Application to Power Transmission
The electricity generated at power stations needs to be transmitted over long distances to towns and cities through transmission cables. So it should be ensured that minimum power losses occur during this transmission otherwise much of the efforts and fuel at the power station would be going to waste. Transformers come to our rescue in this endeavour and this is how it works.
At the power station the power generated is increased in voltage using a step up transformer to the order of several hundreds of kilovolts, depending on the application. This is then transmitted to substations, which are located in proximity to the actual consumers. The substations reduce this voltage using a step down transformer to the normal usage levels, which could be 110, 220 or 440 Volts AC.
If you are wondering how this helps to reduce loss let me explain it for you. The power loss in an electric circuit due to heat is given by the equation:
Power loss = I^2*R ----- (Equation 1) (Note, R is the resistance of the circuit.)
Since Power = Voltage * Current, the above equation can also be written as follows by solving for current and substituting:
Line Loss = P^2 * R/V^2 ---- (Equation 2)
Since power required by a substation is fixed for a given case, as well as R is fixed for a given installation of cables, the line loss is inversely proportional to the square of the voltage from equation 2.
The value of R depends on the material used for transmission cables and their dimensions, so for a given setup of transmission cables, the power dissipated as heat can be reduced by lowering the current, which means by increasing the voltage.
There is another equation which is the power loss fraction and is given by line loss/power transmitted or
Power loss fraction = P * R/V^2 ---- (Equation 3)
Hence we can also see that the fraction of power lost increases with the increase of transmitted power.
There is an upper limit to which the voltage is increased because at very high values of voltages other types of losses such as corona discharge assume significant proportions, and requires extra insulation costs which offset the advantages of less power loss, though their detailed study is beyond the scope of this article
So much for the reasons, but what is the actual mechanism through which the electric power is transferred between the two circuits? Well you need to learn something about magnetic inductance before you could fully understand this concept. In the next article we will learn something about inductance and its uses, especially in context of the workings of the transformer. Also it would be certainly helpful if you could browse though this article about magnetic circuits as well as it pertains to the same theme. Another point worth mentioning is that transformers only work with AC (Alternating Current) and we will talk about the reasons in later articles in this series.