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non-unit protection based on line boundaries is no longer applicable.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.