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. 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.
|1.||Routine test for electric motors & switch gears||small||2 to 3 KV|
|2.||Insulation testing||10 to 20 KVA||50 KV|
|3.||Routine test of cable||50 KVA||10 to 30 KV|
|4.||Extra high voltage transformer & insulators testing||20 to 50 KVA||100 – 200 KV|
|5.||String insulator testing||0.5 to 1 KVA per KV||500 to 2000 KV|
|6.||High voltage cable testing||100 to 500 KVA||100 to 500 KV|
Voltage RegulationThe surges on the high voltage side of the transformer should be avoided. Also for accuracy of voltage measurement, the voltage regulation of transformer should be smooth enough. Sudden variation of voltage during test also to be avoided. A voltage regulator should not distort the voltage wave form during testing. The output voltage of high voltage transformer is regulated by changing input voltage to the primary side. This variation of input voltage to the primary can be done either by
- Variation of alternator field current.
- Inserting resistance or inductance in the supply circuit from alternator.
- Using induction regulator.
- Using tapped transformer.
Variation of Alternator Field CurrentIf one single alternator is used to supply power to the high voltage transformer, the method of variation of alternator field current can be performed. An alternator gives sinusoidal wave form of voltage at no load. But it is also desirable, that, this voltage waveform should not be distorted under load condition. This is achieved by making larger air gap between stator and rotor and by special design of armature winding of alternator. For regulating voltage, no impedance is required to be connected in series with the primary of the transformer, in this case. So voltage wave form distortion due to inserted impedance can be avoided in voltage regulation with variation of alternator field current. The field current of the alternator is varied by a voltage divider, connected across DC supply to the field. In this method zero voltage can be achieved by neutralizing residual magnetism of field by severing required field current.
Voltage Regulation by Inserting Resistance or InductanceWhen there is no provision of using separate alternator for high voltage testing in the lab, this method is applied. The high voltage transformer is fed from AC supply mains in case of testing of small equipments. The variation of supply voltage to the H. V. transformer is obtained by inserting resistance in series with the AC supply. A sliding >resistance is most suitable for achieving smooth regulation of voltage supplied to the transformer primary. Sometimes the resistance can also be connected across main supply and used as voltage divider, to supply variable voltage to the transformer. This method is quite simple but it suffers from power loss problem. The power loss across the resistance is not practically accepted for high power tests. The resistance required for high power application is quite large in size and also not cost effective. Because of these disadvantages this method is limited upto the application for the equipment rated from 2 KVA to 3 KVA. Instead of resistance, voltage regulation can be achieved by connecting a choke coil (inductor) in series with the primary of the transformer. Voltage variation can be obtained by changing the position of iron core in the choke coil. That means, by inserting and withdrawing iron core inside the coil, the voltage variation is achieved. Due to lower power loss, this method is more efficient than resistance method. But still it has some inherent disadvantages.
- For higher power, very large size of this choke coil is required.
- There is always a good chance of voltage distortion due to iron core in the coil.
- Another disadvantage of this method, is in fact that increase of its inductance will increase the primary voltage of the transformer instead of decreasing it if the power factor of the load on the secondary side of the testing transformer is leading as is often the case.