## 4 Results and Discussion  
### Multi-Phase Under Voltage Load Shedding Scheme Strategy  
The Korean power system has a structure in which most of the electric power is generated from non-metropolitan areas and load consumption is concentrated in the Seoul metropolitan area. For this reason, high-voltage interface lines are required to supply power to the metropolitan area from non-metropolitan areas. The interface lines are composed of four routes of 345 kV lines and two routes of 765 kV lines. However, the increase in active power flow via the interface lines due to the economical purpose inevitably leads to voltage instability in cases where reactive power is not adequately supported in the metropolitan area. Also, the Korean power system has a critical constraint with the interface flow, which is mainly determined by voltage stability. Thus, when the most severe 765 kV transmission line faults occur, to prevent voltage instability, the UVLS system might be needed. The UVLS system monitors the voltage of the main substations in the metropolitan area and the tripping of the 765 kV transmission line after faults. It is designed to shed a predetermined 1500 MW \begin{table}
\begin{tabular}{c c c} \hline \hline
**Condition** & **Description** & **Criteria** \\ \hline
**1** &
\begin{tabular}{c} Following the clearing of a fault, time to recover voltage to up to \\ 95\% of their nominal values. \\ \end{tabular} & Recovery Time within 2 s \\ \hline
**2** &
\begin{tabular}{c} Following the successful clearing of a fault, the voltage magnitudes \\ should be no less than 70\% of their nominal values. \\ \end{tabular} & Satisfaction \\ \hline
**3** &
\begin{tabular}{c} Within 20 cycles following the clearing of a fault, the voltage \\ magnitudes should be no less than 80\% of their nominal values. \\ \end{tabular} & (\(\bigcirc\))/Dissatisfaction (\(\blacksquare\)) \\ \hline
**4** &
\begin{tabular}{c} Within 0.5 s following the clearing of a fault, the voltage \\ magnitudes should be no less than 90\% of their nominal values \\ \end{tabular} & magnitudes \\ \hline
**5** &
\begin{tabular}{c} Within 1.5 s following the clearing of a fault, the voltage \\ magnitudes should be no less than the steady-state voltage \\ minimum, typically 92–95\% of nominal. \\ \end{tabular} & magnitudes \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Determination of voltage stability conditions. load in the metropolitan area when voltage falls below a predefined level. However, the UVLS system does not consider the voltage recovery characteristics of the load. Because the load shedding amount is determined under the maximum load conditions, unnecessary load amounts might be shed. This paper proposes a MUVLS scheme to prevent excessive load shedding based on CLM (ZIP plus IM model), and the scheme will improve the short-term voltage stability issue in the Korean power system. The CLM is used to simulate the phenomenon of DVR. The dynamics of IM loads are fully considered, so the voltage stability condition is identified. Considering the IM loads in the power system, the voltage recovery and slip characteristics are influenced by the fault clearing time. The difference in the voltage recovery characteristics can be caused by the different fault clearing time. Even with a short time difference, the load shedding time after the fault has a significant influence on the voltage instability. In order to establish the MUVLS strategy, a review was made on the load shedding amount (LSA) and the load shedding time (LST) for each of the phases 1, 2, and 3. Table 4 shows the case study for the selection of LSA and LST. As it checks whether the voltage has been stably recovered or not, the LSA and LST are changed. To ensure short-term voltage stability, load shedding should be completed within 1.5 s after the fault. For this purpose, to select the LST, the maximum time is set to 1.5 s and the minimum time to 0.9 s. Phase-1 LSA and LST are selected, as LST is increased from 0.3 s, pursuing the lowest LSA and shortest LST.