High Voltage Transformer

The transformers are normally used in high voltage laboratory for extra high voltage testing purpose. This transformer is subjected to transient voltages and surges during their normal operation when the insulation under test breaks down. To withstand these impulse voltages, the insulation of transformer is carefully designed. This high voltage transformer is single phase, core type. This type of transformer is generally oil immersed. Bakelite sheets are used for separating high tension and low tension winding. The high voltage transformers used for HT cable testing purpose are to supply also sufficient electric current and that is why the cooling system of these transformers is very carefully designed. Special cares also to be taken for voltage regulation of transformers. For insulator testing purposes, the required current is very tiny but, while the insulator breaks down during testing, there would flow huge current through the transformer. To limit this current, a high resistance is connected in series with transformer. As the current required is very less during insulation testing, this high voltage transformer needs not to have high KVA ratings. The table below shows, the rating of transformer used for various testing purposes.

Up to voltage 500 KV, generally single one high voltage transformer is used. But for higher voltage more than one transformer, are connected in cascade to produce required high voltage. Actually for getting such high voltage, a single transformer has to be very huge in size which is not at all economical. The figure below shows the typical cascading connection of two transformers. Low voltage is supplied to the low voltage winding of a step up transformer 1 as shown in the figure below. The tank of this transformer is earthed. The secondary of this transformer, is connected to the earthed tank and other end comes out through a high voltage bushing. The bushing is so specially designed and manufactured, that it can withstand full secondary high voltage, in respect of earthed potential of transformer tank. Another tapping terminal also runs through this high voltage bushing. The high voltage end and tapping terminal ends are connected across primary of the second transformer. One end of the secondary winding of second transformer is connected to its tank. The tank of second transformer is not earthed like first transformer. This is isolated and insulated from earth for full secondary voltage of the transformer. One end of the high voltage or secondary winding of second transformer is connected to the earth and other end alone comes out from the high voltage bushing for feeding high voltage to the equipments and insulators under testing.