Valve section capacitance for 660kV HVDC converter valves

HVDC (High Voltage Direct Current) transmission is considered as a mature technology to meet the demand for the bulk power transmission over long distances and provide flexibility for the network interconnections. For ultra-high voltage HVDC applications, the significant effects of the stray capacitance on the voltage imbalance for thyristor converters have to be considered during the transient conditions. Stray capacitances of the valve configuration were obtained using the proposed analytical methods. The valve section capacitors are applied to improve the voltage sharing between different parts of the series-connected valve components at steep front impulse voltage type test conditions. The optimum valve section capacitance for a 660kV HVDC transmission system has been investigated using an EMTDC simulation-based system. The simulation 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, and the application of the valve section capacitor have been verified by the type tests for the valve design.