Dry Type Transformer: Definition, Types, Advantages, and Applications

A dry type transformer is defined as a transformer that uses air or gas—instead of liquid—as the insulating and cooling medium for its windings and core, all contained within a pressurized sealed tank.

Dry type transformers are widely used in various industries and applications because of their safety, reliability, and environmental benefits. They can operate in harsh conditions such as high humidity, fire risk, and seismic events without compromising their performance or posing any hazard to people or property.

In this article, we will discuss the different types of dry type transformers, their advantages and disadvantages, their applications, and the important factors to consider when designing a dry type transformer.

What are the Types of Dry Type Transformers?

There are two main types of dry type transformers: cast resin dry type transformer (CRT) and vacuum pressure impregnated transformer (VPI).

Cast Resin Dry Type Transformer (CRT)

A cast resin dry type transformer (CRT) is a type of transformer that uses epoxy resin to encapsulate its primary and secondary windings. This protects the windings from moisture, dust, corrosion, and other environmental factors that can affect their insulation and performance.

A CRT is suitable for high humidity areas, indoor installations, and fire-risk areas because it is non-hygroscopic, non-inflammable, and maintenance-free. It can also withstand overloads, partial discharges, and low losses, resulting in high efficiency and long service life.

A CRT is available in ratings from 25 kVA to 12,500 kVA, with insulation class of F (90°C temperature rise).

Vacuum Pressure Impregnated Transformer (VPI)

A vacuum pressure impregnated transformer (VPI) is a type of transformer that uses class H polyester resin to impregnate its windings under vacuum and pressure. This eliminates any air gaps or voids in the insulation, enhancing its mechanical strength, dielectric strength, and thermal stability.

A VPI is suitable for outdoor installations, seismic events, and temperature fluctuations because it has a robust construction, a moisture-resistant enclosure, and a low thermal expansion coefficient. It also has easy maintenance, low fire risk, and high resistance to short circuit currents.

A VPI is available in ratings from 5 kVA to 30 MVA, with insulation class of F (155°C) or H (180°C), and protection up to IP56.

What are the Advantages of Dry Type Transformers?

Some of the main advantages of dry type transformers are:

• Dry transformers enhance safety by eliminating flammable or toxic liquids, reducing the risk of leaks or fires.
• They are maintenance-free and pollution-free because they do not require any oil changes, oil tests, oil spills cleanup, or special disposal methods.
• They are easy to install because they do not require any vaults or special foundations. They can be placed close to the load to reduce the need for long and costly low voltage cables.
• They are environmentally friendly because they do not emit any harmful gases or contribute to the greenhouse effect.
• These transformers excellently handle overloads due to their superior heat dissipation and thermal endurance compared to oil-filled types.
• They have reduced costs on civil installation works and fire protection systems because they do not need any oil containment pits, fire walls, fire extinguishers, or sprinklers.
• They have excellent performance in the case of seismic events because they have a rigid structure that can withstand vibrations and shocks.
• They have no fire hazard because they have self-extinguishing properties and do not produce any smoke or flames in case of faults.
• They have excellent resistance to short-circuit currents because they have low impedance and high mechanical strength.
• They are long-lasting due to low thermal and dielectric heating because they have high-quality insulation materials that can withstand high temperatures without degrading.
• They are suited for damp and contaminated areas because they have high moisture ingress protection and corrosion resistance.

What are the Disadvantages of Dry Type Transformers?

Some of the main disadvantages of dry-type transformers are:

• Dry transformers tend to be costlier than oil-filled models with similar power and voltage rating due to higher material and manufacturing costs.
• They are larger and heavier than oil-filled transformers for the same power and voltage rating because they have more air gaps and insulation thickness.
• They are more sensitive to dust, dirt, and vermin because they have open ventilation that can allow the entry of foreign particles that can damage the windings or cause short circuits.
• They are noisier than oil-filled transformers because they have higher magnetostriction and vibration that can produce audible sounds.

What are the Applications of Dry Type Transformers?

Dry-type transformers are widely used in various industries and applications that require high safety, reliability, and environmental compatibility. Some of the common applications of dry-type transformers are:

• Chemical, oil, and gas industry: Dry-type transformers are used to supply power to various equipment and processes that involve flammable or explosive substances, such as refineries, petrochemical plants, pipelines, offshore platforms, etc.
• Environmentally sensitive areas: Dry-type transformers are used to protect the environment from oil spills or leaks that can contaminate water sources, soil, or wildlife habitats, such as water protection areas, forests, wetlands, etc.
• Fire-risk areas: Dry-type transformers are used to prevent fire hazards or minimize fire damage in areas that are prone to fire outbreaks or have strict fire regulations, such as indoor substations, underground substations, hospitals, schools, hotels, shopping malls, etc.
• Renewable generation: Dry-type transformers are used to connect renewable energy sources to the grid or to the load, such as wind turbines, solar panels, hydroelectric plants, etc.
• Other applications: Dry-type transformers are also used in other applications that require high performance, low maintenance, or special features, such as traction systems, marine systems, mining systems, data centers, etc.

What are the Important Factors to Design a Dry Type Transformer?

The design of a dry-type transformer depends on several factors that affect its performance, efficiency, and durability. Some of the important factors to consider when designing a dry-type transformer are:

• Choice of insulation type: The insulation type determines the temperature rating, dielectric strength, mechanical strength, and thermal shock resistance of the transformer. Generally, F and H-class insulation materials are used for dry-type transformers because they can withstand high temperatures (up to 155°C and 180°C, respectively) and have good electrical and mechanical properties. Common insulation materials include varnish, epoxy resin, polyester resin, etc.
• Selection of winding material: The winding material determines the conductivity, resistance, loss, and mechanical strength of the transformer. Generally, copper and aluminum are used as winding materials for dry-type transformers because they have high conductivity and low cost. Copper has better conductivity and mechanical strength than aluminum, but it is more expensive and heavier. For the same current rating, copper requires less cross-section area than aluminum.
• Selection of core material with low hysteresis loss: The core material determines the magnetic flux density, permeability, hysteresis loss, and eddy current loss of the transformer. The core material should have high permeability and low hysteresis loss to reduce the no-load loss and improve the efficiency of the transformer. Common core materials include silicon steel, cold rolled grain oriented steel (CRGO), amorphous metal, etc.
• Regulation: The regulation of a transformer is the ratio of the voltage drop at full load to the no-load voltage. The regulation indicates the ability of the transformer to maintain a constant output voltage under varying load conditions. The regulation depends on the impedance and resistance of the transformer. A low impedance and resistance result in low regulation and better voltage regulation. The leakage reactance of a dry-type transformer should be kept within 2% during design to achieve low regulation.
• Life expectancy: The life expectancy of a transformer indicates how long it can operate reliably before degradation, influenced by factors like temperature, moisture, dust, and corrosion. The insulation class and quality of the dry-type transformer should be chosen to withstand high temperatures and harsh environments without degrading. The temperature rise of the transformer should not exceed the limit specified by the insulation class.
• Losses: The losses of a transformer are the difference between the input power and the output power. The losses consist of no-load losses and load losses. The no-load losses are independent of the load and include core loss and eddy current loss. The load losses are proportional to the load and include copper loss and stray loss. The losses affect the efficiency, heating, and cooling of the transformer. The core material, winding material, insulation material, and design parameters should be selected to minimize the losses and maximize the efficiency of the dry-type transformer.
• Overloading: The overloading of a transformer is the condition when the transformer operates beyond its rated capacity or temperature limit. The overloading causes overheating, insulation breakdown, short circuits, or fire in the transformer. The overloading can be caused by excessive load demand, harmonics, faults, or ambient temperature. The dry-type transformer should be designed with sufficient margin to handle overloads without damaging its components or performance. The dry-type transformer should also be equipped with a fan-cooling system or an air-conditioning system to dissipate the heat generated by overloads.
• K-factor: The K-factor is a measure of the ability of a transformer to withstand the heat generated by non-sinusoidal currents in its windings. Non-sinusoidal currents are caused by various electronic devices that produce harmonics in the voltage and current waveforms. Harmonics increase the losses, heating, and distortion of the transformer. A high K-factor indicates that the transformer can handle higher levels of harmonics without overheating or degrading. The dry-type transformer should be designed with a high K-factor to provide long-lasting life and reliable performance in applications that involve non-sinusoidal currents.

Conclusion

A dry-type transformer is a transformer that does not use any liquid as an insulating or cooling medium for its windings or core. Instead, it uses air or gas as the medium and epoxy resin or polyester resin as the insulation material.

There are two main types of dry-type transformers: cast resin dry-type transformers (CRT) and vacuum pressure-impregnated transformers (VPI). Both types have their own advantages and disadvantages, depending on the application and environment.

Dry-type transformers have many benefits, such as safety, reliability, environmental compatibility, easy installation, low maintenance, excellent overload capacity, reduced cost, excellent performance, no fire hazard, excellent resistance, long-lasting life, and suitability for damp and contaminated areas.

However, dry-type transformers also have some drawbacks, such as higher cost, larger size, heavier weight, higher sensitivity, and higher noise than oil-filled transformers.

The design of a dry-type transformer depends on several factors that affect its performance, efficiency, and durability. Some of the important factors are the choice of insulation type, selection of winding material, the selection of core material with low hysteresis loss, regulation, life expectancy, losses, overloading, K-factor, and insulation level.

Dry-type transformers are widely used in various industries and applications that require high safety, reliability, and environmental compatibility. Some of the common applications are chemical, oil, and gas industry; environmentally sensitive areas; fire-risk areas; renewable generation; and other applications.