A non-unit transmission line protection scheme for MMC-HVDC grids based on a novel distance criterion

Ultra-fast, selective, and reliable protection is significant for MMC-HVDC grids. The methods utilizing the characteristics of fault current limiting reactor (FCLR) are unsatisfactory to detect high resistance faults. In this paper, a novel distance criterion is proposed to form the non-unit protection scheme. The line-zero-mode fault equivalent circuit is built to analyze the characteristics of fault traveling waves at the inductive terminal considering the influence of MMC and the refraction and reflection at the fault point. A multi-resolution morphological gradient algorithm (MMGA) is employed to extract the geometric features of the fault traveling waves. The magnitude of the first modulus maxima of the line-mode fault voltage after MMGA constitutes the start-up criterion. The estimated velocity and the change direction of the line-mode components are adopted to identify if the fault happens in-region. The 4-terminal +/- 500 kV MMC-HVDC grid built in PSCAD/EMTDC demonstrates that the proposed scheme can accurately identify various faults. The sensitivity analysis shows that the proposed scheme is almost immune to the size of FCLR, the equivalent parameters of MMC, the noise environment, and the DCCB breaking process. The proposed scheme has low requirements for sampling frequency and can be utilized to identify the temporary fault.