Abstract:
Transformer insulation is mainly designed based on lightning and switching impulse voltage. The standard lightning impulse waveform is 1.2/50 μsec. Very Fast Transient Overvoltage (VFTO) has a smaller rise time of about 4-100 nanoseconds, followed by some oscillations of about 1 to 50 MHz. The chopped impulse can be classified into two types, if chopping occurs before peak value is reached, the impulse is chopped on front, if it is chopped on the tail, called tail chopped. The chopped impulse and VFTO, if present in the power system network, causes severe degradation of transformer insulation, which may lead to failure or premature breakdown. In this paper, the VFTO and tail time chopped signals are examined on the transformer for inter-turn fault. These test signals are injected into a transformer winding under no-load condition. The frequency response is scrutinized for the healthy and faulty scenario of the transformer in terms of statistical parameters such as Absolute Average Difference (D.A.B.S.), Minimum Maximum Ratio (M.M. absolute), and Comparative Standard Deviation (C.S.D.).These statistical parameters are calculated for a different number of shorted turns on the LV winding of the transformer. Based on these statistical parameters, the variation in the frequency response plots is examined for the healthy and faulty conditions of the transformer.
