14.1 Introduction

Transformers are required to transform the power output from electrical generators up to the voltages used in the transmission system (400 and 275 kV in the UK), to interconnect parts of the transmission system and to step down the voltage at bulk supply points and at various points in the distribution network before reaching the consumer. In addition, special transformers are required for AC/DC convertor stations and for transmission control devices such as quadrature boosters and static VAr compensators.

For a utility, large power transformers are major capital items, costing up to 2M, with construction lead times up to 18 months. Although generally very reliable, when problems occur they are often difficult to diagnose and expensive to correct. For example, just to handle the oil from a transformer to allow an internal inspection, which very often can be inconclusive, can cost over 10 000 for a large transformer.

A utility is therefore concerned, as a customer, to ensure that as far as possible every new transformer purchased is capable of performing to requirements and will continue to do so for a service life of at least 40 years, over the specified operating conditions and without being damaged by the inevitable occasional system abnormalities. The user requires a transformer which will be effective, efficient, reliable and also economical. Key activities in ensuring this are the specification for the transformer, quality assurance during manufacture, effective testing of the transformer before it leaves the manufacturer's works, and appropriate maintenance...