Impact of Surges on Converter Transformer Insulation and Diagnostic Methods: A Survey

Abstract

This paper presents a review on sources and evaluation of converter transformer winding insulation failure under transient and repetitive overvoltage phenomenon. The failure of insulation of converter transformer by accumulative effect due to transient and repetitive overvoltage produced new challenges in the reliable and stable operation of the power system. The review work has been presented with the main focus on winding insulation degradation under composite stresses which have been considered as a major cause of converter transformer failure in HVDC systems. The evaluation of insulation performance and cumulative failure probability can be identified based on voltage –numbers (V-N) characteristics and the Weibull distribution model. The various factors such as amplitude, the time interval of successive impulses, the applied polarity of impulse, etc. produced a considerable impact on the degradation of transformer insulation. Lumped parameter model is the most suitable laboratory method providing excellent discriminative features to identify insulation degradation.  Thus, in this review, the failure of transformer winding and also accumulation effect on the insulation of the converter transformer when subjected to transients and repetitive impulses is discussed.