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Surge Impedance loading in electrical power system

The load at which the reactive power absorbed by inductance of the line is equal to the reactive power supplied by the capacitance of the line is known as Surge Impedance Loading (SIL)
Surge Impedance of a Transmission line     Zs =√L/C

Where L= Inductance of the line per phase
C=Capacitance of the line, phase to neutral

If the line is loaded such that the load impedance is equal to the Surge Impedance (Zs ) of the line, the power carried by the line is called Surge Impedance Loading (SIL)

Pn = V I = V2/ Zs

Line loading is expressed as a multiple of SIL. eg. 1.2 Pn

SIL mainly depends on voltage class and the conductor configuration of the line.

For the load equal to SIL, the voltage of the line does not change along the length of the line.

Hence no extra compensation for reactive power is required.

If the load carried by the line is less than SIL, it is said to be lightly loaded and if the load carried is more than SIL, it is said to be heavily loaded.

Shorter lines are loaded with SIL > 1 and long lines are loaded with SIL < 1.

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