Contents 1 Introduction
Transformers and converter transformers are important equipment for power transmission systems. Oilpaper insulation is the main form of insulation used in large power transformers. The main component of insulation paper is cellulose. With continuous development of the electrical industry, the demand for cellulose insulation materials is ever increasing [1]. The pressboard block is a composite insulation structure made of cellulose pressboard insulation and adhesive. As one of the key parts of the oil-paper insulation system, the pressboard blocks exist in the transformer windings between disks, at the coil ends, the bottom of the iron yoke and other parts, and play a key role in terms of mechanical support, electrical insulation, and oil channel insulation [2]. Although there is little evidence from literature that the adhesives used in the pressboard block will directly cause insulation failure, lots of research results show that the type of adhesive and the orientation of the adhesive layer have an influence on the insulation performance of the pressboard block [3]–[5]. Since the 1980s, a variety of adhesives, including casein, phenolic resin, polyvinyl alcohol, and polyvinyl acetate, etc., have been used in the manufacture of pressboard block. Currently, pressboard blocks made of casein and phenolic resin adhesives are widely used in power transformers in China, but their influencing characteristics and mechanisms on insulation performance are still unclear. Considering that partial discharge is the main indicator to characterize the insulation performance of oil-paper insulation, clarifying the influencing mechanism of different adhesives on partial discharge is an urgent problem to be solved in order to promote the application of adhesive and cellulose insulating materials in electrical industry.
