Practical calculation of SCR of LCC-HVDC considering the role of VSC-HVDC

The accurate quantification of transmission limit and grid strength of the multi-infeed LCC-HVDC （line-commutated converter technology-based high-voltage DC） and VSC-HVDC （voltage source converter technology-based high-voltage DC） systems has become one of the critical measures to ensure the stable operation of new power systems. Through the improvement of the grid strength quantification method of the traditional LCC-HVDC AC/DC systems， a short-circuit ratio （SCR） index for quantifying the grid strength of the hybrid DC infeed systems is proposed to analyze the transmission limit of the AC/DC systems. Firstly， a quasi-steady-state （QSS） model of the hybrid DC infeed systems is built under the flow equation. Secondly， the SCR index for evaluating the transmission limit of LCC-HVDC in the hybrid DC infeed systems is obtained by analysis of the relationship between the Jacobian matrix singularity and the power transmission limit of the DC infeed systems. The sensitivity relationship between the index threshold and the operation mode of the VSC-HVDC system is analyzed. Finally， the effectiveness of the SCR index is verified using PSCAD/EMTDC and MATLAB platform. The analysis results show that the VSC-HVDC system is superior in voltage support under the constant voltage control mode at the receiving end. The operation mode of the VSC-HVDC system that exerts less influence on the transmission limit of the LCC-HVDC system can be quantitatively analyzed by the proposed index.