# Analysis of Six Active Power Control Strategies of Interconnected Grids with VSC-HVDC
## 6 Conclusions  
To date, the frequency control for interconnected grids has been analyzed in several works. However, using frequency control increases the operation burden of one side. Thus, the impact of various types of active power control scheme should be simulated and analyzed. Furthermore, if there is a power increase clause between interconnected grids, this issue will be more critical for grid operators. In this paper, when the receiving side has an emergency event, six applicable active power control strategies are defined and simulated. Before the simulation, our researchers expected that the step control, which is a powerful strength of power electronics, could further improve the angle stability of PGs due to its fast response. Contrary to expectations, the results show that the VSC-HVDC with certain RRSs similar to the other generator output characteristics provides more stable results in several cases. Our findings also confirm that sending power during the frequency recovery stage is more effective for the receiving side, whereas sending power right after the fault degrades the generator output characteristic since the low initial frequency error is measured at the generator side. To prevent this problem, one solution can be applied. If the droop slope of the generator is adjusted when the converter starts the power control, the small initial frequency error caused by the converter support can be corrected at each governor. This increases the generators output, and thus the frequency can reach the nominal value. Furthermore, grid operators have to alter their control strategy based on the SCR of each grid base, as shown in Table 5. These results provide useful information to grid operators, and the advantages and disadvantages of each control scheme are well shown. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Target System** &
\begin{tabular}{c} **Control** \\ **Strategy** \\ \end{tabular} & \(x_{initial}\) & \(x_{F}\) & \(x_{T}\) & \(x_{0}\) \\ \hline \multirow{2}{*}{area 2} & (a) & 93.04 & 102.42 & 88.6 & 0.147 \\  & (e) & 96.21 & 90.82 & 0.05 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Overcorrection results of (a) and (e) in area 2. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Target System** & **(a)** & **(b)** & **(c)** & **(d)** & **(e)** & **(f)** \\ \hline Receiving side with high SCR & \(\Delta\) & \(\Delta\) & O & O & O & O \\ Receiving side with Low SCR & \(\chi\) & \(\chi\) & O & \(\Delta\) & O & \(\Delta\) \\ \hline Sending side with high SCR & \(\Delta\) & \(\Delta\) & O & O & O & O \\ Sending side with Low SCR & \(\chi\) & \(\chi\) & \(\Delta\) & O & O & O \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Suitable control scheme for each TSO (O: Most suitable/\(\Delta\): Suitable/\(\chi\): Not suitable). SCR: short circuit ratio. **Author Contributions:** The main idea was proposed by S.S. and G.J. ; the experiment results were collected and analyzed by S.S. and M.Y. **Funding:** This research received no external funding. **Acknowledgments:** This work was supported under the framework of international cooperation program managed by National Research Foundation of Korea (No. 2017K1A4A3013579) and also supported by \"Human Resources Program in Energy Technology\" of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry and Energy, Korea. (No. 20174030201540). **Conflicts of Interest:** The authors declare no conflict of interest.