Electrical Tree Initiation and Growth in LDPE under Negative HVDC Superimposed with AC Ripples

Understanding the role of power quality in aging of HVDC cable systems is critical to the reliable connection of offshore renewable energy sources, and hence global ambitions to reduce carbon emissions. This work investigates electrical treeing with a needle-plane geometry in low density polyethylene under a high negative DC voltage superimposed with AC ripples (-60 kV DC ± 7 kV AC). Tree initiation showed behavior similar to that widely reported under pure power frequencies. Subsequent tree growth, however, was observed to be confined in a smaller area with limited length and width than seen under pure AC voltages, even after long periods of voltage application. Traditional 2D imaging showed overlapped tree channels which developed to cover the whole area within the tree outline. A distinguishing tree shape resulted, which we have named a ‘slim bouquet’ shape. The volume rendering from XCT showed the structure had a 3D fractal dimension greater than 2, considerably larger than its 2D representation. PD signals during the growth had wing-like PRPD clusters and signal phase concentrations between 10-45° appeared after hours of growth. There was comb-like appearance in maximum PD magnitude variation, which is typical in pure high negative DC fields. Consideration is given to both space charge distribution controlled by high DC fields and continuous degradation by AC fields to explain the slim bouquet tree formation.