Experimental Evaluation and Tests on Ester oil for Non-vacuum Tap Changer

Ester oil filled On Load Tap Changers (OLTC) are introduced in Indian power systems since the last four years. A series of focused experiments done at the authors’ R & D Laboratories prove that ester oil has superior dielectric strength in both uniform and nonuniform fields, considering both pre-breakdown and breakdown processes, when compared with mineral oil. Special test cell and special electrodes were designed to create varying nonuniformities for studying the effect of nonuniformity on the voltage breakdown characteristics. The results indicated satisfactory performance of ester oil filled OLTC for nonuniformities that normally exist in every high voltage product. The current and service duty tests done on many ester oils filled OLTCs indicated satisfactory performance. The paper discusses the experimental study conducted on the natural ester oils and also on the ester oil filled OLTC & concludes that a well-designed OLTC with ester oil provides good performance in the field.