power transformers

Frequently Asked Questions

Why Transformers?

In today’s world, the bulk of electrical power is generated in the form of AC using generators whether thermal, hydro, and nuclear or gas. However due to equipment sizing, insulation requirements, etc. the generating voltage in the present day power stations is limited in the order of 15 to 25 kV though the power generated is in hundreds of mega watts. It is impractical to distribute this much power at the generated voltage due to the magnitudes of currents, which are in the order of about 1000 amps for every 25 MVA at 15 kV. If such high currents are to be transmitted over long distances, it will be necessary to overcome the following:


  • The total power loss in a transmission/distribution system is proportional to I 2 Z where I is the magnitude of current and Z is the impedance of the transmission/distribution system. The larger the current transferred, the larger will be the power loss and lower the transmission efficiency.
  • The cross section of conductors required would increase as the value of current increases and it is impractical to install heavy conductors across a country to carry and transmit higher currents to remote parts.
  • The voltage drop in a distribution system is proportional to I×Z and it is necessary to keep the value of I as low as possible to limit the voltage drops (since the value of Z cannot be reduced very much).


All the above factors necessitate that the transmission current is reduced as much as possible, which is achievable only by increasing the transmission voltage.

A transformer is the answer to the above issues and today’s AC distribution cannot be what it is without the use of transformers. A transformer is an essential device in any Electrical AC power distribution system, which is used to convert (transform) AC voltage magnitudes of any value obtained from 4 Practical Power Transformers: Operation, Maintenance & Testing a power source to any desired value. The development of power transformers dates back to the 19th Century.

The main feature of a transformer is its constant VA rating whether referred to its primary or secondary side. With VA being constant (V refers to the voltage magnitude and A refers to the current magnitude in a transformer winding), it is just possible to get a higher V with lower A OR a lower V with a higher A, by choosing suitable turns ratio for the transformer windings. It is to be noted that the transformers can be used only for changing the magnitudes of AC voltages. They cannot be used for varying DC voltages, without converting them to AC voltages.