Single Line Diagrams

Power systems are extremely complicated electrical networks that are geographically spread over very large areas. For most part, they are also three phase networks – each power circuit consists of three conductors and all devices such as generators, transformers, breakers, disconnects etc. are installed in all three phases. In fact, the power systems are so complex that a complete conventional diagram showing all the connections is impractical. Yet, it is desirable, that there is some concise way of communicating the basic arrangement of power system components. This is done by using Single Line Diagrams (SLD). SLDs are also called One Line Diagrams.


Single Line Diagrams do not show the exact electrical connections of the circuits. As the name suggests, SLDs use a single line to represent all three phases. They show the relative electrical interconnections of generators, transformers, transmission and distribution lines, loads, circuit breakers, etc., used in assembling the power system. The amount of information included in an SLD depends on the purpose for which the diagram is used. For example, if the SLD is used in initial stages of designing a substation, then all major equipment will be included in the diagram – major equipment being transformers, breakers, disconnects and buses. There is no need to include instrument transformers or protection and metering devices. However, if the purpose is to design a protection scheme for the equipment in the substation, then instrument transformers and relays are also included.

There is no universally accepted set of symbols used for single line diagrams. Often used symbols are shown in Fig. 1. The variations in symbols are usually minor and are not difficult to understand.

Concept of Bus


Concept of bus in single line diagrams is essentially the same as the concept of a node in an electrical circuit. Just keep in mind that there is one bus for each phase. Buses are shown in SLDs as short straight lines perpendicular to transmission lines and to lines connecting equipment to the buses. In actual substations, the buses are made of aluminum or copper bars or pipes and can be several meters long. The impedance of buses is very low, practically zero, so electrically the whole bus is at the same potential. Of course, there is line voltage between the buses of the individual phases.



Figure 1 – Graphical Symbols for Single Line Diagrams





Figure 2 – Single Line Diagram showing bus arrangement of a substation.


Single line diagrams like in figure 2 are used to illustrate the layout of buses in a substation. The arrangement of figure two is called a “breaker and a half”. There are three breakers for every two connections of lines or transformers to the bus, i.e. 1 ½ breakers per termination.



Figure 3 – Single line diagram of an electric power system.


Figure 3 shows a small power system. Any information that is required is added to the SLD. In this case connections of generator and transformer windings, as well as the method of grounding the neutral are indicated. This type of SLD has often also specified the size of the equipment in MVAs, voltage levels, and any other relevant information. (see figure 4)


Figure 4 – Single Line Diagram of a 69 kV/12 kV/4.16 kV Substation


                                     3f fault

             B1                    B2

                                     If = 8000 A

         51                 51



Figure 5 Single Line Diagram of Overcurrent Protection of a Radial System



Figure 5 shows a radial system of two lines. The buses represent two distribution stations. The outgoing lines have breakers only on the supply end. There are current transformers on the supply end of each breaker. The current transformers are connected to overcurrent relays. Dashed lines between the relays and the breakers indicate a functional relationship; in this case operation of overcurrent relays causes the associated breaker to trip. This type of SLD is used for calculation of fault current and setting and coordination of the relays.


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