Longitudinal Compensation

the series connection of compensating devices in an AC transmission line for the purpose of changing the reactive parameters of the line. The compensating devices are usually sets of electric capacitors. The connection of these capacitors to the line reduces its total inductive reactance.

Capacitive longitudinal compensation is an efficient means of increasing the carrying capacity of transmission lines that operate at 220–750 kilovolts (kV); such compensation improves the steady-state and transient stability of power systems. Longitudinal compensation is also used to improve voltage conditions in long overhead networks that operate at 6–35 kV, such as power-supply systems for industrial plants and agricultural enterprises. This type of compensation is also used where the loads in the system change rapidly—for example, where motors with large starting currents are switched on frequently or where welding units are used.

The disadvantages of longitudinal compensation are (1) a drastic increase of short-circuit current near the location of the compensating devices and (2) the possibility of resonance effects occurring in the system, such as self-excitation and self-generated load swings. The installed capacity of the compensating devices is limited by the conditions required for the reliable functioning of the relays that protect the transmission lines and by the above-mentioned resonance effects.