Long-distance railways use

Long-distance railways use

Right Answer is:

25 kV Single phase AC


The 25 kV standard at 50 hertz was introduced in 1923 in Hungary and adopted as an international standard in 1950.

Why is electrical energy transmitted at high voltage and low current?

Suppose that power is to be transmitted from a power station to a home or a factory. Since power is the product of current and voltage (P = V x I), a given amount of power can be transmitted either at high voltage and low current or at low voltage and high current. The cables that transmit the power have resistance, and therefore some of the power is bound to be wasted by producing heat in the cables as the current flows through them. If the resistance of the cables is R. the heat energy produced in time t when a current I is flowing through them is I2Rt. Thus the amount of energy that is wasted is proportional to the square of the current in the cables(Ploss=I2R). The most efficient way to transmit power is therefore at high voltage and low current. This is known as high tension transmission. When resistance is low, energy losses are low also. So that faster control for minimizing the arcing of contacts & maintained by a high level of the protective scheme. 

The second advantage of high voltage/low current transmission of electrical power is that low currents require thinner and therefore cheaper cables. A disadvantage is the high cost of the substantial insulation needed when employing high voltages.

Let’s say R = 10 ohms

If you try to send 100W over at 100V, you need to use 1A. Then the power lost in the cable is

I2 x R = 12 x 10 = 10W.

If you try to send the same 100W over at 1000V, you need to use 0.1A. Then the power lost in the cable is

0.12 x 10  = 0.1W

The higher the voltage, the greater is the power it can give. So the transmission lines, railway power lines use very high voltages with thinner copper wires can be used, which is cheaper and apart from that transmission losses also will be reduced.

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