DC faults can easily lead to overcurrent in DC distribution networks; these faults pose serious threats to the safe operation of the system. The blocking of modular multilevel converters based on the full-bridge sub-modules (FBSM-MMC) is mostly utilized to cut off the fault current. However, the blocking causes short-term blackouts in the entire DC distribution network and there are presently no effective solutions to address this problem. In this study, an integrated control and protection scheme based on the FBSM-MMC active current limiting strategy is proposed. The project includes three stages: first, MMC active current limiting strategy is used to limit the output current of the converter to about 1.2 p.u. after the occurrence of the fault (Stage 1); next, faulty lines are identified based on the asynchronous zero-crossing features of the DC currents of the two ends of the line (Stage 2); then, a fault isolation scheme based on the cooperation of converters, DC circuit breakers, and high-speed switches is proposed to isolate the faulty line (Stage 3). The distribution network can restart quickly via control of the converters. Finally, the simulation of a four-terminal flexible DC distribution network in PSCAD/EMTDC demonstrates the effectiveness of the proposed integrated scheme. Index Terms-Flexible DC distribution network, full-bridge sub-modules, converter active current limiting control, current zero-crossing detection, integrated control and protection scheme.
To improve the reliability of power supply, reclosing schemes are required after transient faults, which commonly occur in overhead line based high voltage DC (HVDC) systems. However, in the event of permanent faults, the auto-reclosing scheme may cause a severe strike. To avoid the severe impacts caused by permanent faults, the fault type should be discriminated before activating the reclosing scheme. Therefore, an adaptive reclosing scheme based on phase characteristics is proposed in this paper. Firstly, the modulation of a periodic voltage by actively controlling the hybrid DC circuit breaker (DCCB) is introduced. Then, a cascaded π equivalent model and its decoupling algorithm are presented to analyze the frequency-domain characteristics of the measured impedance of the coupled overhead lines. From the frequency-domain characteristics, the frequency of the periodic detecting voltage is determined to analyze the phase features of the measured impedance at primary frequency. The permanent or transient faults can thus be accurately identified by using these different phase characteristics, with negligible influence on the healthy lines. In addition, the proposed scheme is robust to various fault resistances, leading to improved reliability. The effectiveness of the proposed scheme is verified in PSCAD/EMTDC.
The medium‐voltage DC distribution network with multiple DC feeders is more cost‐effective when equipped with switches than DC circuit breakers. Under such conditions, the converters with fault‐handling capacity are blocked to clear the fault, which will cause the rapid disappearance of the fault current. Therefore, the primary protection has to pursue high speed at the expense of reliability, which threatens the network's safety. To construct a reliable protection system, backup protection with an accelerating strategy based on the fault control of converters is proposed. The current is controlled first by the fault control of converters to ensure the fault characteristics during backup protection. Then the DC switches on the healthy line are blocked by the polarity of the current integral, and the distribution network turns into the close‐loop mode to provide a fault current path. When the DC switches on the faulty line disconnect according to the predefined time delay, the current change time is recorded and thus the switches in the network are accelerated without communication. The proposed scheme only depends on the measured local current and the backup function still exists after acceleration. The effectiveness of the proposed scheme is verified in power systems computer aided design/electromagnetic transient including DC (PSCAD/EMTDC).
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