## 3 Results and Discussion
### Discussion  
The abovementioned analysis results are summarized in Table 4. As shown from these results, the index may be used to monitor the voltage stability, thus reflecting the power system situation in real time with fast computations. As shown in Table 4, voltage instability occurs when the index is 1.0. In theory, voltage instability occurs when the index is 1.0. However, voltage instability might occur even though the index is below 1.0 in a practical situation. Therefore, the system operator requires more absolute values. Hence, the most severe VSI should be added to provide a threshold value to the system operators. Figure 19 shows the results of the measured and threshold values under a severe contingency in the Korean power system. In Figure 19, the red line is the index from the measured data and the blue line is the threshold value. As shown, the system is stable in terms of voltage stability. Consequently, it may be useful to monitor the indicators for deciding the appropriate countermeasures based on the phasor measurement \begin{table}
\begin{tabular}{c c c} \hline
**Classification** & **Voltage Stability Results (Index)** & **Voltage Stability Margin (MW)** \\ \hline Case 1 & Unstable (1.0) & 583 \\ \hline Case 2 & Unstable (1.0) & 246 \\ \hline \end{tabular}
\end{table} 
Table 4: Comparison of the Case 1 and Case 2. Figure 19: Observation for the measured data and threshold value with severe contingency. \begin{table}
\begin{tabular}{c c} \hline
**Time (s)** & **Margins (MW)** \\ \hline
0 & 3075 \\
10 & 2999 \\
20 & 2988 \\
30 & 1873 \\
40 & 888 \\
50 & 291 \\
60 & 246 \\ \hline \end{tabular}
\end{table} 
Table 3: Margins with each load character curve. data. Additionally, because the system operator can actively cope with emergencies using the proposed method, it can prevent enormous economic losses by performing the appropriate countermeasures to prevent a wide-area outage caused by voltage instability. Thus, utilizing the proposed method within the Korean power system might protect the system efficiently. Currently, the Korean electric power system has completed the development of the wide-area monitoring system (WAMS) technology based on PMUs to assess voltage stability. Following the WAMS, the wide-area monitoring and control (WAMAC) technology is being developed. Further, the wide-area monitoring protection and control (WAMPAC) technology is being planned as the final goal of the R&D roadmap. Table 5 shows the stability and control measures for each technology phase. As mentioned above, when major high-voltage transmission lines are tripped after a fault, it might result in a large-scale blackout that may cause catastrophic damage to the entire system. Thus, appropriate countermeasures are required to ensure the frequency and angle stability of a large-scale power system [49; 50]. Hence, a special protection scheme is applied based on the proposed method to secure stability. Depending on each step, load shedding and generator tripping SPS may be used. The comparison results of with and without the SPS for ensuring stability are shown in Figure 20.  
As shown in Figure 19, the system is stable when SPS is applied. When the system experiences low-frequency problems a few seconds after critical events, the UFLS scheme is activated. In the Korean power system, applying the WAMAC technology based on the PMU might effectively ensure its frequency stability. The proposed method of stability index can be used to monitor the frequency state of the system. In addition, PMU applications enable the implementation of the load shedding strategy for emergency situations where load shedding based on the stability margin must be decided. Thus, it is possible to identify a suitable location for the control scheme used as a criterion for UFLS. Further, the generator tripping SPS might be used as a protection strategy in the WAMPAC system by ensuring the angle stability. This final stage is reflected on the grid state for assessing the stability