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To identify the fault area more accurately and improve the reliability and sensitivity of the traditional protection principle on DC lines, an active injection for single-ended protection in the DC grid is proposed. Considering the influence of fault impedance, the effect of frequency-dependent parameters, numerical differential error, and the strength of DC boundary, high controllability of hybrid modular multilevel converter is utilized to inject sinusoidal signals with different frequencies into the line at DC terminal of each converter. Then the single-ended fault identification equation is constructed to preliminarily identify the fault area. According to the amplitude difference of the injection signals in different fault areas, the fault areas of the DC grid can be further distinguished. Simulation results verify the effectiveness of the proposed protection principle, improved reliability and sensitivity of protection, and full length protection of line without DC boundary. [ABSTRACT FROM AUTHOR]

Difficulties in identification of the fault property, for a fault on a single outgoing line of windfarm, arise due to rapid decrease in the electromagnetic energy of the transmission line after the tripping of three-phase breaker. An active solution to such problem is proposed based on low current injection from parallel connected static synchronous compensator (STATCOM). In the reclose sequence after wind turbines’ cut off, the circuit breaker near wind farm and grid-connected circuit breaker of STATCOM are reclosed first. Therefore, based on selected switching operation of power electronic devices of STATCOM converter, a short duration discharge of dc capacitance to the faulty ac line is suggested. Correlation coefficient of resulting current waveform is utilized as an identification criterion for fault presence. Finally, a three-phase adaptive auto-reclosing scheme is proposed for the single outgoing line of 110 kV wind farm. The effectiveness and robustness of the proposed scheme are verified by PSCAD/EMTDC simulation. [ABSTRACT FROM AUTHOR]

After the isolation of a single pole-to-ground fault, the adaptive auto-reclosing concept in high-voltage dc (HVDC) systems is difficult to be applied due to little coupling between the poles. With fault ride through capability, the hybrid modular mult-ilevel converter (MMC) can be advantageous for future HVDC projects. This research proposes an adaptive auto-reclosing operation for hybrid MMC-based bipolar HVDC systems, based on fault identification by characteristic signal injection. The proposed method is based on voltage controllability of healthy pole MMC to inject dc voltage perturbations. Healthy line perturbations result in the induced characteristic signals to the faulty line, which contain information about the fault property. An additional control strategy for hybrid MMC, to inject signals with suitable amplitude and frequency, is also discussed in detail. Then, fault identification is realized by wavelet transform of traveling waves caused by the signal injection. Finally, a dc adaptive single-pole auto-reclosing scheme is proposed to shorten the reclose time and to distinguish between temporary faults and permanent faults. Simulation results verify the effectiveness of the proposed method in PSCAD/EMTDC software. [ABSTRACT FROM AUTHOR]

The half bridge modular multilevel converter (MMC) coordinated with dc circuit breakers (DCCBs) is one of the most promising solutions for the future multi-terminal HVdc system protection. After the isolation of dc line fault, the electrical quantities of the dc system remain stable and no coupling relationship exists between healthy and faulty poles. Thus, ac adaptive reclose scheme is difficult to be applied in dc transmission systems since the information of the adjacent lines cannot be used to assess whether the line fault is cleared or not. An innovative adaptive reclose scheme applied to the MMC-HVdc system is proposed in this paper by injecting voltage pulses to the faulty line. When the breaker is tripped, the converter and dc faulty line are connected by the main breaker of the hybrid HVdc breaker. On the basis of switching control of solid-state devices in the main breaker, the voltage pulse injection to the dc faulty line is realized. Then, the fault characteristic is identified based on the traveling-wave-based fault location algorithm. A fast restart scheme for the MMC-based bipolar HVdc transmission system is also achieved by this method. The simulation results verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

• An active fault identification method for passive networks supplied by MMC-HVDC. • The active injection utilized for both adaptive single/three phase auto-reclosing scheme. • Application to single-circuit and multi-circuit transmission lines. • Repeated injections and extra independent assessments can be supplemented to prevent misjudgement. Modular multilevel converter (MMC) shows prominent advantages on supplying passive networks, which are usually classified by power supply shortage and relatively weak structure. In order to reduce the unnecessary outage time and improve the system stability, an adaptive ac auto-reclosing scheme for passive networks is proposed based on characteristic signal injection. Additional MMC controller is designed to inject the characteristic signals into the healthy line, which in turn induces corresponding signals to the faulty line. Then fault properties are assessed by the travelling wave based fault location method. Finally, an adaptive auto-reclosing scheme is proposed to shorten the reclosing time and avoid the second fault shock to a large extent. The proposed scheme could be applied for both single-circuit and multi-circuit transmission lines. The simulation results obtained from PSCAD/EMTDC verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]