A dry-type transformer is a transformer that relies on air or gas, rather than liquid, for insulation and cooling of its windings and core. These components are housed within a pressurized, sealed tank to ensure protection and efficiency.
Dry-type transformers are widely used across various industries due to their safety, reliability, and environmental advantages. They are designed to function effectively even in harsh conditions such as high humidity, fire-prone areas, and seismic environments, without compromising performance or posing risks to people or property.
In this article, we will explore the different types of dry-type transformers, their advantages and disadvantages, common applications, and the key design considerations for optimal performance.
Working Principle of Dry-Type Transformers
Dry-type transformers operate based on the principle of electromagnetic induction, transferring electrical energy between circuits without a direct electrical connection. The transformer consists of two windings, a primary coil and a secondary coil, both wrapped around a shared magnetic core.
When an alternating current (AC) flows through the primary coil, it generates a magnetic field in the core. This changing magnetic field induces a voltage in the secondary coil, allowing electrical energy to transfer to the connected load. The output voltage is determined by the turns ratio of the primary and secondary windings, following the equation: Vs=Vp×(Ns/Np)
where Vp and Vs are the primary and secondary voltages, and Np and Ns are the number of turns in the primary and secondary windings, respectively.
Unlike oil-filled transformers, dry-type transformers rely on air for cooling instead of liquid insulation. Heat dissipation occurs through a combination of natural convection and forced-air cooling, ensuring efficient temperature management.
To enhance safety and durability, the transformer windings are encapsulated in epoxy resin, which provides excellent insulation while protecting against moisture, dust, and contaminants. Additionally, this resin coating improves thermal dissipation and significantly reduces fire risks, making dry-type transformers a reliable and environmentally friendly option for various applications.
Construction of Dry-Type Transformer
A dry-type transformer is designed without any liquid insulation, relying on solid insulation materials and air cooling to function efficiently. Its construction consists of several essential components, each contributing to its performance, safety, and durability.
1. Magnetic Core
The core is the heart of the transformer, made from high-grade silicon steel laminations to minimize hysteresis and eddy current losses. It provides a path for the magnetic flux generated when current flows through the primary winding. The core is either wound or stacked, depending on the design and application requirements.
2. Primary and Secondary Windings
The transformer has two sets of windings:
- Primary Winding: Connected to the input power source and responsible for creating a magnetic field in the core.
- Secondary Winding: Receives induced voltage from the magnetic field and delivers power to the load.
The windings are made of copper or aluminum, with copper being preferred due to its higher conductivity and mechanical strength.
3. Insulation System
Dry-type transformers use solid insulation materials instead of liquid-based ones. Common insulation methods include:
- Epoxy Resin Encapsulation: Completely seals the windings in a protective layer, preventing moisture and contaminants from causing damage.
- Nomex or Fiberglass Insulation: Used in some transformers for better heat resistance and mechanical durability.
The insulation system determines the temperature rating and life expectancy of the transformer.
4. Cooling System
Since dry-type transformers do not use oil for cooling, heat dissipation relies on:
- Natural Air Cooling (AN/AA): Uses natural convection, where warm air rises and cool air replaces it.
- Forced Air Cooling (AF/FA): Uses external fans or blowers to enhance heat dissipation, allowing for higher power ratings and overload capacity.
5. Enclosure
The transformer is housed in a protective enclosure to shield it from dust, moisture, and mechanical damage. The type of enclosure depends on the installation environment:
- Open Ventilated: For indoor, controlled environments.
- Totally Enclosed Non-Ventilated (TENV): Provides extra protection against contaminants.
- Sealed or Cast-Resin Type: Used in highly humid or hazardous environments for better insulation and protection.
6. Terminal Connections
The transformer has input (primary) and output (secondary) terminals for electrical connections. These terminals are designed to handle the rated current safely and may include additional protection elements like surge arresters and bushings to prevent electrical faults.
Types of Dry Type Transformers
Dry-type transformers are primarily classified into two types: Cast Resin Dry Type Transformer (CRT) and Vacuum Pressure Impregnated Transformer (VPI).
1. 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, providing excellent protection against moisture, dust, corrosion, and other environmental factors.
This makes CRT transformers highly durable and reliable, especially in challenging conditions. Their non-hygroscopic and non-inflammable nature makes them suitable for high-humidity environments, indoor installations, and fire-prone areas. Additionally, CRTs require minimal maintenance as they do not use oil for insulation or cooling, reducing operational costs and environmental risks.
These transformers are designed to withstand short-term overloads, low partial discharges, and low operational losses, which contribute to their high efficiency and long service life. They are widely used in commercial buildings, hospitals, metro stations, industrial plants, and renewable energy installations such as wind and solar farms.
CRTs are available in power ratings ranging from 25 kVA to 12,500 kVA, with an F-class insulation rating (155°C). Their robust construction and energy-efficient design make them an essential choice for modern power distribution systems, ensuring safe, reliable, and environmentally friendly operation.
2. Vacuum Pressure Impregnated Transformer (VPI)
A Vacuum Pressure Impregnated (VPI) Transformer is a dry-type transformer that uses class H polyester resin to impregnate its windings under controlled vacuum and pressure conditions. This process ensures that all air gaps and voids within the insulation are eliminated, significantly improving the transformer’s mechanical strength, dielectric properties, and thermal stability. The enhanced insulation quality makes VPI transformers highly reliable for demanding applications.
VPI transformers are designed to withstand harsh environmental conditions, making them ideal for outdoor installations, areas prone to seismic activity, and locations with extreme temperature fluctuations. Their robust construction, moisture-resistant enclosure, and low thermal expansion coefficient ensure long-term performance even in challenging conditions.
These transformers also offer high resistance to short-circuit currents, reducing the risk of damage during electrical faults. Furthermore, their low fire risk and minimal maintenance requirements make them a cost-effective and safe choice for critical power applications.
These transformers are widely used in industrial plants, marine and offshore platforms, power generation facilities, railway systems, and commercial power distribution networks.
They are available in power ratings ranging from 5 kVA to 30 MVA, with insulation classes F (155°C) and H (180°C), and protection levels up to IP56, ensuring durability against dust, water, and other external contaminants. With their high efficiency, strong insulation, and adaptability to extreme conditions, VPI transformers play a vital role in modern power infrastructure, offering safe, efficient, and long-lasting performance.
Cooling Methods for Dry-Type Transformers
Cooling in a dry-type transformer occurs both internally and externally. Internally, heat naturally moves from hotter to cooler areas, while externally, heat is actively dissipated from the transformer enclosure. There are two primary cooling methods used in dry-type transformers: natural air cooling and forced air cooling.
In natural air cooling (AA), the transformer relies on natural convection to dissipate heat without using fans or pumps. As the transformer operates, electrical losses in the core and windings generate heat, which warms the surrounding air. The heated air becomes less dense and rises, creating a natural convection cycle where cooler air moves in to replace it.
This continuous airflow helps maintain an optimal operating temperature. Since this method does not require mechanical components, it is simple, reliable, and requires minimal maintenance. Additionally, it operates quietly, making it suitable for noise-sensitive environments such as office buildings, industrial and commercial applications, and indoor substations.
However, natural air cooling is typically used for low-voltage and low-power transformers, as it has limited capacity for handling high thermal loads.
On the other hand, forced air cooling (FA) uses fans or blowers to actively push air over the transformer coils, improving heat dissipation. This method is ideal for high-load applications where natural cooling alone is insufficient. Fans, which can be axial or centrifugal, direct airflow over the transformer’s core and windings, effectively dispersing heat. In some designs, ducts or channels guide the airflow for more efficient cooling.
Advanced control systems further enhance this method by integrating temperature sensors that monitor heat levels in real time. When the transformer temperature exceeds a set threshold, the control system automatically activates the fans, ensuring efficient cooling while optimizing energy use. Forced air cooling is highly effective in high-power industrial applications, outdoor and enclosed substations, and transformers operating under variable load conditions.
By selecting the appropriate cooling method, dry-type transformers can operate efficiently, maintain reliability, and extend their service life. Natural air cooling offers a simple, maintenance-free solution for low-power applications, while forced air cooling enhances thermal performance, making it suitable for high-load environments.
Advantages of Dry Type Transformers
Dry-type transformers offer numerous benefits, making them a reliable and efficient choice for various applications:
- Enhanced Safety – Eliminates flammable or toxic liquids, reducing fire and leak risks.
- Minimal Maintenance – No oil changes, tests, spills cleanup, or special disposal required.
- Easy Installation – No need for vaults or special foundations, allowing placement closer to the load.
- Eco-Friendly – No harmful gas emissions or greenhouse effect contribution.
- Superior Overload Handling – Excellent heat dissipation and thermal endurance.
- Cost-Effective – Reduces civil installation and fire protection system costs.
- Seismic Resistance – Rigid structure withstands vibrations and shocks.
- No Fire Hazard – Self-extinguishing properties prevent smoke and flames during faults.
- High Short-Circuit Resistance – Low impedance and strong mechanical structure.
- Long Lifespan – High-quality insulation resists thermal and dielectric stress.
- Ideal for Harsh Environments – High moisture and corrosion resistance make them suitable for damp and contaminated areas.
- Quiet Operation – Produces less noise compared to oil-filled transformers, making them ideal for commercial and residential areas.
- Energy Efficiency – Lower losses lead to reduced energy consumption and operational costs.
- Faster Cooling – Advanced ventilation and insulation materials allow quicker heat dissipation, ensuring stable performance.
Disadvantages of Dry Type Transformers
While dry-type transformers offer several advantages, they also have some drawbacks:
- Higher Cost – More expensive than oil-filled transformers due to higher material and manufacturing costs.
- Larger and Heavier – Require more space due to increased insulation thickness and air gaps.
- Susceptible to Contaminants – Open ventilation makes them more prone to dust, dirt, and vermin, which can affect performance.
- Noisier Operation – Higher magnetostriction and vibrations result in increased audible noise compared to oil-filled models.
Applications of Dry Type Transformers
Dry-type transformers are widely used in industries and environments that demand high safety, reliability, and eco-friendliness. Common applications include:
- Chemical, Oil, and Gas Industry – Powering equipment in refineries, petrochemical plants, pipelines, and offshore platforms, where flammable substances are present.
- Environmentally Sensitive Areas – Preventing oil spills in water protection zones, forests, wetlands, and wildlife habitats.
- Fire-Risk Areas – Ensuring fire safety in hospitals, schools, hotels, malls, indoor and underground substations where strict fire regulations apply.
- Renewable Energy – Connecting wind turbines, solar panels, and hydroelectric plants to the grid.
- Other Applications – Used in traction systems, marine vessels, mining operations, and data centers, where low maintenance and high durability are essential.
Key Design Factors for Dry Type Transformers
The design of a dry-type transformer is influenced by several factors that determine its performance, efficiency, and lifespan. Here are the critical considerations:
- Insulation Type: Determines temperature rating, dielectric strength, and mechanical durability. F and H-class insulation materials (e.g., epoxy resin, polyester) are commonly used, withstanding up to 155°C and 180°C, respectively.
- Winding Material: Copper and aluminum are the primary choices. Copper offers better conductivity and mechanical strength but is heavier and costlier, while aluminum is lighter and more economical but requires a larger cross-section for the same current rating.
- Core Material: Affects magnetic properties and losses. Silicon steel, cold-rolled grain-oriented steel (CRGO), and amorphous metal reduce hysteresis and eddy current losses, improving efficiency.
- Voltage Regulation: A transformer should maintain a stable output voltage under varying loads. Low impedance and leakage reactance (ideally within 2%) ensure minimal voltage drops.
- Lifespan Considerations: Heat, moisture, and contamination impact longevity. The insulation class should withstand high temperatures and harsh conditions without degradation.
- Loss Minimization: Core and winding losses (no-load and load losses) must be optimized. Proper material selection and design adjustments improve efficiency and reduce excess heating.
- Overload Handling: Overloading leads to overheating and insulation damage. Dry-type transformers should include fan cooling or air-conditioning for heat dissipation and thermal protection.
- K-Factor Rating: Measures the transformer’s ability to handle harmonic-rich loads. Higher K-factor designs prevent overheating and ensure long-term reliability in environments with non-sinusoidal currents (e.g., electronic equipment and industrial loads).
Each of these factors plays a crucial role in ensuring safety, efficiency, and durability, making dry-type transformers ideal for modern electrical applications.
Conclusion
A dry-type transformer is a type of transformer that does not use liquid for insulation or cooling. Instead, it relies on air or gas as the cooling medium and epoxy resin or polyester resin for insulation.
There are two primary types of dry-type transformers:
- Cast Resin Dry-Type Transformers (CRT) – Known for their high durability and insulation strength.
- Vacuum Pressure-Impregnated Transformers (VPI) – Offer enhanced moisture resistance and mechanical strength.
Dry-type transformers provide numerous advantages, including safety, reliability, environmental friendliness, easy installation, low maintenance, excellent overload capacity, and long service life. They are particularly suitable for damp, fire-prone, or environmentally sensitive areas.
However, they also have some limitations such as higher cost, larger size, heavier weight, increased sensitivity to dust and contamination, and higher noise levels compared to oil-filled transformers.
The design of a dry-type transformer is influenced by factors like insulation type, winding and core material, voltage regulation, life expectancy, efficiency, overloading capacity, K-factor rating, and insulation level. Proper design ensures optimal performance, energy efficiency, and long-term durability.
Dry-type transformers are widely used in critical industries and applications that demand high safety and reliability, such as:
- Chemical, oil, and gas industries
- Environmentally sensitive locations
- Fire-risk areas
- Renewable energy systems
- Industrial and commercial facilities
With their inherent safety and sustainability, dry-type transformers continue to be a preferred choice for modern electrical power distribution systems.
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