24 Dry-type and cast resin transformers

 Dry-type and cast resin transformers

Dry-type transformers, particularly those using cast resin insulation, are noww idely used in locations where the fi re risk associated with the use of mineral oil is considered to be unacceptable, for example in offi ces, shopping complexes, apartment buildings, hospitals and the like. 

The background to this development and the factors requiring to be considered in installing cast resin transformers within buildings have been discussed at some length in.

This section describes the special features of cast resin transformers themselves. 

Although there are other types of dry insulation systems available, nowadays for most purposes dry type means cast resin, so the following paragraphs are written primarily as referring to cast resin.

In 2004 IEC issued a standard for dry-type transformers as Part 11 of the IEC 60076 series covering power transformers. 

The following discussion of dry-type and cast resin transformers relates to transformers complying with IEC 60076-11, except where indicated otherwise.

Complete encapsulation of the windings of a power transformer in cast resin is an illogical step to take, because, as explained on a number of occasions elsewhere in these pages, one of the main requirements in designing transformer windings is to provide a means of dissipating the heat generated by the flow of load current. 

Air is a very much poorer cooling medium than mineral oil anyway, without the additional thermal barrier created by the resin. 

All air cooled transformers are therefore less efficient thermally than their oil-filled counterparts and cast resin are poorer than most. 

Hence they will be physically larger and more costly even without the added costs of the resin encapsulation process. 

In addition, the absence of a large volume of oil with its high thermal inertia means that cast resin-insulated transformers have shorter thermal time constants which limit their overload carrying capability. 

More will be said about overload capability below.

The incentive to develop an economic design of cast resin transformer was provided by the outlawing of polychlorinated biphenyls (PCBs) in the late

1970s on the grounds of their unacceptable environmental impact. 

Alternative non-flammable liquid dielectrics have all tended to have had some disadvantages, with the result that users have come to recognise the merits of eliminating the liquid dielectric entirely. 

Nevertheless, cast resin does not represent an automatic choice of insulation system for a power transformer. 

Cast resin transformers are expensive in terms of fi rst cost. 

They are less energy efficient than their liquid-filled equivalents.

In their early days there were suggestions that their reliability was poor and even that their fi re resistance left something to be desired. 

In recent times,however, their qualities of ruggedness, reliability and excellent dielectric

strength have come to be recognised as outweighing their disadvantages and

their use has become widespread in situations where these properties are most strongly valued.