Modern insulation condition assessment for instrument transformers

Instrument transformers in service are exposed to the harming effects of water and ageing. The increasing age is leading to a higher risk of explosions, which can damage surrounding parts and cause high follow-up costs. This paper introduces a new approach to use the dielectric response analysis for condition assessment of instrument transformers. The measurement of the dielectric response in a wide frequency range provides information about the insulation condition and especially for oil-paper/pressboard insulations about the water content in the solid insulation. The measured curve is formed by the superposition of the dielectric response of the cellulosic material and the oil. The curve can be used for the assessment of the insulation condition in various ways. Qualitative comparisons, e.g. between instrument transformers of the same type and age, might give general information which of these is in better condition. The dissipation factor and the capacitance trend can also deliver valuable data. Furthermore the water content in the solid insulation can be assessed using an automatic analysis algorithm. To confirm the validity of this assessment, measurements on several instrument transformers in service were performed. Dielectric response curves and the trend of the capacitances for new and aged instrument transformers with different water contents are discussed. Selected examples presented in this paper show, that the dielectric response is well modeled, the water content can be derived and ageing effects can be observed.