Oil and Paper Insulation for DC Converter Transformer

Introduction

In recent year, with the development of the worldwide power industry and voltage levels, the capacity and level of power transformers are increasing. The HVDC system is becoming very popular and its safe operation of high capacity converter transformers is related to the security of entire grid. Meanwhile, transformer oil and paper are vital essential parts of EHV and UHV transformers, and oil–paper insulation is also commonly used in large power transformers. As a bridge between AC and DC transmission systems, the valve side of the main insulation in the converter transformer under normal conditions should have considerable ability to withstand the AC and DC voltages. Due to the imbalance of permittivity and resistivity in mass dielectrics such as the transformer oil, pressboard, and others, the interface between the dielectrics will accumulate charge (Wada et al., 2006; Yang et al., 1997; Xu et al., 2016; Hanaoka et al., 2002). When the power flow of the system is reversed or a sudden outrage occurs, the local electric strength of the dielectrics will have a serious distortion, most likely leading to the occurrence of discharge or insulation damage. Owing to the fact that the flashover breakdown field strength along the oil–paper interface direction is lower than the bulk field strength which is vertical to the interface, surface discharge can occur easily, and will cause power failures. So the effect of the interface charge on oil–paper insulation becomes the topic of focus (Huang et al., 2014).

Oil–paper insulation dielectrics are complex combinations of solid–liquid two–phase morphology, and the injection, transport, accumulation, attenuation characteristics and mechanism of space charge are not entirely clear yet. These characteristics are closely associated with aging and damage mechanisms and directly affect the variation in electrical performance and insulation life of oil–paper insulating dielectrics. Direct-fluorination is a method to modify the chemical constituents of the material which possess the advantage of alternating the surface property only and leaving little modification on the essential property of the matrix, and thus, has been widely applied in many areas (Yi et al., 2015; Saulnier et al., 2013; Charlet et al., 2015; Prorokova et al., 2015; Lee et al., 2009). In recent years, researches have focused on the paper applied in transformers for improved dielectric properties (Chen et al., 2014; Zhuge et al., 2015). However, most studies on direct-fluorination have concentrated on the chemical stability, wettability, mechanical strength, etc. Few researches discussed the effect on electrical property after the treatment. In recent research, efforts have been paid on the surface modification of insulation material for improvement on the dielectric property.

Moreover, the oil applied in transformers is highly refined mineral oil which performs dual functions of insulating and cooling materials. The oil fills up the pores in the fibrous insulation thus affecting the dielectric property of oil-paper insulation system and increasing the breakdown strength. For thermal property the oil carries away the heat generated during normal operation by conduction and convention (IEEE, 1995). The miniaturization and higher voltage of electric devices have brought new challenge to traditional transformer oil. New type of transformer oil with enhanced dielectric property and high thermal conductivity is in urgent need. In recent years, many researchers have been concentrated on preparing nano-oil by dispersing nanoparticles into transformer oil and measuring the electrical property (Mansour et al., 2012; Atiya et al., 2015; Du et al., 2012; Zhong et al., 2013; Liu et al., 2011; Lv et al., 2014). However, most studies on nano-modified transformer oil have been limited to the dielectric property or thermal property only. Very few researchers have considered developing a type of nano-oil with improved properties of both breakdown strength and thermal conductivity, and investigating the relation between the thermal conductivity and the dielectric property.