## 3 Case Study of a Typical Substation
### Functional Unit Division  
Figure 3 shows the electrical energy flow in a path. Only when all six units are working properly can the required functions of the system be guaranteed. According to the voltage carried, these six units can be divided into four parts: the 220 kV part, the transformer part, the 110 kV part, and the bus-bs part. Each part includes various equipment and structures to perform various functions. To visually show the composition of each unit, Figures 4-7 divide the substation into four parts: the 220 kV part, the 110 kV part, the transformer part, and the bus-bs part, according to the voltage carried. Section 1-1 in Figure 4 shows that the electric energy is transmitted in three phases (phase A, phase B, and phase C), but the connection method is the same; therefore, it is represented by a line in Figure 2. There are three TRs in total, the function of which is to convert high-level voltage into low-level voltage. The DS is used to separate different paths, and the CB is responsible for cutting off and connecting the load circuit, as well as cutting off a faulty circuit to ensure safe operation. The DS cannot be used to cut off and connect load currents and can be used only for certain switching operations that do not produce strong arcs; that is, it does not have an arc-extinguishing function. The DS usually performs a wire change after the CB cuts off the power. The function of the potential/current transformer is to convert a primary potential/current with a larger value into a secondary potential/current with a smaller value through a certain transformation ratio for protection, measurement, and other purposes. The LA is used to protect electrical equipment from high transient overvoltage hazards during lightning strikes, especially TRs. The IS, SP, and SF are used to support the equipment. To meet the insulation requirements, all electrical equipment and structures are installed on the SPs and maintain a certain distance from the ground and each other. These devices are usually independent of each other, but SFs are usually shared by different types of equipment. The IS and SP are mainly used to support the tube bus (BUS*) and the abovementioned equipment, and the SF is used to support the wires. Figure 3: Power flow diagram of a single path. Figure 2: Simplified layout of the substation. The composition of each functional unit is shown in Figures 8-11. The line-in unit (Figure 8A) contains a CT, DS, PT, CB, and two SFs. SF 1-1 only supports line-in unit 1, while SF 1-2 supports line-in units 2 to 6. Line output units 1 to 10 (Figure 8B) are independently supported by SF5. The bus connection unit (Figure 9A,B) contains at most two pieces of equipment: the DS and IS. When the DS is far from the next transmission facility, the wire needs to be supported by an IS to ensure that a certain height from the ground is maintained, as shown in Figure 4. Note that the 110 kV line-out unit (Figure 6) has smaller equipment spacing, so an IS could not be installed between Bus I and the equipment, as Bus I is far from the equipment. The bus-bs unit (Figure 9C) contains one CT, one CB, two DSs, one IS, and two SFs. The bus unit includes one row of SPs, one row of ISs, one BUS\({}^{\star}\), one PT, and one LA. There are two bus units in the 220 kV part and in the 110 kV part. Figure 10 shows the bus units in the 220 kV part. The 110 kV bus connection unit is similar in composition to the 220 kV bus unit, except for the fact that the number of supports under the BUS\({}^{\star}\) is different. Figure 11: Transformer unit. Figure 8: Line-in and line-out units. (**A**) Composition of Line-in unit, (**B**) Composition of Line-out unit. Figure 10: Bus unit. (**A**) Composition of 220 kV Bus I, (**B**) Composition of 220 kV Bus II. Figure 7: The 220 kV bus-bs part. Figure 9: Bus connection and bus-bs unit. (**A**) Composition of BC unit 1, (**B**) Composition of BC unit 2, (**C**) Composition of Bus-bs unit. The transformer unit (Figure 11) consists of five SFs, two CBs, two CTs, two DSs, two LAs, and one TR.