Critical voltage limit of a transmission line is increased by

The correct answer is option 3.

Concept:

The critical voltage limit of a transmission line is the same as the critical disruptive voltage. 

Critical Disruptive Voltage is defined as the minimum phase to the neutral voltage required for the corona discharge(corona losses) to start.

The expression for critical disruptive voltage is given by the expression

   .....(1)

Here, r is the radius of the conductor 

d is the interspacing between the conductors 

 is the air density factor

m is the irregularity factor(depends on the atmospheric conditions and decreasing in rough conditions)  

 is the dielectric strength of the air.

Solution:

By equation 1, we can infer that . So, to increase the voltage we need to increase the radius of the conductors.

Additional Information

•  There is another type of rating also for transmission lines known as critical visual voltage.
• The corona glow does not begin at the critical disruptive voltage V_c, but it begins at a higher voltage called the Visual Critical Voltage.
• It is denoted by V_v. The expression for the same is given below

• The power loss due to corona is given by the expression below

Here, V is the phase voltage in kV and other parameters are the same as previous equations. Before calculating the power loss do check that the phase voltage is greater than the , if the phase voltage is less than the  then there is no corona loss and .

From the power loss expression, it is evident that