DC-Side Pole-to-Ground Fault Analysis for Multi-Terminal LCC-MMC Hybrid HVDC System

The multi-terminal hybrid high voltage direct current (HVDC) system that consists of line commutated converter (LCC) and modular multilevel converter (MMC) is an emerging topology for long distance and bulk power transmission. To protect this system, the pole-to-ground (PTG) fault current calculation is of vital importance. However, three obstacles challenge the PTG faut analysis, viz. the nonlinear characteristics of LCC, the asymmetry of PTG fault and the coupling feature between dc transmission lines. In light of this, a transient model is established for LCC with the piecewise linearization method. Then, by employing common- and differential-mode (CDM) transformation, the multi-terminal HVDC system is transferred into CDM networks, which are balanced and decoupled. Based on the derived CDM network, a detailed procedure of fault current calculation is further presented. The calculated fault current can be used to fully understand the regularities that underlying the fault characteristics and properly select the fault protection equipment. Finally, the validity of the proposed calculation method is verified by PSCAD/EMTDC simulation.