Adaptive non unit traveling wave protection algorithm for multi terminal HVDC grids

High Voltage Direct Current (HVDC) systems offer an efficient solution for long-distance power transmission and the integration of remote renewable energy sources. However, protection of the HVDC transmission line poses a significant challenge due to its extremely high short circuit currents and absence of zero crossings. DC faults must be resolved within milliseconds to prevent HVDC grid failure. Thus, advanced protection techniques are crucial for ensuring the efficiency and stability of multi-terminal Voltage Source Converter (VSC) based HVDC systems. This paper proposes a robust and adaptive non-unit protection algorithm for a multi-terminal HVDC grid transmission line, addressing DC fault type discrimination and fault identification under high-impedance fault (HIF) conditions. The algorithm is designed based on distinctive high-frequency components present in the line and pole mode voltage traveling waves (TWs) at various relay units. Daubechies wavelet transform was chosen as the time-frequency tool. The proposed protection scheme accurately detect fault and distinguishes between external and internal DC faults within one millisecond and ensures proper operation during HIF with a 25 dB level of noise interference. A four-terminal meshed HVDC grid is modeled in EMTDC/PSCAD and tested under the positive pole-to-ground (PTG), pole-to-pole (PTP), and negative pole-to-ground (NTG) fault at various locations and times to evaluate the effectiveness of the proposed protection scheme. By exploiting the properties of traveling waves, the algorithm improves the accuracy of DC fault protection, making it suitable for modern HVDC applications. The results indicate that the proposed protection algorithm offers enhanced sensitivity and improved selection performance.