Fast Fault Restoration Control Scheme for LCC AC Side Fault for Hybrid HVDC Transmission Systems Using Flexible DC Voltage Control

Hybrid HVDC transmission systems, which consist of a line commutated converter (LCC) operating in rectifier mode and a voltage source converter (VSC) operating in inverter mode, have a number of advantages: improve inverter immunity against commutation failure, reduced converter cost & semiconductor losses, and enhanced reactive power support capability. However, due to the slower operating speed of LCC, the recovery process after AC fault clearance requires significant time. To improve the recovery speed, this paper proposes a control scheme under which the VSC inverter perceives the LCC-side fault state by its measured instantaneous DC current and coordinate its DC voltage operation. A detailed model of a Hybrid HVDC transmission system is established in Matlab/Simulink to demonstrate the effectiveness, and the simulation results show the proposed scheme reduces the rectifier's grid fault restoration time of the Hybrid HVDC system operation compared with traditional control schemes.