Modeling of Space Charge Accumulation in Oil-Paper Insulation of Converter Transformer Under Polarity Reversed Voltage

Converter transformers serve as critical components within HVDC transmission infrastructure. Under operational service, their oil-paper insulation systems are subjected to multifaceted electrical stresses, including alternating current excitation, direct current biasing, superimposed AC/DC excitation, and transient polarity inversion events. Among which polarity reversed voltage is the worst case where large amount of space charge could accumulate along the oil-paper interface, lead to the distortion of local electric field, and increase the risk of complete breakdown, eventually affect the overall reliability of the insulation system. Therefore, the accurate calculation of space charge amount in the oil-paper insulation is predominant for the insulation design of converter transformer. However, most of the current research on space charge in oil-paper insulation focuses on experimental observations, where the dynamics of space charge pattern and distribution are difficult to be obtained. To solve this problem, a two-dimensional axisymmetric finite element simulation model is constructed in this paper based on the bipolar charge transport equation for simulating the accumulation process of space charge. The whole simulation is divided into three stages: the stage of applying positive DC voltage from 0 to 1200 s, the stage of reversing voltage polarity from 1200 to 1210 s, and the stage of applying reverse DC voltage from 1210 to 3010 s. The simulation results are finally compared and analyzed with the experimental measurements made on naphthenic oil-immersed paper (NOP) and paraffinic oil-immersed paper (POP) under the same conditions. Simulation results in this paper can be reference for the optimal design of oil-paper insulation in converter transformer.