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• The backup protection including fault identification and location methods is proposed for VSC-HVDC transmission lines. • The waveform difference in same/different-mode traveling waves between faulty and unfaulty lines is elaborated in theory. • The protection can identify the double-pole-to-ground fault with unequal positive and negative pole fault resistances. • The waveform difference between 1-mode differential forward and backward currents can locate faults rapidly and accurately. • The methods can improve the protection selectivity, reliability, rapidity, and sensitivity. Due to the low-damping characteristic of the voltage source converter based high voltage direct current (VSC-HVDC) transmission system, the line current will reach the peak value within a few milliseconds after DC fault, threatening the safety of power electronic devices. Therefore, a reliable and rapid protection scheme comprising fault identification and location is necessary. The detection time cost of traditional backup protections is high. Worse still, most existing methods cannot distinguish the double-pole-to-ground fault with unequal positive and negative pole fault resistances. To solve the issues, a backup protection based on the same/different-mode traveling wave, differential forward/backward current, and traveling waveform difference for VSC-HVDC lines is proposed in the paper. The fault characteristics of same/different-mode traveling wave and relationship between fault distance and differential forward/backward current are elaborated. The waveform differences in positive/negative-pole same/different-mode traveling waves and 1-mode differential forward/backward currents are summarized. According to these analyses, the protection is established and the VSC-HVDC is modeled in PSCAD. Based on the sufficient simulation tests, it is verified the proposed fault identification and location schemes have great performance, with superiority in fault resistance and distance tolerance (2,000 Ω and the whole line), strong anti-interference capability, fast fault distinguishment, and accurate fault location. [ABSTRACT FROM AUTHOR]

• The non-unit protection based on the traveling waveform similarity is proposed for the VSC-HVDC transmission line. • The traveling waveform similarity differences between internal and external faults are proved in theory. • The protection scheme can identify the double-pole-to-ground fault with its positive-pole fault resistance unequal to negative-pole fault resistance accurately. • The scheme provides high performance considering the possible influence factors. • The method improves the protection selectivity, reliability, rapidity, and sensitivity. The protection for the voltage source converter based high voltage direct current (VSC-HVDC) transmission lines must detect the faulty DC lines and faulty poles within a few milliseconds, because the fault current rises quickly. The traditional protections are easily influenced by fault resistance and signal noise. What is worse, the double-pole-to-ground (DPG) fault with its positive-pole fault resistance unequal to negative-pole fault resistance cannot be detected accurately. To solve the problems, a non-unit protection scheme based on the waveform similarity and wavelet transform modulus maximum (WTMM) theories for the VSC-HVDC lines is proposed in this paper. A four-terminal VSC-HVDC power network is modeled in PSCAD to verify the protection. Extensive simulations of various fault situations prove that the protection scheme can detect high-fault-resistance, DPG, and remote faults. Moreover, it is insensitive to the sampling frequency, signal noise, smoothing reactor, and start delay. Furthermore, it can meet the rapidity requirement. Therefore, the proposed protection can identify the DC faults accurately, which minimizes the disturbance to power grids and improves system security and stability. [ABSTRACT FROM AUTHOR]

The voltage source converter based high voltage direct current (VSC-HVDC) technology is a feasible solution to realize wide-area complementarity and flexible consumption of the renewable clean energy resources but DC line faults are crucial problems. The traditional traveling wave protection for HVDC transmission lines are easily affected by the fault resistance and signal noise. Besides, it cannot detect the double-pole-to-ground fault. To overcome these problems, a single-ended protection scheme based on the phase-mode transformation and wavelet transform modulus maximum (WTMM) theories for DC lines in the VSC-HVDC transmission system is proposed in this paper. The protection scheme includes the protection startup unit, the internal fault identification unit, and the faulty pole detection unit. Extensive simulation tests carried out in PSCAD/EMTDC verify that the protection scheme has high selectivity in the high fault resistance faults, remote faults, and close-by faults with different fault types. And it does not rely too much on data synchronization. Furthermore, sensitivity analysis and comparisons with other methods indicate that the protection is insensitive to signal noise and suitable for different operational modes. And the effectiveness of the protection is independent of the high sampling frequency. Moreover, it can identify the double-pole-to-ground fault correctly. • A single-ended protection scheme is proposed for VSC-HVDC Transmission Lines. • The method improves the selectivity, reliability, and rapidity of protection systems. • The proposed protection scheme is able to identify the double-pole-to-ground faults. • The different signal noise levels and operational modes do not affect the protection. • The effectiveness of the protection is independent of the high sampling frequency. [ABSTRACT FROM AUTHOR]