Test: Surge Impedance Loading - Electrical Engineering (EE) MCQ

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.

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:

Surge impedance (ZC) = 400 Ω

Voltage (V) = 400 kV

Surge impedance loading 

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 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.

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.

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

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 that, surge impedance (Z_C) = 200 Ω

Voltage (V) = 400 kV

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 50 miles and 25 miles 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.

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.

For overhead line surge impedance is given by 100 - 200  Ω

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

Surge Impedance Loading (SIL):

• It is the loading on the line whenever the load impedance is exactly matching to surge impedance of the transmission line.
• It is also known as characteristic impedance loading or ideal loading or ideal power transfer capability.
• So, at SIL the load impedance is equal to characteristic impedance (ZL = ZC).

Surge impedance loading 

Where, ZC = Characteristic impedance = √(z/y)

Case 1: If loading is equal to SIL

• Load impedance is equal to characteristic impedance.
• The transmission line will act as neither sink nor source of reactive power.
• Receiving end voltage is equal to sending end voltage.
• Sending end current is equal to receiving end current.
• Both sending end and receiving end power factors are unity

Case 2: If loading is greater than SIL

• Load impedance is less than the characteristic equation.
• The transmission line will act as a sink of reactive power.
• Receiving end voltage is less than the sending end voltage
• Receiving current is more than sending end current.
• Receiving end power factor is unity but sending end power factor is lagging.

Case 3: If loading is less than SIL

• Load impedance is greater than the characteristic equation.
• The transmission line will act as a source of reactive power.
• Receiving end voltage is greater than the sending end voltage
• Receiving current is less than sending end current.
• Receiving end power factor is unity but sending end power factor is leading.

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 (SIL):

• It is the loading on the line whenever the load impedance is exactly matching to surge impedance of the transmission line.
• It is also known as characteristic impedance loading or ideal loading or ideal power transfer capability.
• So, at SIL the load impedance is equal to characteristic impedance (Z_L = Z_C).

Surge impedance loading 
Where, Z_C = Characteristic impedance = √(z/y)

Case 1: If loading is equal to SIL

• Load impedance is equal to characteristic impedance.
• The transmission line will act as neither sink nor source of reactive power.
• Receiving end voltage is equal to sending end voltage.
• Sending end current is equal to receiving end current.
• Both sending end and receiving end power factors are unity

Case 2: If loading is greater than SIL

• Load impedance is less than the characteristic equation.
• The transmission line will act as a sink of reactive power.
• Receiving end voltage is less than the sending end voltage
• Receiving current is more than sending end current.
• Receiving end power factor is unity but sending end power factor is lagging.

Case 3: If loading is less than SIL

• Load impedance is greater than the characteristic equation.
• The transmission line will act as a source of reactive power.
• Receiving end voltage is greater than the sending end voltage
• Receiving current is less than sending end current.
• Receiving end power factor is unity but sending end power factor is leading.