Impact of Nonlinear AC Fault Arc Resistance on Commutation Failures in LCC-HVDC Systems

Commutation failures (CFs) in LCC-HVDC systems are typically caused by inverter-side ac line faults. Historically, since the fault arc resistance has been simplified to a fixed resistance in related studies, possible ac arc-induced bus voltage distortions have not been sufficiently studied. This letter investigates the impact of nonlinear ac fault arc resistance on CFs. Specifically, transient expressions for the commutation voltage-time area (VTA) under different fault types are derived. With a black-box arc model embedded in the analysis of the commutation process, periodic voltage distortions resulting from the nonlinear resistance of the fault arc appear in the faulty phase voltages. Notably, these spike-shaped voltage distortions result in a variation in the commutation VTA under single-line-to-ground faults close to the commutation bus. In addition, these impacts of the nonlinear arc resistance on CFs are verified through extensive simulations on the real-time digital simulator. This study reveals a hidden arc-induced feature that impacts CFs and bridges the gap between research on fault transients and the associated CFs.