# Coordinated Control Strategy of CU-MTDC under Abnormal Conditions Considering Power Supply Security
## 4 Case Study  
In this section, the five-terminal CU-MTDC and interconnected CUP grid shown in Figure 7 are studied in PSCAD/EMTDC to verify the feasibility of the proposed coordinated  
Figure 6: Flowchart of the complete calculation procedure of the proposed method. control strategy under abnormal conditions. An actual CUP grid topology in the eastern coast of China and the data of maximum operation mode in summer [23] are applied in the simulations. Three critical lines \(L_{5-7}\), \(L_{13-15}\), \(L_{13-14}\), are facing transmission stress problems (load ratio exceeds 90%). However, additional AC lines are not allowed because of the shortage of transmission corridors and the limitation of short circuit capacity. The short-circuit current of \(Bus_{13}\) and \(Bus_{2}\) is close to the maximum breaking current of a break. Moreover, offshore wind farm A is interconnected with the CUP grid via the HVAC. The random and intermittent characteristics of the wind power will further increase the transmission stress of the critical lines, even lead to serious security problems. Based on the analysis above, a five-terminal CU-MTDC is finally applied in the CUP grid. VSC\({}_{1}\) and VSC\({}_{3}\) are directly connected with a high voltage (HV) grid and generally operate on rectifier mode. VSC\({}_{2}\) and VSC\({}_{4}\) interconnect with the CCUP grid and operate on inverter mode. VSC\({}_{1}\) is the master converter station to keep constant DC voltage control under normal conditions. VSC\({}_{3}\) is the auxiliary converter station to assist the DC voltage control. VSC\({}_{2}\) and VSC\({}_{4}\) are APC stations. The APC converter should turn into droop control under abnormal conditions. VSC\({}_{5}\) is an offshore wind farm converter. The active power capacity of converter 1 and 2 are all rated 500 MVA. The rated active power of converter 3, 4 and 5 are 250 MVA, 200 MVA and 400 MWA, respectively. The length of DC cable between terminals are \(L_{12}=L_{23}=L_{25}=40\) km, \(L_{34}=20\) km. The detailed parameters of the DC/AC system are shown in Tables 2-4. The diagram of the whole system is shown in Figure 8. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Bus Name** & **Short-Circuit Current (kA)** & **Maximum Breaking Current (kA)** \\ \hline B\({}_{13}\) & 43.6 & 50 \\ B\({}_{2}\) & 56 & 63 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Short-circuit current of the critical bus. Figure 7: System structure of the five-terminal CU-MTDC. (**a**) Initial coastal urban power grid. (**b**) Final CU-MTDC applied in a coastal urban power grid. From Table 3, the load ratio of \(L_{13-15}\) and \(L_{13-14}\) are lower than 90%. However, the power flow of the two transmission lines cannot be control independently. To prevent one of the two transmission lines from exceeding the limit value, a limit security value of the \begin{table}
\begin{tabular}{c c c} \hline \hline
**System Component** & **Major Parameter** & **Parameter Value** \\ \hline \multirow{3}{*}{Converter transformers} & \multirow{3}{*}{Transformation ratio} & 525 kV/230 kV (1, 3) \\  & & 220 kV/230 kV (2, 4, 5) \\
1–5 & & Rated capacity & 550, 550, 300 \\ \cline{1-1}  & & Leakage reactance & 0.1 p.u \\ \cline{1-1} Connection reactor & & Inductance & 0.15 p.u \\ \cline{1-1} DC capacitor & & Capacitance & 150 μF \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Parameters of DC simulation model. Figure 8: Simplified diagram of the studied system. two lines' power sum is set at 700 MW by the operation department of the power grid. The security margin of the AC lines power limit value \(\varepsilon\) is set as 5%. Considering the value of \(\varepsilon\), the limit value of the critical lines are 243 MW and 665 MW, respectively. As shown in Figure 7, the CUP grid can be divided into four areas. To reduce the complexity of the description, Figure 8 is shown as a simplified topology. Only some important regions of areas I and II are shown in detail. The DC voltage reference value of the CU-MTDC is selected at 400 kV.