Power travels from the power plant to our houses through a complex system called the power distribution grid.
For power to be useful in a home or business, it comes off the transmission grid and is stepped-down to the distribution grid.
In the distribution grid transformers are essential elements to changing the voltages to reduce how much energy is lost during the electrical transmission process.
A transformer is in fact a static electrical device that uses the electromagnetic induction principle to convert an alternating current signal from one electric circuit to another, often changing or transforming the voltage and electric current.
Without transformers, electricity generation and transmission over long distances would not be possible, nor would it power entire cities and industrial complexes.
Many different transformer designs is encountered in electronic and electric power applications. The transformers are classified based on voltage levels, Core medium used, winding arrangements, use and installation place, etc…
Here are some examples:
STEP-UP TRANSFORMER: in which the secondary voltage is stepped up with a ratio compared to primary voltage.
STEP-DOWN TRANSFORMER: used to step down the voltage level from lower to higher level at secondary side.
AIR CORE TRANSFORMER: where both the primary and secondary windings are wound on a non-magnetic strip where the flux linkage between primary and secondary windings is through the air.
IRON CORE TRANSFORMER: where both the primary and secondary windings are wound on multiple iron plate bunch.
AUTO-TRANSFORMER: where one single winding is used as primary winding as well as secondary winding.
POWER TRANSFORMER: its large size allows to be suitable for high voltage power transfer applications.
DISTRIBUTION TRANSFORMER: which distributes the power generated from the power generation plant to remote locations
MEASUREMENT TRANSFORMER: used to measure the electrical quantity like voltage, current, power, etc.
JAES in its catalogue, offers a wide range of different types of transformers from the major manufacturers.
In this video we will focus on the working principle of a three-phase transformer.
This type of transformer has no moving parts and is a completely static solid state device, which insures, under normal operating conditions, a long and trouble-free life.
According to electromagnetic induction principle, a varying magnetic flux associated with a simple coil loop will induce an electromotive force across it.
An alternate current in a coil produce fluctuating magnetic field. In the specular way, an external fluctuating magnetic field generate a current on a conductive coil.
So, as we said, every current flow produce a magnetic field around it. In this animation you can see the magnetic field produced by one coil.
Due to this relation between current in the coil and its magnetic field, a current fluctuation imposes to the generated magnetic field magnitude a fluctuation as well.
This magnetic flux can therefore be conveniently shared by a secondary winding. To minimize the lines of magnetic force that are not mutually shared, the primary and secondary coils are both winded around a high permeability material, typically ferromagnetic cores.
In the magnetic circuit, where almost all the magnetic flux is confined, the Magnetic field magnitude fluctuates due to the alternating current on the primary winding. In the secondary winding’s coils the current is imposed due to the very same magnetic field.
The windings are a coil series sequence and the current will be the same on all the series, but on the other hand, the total tension will be the sum of the EMF generated in each coil.
Now it is clear that the magnetic field in all primary and secondary coils is virtually the same. In the same way each coil has the same EMF
If we look closer in the primary winding, each coil has an imposed EMF equal to the applied tension, divided by the number of coils.
On the other hand the inducted EMF on the secondary winding is:
We end up to a very easy general rule: we can lower down the tension with a fewer number of coils in the secondary winding than in the primary one.
Of course the reverse situation works as well, we can increase the voltage if the secondary winding has more coils than the primary.
Due to the energy conservation law the currents in the primary and secondary winding follow this rule:
Multiphase and specifically three-phase transformers follow the same rules of the single phase just with a different winding’s and magnetic circuit configurations.
In this configuration, primary and secondary coils are concentric, for a three phase transformer you need to multiply this winding configuration by three.
In case of high power transformers, it is commonly used a special kind of winding known as a Disc type winding where alternatively inner and external coils are alternated
Here you can see how the low voltage windings are connected in a delta configuration. The high voltage windings instead, are connected in a star configuration.
On the high voltage winding the voltage rises up to 3 times more.
From a three-phase transformer you can obtain 3 monophasic lines by using the three-phase lines and one neutral in common.
The electrical energy needs high-voltage insulated bushings to facilitates its passage through the grounded tank of the transformer.
The wavy shape of the bushings is to maximize surface path length and minimize surface leakage, corona, and eventual arcing from exposure to year-round weather conditions, air pollution, dust, etc…
We can notice that the core of the transformer is formed by a layer of thin isulated steel laminations.
More layers of steel laminations form three-phase limbs.
These laminations allow to minimize the eddy current.
Eddy currents in a transformer core represent an unwanted loss. They absorb energy from the supply and reduce the transformer efficiency. Every effort is made to reduce them. They cannot be completely eliminated, but they can be reduced as much as possible.
The low voltage winding are placed near the core.
When the power is transfered from the first coil to the second coil, many kinds of energy loss may occur.
All these energy losses represent heat. For this reason transformers must have some circulation of coolant to remove the waste heat produced by losses.
Three-phase transformers are usually immersed in a cooling coil, to dissipate the heat.
The oil, simply circulates from the coils and the radiator and dissipate heat by natural convection. An oil tank is used to compensate the volume expansion due to the rising temperature.