# Power Stability Analysis and Evaluation Criteria of Dual-Infeed HVDC with LCC-HVDC and VSC-HVDC
## 6 Conclusions  
Based on the concepts of the maximum power curve and the power stability index, this paper has analyzed the influence of VSC-HVDC on the power stability of the hybrid dual-infeed HVDC system and further studied the quantitative relationship between the power stability and the effective short circuit ratio under different system parameters. The main conclusions are summarized as follows:  
1. For the hybrid dual-infeed HVDC system, the strengthening effect of VSC-HVDC on the system is related to its control mode. When the VSC converter operates in constant AC voltage control mode, the system can be strengthened, while when it operates in constant reactive power control mode, the power stability of the system may be weakened. 2. If the VSC converter operates in constant AC voltage control mode, the transmission power of VSC-HVDC has almost no effect on the power stability of the system. If the VSC converter operates in constant reactive power control mode, the transmission power of VSC-HVDC has a significant effect on the stability of the system. The greater the transmission power is, the worse the power stability of the system is. 3. There is a relatively clear quantitative relationship between the power stability and the effective short circuit ratio in the hybrid dual-infeed HVDC system. The greater the ESCR is, the better the power stability of the system is. The ESCR index can still be used to evaluate the power stability of the hybrid dual-infeed HVDC system. 4. If the VSC-HVDC is in constant AC voltage control mode, the CESCR is slightly affected by system parameters, and its variable range is 1.56\(\sim\)1.65. Therefore, the power stability evaluation criteria of the single-infeed LCC-HVDC system can still be used in the hybrid dual-infeed HVDC system. It can be considered that the HVDC system cannot operate stably when ESCR is less than 1.5. 5. If the VSC-HVDC is in constant reactive power control mode, the CESCR is mainly affected by the transmission power of VSC-HVDC, and its variable range is 1.52\(\sim\)2.0. In order to ensure a sufficient stability margin, it can be considered that the HVDC system cannot operate stably when ESCR is less than 2.0 and can operate stably with a high power stability margin when ESCR is greater than 3.0. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Control Mode**} & \multirow{2}{*}{**ESCR**} & \multicolumn{3}{c}{**Powe Stability Index (PSI)**} \\ \cline{3-5}  & & **Numerical Calculation** & **PSS/E Simulation** & **Power Stability** \\ \hline constant reactive & 2.18 & 0.12 & 0.12 & weak \\ power control & 2.32 & 0.20 & 0.21 & weak \\ (\(P_{\text{d2 ref}}\) = 1.5 p.u., \(Q_{\text{d2 ref}}\) = 0 p.u., & 2.60 & 0.31 & 0.32 & weak \\ \(Z_{12}\) = 0.3 p.u.) & 3.02 & 0.43 & 0.43 & strong \\  & 4.27 & 0.60 & 0.61 & strong \\ \hline constant AC voltage & 1.84 & 0.12 & 0.12 & weak \\ control & 1.98 & 0.20 & 0.22 & weak \\ (\(P_{\text{d2 ref}}\) = 1.5 p.u., \(V_{\text{2 ref}}\) = 1.0 p.u., & 2.25 & 0.31 & 0.32 & weak \\ \(Z_{12}\) = 0.6 p.u.) & 2.60 & 0.41 & 0.42 & strong \\  & 3.84 & 0.60 & 0.61 & strong \\ \hline \hline \end{tabular}
\end{table} 
Table 2: LCC-HVDC power stability index under different receiving-end system strengths. These conclusions can be used to guide the planning and operation of the hybrid dual-infeed HVDC system. The proposed evaluation criteria of power stability can be used for the location determination of the HVDC system in the hybrid dual-infeed situation. It should be noted that although the ESCR index can still be used to evaluate the power stability of the HVDC system in the hybrid dual-infeed situation, its calculation and evaluation criteria need to consider two control modes, which will bring inconvenience to the analysis of the actual power system to a certain extent. In future works, new indexes of power stability for hybrid dual-infeed HVDC systems should be studied. Investigation, writing: X.W. ; supervision: [PERSON] and Z.Z. All authors have read and agreed to the published version of the manuscript.