Design of a High Voltage Lead Exit for Power Transformers using Silicone Rubber

Abstract:
High voltage lead exits are used on the high voltage side of power transformers to lead the conductor from the windings to the bushings. These special insulation systems are used in order to increase the dielectric strength of long oil gaps in turrets. Conventionally, high voltage lead exits are composed of paper and pressboard material. They are bulky in construction, and hence, difficult to handle and mount. In this research, silicone rubbers are investigated for their usage as insulation materials for lead exits. Silicone rubbers exhibit excellent thermal, mechanical and chemical stability. They are also flexible materials and have a dielectric constant of 2.8. Hence, with the usage of silicone rubber as the insulation material, the lead exit design can be simplified and the overall size can be reduced. In this contribution, a 400 kV lead exit with a 2-barrier system is taken as the reference lead exit. The cumulative stress at various intervals in the oil gap is determined and is compared to the inception field strength in oil gap curve. The minimum safety factor of the reference design is determined by obtaining the ratio of discharge inception field strength to cumulative field strength at different intervals. Considering the obtained safety factor, the design of high voltage lead exit with silicone rubber insulation is discussed and possible arrangements are presented.