Properties of discharges in liquid and simulation using Whitehead equivalent circuit

Liquid insulations are widely used in high voltage equipments. Partial discharges may occur in liquid insulating materials in the form of streamers. For diagnosis purpose, understanding of discharges in the insulations is important. This paper reports experimental results of partial discharges (PD) in silicone oil under divergent AC electric field obtained using needle-plane electrode system. The discharges occurred were measured by using a phase-resolved partial discharge measurement system. The PD parameters such as magnitude, phase angle of occurrence and number were analyzed. The measurement results indicated that at the initiation stage, negative discharges were observed first before positive discharges occurred at a higher applied voltage. The PD patterns showed the strong roles of instantaneous applied voltage on the PD magnitude as well as PD probability. The PD pulses concentrated around the peak of applied voltage. Overall measured data showed a small unbalance between positive and negative PD. For a deeper understanding, a computer simulation of PD patterns in liquid insulating material was done by using Whitehead PD electrical equivalent circuit. The equivalent circuit contains capacitors and a spark gap. The simulated phase-resolved PD patterns were compared to those experimental results. The simulated PD patterns were similar to those experimental results. The results indicated that Whitehead PD electrical equivalent circuit is suitable for representing PD in liquid rather than in solid.