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Parts of Transformer

Parts of Transformer

Parts of Transformer:

Today, we will discuss the components of a transformer and generally about a power transformer. There are many types of Transformers according to their usage such as,

and many more.

But today we are concerned with the Power Transformer. The reason is that power transformers have more parts than the others, so it is better to discuss them for studying parts of the transformer.

If you are not aware of the working principle of a transformer, please read this article (Working Principle of Transformer).

Power Transformer:

A power transformer is the one that is used in the transmission and distribution of power. It is usually rated in 500KVA or above. It is used for increasing the voltage for transmission and at the distribution end as well.

Parts of Power Transformer:

The main components of the power transformer are,

  • Core
  • Winding
  • Main Oil Tank and Conservator Tank
  • Breather
  • Buchholz Relay
  • Bushings
  • Tap Changer


Core provides less reluctance to the magnetic flux, and it also holds the windings. It is made from Cold Rolled Steel or Hot Rolled Steel. These steel types contain different amounts of silicone. The core is formed by joining thin laminations. These laminations have varnish coating. Due to lamination, eddy current losses are reduced. The lamination is U-shaped, L-shaped, E-shaped, and I-shaped.

Shapes of Lamination a Transformer

These laminations will form the core of the transformer. The part of the core on which windings are attached is called Limb or Leg (Vertical one), and the remaining part is called Yoke (Horizontal one).

Core a of Transformer


Winding is referred to as the laminated wire (conductor), which is wounded on core and given several numbers of turns. The commonly used metal conductors are copper and aluminum. As I have discussed earlier in my article (Working Principle of Transformer), the number of turns depend on the requirement of voltage and is selected on behalf of the required output voltage. The cross-sectional area of the wire depends on the amount of current it needs to bear. Winding is usually placed on a core (a ferromagnetic material) because it has greater potential to link flux of primary to secondary than air. There are commonly two types of windings that are found in a transformer.

  • Concentric Winding
  • Sandwich Winding

Concentric Winding:

In concentric winding, the windings are placed on the two limbs of the core. Consider a core of rectangular shape as in the following picture,

The windings are placed on both the limbs. You may think that the High-Voltage (HV) winding should be on one limb and the Low voltage (LV) winding on the other limb. This arrangement is possible, but it is not done in the concentric winding because there is a distance between the limbs; therefore, some of the flux will not link with the other winding. This loss of flux is termed as Leakage Flux. To minimize the leakage flux, half of the LV winding is placed on the limb (1), and half is placed on the limb (2). The half HV winding is placed above the LV winding on the limb (1), and another half is on the limb (2) above the LV winding. There is insulation between the core and LV winding and LV winding and HV winding. Such type of winding is done on Core-type Transformers.

Concentric Winding

Sandwich Winding:

In Sandwich winding, the HV and LV windings are subdivided into sections, and all the sections are present on the central limb of the core. The windings are in between the outer limbs of the core, which is behaving just like a shell. The sections are placed in such a way that the HV is sandwiched between LV windings. Consider the following picture,

Sandwich Winding

We can see that HV winding is sandwiched between LV winding, and the upper and lower LV winding are the parts of a section. This arrangement reduces the amount of leakage flux. This type of winding is used in the Shell-type Transformers.

Why is LV placed on the core, and HV is above LV in Concentric Winding?

The thickness of insulation depends on the voltage level of the coil. If the voltage is high, then the thickness will be more. If we place HV winding on the core, then it will require more insulation between the core and HV. This arrangement is possible, but it will increase the cost. Therefore, LV is placed on the core.

Main Oil Tank and Conservator Tank:

The main oil tank and conservator tank contain transformer oil.


The oil of a transformer serves as insulating and cooling material. It cools the winding and provides insulation. It also saves winding and core from moisture and dust. Therefore, the oil should have good dielectric strength so that it will not conduct. It should not contain impurities that can form sludge and other corrosive metals. The viscosity should be low so that the heat can be transferred easily.

Main Tank:

The main tank of a transformer contains core, winding, and oil. The tank in small transformers is not completely filled with oil. It contains air as well. There is a vent pipe connected to it to allow in and out of the air. When the load increases, the oil in the transformer expands, and the air is released out.

The tank of the bigger transformer (more than 50KVA) is completely filled. The reason behind this is when a short circuit occurs, the oil and air form an explosive mixture, which is dangerous. In the case of overloading, sludge is formed due to the presence of air. To avoid this, the tank is completely filled, and a conservator tank is provided for the expansion and contraction of oil. As the tank is completely filled, there is no moisture, dust, and air present in the main tank. It is also provided with an explosive vent which contains a glass diaphragm on it. When the pressure of the oil rises above the extreme value, then the oil comes out by breaking the glass diaphragm. The pressure of oil is released, which saves the tank from the explosion.

Conservator Tank:

The transformers that have completely filled main tanks are provided with a conservator. It is a small tank connected with the main tank. The conservator is placed over the main tank. It is not completely filled with oil; nearly half of its space is filled with air. When the oil in the main tank expands, some of the oil is transferred to the conservator tank, and the air in the conservator tank is expelled. It saves the main tank from the exposure of air and moisture. It is provided with a Breather that catches moisture from the incoming air.

Transformer Parts


It is an apparatus that is used to capture the moisture of the air entering the conservator. It contains Silica Gel crystals that are blue in color and turned to whitish-pink when reacted with moisture. A glass is provided with it to observe the color of the Silica Gel. When Silica gel is saturated with moisture, it is replaced by the new one.

Buchholz Relay:

It is a protective device. It protects the transformer from faults. In normal conditions, it is completely filled with oil. When the transformer is overheated due to any fault, the oil evaporates. The vapors leave the main tank and travel to the conservator tank. The Buchholz relay is located between the main tank and the conservator tank. The vapors start to accumulate at the top of the relay. When the float area is filled with vapor, then float hits the mercury switch and completes the alarm circuit. The alarm means there is something wrong. If the vapors accumulate the whole space, then the Trip circuit is completed by the flap, and the transformer supply is broken. There is a release cock present at the top by which the pressure of vapor is released after the operation of the relay. The amount of vapors depends on the severity of the fault.

Buchholz Relay


Bushings are insulations with a conductor in between the insulator body. Their purpose is to insulate the terminals from the body of the transformer. The terminals from windings are taken out with the help of bushings. The bushings are selected on the basis of voltage and current ratings. The insulator can bear specific voltage, and the conductor can pass specific current safely. For low voltage transformers such as (11KV), porcelain bushings are used. In porcelain bushings conductor is surrounded by porcelain.

For High Voltage Transformers (below 66KV), oil-filled porcelain bushings are used. In oil-filled porcelain bushings, there is oil present between the terminal and porcelain, which provides more insulation than a simple porcelain bushing. Gas (usually SF6 gas) insulated bushings are also available for high voltage applications.


Tap Changer:

Tap changers are the devices that are used to change the voltage level. They placed on the High Voltage side of a transformer so that the current through it will be less. I have discussed in my previous article (Click here to read the article), the voltage depends on the number of turns. Tap changers change the voltage by changing the number of turns. There are several tappings taken out from the HV (High Voltage) winding to change the voltage level. There are two types of tap changers on the basis of their use,

  • Off Load Tap Changers
  • On Load Tap Changers

Off Load Tap Changer:

The offload tap changer works when the load is disconnected. It changes the number of turns by moving arm to the required tapping. Consider the following picture,

Off Load Tap Changer

Here the HV winding has six tappings in the picture. When the arm is between 1 and 2, the number of turns is highest.  The lowest number of turns will be at 5 and 6. The arm is moved, which changes the number of turns. Consequently, the voltage level is changed.

Off load tap changer should be operated at offload. To prevent any false attempt, there is a circuit breaker interlocked with it. Whenever anyone tries to change the tap in on load position, the circuit breaker is operated to cut off the load from the transformer.

On Load Tap Changers:

On load tap changer can be operated while the load is connected. It has two switches, the Selector Switch and the Divertor Switch.

Selector Switches are present at the tapping. By this switch, we can choose the tapping we want. This switch doesn’t undergo any transient because it is operated when the current is not passing through it.

Divertor switch is one that helps to change the tap in the on-load condition. It manages the transients by resistors. It is operated when the current is passing through it. Therefore, it is kept in oil for cooling and arc extinction.

With the help of these two switches, the tap changer is operated on-load without damaging the load. Let’s discuss its working. Consider the following picture,

On Load Tap Changer 1

The Divertor Switch is at position AB and Selector switch 4 is closed, which connects the Tap 4. If we want to shift the tap to Tap 3, then we will follow the following steps.

Step 1: We will move the Divertor Switch to B.

On Load Tap Changer 2

The Divertor is at B, and the current will now flow through R, which will reduce the current.

Step 2: The switch of Tap 3 is closed, and Divertor Switch is moved to BC. At this state, the current is divided into Tap 3 and Tap 4. The current is further reduced by passing through two resistors.

On Load Tap Changer 3

Step 3: The Divertor Switch is moved to C, and Tap 4 can be disconnected.

On Load Tap Changer 4

Now the current is flowing from the Tap 3 through resistor R.

Step 4: Finally, the Divertor Switch is moved to the CD.

On Load Tap Changer 5

In this way, we can change the taps while the transformer is loaded.


We have discussed all parts of a transformer in detail. If you still have any questions feel free to ask and share it if you found this helpful.

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Reader Comments

  1. Can u please elaborate on the ‘load current’ variation as the diverter switch turns from A to D(in regards to magnitude).

    1. Yes, I can. It will be a pleasure.
      Consider the given pictures while reading this explanation.
      At first, the divertor switch is at Aand B and the selector switch 4 is selected. The current will follow the less resistant path and will not go through the resistor R. We have to change the selector switch to 3. Then divertor switch is moved to B, the current will pass through the resistor which means the magnitude of the current is decreased. We need to decrease the current in order to control transient. Now the Divertor switch is connected to B and C and both the selector switch 3 and 4 are closed. The current will have two paths and both having resistance R. Hence current will be divided equally. The switch is then moved to C and selector switch 4 is opened. The current will pass through resistance R. Now finally the switch is moved to C and D. The current will face no resistance, therefore, its magnitude is increased.

  2. Making sure that you are able to get the right transformer makes a lot of sense. I know that my uncle has been thinking about getting a power transformer. It might be helpful for him to know that he should check the load tap changers.

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