rkrishna.dignity
2

AC Power is proportional to product of V&I where V is the voltage & I is the current. This power can be transmitted over long distances either by Overhead Transmission Lines or Underground Power Cables. But depending upon various other factors, overhead transmission lines are preferred.

In every case the transmission is effected by line losses on account of the line resistance and line reactance.

These losses can be represented by the equation:-

w is proportional to I(r+jx) where w is the line loss.

I is the line current.

r is the line resistance.

x is the line reactance.

Since the line losses are directly proportional to the current, these losses can be reduced by increasing the voltage of transmission and reducing the current for the same power being transmitted. This is the concept of "High Voltage Transmission".

So normally in India following Voltage are adopted...…….

11KV for short distance power distribution in towns & villages.

33KV for slightly longer distances say upto 50KM.

66KV upto 100KM.

132KV upto 200KM.

220KV upto 500KM.

400KV upto 1000KM.

These voltages however depend upon the quantum of power to be transmitted. High Voltages like 220KV/400KV are adopted even for short distances if the quantum of power to be transmitted is of the order of hundreds of MWs.

With advance in technology of Power Electronics, these days large power is being transmitted over long distances beyond 1000KM efficiently by adopting "High Voltage DC Transmission".

In India, each "State Grid" is interconnected with the "National Grid" mostly by these DC transmission lines.

I know a 800KV DC transmission line which runs from Talcher Power Station in Odisha to Hoody Receiving Station in Bengaluru; a distance of more than 1500KM.

transmitting more than 1000MW.

Conclusion: Power can be transmitted efficiently at high voltage.

Manomay Shravage
1

The losses in a power line fall into two classes.

The first form of loss is that due to the resistance of the wires. This loss is proportional to the square of the current and directly proportional to the resistance. (For this reason, it is often known as the “I-squared R loss”.

It must be remembered that AC can be transformed up and down, whereas DC cannot (or at least not without a lot of very expensive equipment). The higher to voltage, the lower the current, in direct proportion. This means that doubling the voltage will quarter the losses due to resistance for a given gauge of wire.

The second form of loss is that due to leakage from the wires. Aside from actual leakage down the insulators, which can be reduced by regular cleaning, there is leakage directly into the air due to the air being ionised by the high voltage, giving rise to coronal discharge. This leakage gets worse as the effective diameter of the conductor is reduced. Above about 150Kv, this becomes unaccceptably high and 2, 3 or 4 conductors strung together and are used to increase the effective diameter.

With careful design, leakage losses will remain small compared to the resistance losses which would arise at lower voltages.

When carrying high power from a remote generating site (for example a hydroelectric plant in a remote part of the country) to an area where the electricity is to be used (a metropolitan area, for example) the power company will employ as high a voltage as possible. 750kv, or even 1Mv, systems are now commonly used where power has to be transmitted over hundreds of miles.