Page 342 - Handbook of Electrical Engineering

PROTECTIVE RELAY COORDINATION 329

generated by the production facility. Drilling rig DC systems are fed from a 600 V unearthed sys-
tem, which makes the use of a delta winding attractive. This delta winding acts like a tertiary
winding in that it helps to suppress harmonics from being transferred to the primary winding and
HV system.
Transformers used for offshore services will generally have more protection schemes applied,
than those for onshore services. This is due to the cost of lost production caused by failure of main
power supplies, and the difficulties that arise when a failed unit needs to be replaced. Cast resin
transformers are often preferred for offshore services because of their higher reliability, simpler and
safer construction and ease of maintenance.

12.4.1 Overcurrent

An overcurrent situation is more likely to be caused by excessive secondary load or a serious fault at
the downstream switchboard than an internal fault. Overcurrent protection can be regarded as currents
that are above 100% but below about 500% rated current. Currents above about 500% rated current
can be regarded as ‘high-set’ or ‘instantaneous’ currents, and these are protected in a different manner
than for overcurrents. It is feasible, therefore, to treat the overcurrents as a ‘through-fault’ condition
and detect them in either the primary or the secondary winding switchgear, but not necessarily at
both windings. Detecting overcurrents at both windings would appear to be a desirable requirement,
but it can introduce the need to coordinate the protection curves of the two relays. These could often
be different types of relays or even be made by different manufacturers, in which case their curves
may not match satisfactorily. Alternatively the relays, or their curves, could be chosen to be the
same, or nearly the same. In this case their settings could be made the same and whichever relay
operates first can be used to trip the circuit breakers in both windings, either directly or through
intertripping circuits.

The characteristic curve of the overcurrent relay(s) can be chosen from several standard
shapes:-

• Definite time.
• Standard inverse time.
• Very inverse time
• Extremely inverse time.

Definite time relays are chosen when the individual secondary loads are small when compared
with the transformer rating, and when motor run-up times are small i.e. up to 1 second. Care must
2
be taken to match the current–time settings to the thermal characteristic (I T )ofthe transformer. (A
2
good guide to estimating the I t characteristic for a liquid insulated transformer, when data from the
manufacturer are not available, is the ANSI/IEEE standard C57.12–1980 which takes into account
the probability of frequent through faults and the ratings of the transformers. Note, in this connection
plants which do not use or are not fed from overhead power lines would be regarded as having
infrequent through faults.) Definite time relays are the least expensive of the four types and easy to
set up. They are seldom used for offshore platform applications.
When an individual load is a large induction motor which is started direct-on-line and has a
long run-up time, then the standard or very inverse time relays are often chosen.