Numerical analysis on space charge and AC-DC combined breakdown strength in polyethylene

In high voltage direct current (HVDC) power transmission systems, electrical equipment at the valve side usually withstands complex stresses like AC-DC combined voltages. Understanding the charge migration and accumulation characteristics under complex stresses and their influences on the breakdown strength are of great significance. This paper presents an in-depth simulation study on the AC-DC combined breakdown strength using a modified bipolar charge transport model, which takes the deterioration of materials into consideration. Firstly, the simulation results of charge profiles under AC voltages agree well with experimental results, which partly illustrates the appropriate selection of parameters. Then the AC breakdown strengths with different ramping rates and frequencies are studied. Charges accumulated relatively close to the surface of samples can cause severe distortion of the electric field after the change of voltage polarity. After that, the breakdown voltages with different ratios of the AC to DC component are calculated and analyzed, especially on how AC and DC component influence the charge migration and accumulation during the evolution of the aging process eventually leading to breakdown. The simulation results of AC-DC combined breakdown voltages show good agreement with previous experimental results.