In high voltage system

a great harm is caused to the converter transformer and AC and DC power transmission by the DC biasing. The main method to suppress the biasing is current limiting and blocking.At present

the widely used method is series capacitance through the neutral point

but there is little research on the protection of the capacitor. In this paper

the electromagnetic transient simulation platform for 500 k V HVDC is set up by PSCAD/EMTDC

and in case of power frequency overvoltage

the overvoltage of the series capacitor at the neutral point of converter transformer

and also the spark gap of the protection device on both sides are simulated and analyzed

the capacitor with the most ideal capacitive reactance is selected

and the protection of spark gap to the neutral point series capacitor is also discussed. It is shown by the result that the spark gap has the protection function to the neutral point earthing capacitor of converter and the neutral point of transformer.