Abstract
The valve-side single-phase-to-ground (SPG) faults in the half-bridge modular multilevel converter (HB-MMC)-based bipolar high-voltage direct-current (HVdc) systems induce some special fault consequences: overvoltage on the submodule (SM) capacitors in converter upper arms and nonzero-crossing currents in the grid-side ac circuit breaker (ACCB). In this letter, a protection strategy based on converter-embedded double-thyristors is proposed to address these issues. The double-thyristors are installed in parallel with each SM in the lower arms of the converter. By triggering the double-thyristors, the SPG fault is converted into a three-phase short-circuit that ensures current zero-crossings in the grid-side ACCB. Moreover, as the voltages of converter ac buses are clamped by the ground through the triggered double-thyristors, the upper arm overvoltage is mitigated. The effectiveness of the proposed protection strategy has been verified in a bipolar HB-MMC HVdc link built in PSCAD/EMTDC.
Highlights
T HANKS to the extensive research and field applications in the past years, modular multilevel converter based high-voltage direct-current (MMC-HVdc) technology is getting mature and has been deployed in many projects including multiterminal HVdc networks [1], [2]
Current zerocrossings will quickly appear in grid-side ac circuit breaker (ACCB) due to the symmetrical short-circuit created by the triggered double-thyristors
A converter embedded double-thyristor-based protection strategy was proposed to address the problems caused by valve-side SPG faults in bipolar HB-MMC HVdc systems
Summary
T HANKS to the extensive research and field applications in the past years, modular multilevel converter based high-voltage direct-current (MMC-HVdc) technology is getting mature and has been deployed in many projects including multiterminal HVdc networks [1], [2]. According to the studies in [6]–[8], the valve-side SPG faults in bipolar HB-MMCs will lead to grid-side nonzero-crossing currents and overvoltage of the submodule (SM) capacitors in converter upper arms. Li et al [8] employ an LR circuit as the dc-side grounding of the HB-MMC to create grid-side zero-crossing currents. One mixed-SM MMC proposed in [6] uses full-bridge (FB) SMs in the upper arms and HB-SMs in the lower arms This topology can mitigate the upper arm overvoltage, the gridside nonzero-crossing currents still exist. Current zerocrossings will quickly appear in grid-side ACCB due to the symmetrical short-circuit created by the triggered double-thyristors. Given that the ac bus voltages are clamped by the ground through the triggered double-thyristors, the upper arm capacitor overvoltage will be mitigated as well. The proposed protection scheme is compact and cost-effective compared with other solutions
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