The power transformer is the most essential part of electrical transmission and distribution systems. In our current era, most of the electrical systems operate with alternating current or AC. After the production of alternating power in the generating stations, we need to step it up to a higher voltage level. Then we transmit it to the load centers where we again have to step it down to the desired voltage levels for distribution purposes. Power Transformers perform stepping up and stepping down of electrical power during the journey from generating ends to distribution ends. During this journey, the electrical power can be stepped down more than once in different desired transmission voltage levels.
Working of Power Transformers
Like others, general purposed transformers, a power transformer also works based on the principle of mutual induction. Whenever there is a current in a coil, an associated magnetic field exists surround the coil. If the current is alternating the direction and the magnitude of the magnetic field will also be changing according to the alteration of the current in the coil. In other words, there will be a changing field flux exists surrounding the coil. Now if we bring another coil nearer to the first one, the changing field flux links the second coil.
Obviously, the entire field flux produced by the first coil cannot link the second coil. A certain portion of the flux links the second coil. As a result, there will be an induced EMF in the second coil due to the flux linkage. Because according to Faraday’s law of electromagnetic induction, a changing magnetic flux linked with any conductor produces EMF in the conductor.
Transforming of Power
Now if we connect one simple resistance across the second coil, an alternating current starts flowing through the resistance due to the induced EMF in the second coil. Therefore the resistance dissipates a certain power.
What is the ultimate source of this power? Obviously the alternating source connected with the first coil is the ultimate source of this power in the resistance. Therefore we can say the power from the main source has come to the load (resistance) of the second coil without any direct electrical connection. Then how it comes. It is due to the mutual induction between the first and second coils.
This is the most basic principle of a transformer as well as the power transformer. But this model of transformer is not a practical one. In an actual transformer, we refer to the coil connected to the source as the primary winding. Similarly, we refer to the coil connected to the load as the secondary winding. But in our model of the transformer, a very small portion of the primary flux links the secondary winding. The primary flux means the flux created by the primary winding.
Why does a power transformer require a magnetic core?
Hence there should be some means by which the maximum primary flux links with the secondary winding. Therefore, the purpose of the transformer core comes into the picture. The transformer core creates a closed magnetic circuit. In other words, it provides a low reluctance path to the magnetic flux. The most well-known material of low reluctance property is either iron or steel. So the transformer core is generally made of either iron or steel. We place both the primary and secondary windings on the magnetic core. Almost the entire flux is concentrated inside the core body, therefore it links the windings efficiently.
Since the iron core facilitates maximum flux linkage between the windings, the transformer becomes able to transform maximum power from primary to secondary.
Leakage Reactance of Power Transformers
The presence of core in the transformer facilitates the maximum flux linkage but the core cannot concentrate 100% flux inside it. So there will be a small portion of the flux passes outside the core. This portion of the flux does not participate in the flux linkage process. Therefore we call it the leakage flux.
The transformer core also introduces some specific losses in the power transformer. The losses are hysteresis and eddy current loss.