Soft Single-Pole Reclosing Control for MMC-Connected AC Transmission Lines

In high-voltage systems, transmission lines typically use delayed-accelerated relays for fast reclosing during non-permanent faults. Yet, connecting these lines to MMCs can pose security risks due to the control strategy, while adaptive reclosing approaches only target permanent faults. To ensure security during non-permanent faults, single-pole reclosing is often blocked, albeit at the expense of parallel system stability. This study proposes a soft single-pole reclosing control to address this challenge. First, a mathematical analysis identifies the root cause of reclosing inrush, considering two controllable variables among the four influencing factors. These variables are selected to optimize both control dimensions and performance. Then, deep reinforcement learning is introduced to drive the MMC during open-phase operation, which completes a comprehensive regulation within several control cycles. Simulation and experimental studies validate the performance, indicating that the SSRC not only limits the reclosing inrush but also keeps the power system's normal operating.