Per-unit transmission-line constants

For many years, the per-unit or per cent method has been used to define the constants of such equipment as generators, reactors, and transformers. However, the constants of transmission lines still are expressed in terms of ohmic impedances. This fact is not important for short lines where the line impedance is generally small in relation to terminal impedances. However, for long lines, the line constants are one of the principal factors which determine the steady-state and transient power limits of the system. Therefore, it is desirable to use a per-unit system for transmission-line constants which is based upon the power limits of the transmission line.