## 4 Results and Discussion
### Case Studies  
In this study, the objectives of the simulation are to determine the adequate IM load proportion in the Korean power system. In addition, the MUVLS control strategy was applied to a practical system in order to verify the DVR mitigation effectiveness. Case studies on the Korean power system have validated the performance of the proposed MUVLS scheme under severe contingency scenarios. As mentioned earlier, the proportion of IM loads occupied about 30% under summer peak conditions. Therefore, since the proportion of IM loads is expected to increase continuously due to abnormal weather conditions, to perform dynamic analysis simulations of the appropriate proportion at which the power system could stabilize, the IM load proportion was increased by 1% from 30%. Figure 8: Comparison results of bus voltage recovery for existing scheme and proposed scheme. When applying the proposed MUVLS strategy, the process monitors whether to satisfy the voltage stability condition. If this condition is satisfied, it is possible to discuss the adequate proportion at which the dynamic characteristics of the IM loads can restore the voltage of the power system to ensure short-term voltage stability. Figure 9 shows the flowchart of the algorithm for the method proposed in this paper. IM load proportion 30-34% cases did not satisfy voltage stability conditions without performing load shedding. In order to satisfy the criteria shown in Table 3, MUVLS was performed for each case. Table 5 shows the voltage stability condition satisfaction status at post-contingency after the load shedding. Figure 10 shows the stability condition satisfaction status after the load shedding and voltage recovery with MUVLS at severe contingency. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Condition**} & \multicolumn{4}{c}{**IM Loads Proportion (\%)**} \\ \cline{2-5}  & **30** & **31** & **32** & **33** & **34** \\ \hline
**1** & \(\bigcirc\) & \(\bigcirc\) & \(\bigcirc\) & \(\bigcirc\) & \(\bigcirc\) \\
**2** & \(\bigcirc\) & \(\bigcirc\) & \(\bigcirc\) & \(\bigcirc\) & \(\bigcirc\) \\
**3** & \(\bigcirc\) & \(\bigcirc\) & \(\bigcirc\) & \(\bigcirc\) & \(\bigcirc\) \\
**4** & \(\bigcirc\) & \(\bigcirc\) & \(\bigcirc\) & \(\bigcirc\) & \(\bigcirc\) \\
**5** & \(\bigcirc\) & \(\bigcirc\) & \(\bigcirc\) & \(\bigcirc\) & \(\bigcirc\) \\
**Status** & \multicolumn{4}{c}{Satisfaction (stability)} & \multicolumn{4}{c}{Dissatisfaction (instability)} \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Voltage stability condition satisfaction status after the load shedding. Figure 9: Flowchart of the proposed multi-phase under voltage load shedding (MUVLS) strategy. From Figure 10, the cases satisfying all of the voltage stability conditions are those in which the IM load proportions were below 32%. When the proportion of IM loads is greater than 33% of the power system load, voltage instability due to the DVR phenomenon occurs. It can be seen that the power system voltage recovers with proportions less than 33%. As the proportion of the IM loads decreases, the number of buses satisfying the voltage stability condition increases. This dynamic analysis clearly shows that setting the adequate proportion in Korea to about 32% improves the short-term voltage stability. The result can be used as a basic reference for AC load management in Korean power systems. However, it is necessary to consider the diverse situations of Korean power systems. Due to the improved standards of living, the AC, with its convenience of use, is becoming so popular in Korea that the AC loads are continuously increasing. The impact of relatively cheap electric home appliance pricing on the convenience of electrical energy is leading to inefficiency in energy use. The supply and use patterns of AC loads will have a significant impact on power demand in the summer season. As a result, the increased use of AC loads directly affects the increase in maximum power, and this tends to have an important impact on future power systems.