Cast epoxy insulation for high voltage switchgear and power transformers

There are three basic areas of insulation in switchgear of today. These are the Bus Bars, Bushings and Transformers. In all cases the insulation must be able to withstand high voltages and have good mechanical properties at temperature ranges of - 40°C to 130°C. It must also be capable of being readily formed into complex shapes and sizes. Most conventional forms of insulations of today do not possess these qualities. Cast Epoxy, however, has all of these properties-and more. Epoxy resins were first developed in Europe over 16 years ago; however, they were not fully developed until about 10 years ago when a leading European manufacturer of switchgear gave birth to a cast epoxy insulated bar type current transformer with embedded primary conductor and secondary winding. Encouraged by this success, they extended this form of insulation to other parts of their switch units. Today the insulation medium of most of their high voltage switchgear is cast epoxy and all new designs of this range of switchgear are based on cast epoxy insulation. As a result of their bold step forward, many switchgear manufacturers in Europe now use cast epoxy as the main insulation medium on their high voltage switchgear. In the United States, the types of insulations most commonly used in switchgear are Porcelain, Bakelized Paper, Impregnated Fabrics and Glass Polyesters. None of these insulants possess all of the following requirements for good insulation: Good Electrical and Mechanical Properties, Non-Hygroscopic, Flame Retarding and being readily formed into complex shapes of any size at low cost. Many of the conventional types of insulation are often combined. As an example, Figure 1 shows an epoxy insulated bus bar disconnect. This unit would have consisted of a metal flange or housing in which is cemented the porcelains. The conductors from the porcelains would be insulated with impregnated paper or polyester tubes. The joint at the bus bar would be insulated with varnished tapes. All of these steps require assembly time to insulate and assemble the parts. The shape of the connecting conductors is also limited because of the problem of insulating bent or curved conductors. With the use of cast epoxy, it is possible to eliminate all of the pre-assembly operations as well as to reduce the overall size of the units. Switchgear designers are also freed from the problem of fitting conventional shapes and sizes into their units. They are able to let their imaginations run wild as long as they design within the electrical and mechanical properties of cast epoxy. Epoxy resins are not a cheap form of insulation. If it is used to replace an existing component without redesign, the cost could prohibit its use. Components should be designed around epoxy insulation. By doing this, a more sound insulation will be achieved together with a reduction in costs and overall dimensions as compared to conventional type of insulation.