Adaptive DC-Line Reclosing Strategy for VSC-HVDC Transmission from Desert–Gobi–Wasteland Renewable Energy Bases

Reliable fault-nature identification before DC-line reclosing is essential for overhead VSC-HVDC transmission from desert–Gobi–wasteland renewable energy bases because reclosing into a permanent fault may impose severe secondary stresses on converters and DC circuit breakers. To improve post-isolation observability, this paper proposes a coordinated two-terminal active-injection strategy in which MMCs generate step-voltage probing pulses and DCCB transfer branches provide temporary injection paths to the isolated line. Permanent and temporary faults are distinguished using the sum of the first-reflection time delays because the first reflections originate from the fault point under permanent faults but from the remote terminals after temporary-fault clearance. Binary recursive singular-value decomposition and wavelet modulus maxima processing are used to extract candidate wavefronts. To mitigate wavefront misselection caused by renewable power fluctuations and the associated variations in the pulse-tracking response of the sending-end grid-forming MMC, an operating-state-triggered normalized double-edge reliability criterion is applied to MMC1-side wavefront selection. This enhancement improves the estimation accuracy of Δt1 and reduces the influence of MMC1-side wavefront-selection errors on the subsequent two-terminal time-delay-sum decision. The method is validated on an RTDS model of a 200 km bipolar VSC-HVDC system under multiple fault and operating conditions. Across 720 independent validation cases, the proposed enhancement increases the MMC1-side correct wavefront-selection rates at 8% power fluctuation from 30.0% to 91.1% for permanent faults and from 23.3% to 88.9% for temporary faults, while reducing the corresponding mean absolute time-delay errors by 60.0% and 64.4%, respectively.