## 2 Modeling and Problem Statement
### Development of Load Configuration for the Korean Power System  
The analysis of the DVR phenomenon requires dynamic simulation based on appropriate representations of the loads, because the DVR phenomenon may occur due to the highly inductive characteristics of the small heating, ventilation, and air conditioning (AC) units, especially the AC loads. In summer peak conditions, AC loads can be a substantial dynamic factor of the DVR phenomenon, so the trend of gradual increase in the proportion of IM loads can critically impact on the system stability. Thus, determining accurate portion of IM loads of power systems is crucial. From the information on power consumption of system level, a valuable insight on the proportion of IM load can be obtained. In this paper, the IM proportion was decided based on the statistical information on system load [41; 42; 43]. The proportion of IM loads in the system depends on a variety of factors, such as time and weather. As the reference, however, this paper uses the information on the load consumption for peak conditions, because this condition is the most severe in terms of voltage stability. Figure 3 shows the proportion of the IM loads in the total load at the summer peak each year.  
From Figure 3, one can notice that the proportion of IM loads in the system load shows a steady increase over the years. The consumption of IM loads becomes 27.4% in 2016. The increasing tendency of supply and use of IM loads directly affects the maximum demand. Also, an increase in the IM loads might cause an imbalance between the supply and demand, resulting in degrading the energy utilization efficiency and deteriorating the stable supply of electric power.  
The factors for the increase in the IM loads are as follows. One factor contributing to this consumption is the climate conditions. The reliability and load capability of the power systems can be affected by climate change. Another factor exacerbating the problem is the increasing peak electricity demand. The demand on Korean power systems has increased in recent years. Thus, one of the major causes of this problem is AC loads. The third factor is the changes in household type, such as the increase in single-person households, which is the main cause of the increase in electricity power demand. Because AC is regarded as an essential appliance for a basic life in the hot and humid Korean summer, the use of cooling equipment is expected to rise. These factors are expected to further increase in the proportion of future AC loads. Based on the above-mentioned data, it can be seen that the IM load in the Korean Power System could reach more than 30% of the total load. Load modeling considering IM loads in summer peak conditions is reasonable for simulation for DVR problems.  
This paper performs dynamic simulations using the composite load model. Based on the actual load configuration, a recalculated data of the proportion of the ZIP and IM loads was created and applied to the dynamic simulation. The concepts of reflecting the ZIP and IM loads proportion in practical system data can be expressed as shown in Figure 4.  
Figure 3: IM load proportion at summer peak condition.