What is a dry transformer?

Dry transformers do not use any insulating liquid where the coil and magnetic circuit are placed inside. Instead of the coils and magnetic core being placed in a sealed container, they are pressurized by air.

Types of dry transformers

Dry type transformers are of two types. That is:

1.Cast Resin Dry Type Transformer (CRT).

2. Vacuum pressure Impregnated Transformer (VPI).

Dry Resin Type Transformer (CRT) 

Dry resin type transformers (CRTs) are used in areas with high humidity. That's because the primary and secondary coils are coated with epoxy resin. This helps prevent moisture intrusion from affecting the coil material. Complete protection by plastic wrap allows the transformer to operate without interruption in areas with high humidity. Therefore this type of transformer does not absorb moisture. This type of transformer has a capacity of 25 KVA to 12,500 KVA, with insulation class F (90oC Temp. Rise). This type of transformer has several advantages. outstanding. Those are:1. Good overload tolerance.2. Partial discharge with low loss. Therefore the performance is very good.3. Due to the non-flammable coil insulation, there is no fire hazard. Therefore, it is suitable for indoor installation.4. Can be placed outdoors in a case with protection level IP 45.5. And of course not hygroscopic.

Vacuum pressure transformer (VPI)

This type of transformer uses non-flammable materials as winding insulation materials. The coils of this type of transformer are made in the form of continuously layered metal foil. But for higher voltages, the coil is made in the form of a disc and is connected in series or parallel depending on the power level corresponding to the voltage level. The insulation of the coil is made of grade H polyester resin to avoid contamination. The primary and secondary windings with cores are securely bound in a vacuum-protected box. It protects against high humidity and is not affected by humidity. This type of transformer has a capacity from 5kVA to 30MVA with insulation levels F (155oC) and H (180oC). Along with protection level up to IP56. This type of transformer has many advantages. That is:

1. High mechanical strength.

2. No defects in insulation

3. No temperature fluctuations.

4. Easy maintenance.

5. Less risk of causing fire.

Advantages of dry transformers

Main advantages of dry transformers:- Safety for people and equipment.- Maintenance and pollution-free solution.- Easy installation.- Less site clearance.- Environmentally friendly - Perfect capacity to support overloads. - Reduced costs for civil installations and fire protection systems. - High productivity in case of seismic events. - No risk of fire Fire. - Good ability to withstand short circuit currents. - Extended life due to low temperature and dielectric heat rise. - Suitable for contaminated and damp areas.

Disadvantages of dry transformers

But dry transformers also have some disadvantages. These are:- Dry transformers have a long life and are less susceptible to winding problems. But once it is damaged, the entire structure will change, that is, the high voltage and low voltage side windings and the magnetic cylinder will be completely changed. - For the same power and voltage, the transformer Dry type will cost more than oil cooled transformer.
Application of dry transformer

Dry type transformers are widely used in the following industries:

– Chemical, oil and gas industry

– Environmentally sensitive areas (eg water protection areas)

– Areas at risk of fire (e.g. forests)

– Transformer stations in the city

– Underground and indoor transformer stations

– Renewable energy sources (e.g. offshore wind turbines)

Important factors for designing a dry transformer

The important design parameters for a dry transformer are given below.

Select insulation type

In general, insulation classes F and H are used to insulate the primary and secondary windings. This is because these layers can withstand high temperatures, i.e. 155oC for class F and 180oC for class H insulation. Varnish resins and polyester resins are commonly used as the insulation materials of the windings. In addition to temperature resistance and mechanical durability, dielectric strength and thermal shock resistance are also basic factors needed in choosing insulation for coils.

Choice of coil material

Usually copper and aluminum are used to make coils. Although copper wire conducts electricity better, aluminum wire is cost-effective and has a low weight. For the same current, copper conductors with less cross-section are used as materials in transformers. Copper coils have higher mechanical strength than aluminum coils.

Select magnetic core material with low hysteresis loss

Choosing magnetic core material is very important in transformer design. The magnetic core material must have high magnetic permeability and low hysteresis. But neither of those things can be achieved in any type of magnetic core material. Silicon steel, CRGO... are used to give low hysteresis and high permeability.

Voltage regulation

When the voltage on the secondary side of the transformer suddenly drops due to increased load, the voltage regulation is not good. Poor voltage regulation is due to the transformer having higher internal leakage reactance. Therefore, leakage reactance is kept at 2% during design.

Longevity

The life of the transformer depends on the destruction of the internal insulation due to the heating effect of the transformer due to overload. Usually insulation classes B, F and H are more suitable for dry transformers to withstand higher temperatures including ambient temperatures. Therefore, the transformer must be designed with temperature rise under full load operating conditions.

Loss

No-load losses mean core and eddy current losses independent of load conditions. But under load conditions copper losses including iron losses occur which increases the voltage regulation value, meaning poor voltage regulation. The leakage reactance and winding resistance must be within moderate values ​​to minimize these losses and provide better voltage regulation, thus higher efficiency.

Overload

Overloading for a long time will affect the transformer. Overload is the cause of overheating when the transformer must meet the load connection requirements. This causes copper loss inside the transformer, and causes damage to the transformer. To cool the windings, in dry transformers a cooling fan is used.

K coefficient

It is the ability to withstand heat generated by non-sinusoidal current in the transformer coil. Currently, the voltage and current waveforms do not have a pure sine waveform. Due to the use of various electronic devices. In the voltage and current waveforms there are many harmonics. The sturdy design of the transformer inevitably affects the K factor and transformer life.

Insulation level

In transformer design, adjusting the insulation level is an extremely important factor. In general, the insulation level is selected based on the base impulse voltage level and the system overvoltage. The level of insulation certainly increases the life of the transformer.

Source: https://www.electrical4u.com/dry-type-transformer/