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When the load on a transmission line is equal to the surge impedance loading
Receiving end voltage is less than sending end voltage
Sending end voltage is less than receiving end voltage
Receiving end voltage is equal to the sending end voltage
None of above
Receiving end voltage is equal to the sending end voltage
Surge Impedance Loading (SIL) is defined as the power delivered by a lossless transmission line to a load resistance equal to the surge impedance of the line. At this specific loading condition, the reactive power generated by the line capacitance exactly compensates for the reactive power absorbed by the line inductance, resulting in a flat voltage profile where the receiving end voltage is equal to the sending end voltage.
Surge Impedance Loading (SIL) is defined as the power delivered by a lossless transmission line to a load resistance equal to the surge impedance of the line. At this specific loading condition, the reactive power generated by the line capacitance exactly compensates for the reactive power absorbed by the line inductance, resulting in a flat voltage profile where the receiving end voltage is equal to the sending end voltage.
ZcтАЛ=CLтАЛтАЛ тАФ Surge impedance of a transmission line
SIL=ZcтАЛV2тАЛ тАФ Surge Impedance Loading power
In a transmission line, the series inductance L absorbs reactive power (I2XLтАЛ) and the shunt capacitance C generates reactive power (V2XCтАЛ). When the load impedance ZLтАЛ=ZcтАЛ=CLтАЛтАЛ, the line operates in a natural state where the net reactive power flow is zero. Consequently, there is no voltage drop due to reactive power flow along the line, ensuring VsтАЛ=VrтАЛ.
At SIL, the power factor at all points along the line is unity.
Transmission lines are usually operated at or near SIL to maintain stable voltage profiles.
If load > SIL, the line consumes net reactive power, causing voltage drop (VsтАЛ>VrтАЛ).
If load < SIL, the line produces excess reactive power, causing Ferranti effect (VrтАЛ>VsтАЛ).
Maintains constant voltage profile along the line
Minimizes reactive power compensation requirements
Operating exactly at SIL is often not feasible due to load fluctuations
Does not account for line resistance (losses) in practical scenarios
Design and planning of HV/EHV transmission lines
Determining the natural power carrying capacity of long lines
For overhead lines, the surge impedance typically ranges between 350 to 450 ohms.
The Ferranti effect is observed when the receiving end is open-circuited or lightly loaded (load < SIL).
C is correct тАФ When a transmission line is loaded at its surge impedance, the line acts as a matched system resulting in no reactive power exchange with the line, which keeps the magnitude of the sending end and receiving end voltages identical.
Always remember that for a lossless line, SIL represents the equilibrium state where the Ferranti effect and voltage drops due to inductance cancel each other out.