Stray capacitance and compensation for the converter valves for ultra-high voltage HVDC application

HVDC (High Voltage Direct Current) electric power transmission system uses direct current for bulk transmission of electrical power, and conventional HVDC is based on thyristor converters. The main components in a conventional converter valve include thyristors, gate units, monitoring units, and cooling components. In this paper, an EMTDC simulation-based valve section capacitance determination system has been investigated for the converter valves for a 660kV HVDC transmission system, and it is used to improve the voltage sharing between different parts of the series-connected valve components at steep front impulse voltage type test conditions. Stray capacitances of the valve configuration were obtained by using the proposed analytical methods and verified by comparing to the results obtained using the Boundary Element Method (BEM). It was concluded that the analytical methods of stray capacitance calculations, which give matching results comparing to the numerical method, can be used for the steep front impulse voltage simulation to assess the requirement for valve section capacitor. The studies confirmed that a minimum valve section capacitance of 3nF was required to restrict the steep front impulse voltage stresses across the critical components of the valve configuration.