NIE Jingkai ^{1, 2,} HAN Yu ^{1, 2}, ZHOU Mingyu ³, JI Jun ^{1 ,2}, ZHU Zhixiang ³,

LI Yanglin ⁴, MA Jinxu ^{1, 2}, DU Junli ⁵, HE Qiang ^{1, 2}

(1. China Electric Power Research Institute, Beijing 100192, China; 2. State Key Laboratory of Advanced Power Transmission Technology, Beijing 102209, China; 3. Global Energy Interconnection Research Institute Europe GmbH, Berlin DE-10623, Germany; 4. State Grid Jiangxi Electric Power Research Institute, Nanchang 330096, Jiangxi, China;5. State Grid Henan Electric 

Power Research Institute, Zhengzhou 450052, Henan, China)

Extended abstract:[Background and purposes] In extra-high voltage systems, overvoltage and protection have always been one of the key research issues for ensuring the safe and stable operation of power systems. Parallel insertion of closing resistors can convert the L-C oscillating electrical energy in the power grid into thermal energy, which is then dissipated into the external environment, thereby achieving the purpose of suppressing switching overvoltage. Although extensive research has been conducted on materials for closing resistors, the core technology for producing high-voltage closing resistor materials has not been mastered, while there remains a significant gap in key performance indicators between domestically produced resistor discs and foreign products. Carbon-ceramic resistor materials have become the mainstay of high-performance foreign closing resistor products, due to their advantages, such as excellent electrical conductivity, strong pulse resistance, superior overload capacity and efficient heat dissipation. In recent years, research on carbon-ceramic resistor materials has primarily utilized natural minerals as raw materials. However, these raw materials have complex compositions, making the reaction processes during calcination relatively difficult to control. Based on the analysis of a foreign product, a series of samples using high-purity alumina and silica as the main raw materials and graphite as the conductive filler have been designed and prepared.[Methods] Phase composition of the prepared samples was analyzed, while their microstructure is observed and their porosity, bulk density, and electrical resistivity were characterized. Combined with the analysis and test results, the effects of the alumina/silica mass ratio, type of sintering aid, graphite particle size and content of graphite on phase composition and properties of the samples are studied.[Results] When 300-mesh graphite is added at 15% and iron oxide (Fe₂O₃) at 5%, the alumina/silica mass ratio in the range of 2:1 to 3:1 has almost no effect on bulk density of the samples, but significantly influences the open porosity and electrical resistivity. This is mainly attributed to the change in the content of corundum phase with the proportion of alumina. When the alumina/silica mass ratio is fixed at 3:1 and 15% 300-mesh graphite is used as the conductive filler, and the effect of sintering aid types was studied.[Conclusions] As compared with Fe₂O₃, CaCO₃ is more conducive to promote the transformation of quartz to cristobalite and the formation of mullite. Samples prepared with 3% CaCO₃ as the sintering aid are mainly composed of corundum, mullite, cristobalite, and graphite. When the alumina/silica mass ratio is fixed at 3:1, 3% CaCO₃ is used as the sintering aid, 11%–15% of 800-mesh graphite are added as the conductive filler, composite resistor materials with corundum, cristobalite, mullite, and graphite as the main phases are successfully prepared. The prepared samples exhibit an open porosity of 33.2%–35.5%, a bulk density of 1.99–2.10 g·cm⁻³, and an electrical resistivity of 0.11–4.66 Ω·m. The research results of this paper provide data support for the study of carbon-ceramic composite resistor materials.

Key words: graphite; ceramic; phase composition; resistive material; resistivity