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Commercial Transformer Basics
Once wiring is pulled in the ground from the building all the way around the transformer and the pads are down, we go from the high transformer to the low, which is the step-down.
We pull in our high voltage, our low voltage, our ground, we’ve done the 2 high voltage lugs and then we’re ready to ground it. We scrape around where we’re going to land our grounds by scraping the paint off and trying to get down to the metal before grounding it.
Transformers are capable of receiving AC power at one voltage and delivering that power at another voltage. The purpose of transformers is to help achieve better transmission efficiency while transferring the power over longer distances.
In thinking about how a three phase transformer, we can start by looking at it in the most simple terms. At the core, a transformer is simply electromagnetic induction. According to this principle of varying magnetic flux associated with a loop will induce an electromotive force across it.
A fluctuating magnetic field like this can easily be produced by a coil and an alternating EMF system. A current carrying conductor produces a magnetic field around it, while the magnetic field produced by a coil will fluctuate around the coil. Because of the fluctuating nature of the alternating current, the magnetic field associated with the coil will also fluctuate accordingly.
It’s possible to effectively link this magnetic flux to a second winding through a core made up of ferromagnetic material. This fluctuating magnetic field will cause an EMF in the secondary coils due to the electromagnetic induction present. Because the turns are arranged in a series of turns, the sum EMF induced across the winding will include the total of the individual EMFs induced in each individual turn.
Because the same magnetic flux is passing through both the primary coil and the secondary coil, the EMF per turn for both the primary and secondary coils will be the same. The applied input voltage dictates the EMF per turn for the primary coil. As a result, the induced EMF at the secondary coil is expressed as follows.
What this means is that it’s possible to lower the voltage with fewer turns in the secondary than in primary. For the reverse case, you can increase the voltage. Since that energy is conserved, the primary and secondary currents must obey the required relationship. The way three phase transformers work is they use three such single phase transformers, albeit with a slightly different coil configuration.
In that case, the primary and secondary coils sit concentrically. A three phase transformer requires two more of these windings. Transformers that have high power ratings generally use a special winding often referred to as a disk type winding. Separate disk windings are usually connected in series through outer and inner crossovers.
Coils and Windings
The low voltage windings are connected together in a delta configuration and the high voltage windings are connected together in a star configuration. Thus, the line voltage further rises to route three times at high voltage side. What this means is that from a three phase step up transformer, it is possible to draw four output wires. The four wires include three phase power wires and one neutral. High Voltage insulated bushings are required to bring out the electrical energy.
The core of the three phase transformer is made of a thin, insulated steel lamination. Such steel laminations are stacked together as shown to form the three phase limbs. The primary purpose of thin laminations is to reduce the total energy loss due to the eddy current formation.
The low voltage windings usually sit near the core and various kinds of energy loss happens while transferring power from the primary to secondary coil. All these energy losses are dissipated as heat. Transformers like these are usually immersed in a cooling oil to dissipate the heat and keep the temperature low. The oil dissipates the heat via natural convection. Oil in the tank will expand as it absorbs the heat, so a conservator tank helps to accommodate for this volume change.
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