Dry Type Transformers (Explained for Beginners)

A transformer with no oil or liquid is a dry type transformer. The term “dry type” merely refers to the fact that it is cooled by airflow through the air ventilators i.e no liquids are used.

Dry-type transformers are environmentally friendly and have shown to be highly dependable throughout time.

A dry-type transformer’s electrical core and coils are not cooled by a liquid such as transformer oil, silicone, or any other liquid.

This type of transformer needs less maintenance cost. Due to fewer chances of fire, toxic gases, or any other hazardous threats, these types of transformer are installed in hospitals, educational institutions, chemical industries, and other factories.

Traditional transformers use the liquid for insulation of windings and their core.

Why Do We Use Dry Type Transformers?

For locations prone to fires, dry-type transformers are particularly useful there. Moreover, the stations come with more minor regulatory requirements for installation, operation and maintenance, which come with their own set of costs to implement. This includes fewer fire safety guidelines to worry about during process and storage.

Unlike typical liquid transformers, dry-type transformers do not need to be stored in basins or fireproof vaults, and they do not emit poisonous fumes.

Because dry-type transformers are incredibly safe, they may be used to power both indoor and outdoor loads, making the entire system more reliable.

Types of dry type Transformers

Due to different environmental conditions, different types of dry type transformers are in use. Here we will discuss different types of dry-type transformers.

Open Wound Transformer

This type of transformer got its name from its manufacturing process. This process uses the dip and bake method, the transformer’s coils are first heated, then dipped in varnish and baked in the end.

Vacuum pressure Impregnated Transformers

These dry-type transformers’ materials are less inflammable. And its windings made in strips or foil. To make it suitable, the windings are made of disks, These disks are connected either in parallel or in series for the required voltage level and power rating.

The insulation of windings is made of polyester resin. After the insulation, the next process is to place the primary & secondary windings in a vacuum protective box.

These transformers are one of the best alternatives to protect from moisture. While talking about the ratings these are available in the range from 5KVA to 30MVA.

Vacuum pressure Encapsulated Transformers

In this type of transformers, silicon based resin is used instead of polyester. Silicon resin coating completely protects transformers from humidity, salt, and moisture.

This type of transformer works safely in a harsh environment, where there is the susceptibility of alkalis, acid, or chlorides.

Cost coil

The coils of these transformers are solidly cast in epoxy resin. These types of transformers are most suitable for harsh weather conditions and moisture.

Therefore the main installations places are nuclear plants, offshore platforms, production sites, buildings, and tunnels.

Advantages of dry type transformers

Instead of one, there are many advantages of using a Dry Type Transformer listed below:

  1. These types of transformers ensure the safety of people and premises.
  2. Easy installation, maintenance, and environment friendly       
  3. Available in different ratings i.e. from 5KVA to 30 MVA.
  4. Fire and Electric shockproof.
  5. long lasting and excellent performance in harsh weather conditions.
  6. Dry type transformers are eco-safe since they don’t cause pollutant emissions by releasing harmful gases.
  7. Contaminated and damp areas can benefit from their use.
  8. Dry-type transformers are durable due to their dielectric heating and low thermal properties.
  9. They feature a high resistance, which aids in short circuit current management.
  10. They feature outstanding performance and modest side clearance.

Disadvantages of dry type transformers

  • As these type transformers works for a long time, but once if the coil is damaged. The whole assembly will need replacement.
  • These transformers is costly than other liquid containing transformers.
  • Larger in size compared to the same power rating oil immersed transformer.

Difference between Dry type and Oil type Transformers?

Dry type Transformer: To keep dry-type transformers cool, they need an insulating system that encourages air movement. As a result, the units are larger than oil-filled variants. Dry-type variations also require more resources, resulting in increased prices.

Because natural cooling processes are not always consistent and can be readily altered by the surrounding environment, the power distribution units work at higher temperatures in terms of performance. As a result of these flaws, Life expectancy is reduced.

Oil type Transformer: Combustible liquids are used to cool oil-type transformers, making them ideal for outdoor use. Advancements in oil-filled transformer technology have allowed the items to be utilized in countries with stringent environmental regulations by replacing volatile oil with more stable (less flammable) liquids.

Oil and liquid-type transformers provide superior cooling than dry-type transformers, regardless of the kind of liquid employed because, the liquid is a complete cooling medium. Smaller, more compact units are made possible by more effective cooling methods.

For medium-scale power distribution, both types of transformers are widely used. The benefits of each kind are determined by the target location, project/end-user needs, transformer size/capacity requirements, losses, and prices.

Indoor settings will almost certainly utilize dry-type transformers, despite the fact that they are typically bigger (and hence take up more space) because fire safety rules are less rigorous. Oil-filled units are beneficial in areas with greater load needs and outdoor power distribution.

How Do You Test A Dry Transformer?

Among the routine tests that should be conducted on all dry-type power transformers are:

Dielectric Test: 

The applied voltage waveform for single-phase should be roughly sinusoidal. The test must be carried out at the prescribed frequency.

Before disconnecting, the test voltage must be rapidly lowered to 1/3 of the maximum value. All of the windings must be tested. If no failure occurs at the ultimate test voltage, the test is considered successful.

Induced Voltage Test:

While performing this test, the test voltage must be double that of the rated voltage. It should be applied between the terminals of the secondary winding while keeping the primary winding open.

The test period at maximum voltage must be one minute, and the frequency must be double the rated frequency.

Voltage ratio measurement: 

On all tap changer locations, voltage ratio measurements and polarity and connection checks must be done.

It’s also good to double-check the numbers allocated to the taps and the ratings. Measurement of voltage ratio must be done phase by phase between the terminals of matching windings. The voltage ratio is measured using the potentiometric approach.

Load Loss Measurement and No-load Current: 

Supply LV windings are used to perform this test at the rated frequency and voltage. The waveform should be as close to a sine wave as feasible, and the primary windings should be open.

The frequency of the test must not deviate by more than 1% from the rated value. No-load current and loss, as well as the voltage’s mean and effective values, must be monitored.

The average value of three measurements taken by effective value ammeters will be used to calculate the no-load current.

When necessary, instrument transformers and transducers will be utilized to measure the power with three-watt meters.

Measurement of Winding Resistance: 

When the windings are at ambient temperature and without power for a period of time sufficient to reach this State, winding resistance measurement should be done.

According to the measurements must be done in direct current between terminals. Temperatures in the surrounding environment must also be taken into account. It will be calculated as the average of three thermal sensor values.

Short Circuit Impedance: 

The transformer’s performance is shown by the short-circuit loss and short-circuits voltage. The transformer’s HV windings are powered while the LV windings are short-circuited.

During the measurement, the current must be at IN or as near to this value as feasible. During the measurement, each phase’s voltage, current, and short-circuit losses should be monitored.

The measurement must be done quickly in order to prevent raising the winding temperature due to the applied current, and the measuring current must be kept between 25% and 100% of the rated current; by doing this, the measurement inaccuracies are caused by the rise in winding temperature will be reduced.

By performing all upper mentioned tests, you can ensure the proper operations of your transformer; if test values are not according to the correct operations values of the transformer conducted test will be considered failed, and you need to take the necessary measures to fix the fault to avoid any loss or incident.

How Long Do Dry Type Transformers Last?

Dry-type transformers can last much longer than 25 years. However, do not allow them to become damp, unclean, or buried beneath “things.” They despise both water and heat. If the airflow is obstructed, they may overheat, reducing their dependable Life expectancy.

So, if you keep your general-purpose, dry-type, air-cooled transformer dry, clean, and aired, it may survive 30, 40, or even 50 years.

For many electricians and plant engineers, the new criterion is that it should survive until you retire.

Also, while these transformers appear harmless, they are highly hazardous and can be lethal if they blow up or something gets trapped within the enclosure.

What is Protection in Dry Type Transformers?

Overheating Protection :

With a specific relay that monitors the sensors within the winding’s of a dry type transformer, they are protected from overheating due to the overloads.

Internal faults and short circuits:

In dry type transformers, primarily inter turns and phase-to-earth short circuits, are cleared using circuit breakers or fuses installed on the main section of the transformer.

When circuit breakers are triggered, the phase-to-phase and phase to Earth over current safeguards start the tripping process.

IP protection:

Depending on the climatic parameters and protection needs, dry-type transformers can employ a variety of shells. The IP20 protective cover is often used to keep solid foreign objects more prominent than 12mm in diameter out of the live parts while also providing a safety barrier.

It can keep tiny creatures out (rats, snakes, cats, birds, and so forth), resulting in dangerous problems such as short circuit power failure. The IP23 protective shell can be utilized if the transformer is positioned outside. It may also prevent water droplets from entering within 60 degrees of the vertical line and the IP20 protection.

However, the transformer’s cooling capability will be affected by the IP23 casing. When making your choice, keep in mind that the operational capacity has been lowered: the capacity has been reduced by roughly 5%. A stylish and robust aluminium alloy cover is usually provided by the manufacturer.

Unlike oil immersed transformers, Buchholz relay is not used because it’s a gas actuated relay.

Read my other article, What is transformer mechanical protection.

What Causes a Dry Type transformer to Fail?

Locking bolts that are loose could result in core laminations becoming loose and the coil moving or coil displacement, both of which could cause destruction to the structural integrity of the transformer, make the transformer noisy, and ultimately lead to failure.

Instead of Loose Locking Bolts, three primary reasons are listed below that lead dry-type transformers to failure:

Salt-laden atmosphere

This can lead to corrosion of the copper or aluminium conductors used in dry type transformer windings, the terminals, the silicon steel core and the carbon steel clamping structure that can be the failure of dry type transformer.

Effects of moisture

Moisture is the biggest enemy of any electrical device, especially the dry type transformer, and the negative consequences of moisture are more dangerous in offshore installations.

Moisture decreases dry type transformer dielectric property of insulation and is especially difficult for their coils.

The moisture on the surface can cause being tracked over the insulation surfaces, especially for medium voltage dry-type transformers. Read my article, Moisture in transformers causes and solutions.

Chemical Reactions

On drilling rigs and industrial locations, a variety of chemicals are used, and these chemicals can harm transformer insulation and conductors.

Exhaust fumes from engine-driven generators, for example, may penetrate the transformer enclosure and deposit carbon on coils, causing tracking and eventual Damage to the transformer insulation system.


If the transformer has weak or no overcurrent protection, any overcurrent will cause the transformer to heat up and can burn it out.


Overvoltage decreases the insulation resistance of the winding. The lower the resistance the higher the internal short circuit chance. As you know internal short circuit is a destructive fault for transformers, motors, generators and all other electrical devices. I have written a detailed article about electrical short circuit, and why is it dangerous. You can read it for more information.

Read my other article about Transformer faults, for more information.

Can Dry Type Transformer Explode?

Because a dry-type transformer does not utilize harmful fluids to cool its system, it does not require any gas ventilation.

A dry solution is less likely to catch fire or explode when overloaded since it contains no combustible substances.

Because of their proven safety features, air-cooled transformers are the recommended equipment for interior applications.

They’re perfect for hospitals, schools, and other public buildings where security is paramount.

Components of Dry Type transformers

A dry transformer is an appliance that allows for the transfer of high-voltage electric energy with little power loss.

Active parts of dry type transformer are the iron core, low voltage winding, and high voltage winding.

These are the three main components of any type of transformer. Other components in transformers include insulation, cooling fans, protective relays, and enclosures. Next, let’s look at each of these components in more detail:

Transformer Core

three phase iron core transformer
3 Phase Iron Core Transformer Winding

The core supports the primary and secondary windings by providing a low resistance channel for electromagnetic flux. It’s constructed by stacking thin sheets of high-grade grain-oriented steel separated by insulating material.

The carbon Content of the core steel is kept below 0.1 per cent to keep hysteresis and eddy currents to a minimum. Eddy currents can be decreased when it is alloyed with silicon.

Transformer Winding

Primary and secondary windings are carried by the transformer for each phase. This winding comprises several turns of aluminium or copper conductors, which are separated from one other and the transformer’s core.

The form and configuration of transformer winding are determined by the current rating as well as the short circuit’s capacity, the strength of the circuit, rise in temperature impedance, and surge voltages.

Transformer Insulation

Between the windings and the core, between the primary and the secondary windings, between each turn of the winding, and between all current-carrying elements and the tank, insulation is necessary.

It is imperative that the insulators have high dielectric strength, good mechanical properties, and be able to withstand high temperatures.

Among the materials used to insulate transformers are synthetic materials, paper, cotton, and others.

Transformer Tank

The main tank is a component of a transformer. It serves two functions: one is to protect the core and windings from external influences, and secondly, it provides support to the other accessories of transformers.

Fabrication of rolled steel plates into containers is used to create tank bodies.

As part of the package, lifting hooks and cooling tubes are included. Aluminium sheets are utilized instead of steel plates to save weight and prevent stray losses. On the other hand, Aluminium tanks are more expensive than steel tanks. 

Transformer Bushings and Terminal 

Transformer bushings
Transformer Bushing from Inside The Transformer

Bushings serve as insulators between the terminals and the tank.

They’re affixed to the tops of the transformer tanks. They provide a safe path for the conductors that link terminals to windings.

Porcelain or epoxy resins are used to construct them.

Read my detailed article, Transformer bushing, what you should know.

Transformer Tap changers

Transformer secondary voltage is adjusted via tap changers. The transformer’s turns ratio is capable of being adjusted by these devices as needed.

Two types of tap changers exist: On-load tap changers and off-load tap changers.

On-load tap changers can work without stopping the current flow to the load, but off-load tap changers can only operate when the transformer is not providing any loads.

There are also tap changers that change the taps automatically as per requirement. Transformer Air Ventilators.