Part winding resonance: demerit of interleaved high-voltage transformer winding

Interleaved transformer winding is a new and promising development in the field of design of high-voltage transformer windings. Although the strengths and weaknesses of the interleaved winding concept against surge voltages are theoretically known, implementation in a practical multiwinding EHV transformer may lead to various complications and the winding can show unpredictable behaviour contrary to existing knowledge. A comparative assessment of interleaved transformer winding has been made by EMTP simulation of two identical multiwinding EHV transformer models having interleaved and noninterleaved conventional disc windings. The observations regarding relative surge performance of interleaved and noninterleaved windings may throw some light on optimum interleaving of the disc coils in transformer winding. The tap changer or regulating winding forms a weak point in HV transformers and is found to be responsible for the majority of failures. Distribution of impulse voltage in the regulating or tap-changer winding has also been studied. The results of these observations may be useful in finding suitable explanations for the failure of tap-changer windings in EHV power transformers