Dry Type Transformers: Types and Applications

 Dry Type Transformer

A dry type transformer  does not use any  liquid for its cooling and insulation. A oil filled transformer uses a mineral insulating oil for its cooling and insulation. For cooling of dry type trnsformer  air is forced into the cabinet of the transformer. Many transformers are cooled with oil or a fire resistant dielectric. This requires regular maintenance and continuous monitoring of oil. However, dry type transformers do not use oil and thus it requires very less maintenance.

Types of Dry Type Transformers

There are  two types of dry type transformers. They are
  1. Cast Resin Dry Type Transformer (CRT)
  2. Vacuum pressure Impregnated Transformer (VPI)

Cast Resin Dry Type Transformer (CRT)

The primary and secondary of the transformer is encapsulated with epoxy resin. This encapsulation of the winding prevents the possibility of penetration of the moisture in the winding. Thus, Cast  resin dry type transformer (CRT) can be safely used in the high moisture prone areas.
This type of transformer has following advantages. They are-
  1. High short time over load capacity.
  2. Lower load losses  and higher efficiency
  3. No use of oil – Zero risk to fire hazard make it  suitable for indoor installation.
  4. Can be fitted outdoor in IP 45 enclosure.
  5. Does not absorb moisture- non hygroscopic.
  6. Eco-friendle and safe

Vacuum Pressure Impregnated Transformer (VPI)

In a vacuum impregnated transformer the  primary and secondary winding is impregnated with resin within a vacuum chamber. The winding is made in the form of foil or strip. For higher voltage application the winding is made in the form of disc.
The winding can be physically seen by opening the door of the transformer. But in a  CRT dry type transformer the primary and secondary winding is moulded. The winding of CRT dry type transformer becomes a solid mass after mould casting. Thus opening the door of the transformer one can see three moulds for three different limbs of the transformer.
VPI gives a chance for repair of the winding in case there is some winding fault but CRT is a solid mass and it can not be repaired. CRT has higher mechanical rigidity than VPI which gives a higher short circuit withstand capability. The insulation of the winding is of grade F(155oC) and H(180oC). This type of transformer is available from 5KVA to 30MVA rating.
This type of transformer has following advantages.
  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 Transformers

The important design parameters for a dry type transformer are given below.
1. Choice of Insulation Type
The insulation class F and H have temperature withstand capacity  155oC  and 180oC respectively, and generally F and H class of insulation of insulation is used to insulate the primary and secondary winding.The other factors like mechanical strength, thermal shock and dielectric strength of insulation must be considered while designing the dry type transformer.
2. Selection of Winding Material
The copper and aluminum are  used  used to make the winding or coil. Even though copper is better conductor, the aluminum conductor wound transformer has lower  cost and weight. The transformer winding with copper conductor has less cross section area for the same current rating as compared to the aluminium conductor. The copper coils also provides more mechanical strength compared to aluminium coil.

3. Selection of Core Material with Low Hysteresis Loss

The core of the transformer must have high permeability and less hysteresis loss in order to have better  efficiency. The CRGO stell is used for the core material . The CRGO steel offers higher permeability and the minimum hysteresis loss.

4. Regulation

The leakage reactance of the transformer must be as minimum as possible to get better voltage regulation of the transformer.The regulation of the transformer shows drop in the secondary voltage with increase of loading on transformer. The leakage reactance is generally kept within 2 % during transformer design.

5. Life Expectancy

The life of the transformer depends upon the useful life of the insulation. The life of the insulation is dependent on the temperature. The rise in temperature cause deterioration in the insulation value which may further cause breakdown of the insulation. Insulation class B, F and H is preferred for dry type transformer to withstand higher temperature. The temperature rise must be calculated during desin stage of the transformer to get higher life expectancy of the transformer.

6. Losses

The no load losses of the transformer depends on the eddy current and hysteresis loss. The no load losses can be minimized by use of the CRGO core and by keeping the leakage reactance as minimum as possible. The load losses depends on the resitance of the conductor. Thus, by keeping the winding resistance within moderate value losses, voltage regulation and the efficiency of the transformer can be improved.

7. Overloading

If the load on the transformer increases above its rated kVA, the transformer is said to be overloaded. The overloading cause rise in temperature of the transformer which reduce the useful life of the transformer. The cooling system should be designed to cater the temporary overloading of transformer.

8. K-factor

If the dry type transformer is to be used for supplying the current to non-linear loads the temperature rise takes place due to harmonics in the current waveform. The transformer for such application must be assigned K-factor.

9. 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.