Investigations of High-Frequency Overvoltages in Cast-Insulated Windings of Distribution Transformers
Abstract
Over the past 20 years, oil-filled power transformers and dry transformers with air-barrier insulation used for power supply to electric power grid auxiliaries (up to 100 kV·A) in the Russian Federation have been replaced with dry transformers with cast epoxy resin insulation. Unfortunately, field experience shows that some of dry transformers with cast insulation fail before the end of their specified service life. The typical features of these incidents were the selectivity of damaged transformers, repeatability, and a significant service life before failure (sometimes two or more times the warranty period), which excludes the version of a manufacturing defect. The article presents the results of studies carried out on structural samples produced by JSC Sverdlovsk Plant of Current Transformers (SZTT). For revealing the causes of such failures and achieving higher reliability of its products, the manufacturer initiated a set of activities to investigate the causes of transformer damages, improve the winding design, and conduct a comparative analysis of the stability of old and new winding designs to switching overvoltages. Prototype 10 kV voltage class transformers for capacities ranging from 10 to 63 kV·A were studied, including experimental determination of natural frequencies and high-frequency resonance overvoltage ratios in high-voltage windings of various designs. The article provides brief information on the results of the transformer failure investigation and presents the main results obtained from the studies of high-frequency resonance overvoltages in high-voltage cast-insulated windings.
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