Protection of Hybrid VSC-MTDC System Against DC Fault

• Author / Creator

  Zhang, Jialin

• Nowadays, the recent trend of high voltage direct current (HVDC) industry is to employ modular multi-level voltage source converter (MMC-VSC) and to build multi-terminal DC (MTDC) systems. The MMC-VSC technology allows VSCs to achieve a higher power rating and the modularity of VSCs provides more flexibility. MTDC systems offer more flexibility and expandability compared with individual point-to-point HVDC connections. It is possible that different types of converters from different manufacturers will be working together in a future MTDC system. In general, MMC-VSCs can be divided into two main categories, full-bridge (FB) VSCs and half-bridge (HB) VSCs depending on the backward current blocking capability. The two kinds of converters have different fault transient responses and require different protection procedures. This thesis focuses on the protection of hybrid MTDC systems that contain VSCs with different fault blocking capabilities against DC faults.
  The proposed protection algorithm considered the impact of using the two types of converters in one MTDC system. The DC fault protection algorithm includes a selective fault detection and isolation algorithm and a universal restoration algorithm. The fault detection unit uses DC current derivatives as the criteria to detect potential DC faults. When a pole-to-pole fault occurs, converter-level controllers would block the converters immediately if the preset current derivative thresholds are exceeded. The central-level controller compares the current directions of all the transmission lines and locate the fault. In pole-to-ground fault situations, the central-level controller receives the current measurements from all the converters and determines a pole-to-ground fault by current magnitude analysis. The central-level controller can also analyze the current magnitudes of all the transmission line terminals and locates the pole-to-ground fault.
  After the fault isolation, the healthy part of the MTDC system should be restored quickly. A universal restoration algorithm that fits MTDC systems with different types of converters is proposed, which is simpler and faster compared with the ones proposed in the literature. All the converter stations are controlled individually during the restoration process, avoiding any communication delay. First, fault clearance test is performed by closing an AC breaker which is connected to a HB converter. Second, DC voltage-controlled converters are restored when the voltage of the DC grid reaches the preset values. Finally, the rest of the converters are restored when the DC terminal voltage reaches a stable value. With the proposed restoration algorithm, the four-terminal MTDC test system of this thesis can restore its steady-state operation in one second after isolating the DC fault.

• Subjects / Keywords

  • DC fault
  • Restoration
  • VSC-HVDC

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R36W96R5V

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.