In this article, We are discussing the concept of Surge Impedance loading but, before discussing the idea of Surge impedance loading First, we have to understand the concept of Characteristic Impedance and Surge Impedance.

### What is Characteristic Impedance (Zc)?

It is the square root of the ratio of series impedance and the shunt admittance. It means characteristic is actually the impedance of the transmission line at any point.

Zc = √Z/Y

Where Zc = characteristic impedance

Z = series impedance

### Surge Impedance

Surge impedance is the impedance offered to the traveling wave or the surge in the transmission line.

Surge impedance is nothing but the characteristic impedance for a lossless line or loss-free line.

We know that the characteristic impedance of a transmission line is given by the formula,

Zc =√Z/Y        where Z= series impedance    & Y= shunt impedance

Zc= √R+jXL/G+jXC

And we know that for a lossless transmission line R=G=0

Then Zc= ZS=√0+jXL/0+jXC

ZS=√0+jwL/0+jwC

ZS=√L/C

Thus from the above formula, we can clearly note two important conclusion

(1). The value of surge impedance depends on both the inductance and the capacitance of the transmission line.

(2). Surge impedance is independent of the length of the transmission line.

### Important facts about surge impedance

1. The value of Surge impedance for an overhead transmission line is 400Ω i.e. (Zs=400Ω).
2. The value of surge impedance for underground cable is 40Ω i.e. (Zs=40Ω).
3. For a pure Inductor, the value of surge impedance is infinite (Zs=∞) because, for a pure inductor, the value of capacitance is zero.
4. For a pure capacitor, the surge impedance is zero(Zs=0) because, for a pure capacitor, the value of inductance is zero.
5. The value of surge impedance for a transformer is approximately equal to 5000Ω.

Surge impedance loading (SIL) is defined as the amount of power transferred when the value of the load connected to the transmission line is equal to the surge impendence (Zs).

Where, Vs = source voltage or operating voltage and Zs = Surge Impedance

In other words, we can also say that SIL is the mega-watt power delivered by the load whose impedance is equal to the transmission line’s surge impendence (Zs).

Hence we can write,

SIL = 3VphIphcosφ

And cosφ = 1 because the load is resistive.

IPh=Vph/R =  Vph/zs= VL/√3Zs

SIL = 3×VL/√3×VL//√3Zs= VL2/ZS

SIL = VR2/ZS= VS2/ZS =VS2√C/L

Where VS, VR =line voltage, and C and L have their phase value.

1. Surge impedance loading or SIL is directly proportional to the square of the source or the line voltage.

i.e SIL∞VS2

1. SIL is directly proportional to the square root of the capacitance.

i.e SIL∞ √C

1. SIL is inversely proportional to the square root of the inductance.

i.e SIL∞ 1/√C

### Different ways to reduce L are as follows

Q. If the surge impedance is 300 for a 400km transmission line then what will be the surge impedance for an 800km transmission line?

(a) 300    (b)30   (c)150Ω  (d) none

Ans- (a)300Ω

Explanation- As the surge impedance is independent of the length of the transmission line, therefore, the value of surge impedance for the 800km transmission line same as the surge for the 400km line.

Q. What will be the value of SIL for a 400kv ac transmission line?

Ans–   SIL = VS2/ZS but in this question, the value of surge impedance is not given so the default value of surge impedance (ZS) will be taken as 400Ω.

Then, SIL = (400kv)2/400Ω

= 400×400×103×103/400

= 400×106 watt

= 400 megawatt