26-03-2012, 04:26 PM
Single Line Diagrams
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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.
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.