One-Line Diagram

UW - Electrical Engineering

 APT

Power Systems are often represented by one line diagrams. In a one line diagram, lines on paper (or on a computer screen) represent wires. Rather than Drawing all three wires in a three-phase system, it is normal to simplify things by representing all three phases with one line - thus the name One-Line Diagram.

The drawing below represents a typical one line diagram.

The diagram at the left is a very simply three bus one line diagram.

  • The circle at the top represents a Generator
  • The heavy lines represent buses - a bus is typically located at a Substation of generating station. Buses are heavy copper or aluminmum structures used for carrying large amounts of current and distributing it to different loads. (See image in the upper right)
  • The thin lines connecting the buses are the power lines. They may represent either transmission or distribution lines
  • The arrows represent loads.
One Line Symbols - various symbols are used on one line diagrams. Below are some common symbols that a often seen on one-line diagrams used for power flow analysis.

Transformers

The two symbols at the left are typically used to represent transformers.

Protective Devices

The two symbols at the right represent protective devices

1. Item 1 represents a fuse.

2. Item 2 is a Circuit Breaker.
Below is a fairly complicated one line diagram. In this case there are 33 buses. It stared out as a 30 bus system, but due to expected load growth and the addition of a new industrial plant, it grew to 33. The system being represented is a typical medium sized city transmission and sub-transmission system. Bus work at locations such a Cloverdale, Hancock, Fieldale ect... represent substation bus work. Glen Lyn and Claytor represent power plants.