PEA–Kerr Combined Measurement Method for Space Charge and Electric Field Distribution of Oil–Paper Composite Insulation

The distribution of space charge and electric field of oil–paper composite insulation is of great importance to the operation and design of converter transformers. However, the widely used pulsed electric-acoustic (PEA) method and Kerr method are difficult to measure the structure of thick pressboard + oil independently. In this article, a PEA–Kerr combined measurement method is proposed. It can not only compensate for each other’s shortcomings but also correct the effects of ultrasonic attenuation on space charge measurement by combining the two measurement signals. By integrating and optimizing the system structure and signal processing methods of PEA and Kerr measurement, the PEA–Kerr combined measurement system is built. The test results show that the system has good linearity, the spatial resolution of the electroacoustic measurement part is 8.8%, and the maximum error of the electrooptic measurement part is 2.5%. Finally, the system is used to measure the structure of 2-mm pressboard + 3-mm oil gap. The relationship between the distribution of space charge and the interface charge density under different voltage amplitudes and polarities is analyzed, and the influence of electric field distortion caused by space charge on the interface charge measurement of oil–paper composite insulation is revealed. This article can provide a basis for measuring and analyzing the distribution of space charge and electric field on thick and complex structures.

I. Introduction

HIgh-voltage direct current (HVDC) transmission technology has outstanding advantages in long-distance and large-capacity power transmission and has been greatly developed in recent years. Converter transformer is one of the core equipment in dc transmission system, with oil–paper composite insulation as the main insulation material, which shall always withstand the effects of dc electric field during operation. Under the dc electric field, the oil–paper composite insulation material faces an outstanding problem, namely, the accumulation of space and interface charge, which will cause severe electric field distortion, partial discharge, and even breakdown, thus posing a great safety hazard. Therefore, it is important to study the space charge accumulation characteristics and electric field distribution characteristics of oil–paper composite insulation under dc voltage.