Life evaluation method of cast-resin transformer by tan delta measurement: a cast study

The life cycle of an electrical equipment is determined by the life cycle of dominant components. “Electric Facility Diagnosis and Diagnosis Technology Report” published by Society of Electrical Cooperative Research of Japan suggested solid insulation components as key components that determine the lifetime of a cast-resin transformer. Solid insulation components in electrical equipment requires electrical insulation and structural support performances and exhibits irreversible degradation characteristics due to various stresses during its operation unlike gas or liquid insulation. The life cycle of cast-resin transformers is 15-20 years proposed by JEMA (Japan Electrical Manufacturers' Association) and 20 years proposed by KESCO (Korea Electric Safety Corporation) despite the lack of technical evidence to support. In the past, failures in cast-resin transformer have primarily caused by poor manufacturing skill, but now environmental factors and deterioration are the main causes of failure of cast-resin transformer. It is necessary to redefine the life cycle of cast-resin transformer. Cast-resin transformers in operation have limited test and inspection methods compared to oil-immersed transformers due to their structural characteristics, so only a few measurement tests are allowed such as visual inspection, temperature measurement using thermography, vibration and partial discharge, but there is insufficient technical basis for their criteria. Doble Engineering, a measurement device manufacturer, presents the evaluation method through tan delta measurement and some organizations refer to the criteria. The tan delta measurement is useful method to evaluate the overall insulation condition and to indicate the degradation of the cast-resin transformer though the assessment criteria need to be verified because of the insufficient amount of field data by the limited conditions to test cast-resin transformer in operation. This paper describes that the lifetime of the cast-resin transformer is determined by an epoxy insulation, the lifetime of the cast-resin transformer is derived through accelerated degradation tests of the epoxy insulation, and the tan delta measurement method is used to evaluate the degradation of the cast-resin transformer. This paper also suggests case studies conducted to verify this method.