Experimental studies for surge voltage response of a power transformer model winding provided with metal oxide varistors

Surge voltage distribution across a power transformer winding due to appearance of very fast rise transient overvoltages (VFTO) such as lightning surges consists of high voltage oscillations and the voltage distribution along the length of the winding can be highly non-uniform. In order to make the voltage distribution more uniform along the length of winding under these conditions, a method has been explored as an alternate to the conventional methods that are being used presently. This method consists of providing suitably designed metal oxide varistors (metal oxide surge absorbers) across sections of winding. The voltage - current characteristic of metal oxide varistor (MOV) is given by V = KIβ, where K and β are the constants of MOV. Experimental investigations have been carried out on a transformer model winding with MOVs provided across fifty percent and hundred percent of winding length to investigate surge voltage performance of these types of windings for appearance of lightning overvoltage (full impulse voltages) at line terminal of the model winding. Transformer windings with α values 5.6, 11.8 and 18.9 have been analyzed. The oscillographic records obtained with presence of identical MOVs (MOVs having similar characteristic constants K and β) or non-identical MOVs (MOVs having different K value and similar β) across transformer winding sections show appreciable reduction in surge voltages across sections of winding as compared to that obtained without MOVs.