Novel setting-less pilot protection for MMC HVDC grids

• The measured voltage is proportional to the derivative of measured current when forward fault occurs, the two satisfy proportional relationship when backward fault occurs. • The identification of fault can be realized by recognizing relationships of voltage and current instead of traditional comparison with a fixed threshold. • A novel setting-less pilot protection is proposed, which has the merits of high operation speed, high tolerance to transition resistance, and no need to set the threshold value. • The proposed protection is applicability to various system states and fault types. • The proposed protection has simple algorithms and has no need for high sampling frequency. The determination of a suitable threshold is challenging for most of existing pilot protections of MMC HVDC systems. There are two kinds of setting-less pilot protection among the existing research: current polarity-based and inductor voltage-based pilot directional protections. However, these methods have inherent flaws in certain fault situations accompanied with sharp fluctuations. To address these issues, a novel setting-less pilot protection is proposed based on the idea of pattern recognition. Firstly, the relationship between one-mode fault component voltage and current is analyzed. It can be concluded that Δ u ∝ dΔ i /d t when forward fault occurs, and Δ u ∝ Δ i when backward fault occurs. Therefore, the correlation coefficients ρ R (Δ u , Δ i ) and ρ L (Δ u , dΔ i /d t ) are defined. | ρ R | < | ρ L | indicates forward faults, and | ρ R | > | ρ L | indicates backward faults. The proposed protection can recognize faults within less than 1 ms, and have higher tolerability against transition resistance. It avoids complex threshold setting, and can be applied to various system states and fault conditions. It performs well in those situations where existing setting-less protections may fail. Both simulations and physical experiments have validated the effectiveness of the proposed protection, and have shown that the proposed method is simple for application and of high reliability.