Partial Discharge Characteristics in Liquid Nitrogen /Polypropylene-Laminated-Paper Insulation System under an DC Superimposed AC Voltage for HTS Cable

This paper aims at studying the partial discharge (PD) characteristics of the LN
₂
/PPLP insulation system under DC superimposed AC voltage for high temperature superconducting (HTS) cables. The pulse current method was adopted to measure the PD characteristics under the superimposed voltage with different DC components. The voltage virtual value (V
_rms
) of AC-DC superimposed voltage is set as 7 kV to analyze the effects of the DC component on the phase distribution of discharges times and amplitude. The experimental results show that: the number of discharges and the discharge amplitude decreases with the increases of DC component. According to the distribution characteristics of the phase spectrum of discharges times, the discharges with different ratio of DC components can be divided into three stages. The partial discharge pulse repetition rate is highest when AC voltage was applied. With the increase of DC components of the superimposed voltage, the total discharge times and the maximum discharge amplitude decrease, together with the widening of the PD phase distribution in stage I and II. When the DC component is up to 6 kV, the discharge times is minimized, and the PD is concentrated in the first quadrant in stage III. As a result, with the increase of the proportion of positive DC in the AC-DC superimposed voltage, the total discharge times and discharge amplitude decrease throughout the process of the PD. As the positive DC proportion increases, the discharge pulse in the positive period gradually exceeds that in the negative one.