Volt-VAR secure operational range assessment for preventing initial commutation failures of multi-infeed HVDCs

High-voltage direct current (HVDC) technology has been applied in many countries. Due to the power imported via HVDCs, many synchronous generators have been shut down in receiving-end grids. Under this circumstance, simultaneous commutation failures (CFs) of multi-infeed HVDCs are likely to occur under AC faults that may cause a blackout. Thus, it is crucial to enhance the dynamic var support in receiving-end grids. Many studies mainly focus on the fast action of power electronic dynamic var sources and synchronous machine (SM) AVRs' control, while the spontaneous var output (SVO) of SMs may be underestimated. However, due to the time delay of power electronic and AVR control, initial CFs are still likely to occur under short-circuit faults. To address this problem, this paper analyzes the mechanism of the SVO and its impact on initial CFs. Then, the concept of the Volt-VAR secure operational range (VVSOR) is proposed to ensure the sufficiency of SVOs for preventing initial CFs. An optimization model for evaluating the VVSOR is established accurately without iteration with time-domain simulations. A heuristic solution method based on the decomposition algorithm is proposed. Finally, the validity and efficiency of the evaluation method are verified based on a modified IEEE 39-bus system and a real system in eastern China.