DC-side pole-to-ground fault current calculation for multi-terminal LCC-MMC hybrid HVDC system

The multi-terminal hybrid high voltage direct current (HVDC) system integrating both line commutated converter (LCC) and modular multilevel converter (MMC) is becoming an emerging solution for long distance and bulk power transmission. To protect this system from short circuit, the pole-to-ground (PTG) fault current calculation is of vital importance. However, three obstacles exist in the PTG fault analysis, viz. the nonlinear characteristics introduced by LCC, the asymmetry of PTG fault and the inherent coupling between dc transmission lines. In this paper, in light of the nonlinear obstacle, the dc characteristics of LCC and its specific nonlinear form is analyzed at first. Then, for system-level fault current calculation, a linear transient model is established for LCC with the piecewise linearization method. In light of the asymmetry and coupling issues, a common- and differential-mode (CDM) transformation-based analysis method is proposed. By transforming the multi-terminal hybrid HVDC system into CDM networks, analytical expressions of the PTG fault currents are presented in this paper. The analytical expression of the PTG fault current reveals the underlying fault mechanism, and can be used to select the proper fault protection equipment, the validity of which is fully verified by PSCAD/EMTDC simulation.