Applicability analysis of travelling wave protection for the hybrid HVDC system

The hybrid high-voltage direct current (HVDC) system, in which the rectifier is made up of the line commutated converter (LCC) and the inverter is composed of the modular multilevel converter (MMC), has quite different fault characteristics from the symmetrical LCC–HVDC and MMC–HVDC systems. Therefore, it is necessary to analyse the applicability of the existing line protection principles to the hybrid HVDC system. First, the fault characteristics of the travelling wave on both ends of the hybrid HVDC line are obtained by analysing the boundary characteristics. The applicable conditions of travelling wave protection to the hybrid HVDC line are given based on the criteria of travelling wave protection and the fault characteristics. Second, an electromagnetic transient simulation model of the hybrid HVDC system is established in Power System Computer-Aided Design/Electromagnetic Transients including DC (PSCAD/EMTDC) in order to verify the theoretical analysis results. A large amount of simulation results show that the travelling wave protection has good applicability in the hybrid HVDC system, but it has poor sensitivity for high-resistance grounding faults; an appropriate sampling rate should be chosen according to the parameters of the boundary element for a better performance of the protection