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PGCIL DT Electrical 13 Aug 2021 Official Paper (NR I)

Option 4 : There is an increase in surge impedance loading as it is directly proportional to the transmission voltage.

__Advantage of Extra High Voltage (EHV) Transmission:__

Consider the transmission of electric power by a three-phase line

Let,

P = power transmitted in watts

V = line voltage in volts

cos φ = power factor of the load

The current (I) flowing through the system is given as,

\(I = \frac{P}{{\sqrt 3 \;V\;cos\phi }} \)

Resistance per conductor, \(R = \frac{{\rho l}}{a} \)

Total Power loss (W) is given as,

\(W = 3{I^2}R = 3{\left( {\frac{P}{{\sqrt 3 \;V\;cos\phi }}} \right)^2} \times \frac{{\rho l}}{a} = \left( {\frac{{{P^2}\rho l}}{{\;{V^2}{{\cos }^2}\;\phi \;a}}} \right) \)

\(W\propto \frac{1}{{{V^2}}} \)

Where,

l = length of the line in meters

R = resistance per conductor in ohms

ρ = resistivity of conductor material

a = area of X-section of conductor

**From the above equation, we can say that,**

- Voltage increased then line loss will decrease and vice versa.
**The installation cost of transmission lines per kilometer decreases as the volume of the conductor decreases**, and hence the cost of line support reduces.**There is improved performance of transmission lines, i.e., efficiency and regulation**increases with an increase in transmission voltage.**With an increase in operating voltage, the number of circuits and the requirements of land are reduced considerably.**

**Limitations of EHV:**

- The increased cost of insulating the conductors.
**There is an increase in surge impedance loading as it is directly proportional to the transmission voltage.**- The increased cost of transformers, switchgear, and other terminal apparatus
- The surface-voltage gradient on conductors becomes larger as the voltage increases.