Potential Compensation Method for Restraining the DC Bias of Transformers During HVDC Monopolar Operation

The earth return currents (ERCs) of high-voltage direct current transmission can cause severe half-cycle saturation of transformers. This study presents a novel and highly beneficial potential compensation method (PCM) to mitigate the detrimental effects of ERCs. The simulation model of the PCM was first derived and verified through a laboratory test. Analysis has suggested an association of network configuration and mutual resistance to the compensatory factor. The proposed method has been applied in two representative substations with different types of transformers in various operating modes. Results show that as opposed to the neutral blocking device (NBD), the PCM can adjust the dc voltage equilibrium of ac systems rather than intervene in the neutral path of transformers. Moreover, the PCM can eliminate the direct magnetomotive force of autotransformers with proper selection of the injected current. An application shows that the PCM can outperform NBDs and line-series capacitors in restraining the dc bias of autotransformers.