Development of a transformer oil tester: Capacitance a testing criterion

Transformers are used extensively for power generation and distribution. Core and winding of these transformers are immersed in mineral oil that works both as an insulator and coolant. But as transformer continues service, its oil is subjected to degradation and should be checked regularly. When a potential difference exists between the winding and tank, which are separated by the dielectric- oil, transformer acts as a capacitor. In this paper, influence of oil degradation level on transformer capacitance is analyzed. Based on analysis and mathematical modeling, correlation between power rating of unused transformer and its capacitance is established. Then a non-destructive oil testing kit is developed which utilizes the obtained relation to determine the insulating oil quality based on the measured value of transformer capacitance. This device is tested on several transformers and results suggest that power transformer capacitance is a potential indicator of its oil quality.