Estimation of apparent charge and inception voltage of partial discharges produced in vegetable oil for power transformers

The electrical field causes a series of stresses on the power transformers´ insulation systems, respectively over voltages, surface electrical discharges on the solid components´ surfaces, partial discharges etc. All these stresses (permanent and temporary) are more important for high electrical field strength and they cause insulation´s degradation and loss of life. Among these, partial discharges (PD) are one of the most harmful and, usually, in the case of power transformers, they take place in oil. This paper presents the measurement and estimation of the values of two important parameters which characterize partial discharges produced in vegetable transformer oil: partial discharge inception voltage (PDIV) and apparent charge Q_a. The results obtained with a measuring cell which consists of a point-plane electrodes system indicate that for voltages lower than 20 kV the apparent charge measurement is no longer possible because the values indicated by the measuring device corresponds to the set-up´s noise. Therefore, in order to obtain the PDIV and estimate Q_a for voltages lower than 20 kV, the paper presents a method based on electrical field computation in the measuring cell used for the experimental study.