Online and offline power reversal methods for bipolar LCC-HVDC systems

The high penetration of renewable energy sources (RESs) has increased the utilization of high-voltage direct current (HVDC) technology for power transfer purposes. However, there are emerging requirements for HVDCs to provide grid flexibility due to the inherent variability of RESs. Line-commutated converter-based HVDC (LCC-HVDC) systems hold the highest installed capacity among all HVDC technologies and are well-suited to offer the much-needed grid flexibility by providing fast and accurate power flow controls. This paper focuses on one of the key aspects in providing such flexibility - power reversal function of bipolar LCC-HVDCs. This paper categorizes the power reversal methods for bipolar LCC-HVDCs into offline and online types and conducts a comparative analysis of these methods from the perspectives of regulating capacity and reversal duration. Both types of power reversal methods are validated using PSCAD/EMTDC. The simulation results show that the online method can provide continuous active power control without a dead-band, which enables greater controllability in terms of regulating capacity and reversal duration compared to the offline method.