1. School of Electric Power, South China University of Technology, Guangzhou 510640, China;2. Power Dispatching and Communication Center of CSG, Guangzhou 510623, China

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.