The Effect of the Non-uniformity in Dielectric Material on Spatial Distributions of Electric Field in a Parallel Plain Electrodes with Oil-pressboard Composite Insulation under DC Voltage

The safe operation of converter transformer, whose major insulating structures is comprised of oil-pressboard composite insulation, could be the important guarantee for the reliable power transmission of HVDC system. Due to the presence of DC voltage operating condition, the interface charge accumulation could fail the validity of RC (Resistivity-capacitance) simulation model in the insulation design and optimization. Thus, there has been arising the research focus for the electric field and interface charge characteristics of oil-pressboard insulation structures. Kerr electro-optic effect method, benefiting from the non-contacting laser measuring approach, has been widely adopted for the electric field measurement. However, the common experiment platforms by using Kerr electro-optic effect are mainly based on single-point photodetector indicating. The present paper developed a high-sensitivity photodetector array to accomplish the synchronous and regional measurement for electric field distribution in oil-pressboard insulation with a parallel-plain electrode under DC voltage. The electric fields in oil show a quite uneven distribution characteristics when voltage is applied, and the maximum deviation from different measuring photodiodes could reach 16.6%. With the time prolonging, the electric field in oil tended to decrease, but there witnessed different field strength reduction in different positions in oil, the maximum rate could reach 41.1% with a saturation time of 2960s, while the minimum one is 32.1% and 2530s, respectively. Such witnessed great differences in both distribution and time-dependent characteristics of electric field in different measuring photodiodes were explained by the non-uniformity and anisotropic of resistivity of pressboard. Hopefully, the experiment results in this paper could provide meaningful technical supports for the insulation design of converter transformer.