Experimental Investigations on IEC Suggested Methods for Improving Waveshape During Impulse Voltage Testing

The objective of Lightning Impulse Voltage (LI Voltage) test on power transformer is to check the dielectric strength of the transformer against over voltages of atmospheric origin. Mostly when transformers of high transformation ratio is subjected to LI Voltage test, the standard LI voltage waveshape distorts and consequently the wavetail falls much below the specified tolerance and swings to opposite polarity. In these cases, IEC has suggested some waveshape improvement techniques in the test circuit or energy enhancement of the impulse generator. This paper elucidates the experimental investigations carried out on a 315kVA, 11kV/433V, Dyn 11, three phase transformer. Investigations was carried out based on the voltage and stress analysis on the transformer windings. For this analysis, tapping are provided on one of the HV winding at 100%, 87.5%, 62.5%, 37.5%, and 12.5%. The investigations include voltage and stress distribution on the windings, voltage across the waveshape component and analysis of stress across major and minor insulation. Apart from voltage and stress distribution analysis, a few novel techniques of waveshape improvement are attempted and a comparative analysis with IEC methods is also presented in this paper.