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A dry-type transformer, also known as a cast resin transformer or a non-liquid-filled transformer, is an electrical transformer that does not use any liquid coolant like oil.

Instead, it uses air or a solid insulation material to cool and insulate its components. The main parts of a dry-type transformer include:

In the upcoming sections of this article, we will delve into a detailed discussion of the main parts of a dry-type transformer. However, for now, here’s a brief overview.

1. Core: Provides a magnetic path for the transformer’s operation.

2. Windings: Consist of primary and secondary windings for input and output voltage transformation.

3. Insulation: Insulates the windings and core to prevent electrical breakdown.

4. Enclosure: Protects the transformer and contains the components.

5. Terminals and Bushings: Connect the transformer to the electrical system.

6. Cooling System: Allows for heat dissipation, either through natural convection or forced-air cooling.

7. Tap Changer (optional): Adjusts the output voltage if necessary.

8. Grounding System: Safely directs fault currents to the ground for safety.

These components collectively enable the dry-type transformer to function effectively in various electrical applications.

Why Do We Use Dry Type Transformers?

Types of dry-type Transformers

Difference between Dry type and Oil type Transformers?

Dry-type transformers and oil-type transformers are two common types of electrical transformers used in various applications.

They differ primarily in their cooling and insulation methods, making them suitable for different environments and applications. Here are the key differences between dry-type transformers and oil-type transformers:

1. Cooling Method:
   • Dry-Type Transformer:
     • Dry-type transformers use air or a solid insulation material (such as epoxy resin) to cool and insulate the windings.
     • Cooling is achieved through natural convection (air circulation) or forced convection (using fans) inside the transformer enclosure.
     • They are suitable for indoor applications where the risk of oil leaks or fires is a concern, as they do not contain flammable liquids.
   • Oil-Type Transformer:
     • Oil-type transformers use mineral oil or synthetic oil as a coolant and insulating medium.
     • The oil serves both as a coolant and insulator, dissipating heat generated during operation.
     • They are often used in outdoor or high-power applications where the heat dissipation requirements are greater.
2. Fire Safety:
   • Dry-Type Transformer:
     • Dry-type transformers are considered safer in terms of fire hazards because they do not contain flammable oil.
     • They are commonly used in buildings, tunnels, and other indoor environments where fire safety is a concern.
   • Oil-Type Transformer:
     • Oil-type transformers use flammable mineral oil as a coolant and insulating medium, which poses a higher fire risk.
     • Additional fire protection measures, such as containment systems and fire-resistant barriers, are often required when installing oil-filled transformers.
3. Environmental Impact:
   • Dry-Type Transformer:
     • Dry-type transformers are more environmentally friendly as they do not contain hazardous oils that can potentially leak and harm the environment.
     • They are easier to handle and dispose of when they reach the end of their operational life.
   • Oil-Type Transformer:
     • Oil-filled transformers require special care and precautions to prevent oil leaks, spills, and contamination of soil and water.
     • Spills or leaks of transformer oil can have adverse environmental impacts and may require costly cleanup efforts.
4. Maintenance:
   • Dry-Type Transformer:
     • Dry-type transformers generally require less maintenance compared to oil-filled transformers.
     • There is no need to monitor oil levels or deal with oil leaks and oil quality.
   • Oil-Type Transformer:
     • Oil-filled transformers require regular monitoring of oil levels, quality, and insulation integrity.
     • Maintenance includes oil testing, filtration, and occasional oil replacement to ensure proper operation and longevity.
5. Size and Weight:
   • Dry-Type Transformer:
     • Dry-type transformers are typically larger and heavier than oil-filled transformers with the same power rating due to the need for larger cooling surfaces and insulation.
   • Oil-Type Transformer:
     • Oil-filled transformers are more compact and lightweight for the same power rating, making them suitable for applications with space constraints.

The choice between a dry-type and an oil-type transformer depends on factors such as the application, environmental considerations, fire safety requirements, maintenance capabilities, and available space.

Each type has its advantages and limitations, and the selection should be based on the specific needs of the electrical system and safety regulations in place.

How Do You Test A Dry Transformer?

Among the routine tests that should be conducted on all dry-type power transformers are:

Dielectric Test: 

The applied voltage waveform for single-phase should be roughly sinusoidal. The test must be carried out at the prescribed frequency.

Before disconnecting, the test voltage must be rapidly lowered to 1/3 of the maximum value. All of the windings must be tested. If no failure occurs at the ultimate test voltage, the test is considered successful.

Induced Voltage Test:

While performing this test, the test voltage must be double that of the rated voltage. It should be applied between the terminals of the secondary winding while keeping the primary winding open.

The test period at maximum voltage must be one minute, and the frequency must be double the rated frequency.

Voltage ratio measurement: 

On all tap changer locations, voltage ratio measurements and polarity and connection checks must be done.

It’s also good to double-check the numbers allocated to the taps and the ratings. Measurement of voltage ratio must be done phase by phase between the terminals of matching windings. The voltage ratio is measured using the potentiometric approach.

Load Loss Measurement and No-load Current: 

Supply LV windings are used to perform this test at the rated frequency and voltage. The waveform should be as close to a sine wave as feasible, and the primary windings should be open.

The frequency of the test must not deviate by more than 1% from the rated value. No-load current and loss, as well as the voltage’s mean and effective values, must be monitored.

The average value of three measurements taken by effective value ammeters will be used to calculate the no-load current.

When necessary, instrument transformers and transducers will be utilized to measure the power with three-watt meters.

Measurement of Winding Resistance: 

When the windings are at ambient temperature and without power for a period of time sufficient to reach this State, winding resistance measurement should be done.

According to the measurements must be done in direct current between terminals. Temperatures in the surrounding environment must also be taken into account. It will be calculated as the average of three thermal sensor values.

Short Circuit Impedance: 

The transformer’s performance is shown by the short-circuit loss and short-circuit voltage. The transformer’s HV windings are powered while the LV windings are short-circuited.

During the measurement, the current must be at IN or as near to this value as feasible. During the measurement, each phase’s voltage, current, and short-circuit losses should be monitored.

The measurement must be done quickly in order to prevent raising the winding temperature due to the applied current, and the measuring current must be kept between 25% and 100% of the rated current; by doing this, the measurement inaccuracies are caused by the rise in winding temperature will be reduced.

By performing all upper mentioned tests, you can ensure the proper operations of your transformer; if test values are not according to the correct operations values of the transformer conducted test will be considered failed, and you need to take the necessary measures to fix the fault to avoid any loss or incident.

How Long Do Dry-Type Transformers Last?

The lifespan of a dry-type transformer can vary significantly depending on several factors, including its design, quality, operating conditions, and maintenance.

On average, well-maintained dry-type transformers typically have a service life of 20 to 40 years or more. However, some transformers can last even longer under favorable conditions, while others may fail prematurely.

Here are some key factors that can influence the lifespan of a dry-type transformer:

1. Design and Quality: The quality of materials and manufacturing processes used in the construction of the transformer can have a significant impact on its lifespan. High-quality transformers are likely to have longer service lives.
2. Operating Conditions: The operating conditions of the transformer, such as load levels, temperature, and voltage stability, play a crucial role. Transformers operated within their rated capacity and under stable conditions are more likely to last longer.
3. Maintenance: Regular maintenance, including inspections, testing, and cleaning, can help detect and address issues early, prolonging the transformer’s life. Neglected transformers are more prone to failure.
4. Environmental Factors: The environmental conditions where the transformer is installed can affect its lifespan. Transformers exposed to extreme temperatures, humidity, corrosive atmospheres, or other harsh conditions may experience accelerated aging.
5. Electrical Stress: Overloading, voltage surges, and other electrical stress factors can cause insulation breakdown and shorten the transformer’s life.
6. Contaminants: Accumulation of dust, dirt, and contaminants on the transformer’s components can reduce its cooling efficiency and lead to overheating and premature failure.
7. Manufacturing Quality: Transformers with manufacturing defects or quality control issues may be more prone to early failures.

It’s important to note that dry-type transformers can be rebuilt or refurbished in some cases, which can extend their service life. However, this may not always be a cost-effective option, depending on the extent of the damage or wear.

To maximize the lifespan of a dry-type transformer, it’s essential to follow manufacturer recommendations for maintenance, operate it within its rated capacity, protect it from adverse environmental conditions, and monitor its performance regularly. Routine inspections and testing can help identify potential issues before they lead to major failures.

What is Protection of Dry Type Transformers?

The protection of dry-type transformers is essential to ensure their safe and reliable operation. Protection measures are put in place to prevent or mitigate various electrical and mechanical faults that can occur in transformers. Here are some of the key aspects of protection for dry-type transformers:

1. Overcurrent Protection: Overcurrent protection devices, such as fuses and circuit breakers, are used to safeguard dry-type transformers from overloads and short circuits. These devices are installed on the primary and secondary sides of the transformer. They interrupt the current flow when it exceeds a predetermined threshold, preventing damage to the transformer.
2. Temperature Monitoring: Transformers generate heat during operation, and excessive temperatures can lead to insulation breakdown and other issues. Temperature monitoring devices, such as temperature sensors or thermocouples, are placed within the transformer windings or on its surface to measure temperature. Alarms or protective actions are triggered if the temperature exceeds safe limits.
3. Buchholz Relay: Buchholz relays are used in oil-filled transformers, but they are not applicable to dry-type transformers since dry-type transformers do not use oil as a coolant or insulating medium.
4. Pressure Relief Devices: Dry-type transformers are typically sealed, and pressure relief devices are not commonly used. However, in some designs, pressure relief devices may be employed to relieve internal pressure in the event of a fault, preventing the transformer from rupturing.
5. Grounding and Ground Fault Protection: Proper grounding of the transformer and the use of ground fault protection devices are essential to detect and clear ground faults promptly. Ground fault protection devices can include ground fault relays and sensors.
6. Differential Protection: Differential protection is used to detect internal winding or core faults in a transformer. Current transformers (CTs) are used to measure the differential current between the primary and secondary windings. Any imbalance in current indicates a fault, and the protection system initiates a trip action.
7. Voltage Protection: Voltage protection devices, such as under-voltage and over-voltage relays, can be used to protect the transformer from voltage fluctuations and deviations beyond acceptable limits.
8. Surge Arresters: Surge arresters or lightning arresters are employed to protect the transformer from voltage surges caused by lightning strikes or switching events. They divert excess voltage to ground, preventing it from reaching the transformer.
9. Insulation Monitoring: Periodic insulation resistance testing can help identify deterioration in the insulation system of the transformer, allowing for proactive maintenance and preventing insulation-related faults.
10. Remote Monitoring and Control: Remote monitoring and control systems can provide real-time data on the transformer’s performance, allowing operators to respond quickly to abnormal conditions and implement protective actions.

The specific protection measures and devices used for a dry-type transformer depend on its application, size, and the criticality of the load it serves.

Proper protection design and regular maintenance are crucial to ensure the safe and reliable operation of dry-type transformers and to minimize the risk of failure or damage.

Read my other article, What is transformer mechanical protection.

What Causes a Dry-Type Transformer to Fail?

Dry-type transformers are designed to be durable and reliable, but they can still fail for various reasons. Here are some common causes of failure in dry-type transformers:

1. Overloading: Operating a transformer at a higher load than its rated capacity can lead to overheating and insulation breakdown, ultimately causing the transformer to fail. Overloading can occur due to changes in load demand or improper sizing of the transformer for the application.
2. Insulation Degradation: Over time, the insulation materials used in transformers can degrade due to factors like moisture ingress, temperature extremes, and chemical contaminants. As insulation deteriorates, it can lead to short circuits and other electrical faults.
3. Moisture Ingress: Moisture is one of the primary enemies of transformer insulation. If moisture enters the transformer, it can compromise the insulation properties and cause arcing, short circuits, and reduced dielectric strength. This can result from improper storage, exposure to humid environments, or damaged seals.
4. Overvoltage or Voltage Surges: Excessive voltage levels, voltage spikes, or transient voltage surges can stress the insulation and windings of a transformer. This can occur due to lightning strikes, switching events, or faults in the power distribution system.
5. Contamination: Dust, dirt, and other contaminants can accumulate on the transformer’s core and windings, reducing its cooling efficiency and potentially causing hotspots. These hotspots can lead to insulation breakdown and eventual failure.
6. Aging: Transformers have a finite lifespan, typically ranging from 20 to 40 years or more depending on their design and operating conditions. As they age, the insulation and other materials can deteriorate, increasing the risk of failure.
7. Mechanical Damage: Physical damage to a transformer, such as from impact or vibration, can damage the winding or core, leading to internal faults and failure.
8. Poor Maintenance: Inadequate or irregular maintenance can contribute to transformer failure. Regular inspections, testing, and maintenance procedures help identify and address issues before they escalate into major problems.
9. Manufacturing Defects: In rare cases, transformers can have manufacturing defects or quality control issues that lead to premature failure. This can include problems with winding connections, core alignment, or insulation quality.
10. Environmental Factors: Extreme environmental conditions, such as high temperatures, corrosive atmospheres, or seismic activity, can stress the transformer and contribute to its failure.

To mitigate the risk of dry-type transformer failure, it’s essential to follow proper maintenance practices, operate the transformer within its specified load and voltage limits, protect it from environmental hazards, and address any issues promptly through inspections and testing. Regular maintenance and monitoring can help extend the life and reliability of a dry-type transformer.

Read my other article about Transformer faults, for more information.

Can Dry Type Transformer Explode?

Dry-type transformers are less prone to catastrophic failures like explosions, they are generally designed to be safe and do not contain flammable liquids or gases like oil-filled transformers.

However, while dry-type transformers are less prone to catastrophic failures like explosions, they can still experience mechanical and electrical faults that may lead to certain hazards.

These hazards are typically not explosive in nature but can pose risks to equipment and personnel. Here are some potential issues associated with dry-type transformers:

1. Overheating: Dry-type transformers can overheat if they are overloaded or subjected to excessive ambient temperatures. Prolonged overheating can cause insulation breakdown and damage to the transformer, but it typically does not result in an explosion.
2. Short Circuits: Short circuits can occur in dry-type transformers due to insulation failure or other electrical faults. While short circuits can cause fires and damage to the transformer, they do not lead to explosions like oil-filled transformers.
3. Mechanical Failure: Mechanical failures, such as winding deformation or core damage, can occur in dry-type transformers. These failures can disrupt the transformer’s operation, but they do not result in explosive events.
4. Pressure Relief: Some dry-type transformers are equipped with pressure relief devices to release internal pressure in the event of a fault, preventing the transformer from rupturing. However, this is not an explosive event; it is a controlled release of pressure.
5. Arcing and Fire: In cases of severe electrical faults, such as internal arcing or prolonged short circuits, dry-type transformers can catch fire. While transformer fires can be dangerous, they are not explosions.

It’s important to note that the design and construction of dry-type transformers aim to minimize the risks associated with electrical and mechanical faults.

Proper protection, maintenance, and monitoring are essential to ensure the safe operation of dry-type transformers and to prevent faults from escalating into hazardous situations.

In contrast, oil-filled transformers, which use flammable oil as an insulating and cooling medium, pose a greater risk of explosion if there is a catastrophic failure that results in the release of flammable oil and its ignition.

Dry-type transformers, being solid-state and oil-free, are considered safer in this regard. However, safety precautions should still be taken to prevent electrical and thermal issues in dry-type transformers.

dry type transformer parts

A dry-type transformer, also known as a cast resin transformer or a non-liquid-filled transformer, is an electrical transformer that does not use any liquid coolant like oil.

Instead, it uses air or a solid insulation material to cool and insulate its components. The main parts of a dry-type transformer include

Transformer Core

The core supports the primary and secondary windings by providing a low resistance channel for electromagnetic flux. It’s constructed by stacking thin sheets of high-grade grain-oriented steel separated by insulating material.

The Carbon content of the core steel is kept below 0.1 percent to keep hysteresis and eddy currents to a minimum. Eddy currents can be decreased when it is alloyed with silicon.

Transformer Winding

Primary and secondary windings are carried by the transformer for each phase. This winding comprises several turns of aluminum or copper conductors, which are separated from one other and the transformer’s core.

The form and configuration of transformer winding are determined by the current rating as well as the short circuit’s capacity, the strength of the circuit, the rise in temperature impedance, and surge voltages.

Transformer Insulation

Between the windings and the core, between the primary and the secondary windings, between each turn of the winding, and between all current-carrying elements and the tank, insulation is necessary.

It is imperative that the insulators have high dielectric strength, good mechanical properties, and be able to withstand high temperatures. Among the materials used to insulate transformers are synthetic materials, paper, cotton, and others.

Transformer Tank

The main tank is a component of a transformer. It serves two functions: one is to protect the core and windings from external influences, and secondly, it provides support to the other accessories of transformers. Fabrication of rolled steel plates into containers is used to create tank bodies.

As part of the package, lifting hooks and cooling tubes are included. Aluminum sheets are utilized instead of steel plates to save weight and prevent stray losses. On the other hand, Aluminium tanks are more expensive than steel tanks. 

Transformer Bushings and Terminal 

Bushings serve as insulators between the terminals and the tank. They’re affixed to the tops of the transformer tanks. They provide a safe path for the conductors that link terminals to windings.

Porcelain or epoxy resins are used to construct them.

Read my detailed article, Transformer bushing, what you should know.

Transformer Tap changers

Transformer’s secondary voltage is adjusted via tap changers. The transformer’s turns ratio is capable of being adjusted by these devices as needed. Two types of tap changers exist On-load tap changers and off-load tap changers.

On-load tap changers can work without stopping the current flow to the load, but off-load tap changers can only operate when the transformer is not providing any loads. There are also tap changers that change the taps automatically as per requirement. Transformer Air Ventilators.

Cooling System:

While dry-type transformers do not use liquid coolants, they rely on air for cooling. Some may have natural air cooling, where air circulates freely around the windings, while others may have forced air cooling, where fans are used to enhance the cooling process.

Grounding System:

A grounding system is essential for safety reasons. It ensures that any fault currents are safely directed to the ground, reducing the risk of electrical shock or fires.

Temperature Sensors and Protection Devices:

Dry-type transformers may be equipped with temperature sensors and protection devices to monitor the transformer’s temperature and respond to abnormal conditions, such as overheating or short circuits, by disconnecting power or issuing alarms.

These are the main parts of a dry-type transformer. The specific design and components may vary depending on the transformer’s size, voltage rating, and application.

Dry-type transformers are commonly used in indoor applications where safety, environmental concerns, and space constraints make them a preferred choice over oil-filled transformers.

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