Surge impedance Loading

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Surge Impedance Loading:

A transmission line may considered as generating capacitive reactive volt-amperes in shunt capacitance and consuming (absorbing) inductive reactive volt-amperes in its series inductance. The load at which inductive and capacitive reactive volt amperes are equal and opposite is called the surge impedance Loading or (nature load of the line).

"The power delivered by the line to purely resistive load equal to its surge impedance the load at which the inductive and capacitive reactive volt-amperes are equal and opposite, is called surge impedance Loading of the line."

Capacitive volt-amp. generated in the line = V *I = V / Xc = V² / ωL

Inductive volt-amp.  absorbed in the line = V * I = I *XL = I² *ωL

Under natural load conditions,

Capacitive volt-amp. = Inductive volt-amp.

            V² * wc   =    I² * ωL

   or,  V / I   = √(L/C) =  Zo = Surge impedance

At this load V and I are in same phase. At this Zo is purely resistive. There is not present imaginary value.

If P is the natural load of 3- Φ line,

       P = 3 Vp * Ip cos Φ

Since, load is purely resistive cos Φ = 1

    P = 3 Vp * Ip = 3 Vp * Vp / Zo = 3 Vp² / Zo

Therefore, 

           SIL = 3 Vp² / Zo Watt (for single phase)

or, SIL = 3 * {VL /√(3)} ² / Zo

Therefore, SIL  =  VL ² / Zo   ----------(1)

                                          ( for three phase)

                Where, VL = Line Voltage

The equation is also called ideal power transfer capability.

Thus, surge impedance load of a line may be defined as the power delivered by it to a purely resistive load equal to its surge impedance.

In case of surge impedance Loading;

I.) |VR| = |Vs| and |IR| = |Is|

II.) Load power factor = unity

III.) Voltage regulation = zero

IV.) Losses = zero

V.) Efficiency = maximum

**Surge impedance Loading is higher in underground cable than overhead transmission line because surge impedance of cable and overhead transmission is 50 ohm and 400 ohm respectively.**





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