Over-voltage Study of AC System at Converter Station of A Multi-terminal UHVDC Project

The over-voltage level directly affects the selection and insulation design of the main equipment in power station. This paper calculates the over-voltage condition of AC lines and equipment in the converter station of Yunnan-Guangxi-Guangdong UHVDC (ultra-high voltage direct current) project. The suppression method and equipment insulation parameters are proposed. The calculation shows that the shunt reactor has obvious effect on limiting the power frequency over-voltage (PFO) of long-distance line. In order to suppress the resonant over-voltage and the secondary arc current, the installation of the neutral point reactor is recommended. The maximum switching over-voltage (SO) can be limited by installing a $400\Omega$ closing resistor to a AC line breaker. The switching over-voltage can be very large during bi-pole blocking at converter station when DC system passively enters into isolated mode. Multiple measures, such as installing metallic oxide arrester (MOA), control and protection optimization, are taken to reduce the overvoltage. The SO is limited below 1.64 p.u. afterwards. The maximum power frequency transient overvoltage level during bi-pole blocking at converter station is calculated as 1.49 p.u., which does not meet the requirements of the over-voltage regulations and should be specified when selecting equipment. The maximum absorption energy of A type arrester is 4245kJ, which is lower than its rated capacity. This paper studies the over-voltage and its suppression method of AC system and provides the basis for the insulation design of transmission line and the selection of main equipment in converter station.