The surge impedance loading or SIL of a transmission line is the MW loading of a transmission line at at which a natural reactive power balance balance occurs. The following brief article will explain the concept of SIL. Transmission lines produce reactive power (Mvar) due to their natural capacitance. The amount of Mvar produced is dependent on the the transmission lines capacitive reactance (!") and the voltage (#$) at which the line is energi%ed. In e&uation form form the Mvar Mvar produced is'
Transmission lines also utili%e reactive power to support their magnetic fields. The magnetic field strength is dependent on the magnitude of the current flow in the line and the lines natural inductive reactance (! L). It follows then that the amount of Mvar used b a transmission line is a function of the current flow and inductive reactance. In e&uation form the Mvar used b a transmission line is'
transmission lines surge impedance loading or SIL is simpl the MW loading (at a unit power factor) at which the lines Mvar usage is e&ual to the lines Mvar production. production. In e&uation form we can state that the SIL occurs when'
If we ta#e the s&uare root of both sides of the above e&uation and then substitute in the formulas for ! L (*+pfL) and !" (*,-+pf") we arrive at'
The term
in the above e&uation is b definition the surge impedance/.
The theoretical significance of the surge impedance is that if a purel
resistive load that is e&ual to the surge impedance were connected to the end of a transmission line with no resistance0 a voltage surge introduced to the sending end of the line would be absorbed completel at the receiving end. The voltage at the the receiving end would have the same magnitude magnitude as the sending end voltage and would have a voltage phase angle that is lagging with respect to the sending end b an amount e&ual to the time re&uired to travel across the line from sending to receiving end. The concept of a surge impedance is more readil applied to telecommunication sstems than to power sstems. 1owever0 we can extend the concept to the power transferred transferred across a transmission line. The surge impedance loading or SIL (in MW) is e&ual to the voltage s&uared (in #$) divided b the surge impedance (in ohms). In e&uation form'
2ote in this formula that the SIL is dependent onl on the #$ the line is energi%ed at and the lines surge impedance. The line length is not a factor in the SIL or surge surge impedance calculations. Therefore the SIL is not a measure of a transmission lines power transfer capabilit as it does not ta#e into account the lines length nor does it consider the strength of the local power sstem. The value of the SIL to a sstem operator is reali%ing that when a line is loaded above its SIL it acts li#e a shunt reactor3absorbing Mvar from the sstem3and when a line is loaded below its SIL it acts li#e a shunt capacitor 3suppling Mvar to the sstem. 4igure , is a graphic illustration of the concept of SIL. This particular line has a SIL of 567 MW. MW. Therefore if the line is loaded to 567 567 MW (with no Mvar) flow0 the Mvar produced b the line will exactl balance the Mvar used b the line.
4igure ,' Surge Impedance Impedance Loading of a Transmission Loading Loading
