The electric power system is divided into two systems by the network shape. Figure 5.1 shows the radial and loop types of network of an electric power system in Japan.

5.1.1.1 Radial System

Figure 5.1a shows a radial type of electric power system, which consists of radially connected components including the power plant, the substation, and the transmission lines. Because the composition of the radial system is simple, it is easy for the system operators to control the power flow. As a result, when an accident occurs in a radial system, it is easy to isolate the damaged components in the whole system to localize the effect of the damage. On the other hand, when a route cutoff accident (route fault accident) occurs in the loop system, it may be difficult to prevent power failure because of less redundancy in the system.