Unbalanced current analysis and novel differential protection for HVDC transmission lines based on the distributed parameter model

The current differential protection presently being used for high-voltage direct-current (HVDC) transmission lines requires increases in time delay and threshold to avoid misoperation, which is induced from a distributed capacitance current along the transmission line, leading to low operation speed and disabling the device from distinguishing between internal and external faults. The unbalanced currents of the non-fault pole are studied in two cases: under an external fault as induced by the distributed characteristics and under a single-pole-to-ground fault as induced by the coupling characteristics of the transmission line parameter. To overcome its defects, this paper develops a new differential protection algorithm based on the distributed-parameter transmission-line model. The fault identification criterion using the compensated current of the common mode and fault pole selection criterion using the compensated current of the differential mode are introduced. Unaffected by the distributed capacitance current, this differential protection correctly operates with high sensitivity through the entire process during the fault. The feasibility and validity of the differential protection algorithm are verified.