# Decision Optimization for Power Grid Operating Conditions with High- and Low-Voltage Parallel Loops
## 5 Case Study
### Actual AC/DC Interconnected Power System
5-6) are selected as the candidate opening schemes, whose filtering indices and priority indices are shown in Table 5. In addition, the original network with no branch being switched off acts as the control group (No. 7) to highlight the effect of optimal opening scheme. In Table 5 the filtering index and priority index of the first four opening schemes generated by proposed method are overall larger than two experiential schemes and original network. Based on the analysis above and results in Table 5, it can be concluded that the proposed parallel loop opening method can effectively find optimal opening schemes in AC/DC interconnected power systems, which is usually more effective than the method based on engineering experience. Additionally, the comparison between scheme 6 and scheme 7 indicates that the experiential method cannot always guarantee a better opening scheme. According to the priority index in Table 5, scheme 1 should be the optimal opening scheme, with the original network divided into two partitions. \begin{table}
\begin{tabular}{c c} \hline
**Inverter Station** & **MISCR** \\ \hline QDC-L & 3.39 \\ QDC-H & 3.88 \\ LDC-L & 3.40 \\ LDC-H & 3.84 \\ NDC & 4.32 \\ \hline \end{tabular}
\end{table} 
Table 4: MISCR of Each Inverter Station before Optimization. Figure 6: Basic partitions of actual power system. As an AC/DC interconnected power system, the system stability (evaluated by MISCR) should be checked after any opening scheme is employed. The MISCR calculation results after scheme 1 is employed, are shown in Table 6.  
Compared with the MISCR results in Table 4, it can be concluded from Table 6 that system stability is high enough after scheme 1 is employed. From the perspective of time-domain simulation, transient voltage curves of inverter buses of QDC and LDC inverter stations also validate this conclusion, which are shown in Figure 7. The assumed fault is that QCT-CLT double lines are disconnected abruptly. After the optimal scheme is employed, the voltage curves of all of four inverter buses are able to recover to stable state faster, which indicates a more stable power system. \begin{table}
\begin{tabular}{c c} \hline
**Inverter Station** & **MISCR** \\ \hline QDC-L & 3.50 \\ QDC-H & 3.90 \\ LDC-L & 3.61 \\ LDC-H & 3.88 \\ NDC & 4.30 \\ \hline \end{tabular}
\end{table} 
Table 6: MISCR (Multi-Infeed Short Circuit Ratio) of Each Inverter Station with the Optimal Scheme. Figure 7: _Cont._ \begin{table}
\begin{tabular}{c c c c c c} \hline
**No.** & **Candidate Opening Scheme** & \(Q^{\prime}\) & \(M^{\prime}\) & \(\xi\) & \(P\) \\ \hline
1 & QCT+CLT, LYT+ZZT+HZT & 0.6306 & 0.7681 & 0.69935 & 0.5698 \\
2 & QCT, CLT, LYT+ZZT+HZT & 0.6507 & 0.6528 & 0.65175 & 0.5417 \\
3 & QCT+HZT+ZZT, CLT+ LYT & 0.5291 & 0.6823 & 0.6057 & 0.5206 \\
4 & QCT+HZT, CLT+ LYT +ZZT & 0.5213 & 0.7214 & 0.62135 & 0.5257 \\
5 & Open lines of YC-SG, GC-SG, JN-ZB, & & & & \\ ZC-GZ, XT-LZ, ZXP-YM, ZZ-YM, YH-LL & 0.5118 & 0.6924 & 0.6021 & 0.4798 \\
6 & Open lines of TY-TS, TS-ZC, ZC-ZXP, & & & & \\ LZ-LQ, YN-MZ, RZ-LY & 0.4987 & 0.7422 & 0.62045 & 0.5175 \\
7 & Original Network & 0.4533 & 0.7825 & 0.6179 & 0.5023 \\ \hline \end{tabular}
\end{table} 
Table 5: Filtering Index and Priority Index of Candidate Schemes. Figure 7: Transient voltage curves of inverter buses: **(a)** QDC-L; **(b)** QDC-H; **(c)** LDC-L; **(d)** LDC-H.  
All of the above simulation results in this actual power system case show that the proposed method can effectively be applied in AC/DC interconnected power systems and successfully finds the optimal opening scheme. The derived optimal scheme has good parallel loop opening effects and strong support ability for multiple HVDC links.