A DC fault protection method for MMC-HVDC grids using modal voltage adaptive thresholds.

• The 1-mode and 0-mode transient voltage integrals are analyzed in the pure-fault network. • A single-end protection method using adaptive thresholds is proposed for the MMC-HVDC grid. • The proposed method is sensitive to single-pole high-resistance faults and suitableness for weak boundaries. For the bipolar modular multilevel converter-based high voltage DC (MMC-HVDC) grids, single-end protection methods suffer from low sensitivity in detecting high-resistance faults (HRF). The performance further deteriorates in a weak boundary configuration. To improve the deficiency, this paper proposes a protection method with adaptive thresholds. First, an MMC simplified circuit, an internal fault analysis network, and a DC bus fault analysis network are presented in pure-fault networks. The integral characteristics for 1-mode and 0-mode voltage fault components are studied under different locations. It is discovered that the relative size for two modal voltage integrals is different for internal faults and DC bus faults. A new form of mathematical morphology filter is proposed to select the required integral time. With 1-mode and 0-mode voltage integral, the fault identification method using adaptive threshold is proposed as well as its setting method. Simulations of a four-terminal MMC-HVDC system built in PSCAD/EMTDC verify the effectiveness of the proposed method. The method can fast detect internal single-pole HRFs of different locations and operate reliability under active power reversal, AC-side faults, and noise interference. Compared to some existing methods, the proposed method can detect high-resistance faults under weak boundary configurations. [ABSTRACT FROM AUTHOR]