STUDY OF TRANSFERRED SURGES ON TERTIARY OF 220 KV CLASS POWER TRANSFORMER AND ITS PROTECTION

Abstract

This dissertation is based on an experimental approach to the study of transferred surge phenomena on 11 kV delta connected tertiary of 220 kV Class, 3 Phase or 3 Phase bank of single phase 3 winding transformers and auto transformers using recurrent surge generator and CR0. The magnitude of transferred surges on tertiary at different terminal conditions are determined when a lightning or switching impulse wave applied either -on high voltage or medium voltage terminal of 220 kV transformer. The transferred surge voltage on tertiary are found to exceed considerably the specified value of BIL for 11 kV syatem voltage when tertiary is kept unloaded or connected to high impedance circuit or one terminal of tertiary is earthed and other two are kept isolated in service. Thus while designing the insulation for this type of winding whether loaded or used for stabilising purpose in the transformer, the magnitude of transferred surges on it under unloaded conditions should be taken into account. The impulse insulation levels for 11 kV tertiary of 220 kV Auto Transformer or system step down transformer are recommended by analysing the test results of the transformers. The effect of an earthed shield placed outside the tertiary coil on the magnitude of transferred surges on tertiary has been studied. The relative advantages and disadvantages of this shield in transformers are discussed. In the case of loaded tertiary, the clearances for tertiary bushings and associated bus bars and the BIL of tertiary load would have to be kept more than the value specified in the I.S. for 11 kV system in absence of proper protective measures at tertiary load circuit. The necessary protective measures are recommended in the dissertation depending upon the nature of associated impedance or -load. Suggestions for the selection of protective levels and energy rating for the tertiary side arresters are mentioned.