Dry Type Transformer

What is Dry Type Transformer?

Dry type transformer never uses any insulating liquid where its winding with core be immerged. Rather windings with core are kept within a sealed tank that is pressurized with air.

Type of Dry Type Transformer

The dry type transformer is of two types. They are

1. Cast Resin Dry Type Transformer ( CRT)
2. Vacuum pressure Impregnated Transformer ( VPI)

Cast Resin Dry Type Transformer ( CRT)

Cast resin dry type transformer (CRT) is used in the high moisture prone areas. It is because of its primary and secondary windings are encapsulated with epoxy resin. This encapsulation helps to prevent moisture to penetrate to affect the winding material. Complete protection is achieved by this cast resin encapsulation so that the transformer can work without disruption in highly moisture prone area. Thus this transformer is non hygroscopic.

This type of transformer is available in ratings of 25 KVA to 12,500 KVA. with insulation class of F (90^oC Temp. Rise).
This type of transformer has some featured advantages. They are-

1. Better over load capacity.
2. Low partial discharge along with low loss. Hence efficiency is very good.
3. As it is with non inflammable winding insulation, it offers zero risk to fire hazard. So it is suitable for indoor installation.
4. Can be fitted outdoor in IP 45 enclosure.
5. And off course non hygroscopic.

Vacuum Pressure Impregnated Transformer (VPI)

This type of transformer is made with minimum flammable material as insulation of windings. The windings of this transformer are made in foil or strip in a continuous layer. But for higher voltages, the winding is made of disks that are connected in series or parallel as per power rating with respect to voltage level.
The insulation of the winding is void free impregnation that is made with class H polyester resin. The primary and secondary winding with core are laced safely within a vacuum protective box. Moisture Ingress Protection is high and it never gets affected by moisture.
This type of transformer is available from 5KVA to 30MVA with insulation grade F(155^oC) and H(180^oC). It’s with Protection up to IP56.
This type of transformer has several advantages. They are-

1. High mechanical strength.
2. Void free insulation.
3. No temperature fluctuation.
4. Easy maintenance.
5. Less prone to fire hazard.

Advantages of Dry Type Transformer

The main advantages of dry type transformer are given below.

• Safety for people and property.
• Maintenance and pollution-free solution.
• Easy installation.
• Side clearance is less.
• Environmentally friendly.
• Excellent capacity to support overloads.
• Reduced cost on civil installation works and fire protection systems.
• Excellent performance in case of seismic events.
• No fire hazard.
• Excellent resistance to short circuit currents.
• Long lasting due to low thermal and dielectric heating.
• Suited for damp and contaminated areas.

Disadvantages of Dry Type Transformer

But there are some disadvantages of dry type transformer. They are-

• Dry type transformer is long lasting and with less chance of winding failure. But once it fails whole set up is to changed, i.e. complete change of high voltage and low voltage winding with limb.
• For same power and voltage rating, dry type transformer is costlier than oil cooled transformer.

Application of Dry Type Transformer

Dry type transformer are widely used in-

• Chemical, oil and gas industry
• Environmentally sensitive areas (e.g. water protection areas)
• Fire-risk areas (e.g. forests)
• Inner-city substations
• Indoor and underground substations
• Renewable generation (e.g. off-shore wind turbines)

Important Factors to Design a Dry Type Transformer

The important design parameters for a dry type transformer are given below.
Choice of Insulation Type
Generally F and H class of insulation of insulation is used to insulate the primary and secondary winding. It is because these classes have high temperature withstand property, i.e. 155^oC for F and 180^oC for H class of insulation. Generally varnish and polyester resin are used as insulation of the winding. Apart from the temperature withstand capability, mechanical strength; dielectric strength and resistance to thermal shock are the basic capabilities that have to be fulfilled by the insulation selected for the windings.
Selection of Winding Material
Generally copper and aluminum are used to make the winding or coil. Though copper is better conductor, aluminum conductor wound transformer possesses low cost and weight. For the same current rating, copper conductor with less cross section is used as the winding material in the transformer. Copper coil provides more mechanical strength than aluminum coil.
Selection of Core Material with Low Hysteresis Loss
Core material selection is very important in transformer design. Core material must possess high permeability and less hysteresis loss. But both cannot be achieved in any core material. Generally silicon steel, CRGO etc are used to allow minimum hysteresis loss with higher permeability.
Regulation
When transformer secondary voltage drops abruptly due to increase in load, this regulation is called poor regulation. Poor regulation is due to transformer higher internal leakage reactance. So, leakage reactance is kept within 2% during design.
Life Expectancy
Life of the transformer depends on the breakdown of winding insulation due to temperature rising effect in winding due to overloading. Normally class B, F and H insulation is preferred for dry type transformer to withstand higher temperature gradient including ambient temperature. So design of the transformer must be done with respect to the rise of the temperature of operating full load condition.
Losses
No load losses mean core loss and eddy current that is independent of loading condition. But in loading condition copper loss occurs that includes the iron loss to increase the value of voltage regulation, i.e. poor voltage regulation. Leakage reactance and winding resistance must be within moderate value to minimize this loss and better voltage regulation, thus higher efficiency.
Overloading
Over loading condition is harmful for transformer for long time span. Overloading is the cause of overheating when transformer has to fulfill its connected load demand. Hence huge amount of current makes copper loss in the winding, and that causes transformer damage. To cool the winding, fan-cooling system is provided in dry type transformer.
K-factor
It is the ability of withstanding of heat generated by non sinusoidal current in the transformer winding. Pure sine wave is not obtained in voltage and current wave form. It is due to today’s various electronic devices uses. Several harmonics are present in the voltage and current wave form. Robust design of a transformer off course bothers k-factor to provide transformer long lasting life.
Insulation Level
In transformer design, insulation level adjustment is an important factor. Generally insulation level is chosen as per basic impulse level and system over voltage. Strong insulation level increases the life of a transformer.