Surge impedance loading, commonly called SIL, is a quantity used by system operators as a benchmark to determine whether a transmission line is acting as a capacitance that injects reactive power (VARs) into the system or as an inductance that consumes VARs, thus contributing to reactive power losses in the system. SIL is measured in terms of real power (MW).
SIL is a measure of the amount of real power that would exist in a transmission line where the line neither produces nor consumes VARs. By comparing the SIL to the actual real power flow, it can be determined whether a line is consuming or producing VARs:
On highly loaded transmission lines, the SIL is most often less than power flow meaning that the system operator must provide VARs to support voltage. SIL is calculated as the square of transmission voltage divided by the line’s surge impedance: