08-16-2017, 10:57 PM
High Voltage Transformer
Such transformers are normally used in high voltage laboratory for 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 less 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 unit of high voltage transformer is used. But for higher voltage rating 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.
High Voltage Testing Procedure
Electrical equipment must be capable of withstanding overvoltages during operation. Thus by suitable testing
procedure we must ensure that this is done.
High voltage testing can be broadly classified into testing of insulating materials (samples of dielectrics) and
tests on completed equipment.
The tests carried out on samples of dielectric consist generally of the measurement of permittivity, dielectric loss
per unit volume, and the dielectric strength of the material. The first two can be measured using the High
Voltage Schering Bridge.
The tests carried out on completed equipment are the measurement of capacitance, the power factor or the total
dielectric loss, the ultimate breakdown voltage and the flash-over voltage. The breakdown voltage tests on
completed equipment is only done on a few samples since it permanently damages and destroys the equipment
from further use. However since all equipment have to stand up to a certain voltage without damage under
operating conditions, all equipment are subjected to withstand tests on which the voltage applied is about twice
the normal voltage, but which is less than the breakdown voltage.
