Electrical Tree and Partial Discharge Characteristics in Silicone Rubber Under Lightning Impulse Superimposed DC Voltage

The cable joints in high voltage direct current (HVDC) systems are mostly made from silicone rubber (SiR) and possess excellent electrical and mechanical properties. Moreover, they are easy to install and resilient to oxidization. However, HVDC cable joints may experience strain from external and internal transient sources, such as lightning and switching impulses, which could cause severe damage to the HVDC system. To date, the effect of the strain from lightning impulse (LI) superimposed DC voltage to HVDC cable joints, especially SiR, has remained uncertain and requires further study. Thus, this study investigates the electrical tree and partial discharge (PD) characteristics in SiR under 1,000 shots of LI superimposed 4 kV DC. The LI voltages of 15, 18, and 21 kV of positive and negative polarities were applied to the needle electrode superimposed 4 kV DC (50% of PD inception voltage [PDIV]). Results reveal that the LI superimposed DC can initiate PD and electrical tree in SiR even if the DC voltage is lower than the PDIV. The inception of PD and electrical tree have a polarity dependence of LI voltage. The magnitude and numbers of LI superimposed DC voltage for both polarities have a considerable effect on the tree progression and local electric field (EL) at the end of the tree tip. The branch type tree with a long and broad tree structure is typically observed at 1,000 shots of positive LI. Nonetheless, the tree structure appears in the branch type tree with a short tree length and tiny tree width under negative polarity. In addition, PD pulses are mostly triggered by dominant magnitude at the rising front of the superimposed waveform for both polarities and occur prior to tree inception. Single and numerous PD pulses are observed during positive and negative LI superimposed DC, respectively.