# Coordinated Control Strategy of CU-MTDC under Abnormal Conditions Considering Power Supply Security
## 4 Case Study
### Simulation under Priority Control Mode  
In this case, the load condition is selected No.6 in Table 5. The active power of the \(Load_{18}\) and \(Load_{7}\) are increased by 100 MW and 50 MW, respectively. The maximum regulation capacity of the APC converters are \(\Delta P_{dc2 max}\) = 370 MW and \(\Delta P_{dcmax}\) = 150 MW under this condition. The initial active power of converter 2, 4 and 5 are 500 MW, 200 MW and 220 MW. When \(t\) = 0.2 s, a temporary inner fault occurs in master converter 1 and lasts for 0.3 s, which results in the converter 1 blocking. At \(t\) = 0.9 s, the active power of the offshore wind farm increases to 400 MW. When \(t\) = 1.2 s, converter 1 is out of operation because of a serious converter fault. The simulation results are shown in Figure 9.  
As shown in Figure 9,b, when \(t\) = 0.2 s, the master converter is blocked, and active power is decreased to zero immediately. DC unbalance power occurs when the auxiliary converter reaches its maximum capacity. The final unbalanced power is 251 MW, and the operation mode will turn into VSC\({}_{2}\) priority mode. According to equation (15), \(U_{dc2}\) of VSC\({}_{2}\) is 391 kV. Then \(U_{dc2}\) of the VSC\({}_{2}\) is set at 379 kV as the result of a security margin for DC voltage control of VSC\({}_{4}\). The DC unbalanced power is absorbed by VSC\({}_{2}\) independently. Under VSC\({}_{2}\) priority mode, the DC voltage will rapidly reduce until VSC\({}_{2}\) turns into droop control, and then DC voltage is always within the security range.  
At \(t\) = 0.9 s, the offshore wind farm outputs the maximum active power and the output of VSC\({}_{I}\) and VSC\({}_{3}\) are decreased to 181.7 MW and 130.8 MW, respectively. As a result, DC unbalanced power will be changed to 60 MW if VSC\({}_{I}\) is out of operation.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Name**} & \(\Delta\)**Load\({}_{18}\)** & \(\Delta\)**Load\({}_{7}\)** & \(\Delta\)**P\({}_{dc2 max}\)** & \(\Delta\)**P\({}_{dcmax}\)** & **Total Capacity** \\  & **(MW)** & **(MW)** & **(MW)** & **(MW)** & **(MW)** \\ \hline Case 1 & 0 & 0 & 460 & 185 & 645 \\ Case 2 & 20 & 10 & 44  
When \(t=1.2\) s, VSC\({}_{1}\) will be out of operation for a long time because of a serious fault in the converter. The operation mode will change into VSC\({}_{4}\) priority mode, and the DC unbalanced power is absorbed. The DC voltage is within the normal range during the process.  
From Figure 9c, it is observed that the power supply security of critical lines is confirmed during the simulation process. Under VSC\({}_{2}\) priority mode, the sum of active power value of \(L_{13-15}\) and \(L_{13-14}\) is 616.4 MW, and the active power of \(L_{5-7}\) is kept around 199 MW. Under VSC\({}_{4}\) priority mode, the active power of \(L_{5-7}\) is 220 MW, and the active power variation of \(L_{13-15}\) and \(L_{13-14}\) are quite small. The influence on the AC system power flow can be limited in one area.  
It can be concluded that, under the priority mode, the APC converter can absorb the DC unbalanced power independently. Meanwhile, power supply security is confirmed, and the influence on power flow can be limited in a small area. In addition, the transmission security of the important power supply area is guaranteed because the important power  
Figure 9: Simulation under priority control mode. (**a**) Active power of converters. (**b**) DC voltage of converters. (**c**) Active power of critical AC lines.  
supply area usually has greater power supply demand. According to the priority control method proposed in this paper, these converters have lower priority. Therefore, they are the last to participate in power regulation when the DC unbalanced power is small.