Commutation Failure Immunity Mechanism and State-Aware Coordinated Protection Strategy for SLCC-HVDC in Weak Receiving-End Grids

In weak receiving-end grids, the active support of the valve-side static var compensator and filter (SVF) extends the commutation failure (CF) boundary of LCC-HVDC. However, SVF control state transitions reshape valve-side voltage and harmonic characteristics, causing conventional fixed threshold protection to exhibit concurrent misblocking and failure to operate risks, while SVF zone internal faults are prone to excessive pole-level escalation. This paper proposes a state-aware coordinated protection strategy for symmetric single-pole SLCC-HVDC systems. A normalized commutation margin index, derived from the commutation voltage time integral, characterizes the nonlinear CF boundary under SVF support. SVF control mode, health status, and reactive power margin serve as conditioning variables for adaptive threshold and time window modification. A three-level escalation strategy—local isolation, derated ride-through, and pole-level action—is further designed for SVF zone faults. Validation via RTDS sequence of events records and EMT–protection logic replay co-simulation shows that the proposed index achieves a 100% CF risk prediction rate across five fault scenarios, versus 40% for conventional indices. The method maintains zero failure to operate with a misblocking rate ≤ 10.1% at SNR ≥ 30 dB. The staged response correctly escalates all four SVF zone fault types to the required level, compared with two of four for the fixed threshold baseline. These results confirm effective enhancement of protection robustness, fault ride-through capability, and operational continuity for SLCC-HVDC in weak receiving-end grids.