Test: Surge Impedance - Electrical Engineering (EE) MCQ

Surge impedance:

The surge impedance or characteristic impedance of a long transmission line is given by,

Where,

Z is series impedance per unit length per phase

Y is shunt admittance per unit length per phase

In case of a lossless line (i.e. R = 0, G = 0), the surge impedance is given by

• Surge impedance does not depend upon the length of the transmission line.
• Surge impedance depends on the characteristics of the transmission line like diameter & spacing between the conductors.
• So that surge impedance of 100 km and 50km long underground cable is 50 Ω.
   

Important Points

​Surge impedance loading (SIL):

Surge impedance loading is defined as the power load in which the total reactive power of the lines becomes zero. The reactive power generated by the shunt capacitance is consumed by the series inductance of the line.

The characteristic impedance of a wave travelling in a lossy medium is given by

Put σ = 0 (free space and lossless medium)

Where,

μ₀ = Permeability of free space = 4π x 10^-7 H / m

ϵ₀ = Permittivity of free space = 8.85 x 10^-12 F/m

When a loss-free overhead transmission line is terminated through a resistance equal to surge Impedance of the line, there will be no reflection of travelling wave hence the value of R is 400 Ω.

Important Points:

The surge impedance or characteristic impedance of a long transmission line is given by,

Z is series impedance per unit length per phase

Y is shunt admittance per unit length per phase

Surge Impedance for the transmission line is about 400 ohms it is around 40 ohms for underground cables.

Surge impedance loading (SIL):

Surge impedance loading is defined as the power load in which the total reactive power of the lines becomes zero. The reactive power generated by the shunt capacitance is consumed by the series inductance of the line.

The surge impedance or characteristic impedance of a long transmission line is given by,

Surge impedance loading (SIL):

Surge impedance loading is defined as the power load in which the total reactive power of the lines becomes zero. The reactive power generated by the shunt capacitance is consumed by the series inductance of the line.

The surge impedance is always same in any length of the transmission line. It does not depend on the length of the transmission line

Here

L = transmission line inductance,

C = transmission line capacitance,

Zs = surge impedance

For a 200 km length of the same line, the surge impedance

= 400 ohms

The equation for surge impedance is given by

Where, L = inductance in henry

C = capacitance in farad

An impedance which renders the line as the infinite line is known as surge impedance. It has a value of about 400 ohms and a phase angle varying from 0 to -15 degrees for overhead lines and around 40 ohms for underground cables.

For overhead line surge impedance is given by 400-600 Ω

For underground cable surge impedance is given by 40-60 Ω

Concept:

Surge impedance loading: It is defined as the power load in which the total reactive power of the lines becomes zero.

The reactive power generated by the shunt capacitance is consumed by the series inductance of the line.

Surge impedance is used in the prediction of maximum loading capacity of transmission lines and is given by,

Calculation:

l = 1, X_L = 1, Z = 1

Where,

l = length of line

X_L = Inductance of line per unit length

X_C = Capacitance of line per unit length

Z = Impedance of line per unit length

Z_s = Surge impedance

Therefore, magnitude of surge impedance = 0 Ω 

The equation for surge impedance is given by

Where L = inductance in henry

C = capacitance in farad

An impedance which renders the line as the infinite line is known as surge impedance. It has a value of about 400 ohms and phase angle varying from 0 to -15 degrees for overhead lines and around 40 ohms for underground cables.

For overhead line surge impedance is given by 400-600 Ω

For underground cable surge impedance is given by 40-60 Ω

Concept:

Characteristic impedance 

Where, L is per phase line inductance

C is per phase line capacitance

Power transfer capability under surge impedance loading is given by,

Calculation:

Given that,

Line inductance (L) = 10 mH

Line capacitance (C) = 1 μF

Concept:
The surge impedance or characteristic impedance of a long transmission line is given by,

Z is series impedance per unit length per phase

Y is shunt admittance per unit length per phase

• The surge Impedance for the transmission line is about 400 ohms it is around 40 ohms for underground cables.

Surge impedance loading (SIL):

• Surge impedance loading is defined as the power load in which the total reactive power of the lines becomes zero.
• The reactive power generated by the shunt capacitance is consumed by the series inductance of the line.

Calculation:

Given:

Surge impedance (Z_C) = 400 Ω

Voltage (V) = 200 kV

Surge impedance loading =