What is surge impedance loading (SIL) of a transmission line?

Surge impedance loading is the power level at which the line's inductive and capacitive reactive powers are equal and cancel out, requiring no external reactive compensation. SIL = V^2/Zc where Zc is the characteristic (surge) impedance, typically 250-400 ohms for overhead lines. At SIL, voltage is flat along the line. Below SIL, the line generates VARs (voltage rises); above SIL, the line absorbs VARs (voltage drops). SIL is about 1.0 MW per kV of line voltage.