in the region of 12 times full load for 15 ms.

OVERCURRENT PROTECTION
A common arrangement offshore is to protect the primary winding of the transform-
ers between the main (medium voltage) and the production (LV) switchboards with
inverse definite minimum time overcurrent relays graded with the main generator
overcurrent protection. The relay may have an earth fault element that detects earth
faults in the primary winding only.

RESTRICTED EARTH FAULT PROTECTION

With the restricted earth fault scheme, the residual current obtained from a current
transformer (CT) in each line is balanced against the current from a CT in the neutral.
The neutral is usually solidly earthed and therefore a healthy fault current will be
produced even from a fault at the last turn of the winding (i.e., closest to the neutral).
As faults are only detected between the line CTs and the neutral CT, only the star
secondary is protected, using a sensitive instantaneous relay.

DIFFERENTIAL PROTECTION
Differential protection is rarely used on transformers offshore. However, it does have
the advantage over restricted earth fault protection in that both primary and secondary
windings are protected from both earth and phase-to-phase faults. The relay will only
operate for faults appearing in the protection zone between the sets of sensing CTs at