What is War of Currents?

Why mostly the Power Distribution systems uses AC over DC?

Actually, at the time of development in Electrical Systems and Electrical Distribution, Both AC and DC were used for the Power Distribution.

AC Generator

Thomas Edison Promoted DC Power Distribution System and Nikola Tesla and George Westinghouse, promoted the AC Power Distribution System.

This is known as War of currents! (1880s)

Finally AC won over the DC Power Distribution System.

The main Reasons for use of AC in Power Distribution System are:

1. When Electricity passes through a Conductor, there is Transmission loss in the form of Heat. It is also known as Ohmic Loss.

Power Loss = I^2 Rt

Where I  = Current

           R = Resistance of the Conductor

            t = time

So if we transmit larger currents, the loss will be more.

However, in AC, there is a relation for the Transformers

I1V1=I2V2

ie, when the Voltage at one end of the transformed increases, Current decreases.

This is because, we have to keep the product of Current and Voltage constant.

So if we can transmit the Current at Higher Voltage, it will reduce the current through the Conductor and in effect the Ohmic Power loss will decrease.

2. Long distance Power Transmission is possible with Higher voltage. This can be easily achieved using the Transformers in AC, However Stepping up and Stepping Down using Transformers are not possible in DC.

AC Power Distribution

However there are some advantages for DC over AC

DC power maintains a constant direction of current. One advantage of DC power is there is no reactance in the line. 

This allows higher power transfer capability, higher capacity utilization of generators, and less of a voltage drop along the line. 

DC also has a lower line resistance than AC because of the “skin effect” in AC. This is when charge is carried mostly near the outside of the wire.

In the DC system, power is just the real component. This means that the transmission system operator need not worry about the sufficiency of reactive power to maintain the security and stability of the system.

In DC, there is no frequency, so generators connected to the transmission grid do not need to be synchronized.

The DC system does not introduce susceptance along the line thus removing the effect of changing current and over voltages in the system.

Analysis of DC systems only involves real numbers, while AC systems involve complex numbers. (Think about a world without AC; How easy will be the calculations in Electrical Engineering :-) )

A good Resource for studying the AC Theory is available Here