Normalized Dynamic Time Warping Distance Based Auxiliary Restraint Scheme for Restricted Earth Fault Protection of Converter Transformers

Converter transformers are one of the most important and costly equipment in High Voltage Direct Current (HVDC) transmission systems. They require sensitive and reliable protections. However, the maloperation of restricted earth fault (REF) protection of converter transformers caused by unloaded energization of converter transformers has been reported. Aiming at this problem, the saturation and measurement distortion of the current transformer on neutral line (CTN) and the resulting maloperation mechanism of REF protection are analyzed firstly. Then, an auxiliary restraint scheme of REF protection of converter transformers is proposed exploiting the similarity between the waveforms of the zero-sequence current on neutral line and the calculated zero-sequence current from measured phase currents under magnetizing inrush and fault conditions. The proposed auxiliary restraint scheme is based on normalized dynamic time warping distance (DTWD), which has strong robustness against CT saturation. The proposed auxiliary restraint scheme is verified by simulations and practical data recorded on-site.