Transformers are capable of receiving AC power at one voltage and delivering it at another voltage. In this article, we will go through the working and construction of a 3 phase transformer by starting from its simplest form. We will also understand what is power transformer and how it is constructed.
A detailed webpage version of the video is given below.
Why Transformers are used ?
Transformers are ubiquitous devices. They are used to either step-up the A.C voltage or to step-down it. But, why should we do this voltage transformation ?. It is a science fact that a stepped-up voltage is associated with a reduced current. A reduced current leads to low eddy current energy loss. In this way, transformers help achieve better transmission efficiency while transferring the power over longer distances.
The Basic Working Principle
The basic working principle of a transformer is simple, electromagnetic induction. According to this principle, a varying magnetic flux associated with a loop will induce an electromotive force across it. Such a fluctuating magnetic field can easily be produced by a coil and an alternating E.M.F (EP) system. A current carrying conductor produces a magnetic field around it. The magnetic field produced by a coil will be as shown in the first part of Fig.2. With the fluctuating nature of the alternating current, the magnetic field associated with the coil will also fluctuate.
This magnetic flux can be effectively linked to a secondary winding with the help of a core made up of a ferromagnetic material. The linked magnetic flux is shown in the second part of Fig.2. This fluctuating magnetic field will induce an E.M.F in the secondary coils due to electromagnetic induction. The induced E.M.F is denoted by ES.
But since energy is conserved, the primary and secondary currents have to obey the following relationship.
3 Phase Transformer
Three phase transformers use 3 such single-phase transformers, as shown in the figure below.
Power Transformer - Construction Features
The transformers which are used in high voltage applications are referred as 'Power Transformers'. They handle voltage in the range of 33 to 400 kV. The winding of a power transformer is quite different from that of a low voltage transformer (Distribution Transformer). We will explore the construction and connection details of the power transformer winding in this session.
The power transformers generally employ a special kind of winding, known as a disc-type winding, where separate disc windings are connected in series , through outer and inner cross-overs.
The low-voltage windings of a power transformer are connected in a delta configuration and the high-voltage windings are connected in a star configuration. The winding connections are shown in the Fig. 6 and Fig.7 respectively.
High voltage insulated bushings are required to bring out the electrical energy. It is clear from the Fig.8 that, the bushings at the high voltage side are quite bigger compared to the low voltage bushings.
The Core Construction
The core of the transformer is made of thin, insulted, steel laminations. Such steel laminations are stacked together, as shown in the Fig.9, to form 3 phase limbs. The purpose of thin laminations is to reduce energy loss due to eddy current formation. Pleas note here that, the separated out layer blocks in the first part of Fig.9 is a stacked layer of much thinner steel laminations. The thickness of each steel laminations varies from 0.25 - 0.5 mm.
The output voltage of a transformer will undergo minor fluctuations due to the reasons like load variation and change in power input supply. A tapping mechanism in the secondary coil helps in regulating the output voltage to the specified limit. The tapping mechanism simply changes the number of active coils in the transformer action, thus controls the output voltage. Since more number of turns are there in the HV windings, voltage fine tuning can be more accurately controlled by providing the tapping on the HV side. This is another reason why HV windings are not placed near to the core. If they were placed near to the core, movement of tapping mechanism would have been more difficult, causing the tapping design more complex.
Energy losses in a Transformer
Various kinds of energy loss happen while transferring power from the primary to secondary coil. Following are the major source of energy losses.
- Eddy current loss
- Hysteresis loss
- I2R loss
Use of the Conservator Tank
Oil in the tank will expand as it absorbs the heat. A conservator tank helps to accommodate for this volume change. As can been seen in Fig.11, there is a free space above the oil, in the conservator tank. When the oil expands, this space shrinks and accommodate for the volume rise.