Page 226 - Offshore Electrical Engineering Manual

Parallel Transformers and Intertripping 213

the primary and secondary terminals. Under normal load conditions the CT secondary
currents are equal and no current flows in the relay operating coil. If these currents
become unequal because of a fault current being sensed in the transformer windings,
the resulting current energises the operating coil. The relay contacts close when the
ratio of this differential current to the through current exceeds the slope of the relay
operating characteristic. This characteristic may be altered by adjusting the turns ratio
of the operating and restraint coils. The bias slope is chosen so that the relay is insensi-
tive to unbalanced external lead burdens which normally give a lower ratio of differen-
tial current to through current than an internal fault. In addition, a fairly high bias slope
is required to prevent maloperation by CT differential currents arising from

1. tap changing on transformers giving CT mismatch,
2. different CT ratios and hence saturation levels giving differential currents under
through-fault conditions,
3. magnetising inrush giving secondary currents in one set of CTs only.

To prevent maloperation by magnetising inrush, the relay operation is delayed by
a selected time delay which allows the initial current peaks to decay below the relay
setting value determined by the percentage bias.

OIL AND GAS-OPERATED DEVICES

Pressure or flow switches of various kinds may be fitted to oil-filled transformers in
order to detect faults in the windings which give rise to sudden pressure increases
in the tank or sudden flow to the conservator (i.e., Buchholz devices). As conserva-
tors are rarely fitted offshore, because sealed silicon oil designs are now preferred
to reduce fire risk, a rate-of-rise pressure sensing device is usually fitted on the tank.
Protection of this kind which minimises the risk of transformer fires should be con-
sidered for all but the smallest of offshore distribution transformers because of the
serious consequences such a fire would have.

PARALLEL TRANSFORMERS AND INTERTRIPPING
MOMENTARY PARALLELING SCHEMES
In order to allow for transformer and associated equipment maintenance and for
unplanned outages, it is usual to provide two transformers in parallel as feeders to
the production LV switchboard. In most systems, each transformer is rated for the
full production switchboard load, but the production switchboard is only fault rated
for operation with one transformer connected. In such a case, interlocking and paral-
leling facilities are provided which only allow momentary paralleling as the load is
transferred from one transformer to the other. The risk that a fault occurs during the
changeover period is usually very small and therefore acceptable.