A new scheme of zero-sequence overcurrent protection of converter transformer based on zero-mode inrush current characteristics in station area

Under the development trend of AC/DC hybrid power grids, converter transformers are indispensable as AC/DC conversion equipment, whose safety and stability are particularly important. There have been many serious accidents where the zero-mode inrush current(abbreviated as ZMIC) of high-impedance transformers caused the zero-sequence backup protection of the transformer and even the upper-level line to malfunction. As a type of high-impedance transformer, the converter transformer may cause the existing zero-sequence protection(abbreviated as ZSP) to malfunction due to the ZMIC generated during no-load operation. However, its winding connection form and no-load operation method are different from ordinary high-impedance transformers. There may be sympathetic interaction between multiple transformers. So, it is necessary to analyze the ZMIC separately. Through the existing ZMIC equivalent circuit of converter transformer, the amplitude relationship and attenuation characteristics of four bipolar converter transformer ZMIC are clear, which lay the foundation of the following protection scheme. The fact that ZMIC may cause the zero-sequence overcurrent protection to malfunction is found through PSCAD (Power Systems Computer-Aided Design) simulation. An inrush current identification scheme based on the station area information of the relationship between zero-mode inrush fundamental wave and harmonic amplitude is proposed. Considering the system resistance, closing angle, CT saturation, and fault conditions, the effectiveness of the proposed scheme and its superiority compared with the existing inrush current identification methods are verified.