Single line diagram and its significance

A single line diagram (SLD), or one-line diagram, shows a balanced three-phase power system using one line to represent all three phases. Standard symbols show the generators, transformers, lines, breakers, buses and loads, together with their ratings and connections.

Typical SLD

  G1                                  M
 (~)--[x]--8|8--+===== line =====+--8|8--[x]--(M)
       CB   T1  |                |   T2
               Bus 1           Bus 2
                |                |
               Load            Load
  (~) generator   8|8 transformer   [x] breaker

Information shown

• Ratings: MVA, kV and % reactance of machines and transformers
• Transformer winding connections (Y, Δ) and neutral earthing
• Line impedances, buses, and the location of breakers, CTs and PTs
• The points where loads and generators connect

Significance in power system analysis

1. Simplicity: A balanced three-phase system is solved on a per-phase basis, so one line gives all the needed information without clutter.
2. Starting point for analysis: The impedance and reactance diagrams, the bus admittance matrix, load flow, fault studies and stability studies are all built from the SLD.
3. Planning and operation: It shows the system layout clearly to planners, operators and protection engineers.
4. Protection coordination: It shows the locations of breakers and relays and the fault paths.
5. Data in one place: All the ratings needed for p.u. calculation are on one drawing.

A simple example is a generator, a step-up transformer, a transmission line, a step-down transformer and a load in series, each shown by its symbol and rating.