Use of Impedance and Phase Measurements to Define Thresholds of Power Transformers Insulating Mineral Oil Capability When Subjected to Successive Discharges

Power transformers represent the most expensive and vital equipments in power distribution and transmission networks. Their real time diagnosis is of great importance in order to ensure them a safe and permanent operating. The performance and reliability of these equipments is directly dependent on the electrical discharges activity that could occur inside the mineral oil that is filled inside them as an insulation and cooling fluid. This paper discusses the effect of applying simultaneous electrical discharges of low energy under AC voltage on the characteristics of the impedance and phase of the insulating mineral oil with frequency ranging from 20 Hz to 10 MHz. The impedance and phase of the oil are measured with a high precision LCR-meter in a point-plane electrode system and a gap of 2.5 mm. The performed measurements provide a well correlation with the degree of degradation of the mineral oil and hence with its capacity and resistivity when submitted to AC high voltage discharges. Threshold values can be defined to judge whether the transformer must be stopped and repaired.