A new protection method for UHV converter transformers suitable for no-load closing andinter-turn faults

The ultral high voltage direct current (UHVDC) transmission system has a high voltage level and is a mixed AC and DC connection, which makes the coupling relationship between them more closely, so that the converter transformer has different characteristics from ordinary transformers in the case of excitation inrush current and internal faults, which causes traditional protections cannot be well identified. In this paper, according to the Spearman coefficient and the time coefficient of absolute value of zero-sequence current, a new method for identifying excitation inrush current of UHV converter transformer is proposed. This method first calculates the Spearman coefficient of the absolute value waveforms of the excitation inrush current and the internal fault in the front and back half-week waves. In view of the waveform characteristics of no-load closing with inrush current characteristics under inter-fault, the time coefficient of the absolute value of the zero-sequence current on the grid side is used as the auxiliary criterion. Through PSCAD/EMTDC software simulation, it is proved that the method proposed in this paper can effectively distinguish the excitation inrush current from the internal fault, and can effectively avoid the protection fault braking caused by the no-load closing on the inter-turn fault. Thus, the safety of converter transformer system can get obvious improvement.