Influence of Polarity Reversal on the dielectric strength of extruded HVDC Cables

The number of HVDC transmission lines has been increasing in recent years due to the need to integrate renewable resources, countries’ cross-border interconnectors, and to reinforce the electricity network. In LCC-based HVDC systems, the power flow direction is governed by reversing the operating DC voltage polarity. In the presence of space charge, voltage polarity reversal can locally enhance the electric field, thereby accelerating the aging of HVDC cable insulation and ultimately leading to premature dielectric breakdown. In the literature there has been a significant focus on lifetime models, space charge measurement and consequently electric field calculation via numerical or analytical methods. However, less attention has been paid to experimental dielectric strength measurements and their deterioration as a result of fast or slow polarity reversal. In this paper, the authors introduce two test setups to perform the polarity reversal on flat samples, and afterwards executes breakdown tests to understand the influence of polarity reversal on dielectric strength and the lifetime of cable insulation. After breakdown tests, statistical analysis has been carried out using Weibull distribution. To define the PR voltage profile, five test parameters were identified, and different values were assigned to a subset of these parameters for the present study. In addition, the effect of constant electrothermal stress, and only thermal stress on the dielectric strength of samples has been investigated. The experimental results have illustrated that high temperature and electric field are the most critical parameters among parameters of PR voltage profile. The combination of both could cause significant dielectric strength reduction in the insulation. Under such conditions, slow polarity reversal resulted in slightly lower dielectric strength compared to a fast polarity reversal. In addition, under constant electro-thermal stress, or under solely thermal stress, increasing the temperature from 25 °C to 70 °C increases the dielectric strength.