Transient numerical calculation and differential protection algorithm for HVDC transmission lines based on a frequency-dependent parameter model

Being free from the influences of the transmission characteristics, the current and voltage near a fault point can better reflect the fault features. Thus, these electrical quantities along a transmission line can be used to formulate the protection criteria to improve the sensitivity and selectivity of the protection. A numerical calculation method for transient electrical quantities along a transmission line based on a frequency-dependent parameter transmission line model is studied. Differential protection uses currents from both terminals and can protect transmission lines with excellent selectivity. However, the differential protection currently used for high-voltage direct-current (HVDC) transmission lines requires an increase in the time delay and the threshold to avoid mis-operation induced by the distributed capacitance current along the transmission lines, which results in low operation speeds and poor sensitivity. This paper presents a new differential protection algorithm using the calculated current at compensation points to overcome this problem. Because the parameters involved in the differential protection can be precalculated offline, this algorithm has low computational complexity and can be easily implemented. Unaffected by the distributed, frequency-dependent, and coupling characteristics of the transmission line parameters, the proposed differential protection approach can operate correctly with high sensitivity throughout the entire process under fault. Simulations demonstrate the feasibility and validity of the differential protection algorithm.