A Pilot Protection Scheme for VSC-MTDC Grids Based on Polarity Comparison Using A Combined Morphological Technique

This paper proposes a pilot protection scheme for voltage source converter-based multi-terminal high-voltage direct current (VSC-MTDC) grids based on polarity comparison of currents between different relay sections. The proposed strategy compares the polarity of currents in normal system operation and the rate of change of current (ROCOC) in fault condition, which accurately determines the faulty zone regardless of the system power flow, fault type, and fault resistance. It consists of a fault-type detector and a fault locator using two-sided measurements in the faulty zone. A combined series multi-resolution morphological gradient and morphological undecimated wavelet method (SMMG-UDW) is used to detect the fault traveling wave (TW) arrivals. The proposed signal-processing scheme processes the signal in time domain and it is significantly fast due to using simple mathematical operators. It consists of a de-noising sub-process to discriminate between small noise disturbances and fault events. CIGRE DCS2 VSC-MTDC grid is simulated in PSCAD and the signal-processing is investigated in MATLAB. The method is also tested in real-time hardware-in-the-loop (HIL) setup. The results prove the accuracy of the SMMG-UDW scheme in different fault events, it’s robustness in a noisy environment and the feasibility for lower sampling frequencies.