Non-Unit Protection Scheme Based on Cooperation with Control Strategy for Multiterminal Hybrid HVDC System Without Line Boundary

Multiterminal hybrid HVDC systems, combining the advantages of conventional and flexible DC systems, are considered important development trends for future power networks. With the ability to suppress DC fault current, hybrid HVDC systems eliminate the requirement of current limiting reactors at both ends of DC lines. Thus, non-unit protection based on line boundaries is no longer applicable. In this paper, a non-unit protection scheme in cooperation with converter control is proposed to ensure protection speed, reliability, and sensitivity for multiterminal hybrid HVDC systems without line boundaries. In the proposed protection scheme, the protection criterion based on the transient energy of the DC voltage and the direction criterion based on the travelling wave are used to identify whether the fault occurs on the DC line. When the fault is detected as a DC-line fault, the relevant converter station is quickly blocked. After a specified hold-off duration, the converter station interfacing with multiple DC lines is prioritized for restart, while the remaining converter stations are restarted once the DC voltage is stabilized. Simulations based on PSCAD/EMTDC validate the feasibility of the proposed protection scheme and its superiority over competing approaches.