Power operation region modeling and transient overvoltage suppression strategy of hybrid cascaded HVDC

The hybrid cascaded HVDC system(HC-HVDC) in sending end system is composed of an Line Commutated Converter(LCC) connected in series with a parallel group of Modular Multilevel Converters (MMC). This system offers high flexibility and strong adaptability to weak systems. However, since the commutation failure (CF) is easy to occur at the inverter side in the LCC, which can lead to the transient overvoltage at the rectifier side. The mechanism and voltage characteristics of the transient overvoltage are not yet clear. To handle this problem, firstly, the mechanism of transient overvoltage caused by CF is analyzed. Then, the power operation region of the sending end HC-HVDC considering transient overvoltage constraints is established. Based on this analysis, a transient overvoltage suppression strategy is proposed. It determines the increment in reactive power consumption of MMC and LCC based on different power operating points. The reactive power surplus at the sending end system can be reduced, and the transient overvoltage can be suppressed. Finally, a simulation model in PSCAD/EMTDC platform to verify the mechanism of transient overvoltage and the transient overvoltage suppression effect of the proposed strategy in the situation of different fault types, fault duration, fault severity and short circuit ratio is verified.