328 HANDBOOK OF ELECTRICAL ENGINEERING
Figure 12.11 Protection devices for a three-winding transformer.The number of these protection schemes applied to a particular transformer depends upon several
factors e.g.:-
- Critical nature of the load e.g. emergency, essential, normal production.
- Single or duplicated feeder.
- Nominal kVA rating of the transformer.
- Secondary winding voltage.
- Primary winding switching device.
- Physical location and availability of spare complete units or parts.
Figure 12.9 applies to a delta-star transformer that feeds a high or low voltage switchboard.
In this case the secondary winding star point is earthed through an impedance, shown as a resistance.
The NER could also be an earthing inductor or a transformer with a secondary winding feeding
a resistance load, as explained in sub-section 13.3.1. The use of earthing transformers in the oil
industry is seldom encountered, it is more commonly found in EHV systems operated by power
utility companies.
Figure 12.10 is almost the same as Figure 12.9, the difference being that the star point of the
secondary winding is solidly earthed, which is usually the case with low voltage secondary systems.
Figure 12.11 is again similar to Figure 12.9 but applies to a three-winding transformer. Three-
winding transformers are occasionally used in the oil industry. The most common application is
with drilling rigs that are located at offshore production platforms, where they share the power