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Transformer Science

20/25 MVA Oil-filled Transformer

Transformer is an electrical static machine whose aim is to increase and decrease an Alternative Voltage.

Runs on magnetic affect, without any part being in motion.

Usefulness of Transformers

The transport of electrical energy at high voltage allows to convey less current in the lines (P = 3 V I cos ).

For given amount of losses, transportation under limited current, allows to use conductors with lower cross sectional areas, i.e. cheaper! Also structures that can stand these conductors will be less restrictive because a lower cross sectional area also means a reduced weight.

That is why it is cheaper to transform the voltage and use intermediate voltages, especially when distances become longer…

The transformer also provides a galvanic insulation, i.e.. the primary and the secondary are physically interconnected.

It is an important property of the transformer, which enables to completely insulate two circuits; This is essential if, for example, the ground connection diagram needs to be changed.


The single-phase transformer is never used for power distribution, because energy is distributed in three-phase current. It is described in order to explain the principle of a transformer.

The magnetic circuit is made up of a soft iron – silicium sheet stack, isolated among them in order to avoid any energy loss by eddy current flows. The use of a massive block would generate much more losses.

The primary includes a winding made up of N1 turns. This winding is subject to the U1 voltage and an (alternative) current I1 passes through it. This current induces an alternative magnetic flux Φ1 in the magnetic circuit. It can be demonstrated that the primary voltage is proportional to the number of turns, flux, and the frequency. The secondary being located on the same magnetic circuit as the primary, it receives the flux Φ1 which creates a voltage U2 at the secondary where U1/U2 = N1/N2.

If a receptor is connected to the secondary, a current I2 flows. This current I2 creates a flux Φ2 which is opposed to the flowΦ1. But U1 sets the complete flow in the magnetic circuit.

And as U1 keeps the same value (network), the complete flux should stay constant, so there is a need for current at the primary, which will compensate the flow decrease generated by I2. By considering a perfect transformer (without losses) and the power conservation principle, therefore U1.I1 = U2.I2, therefore I1/I2 = N2/N1

In fact, the transformer involves Joule effect losses and magnetic losses which are responsible for its overheating and voltage drops.

The ratio of the secondary tension to the primary no-load voltage U1/U2 = N1/N2 = I2/I1 is called transformation ratio.

Here is shown a wye coupling. In this case the primary and secondary neutral are accessible, but not necessarily outside.

The couplings could be delta or zigzag (see next slides). The delta couplings do not give acces to the neutral.


Rated power


The passage of active power through the transformer will generate losses (which must be removed (air, oil cooling, combination of the two, etc.).

We will therefore have an active power supplied to the primary a greater than the power at secondary.

Copper losses: these are the losses by joules effect in the conductors of the primary and secondary windings which have a certain resistivity and will therefore dissipate heat. They depend on the current, and therefore on the secondary load. Iron losses: these are the magnetic losses in the magnetic circuit which will also contribute to the heating.

A distinction is made between eddy current losses originating from the currents induced in the magnetic circuit. They are limited by the laminated structure of the magnetic circuit. Then there are the hysteresis losses due to the “core” of the magnetic circuit. In a simple way, we can imagine that magnetic induction wants to remain at the value it had and that it is necessary to provide energy to vary it (we are in alternative).

They practically depend only on voltage and frequency and are therefore considered to be constant. The yield will therefore be logically better for a high cos , since in this case, all the power passing through the transformer is transformed into usable power. If, for the same apparent power of the load, its cos  is bad, there will be as many losses to the transformer, but less active power will be used by the load. The magnetization of the magnetic circuit and the losses of magnetic flux are at the origin of the reactive power consumption specific to the transformer, in addition to that of the load; in general, the ratio between the reactive power consumed and the rated power increases with the size of the transformer.


Short-circuit voltage

Values of efficiency and voltage drop

Voltage setting

No-Load or Dead Setting
1. Generally 5-position switch
2. Can be handled while DEAD
3. Standard range : -5%; -2,5%; 0; +2,5%; +5%
4. Setting is done at primary

Voltage Control

Click format phase

Cooling methods



Other cooling method

Transformer Classes

Filling liquids

1. Mineral Oil : reduced cost, but flammable
2. Silicon Oil : lowly flammable
3. LIHT : nonflammable synthetic liquid, without PCB


1. The temperature of the dielectric changes continuously : it expands and it contracts.
2. The volume variation of the dielectric should be absorbed.
Two principles :
a. Free breathing Transformer
b. Sealed Transformer

Dry-type Transformer

Designation Examples


The homopolar generator

1. When the secondary of the main transformer is delta coupled, there is no access to the neutral
2. An artificial neutral artificial with the aid of a homopolar generator is created.
3. Frequently, the Y-Δ (Wye-Delta) transformer is used.
4. Y ’s neutral is grounded and a resistor connected in series in the Δ.

To avoid the passage of harmonic losses through the transformers, we install, if possible, a different coupling at secondary and primary. If a system is supplied from a star delta transformer, it is not possible to remove the neutral from the secondary. It is then necessary to create an artificial neutral. We use for this a zero sequence generator which is nothing other than a star triangle transformer.

There are several ways to use transformers as zero sequence generators. In other installations, the procedure is as follows: the primary of the zero sequence generator is connected to the network and is wired in STAR. We therefore have access to the neutral of this star. We connect it to the earth. The secondary is coupled in a triangle. We insert a resistor in series in the triangle. If the network is balanced and there is no fault, no current flows through the resistor, the secondary being empty. When there is a fault, a zero sequence current flows in the network and the current passing through the resistor is an image of the zero sequence current. The zero sequence generator also plays the role of fault current limiter thanks to the impedance of the primary. When an earth phase fault appears, it goes back up through the neutral of the primary of the zero sequence generator and loops through the network.

Protection – Control – Signalling

1. Buchholz relay

2. Overpressure Valve
Detects and eliminates overpressures in the tank

3. Fast working
The Protection Relay of the switch
Transformer with on-load tap-changer
Protects the on-load tap-changer and the transformer.

Pressure relief valve: This equipment is sensitive to a sudden overpressure in the tank of the device and immediately eliminates it thanks to its rapid opening (a few milliseconds). It is usually placed on the cover of the device it protects. Operation: During an incident, the opening pressure (0.2 to 0.7 bar) is reached.

The valve opens. The dielectric is ejected as long as the valve is open. Switch protection relay: In the event of a fault in the on-load tap-changer, this relay must protect the latter and the transformer. The operation of the protection relay necessarily causes the transformer to be de-energized.

4. The automatic valve
Combined with the BUCHHOLZ, the valve saves the loss of dielectric liquid in case of rupture of the connection piping.

5. Drier
Absorbs the humidity of the aspirated air when the liquid is in contraction stage.

Automatic valve: In the breathable transformer, in the event of a rupture of a pipe, a joint, or essentially a terminal, the valve prevents the loss of dielectric liquid contained in the expansion tank.

Dryer (air dryer): In the breathable transformer, when the liquid contracts, there is a call for air. The desiccant absorbs moisture from the air through a dehydrating product. If the volume variations are small, the drier isolates the air intake to avoid permanent contact of the desiccant with atmospheric air.

6. Thermostats
Start ventilation (ONAN / ONAF)
Alarm and triggering

7. Level
Checks the expansion of the tank’s level

8. Thermal image
Gives the highest temperature of the winding (temperature & current control)

Thermostats: Measuring the temperature of the windings and the dielectric. Used for Switching on the ventilation of radiators or air coolers of power transformers; The “alarm” signaling and triggering These devices consist of a thermometric assembly comprising a probe which can be mounted in a thermowell.

The thermal image: This device includes thermometric equipment associated with a heating resistance traversed by a current which is a reflection of the current passing through the winding whose temperature is to be controlled.

9. Surge Arresters
Connected between phase and ground
Protection against over-voltage due to power cuts and closings of the circuits

10. Spark Gap
Limits the voltage to a known value.

11 DGPT Relay (Sealed transformers)
Detection of Gas Pressure Temperature
For small-sized transformer.
Same functions as the Buchholz relay (gas detection), additionally
A. Indication of any excessive pressure
B. Indication of any temperature increase.

The temperature detection can be on two levels: triggering of an alarm or triggering of an upstream protection.

Types of connectors

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