Protection CT Polarity Checking Method for MMC-HVDC Converter Station Transformer

Aiming at the difficulty of CT polarity checking of MMC-HVDC converter station transformer, a method for CT polarity checking based on waveform comparison and instantaneous power theory is proposed. The transient current during the process of the converter transformer and the converter valve charging is analyzed, and the polarity of the zero-sequence differential protection and the longitudinal differential protection is checked by comparing the crossing transient current on both sides of the protection based on the longest common subsequence (LCSS) distance. At the same time, the instantaneous active power during the converter valve deblock is used to check the polarity of directional protection. This method can not only check the polarity of longitudinal differential protection and zero-sequence differential protection, but also can check the polarity of directional protection, and the reversed phase of CT can be found out. Moreover, it can withstand a certain degree of ratio error and phase error of CT. The simulation verifies the effectiveness and applicability of the method.