A number of DC 50 Hz protection malfunctions have occurred in recent years in China Southern power grid due to no-load closing of the converter transformers. The main cause is that the sympathetic inrush current of the operating converter transformer contains abundant positive-sequence second harmonic

which has been transformed to 50 Hz harmonic by converter. This paper analyzes the impact of the DC operation conditions

such as the AC system voltage and DC transmission power

on the tap of converter transformer and AC filters

which changes the positive-sequence second harmonic of sympathetic current finally. It also analyzes the imbalances of positive-sequence second harmonic of the double circuit DC transmission lines from the view of the positions of poles

current interconnection and so on. To avoid the malfunction of 50 Hz protection

some suggestions are proposed

such as decreasing the impedance of AC system

increasing the voltage of AC bus

limiting the transporting power of running pole

etc. The sympathetic inrush current of the other DC transmission line on the same pole as no-load closing pole should be concerned seriously.