Transformer Bushings: What You Should Know About

Transformers have active parts and auxiliary parts, One of the most important auxiliary parts is Bushings. In this article, I will answer important questions about bushings. Let’s get started.

Transformer bushings
Transformer Bushing From the Inside of the Transformer

An electric transformer bushing is a component used to insulate and connect conductors passing through the transformer tank to the external high-voltage (HV) and low-voltage (LV) electrical systems.

The primary purpose of transformer bushings is to provide electrical insulation and support for the conductors, preventing electrical breakdown and ensuring the safe and reliable operation of the transformer.

Here are the main reasons why transformers have bushings:

  1. Insulation: Transformer bushings are designed to insulate the internal conductors from the grounded tank and from each other. This is crucial to prevent electrical arcing and short circuits, which could damage the transformer and interrupt the flow of electrical power.

  2. Support: Bushings provide mechanical support for the conductors that pass through the transformer tank. They help secure the conductors in place and prevent them from vibrating or moving, which could lead to mechanical failure.

  3. Sealing: Bushings also serve as a means to seal the transformer tank, preventing the ingress of moisture, contaminants, and other environmental factors. This is essential for maintaining the dielectric properties of the insulating oil and ensuring the long-term reliability of the transformer.

  4. Voltage Tapping: In some transformers, bushings are used to provide access points for tapping into different voltage levels. This allows for flexibility in the transformer’s output voltage, catering to various requirements in the electrical grid.

There are different types of transformer bushings, including oil-filled bushings and dry-type bushings.

Oil-filled bushings are immersed in the insulating oil of the transformer, providing additional cooling. Dry-type bushings, on the other hand, use solid insulation materials and do not rely on oil for cooling.

Overall, transformer bushings play a critical role in maintaining the electrical integrity and reliability of transformers, which are essential components in electrical power systems for voltage transformation and distribution.

Transformer Bushing Principle

11KV transformer bushings
11KV transformer bushings

The bushing principle is straightforward, It keeps the high and low-voltage wires insulated from the transformer body.

To prevent corona discharge and short circuits with other live wires bushings are made of insulation material and designed to handle the voltage level of the transformer’s low and high voltage sides.

Bushing passes the conductor through itself saving it from any kind of faults that may occur.

Can Bushings Cause Transformer Oil Leakage?

Yes, in some cases, bushings can contribute to transformer oil leakage. Transformer bushings are designed to provide electrical insulation and mechanical support to the conductors passing through the transformer tank.

They typically have seals and gaskets to prevent the escape of oil. However, over time, due to factors such as aging, mechanical stress, or manufacturing defects, the seals on the bushings may degrade or develop leaks.

For more details about Transformer Oil Leakage, read my other article here.

Difference Between Insulators And Bushings

overhead power line insulators
Overhead Power Line Insulators

Insulators and bushings are related components used in electrical systems, particularly in transformers and other high-voltage equipment, but they serve different purposes.

  1. Insulators:

    • Function: The primary function of insulators is to provide electrical insulation and prevent the flow of electric current between two conductive parts with different potentials. They are used to support and separate electrical conductors, preventing unintended electrical contact or short circuits.
    • Design: Insulators are typically made of materials with high electrical resistance, such as ceramics, glass, or polymer composites. These materials are chosen for their ability to resist the flow of electricity.
    • Applications: Insulators are used in various parts of electrical systems, including overhead power lines, insulator strings on transmission towers, and insulating supports for electrical components.
  2. Bushings:

    • Function: Bushings serve as a combination of electrical insulators and mechanical supports for conductors passing through barriers, such as transformer tanks. They prevent electrical breakdown, support the conductors, and seal the transformer from environmental contaminants.
    • Design: Bushings can be oil-filled or dry-type and are designed to provide insulation and support for the conductors while ensuring a sealed environment. They often have seals and gaskets to prevent oil leakage and maintain the integrity of the insulating oil.
    • Applications: Bushings are commonly used in transformers, circuit breakers, and other high-voltage equipment where electrical conductors need to pass through insulating barriers.

In summary, while both insulators and bushings contribute to electrical insulation, insulators are generally standalone components used to separate and support conductors. In contrast, bushings are specialized components designed to insulate, support, and seal conductors passing through barriers in equipment like transformers.

The choice between insulators and bushings depends on the electrical system or equipment’s specific requirements and design considerations.



Insulators are made of porcelain, glass, and other composite materials Bushings are also insulators made of composite materials that can withstand a certain level of voltage and current.
Insulators can be hollow or filled. Bushings are hollow to pass the wires.
Insulators can be used in many equipment. Bushings are used in Transformer and shunt reactor circuit breakers.
Insulators can be made of rubber also. Bushings can not be made out of rubber
Insulators can’t be filled with dielectric. Di-electric is mandatory in insulators.
Insulators are used in daily life electrical equipment to protect the equipment’s circuitry or wiring. Bushings can’t be used in daily life electrical equipment.

How Many Bushings Should A Transformer Have?

The number of bushings a transformer has depends on its design and purpose. Transformers can have varying numbers of bushings based on factors such as the transformer’s configuration, voltage rating, and whether it is single-phase or three-phase. Here are some common scenarios:

  1. Single-Phase Transformers:

    • Single-phase transformers typically have two bushings—one for the high-voltage winding and one for the low-voltage winding. These bushings are used to connect the transformer windings to the external electrical system.
  2. Three-Phase Transformers:

    • Three-phase transformers can have a different number of bushings based on their configuration. A common arrangement is for each phase to have two bushings—one for the high-voltage winding and one for the low-voltage winding. Therefore, a three-phase transformer might have six bushings in total.
  3. Auto-Transformers:

    • Auto-transformers, which have a common winding for both the high and low voltage, may have fewer bushings than a traditional two-winding transformer. They might have only three bushings—one for the common winding and one for each of the high and low-voltage windings.
  4. Specialized Transformers:

    • Some transformers, especially those with specific applications or designs, may have additional bushings. For example, transformers with multiple voltage tapping points may require additional bushings for each tapping point.

It’s important to note that the design of transformers can vary based on factors such as voltage levels, power ratings, and specific application requirements.

Transformer specifications, as provided by the manufacturer or in engineering documents, will specify the number and type of bushings required for a particular transformer.

Always refer to the transformer’s documentation for accurate information on the number and configuration of bushings it should have.

Transformer Bushings Material And Classifications?

Transformer bushings are made from various materials to ensure electrical insulation, mechanical strength, and resistance to environmental conditions.

The material used often depends on factors such as the transformer’s voltage class, application, and design. Here are common materials and classifications for transformer bushings:

  1. Porcelain Bushings:

    • Material: Porcelain is a traditional and widely used material for transformer bushings. It offers good electrical insulation properties and is resistant to environmental factors.
    • Classifications: Porcelain bushings are often classified based on their design and application, such as solid-core porcelain bushings, oil-impregnated porcelain bushings, and condenser-type bushings.
  2. Composite (Polymeric) Bushings:

    • Material: Composite materials, often based on polymers or silicone rubber, are used to make bushings. These materials provide advantages such as lighter weight, resistance to pollution, and flexibility.
    • Classifications: Composite bushings may include categories like cycloaliphatic epoxy resin bushings or silicone rubber bushings. They are often classified based on their construction and insulating properties.
  3. Oil-Filled Bushings:

    • Material: Bushings designed to be immersed in transformer oil are typically made of materials like porcelain or composite, with the oil providing additional insulation and cooling.
    • Classifications: Oil-filled bushings can be further classified based on their design and construction, such as cap-and-rod type, radial-fin type, or other configurations.
  4. Dry-Type (Resin-Impregnated) Bushings:

    • Material: Dry-type bushings are often made with a solid insulating material, such as epoxy resin, and are not immersed in oil.
    • Classifications: Depending on the design and application, dry-type bushings can be categorized into various types, including capacitance-graded, capacitance-graded with a condenser core, or other configurations.
  5. Capacitor Bushings:

    • Material: These bushings are designed with a condenser core to improve the capacitance grading and voltage distribution.
    • Classifications: Capacitor bushings are classified based on factors like the presence of a condenser core, which enhances their performance.

When selecting transformer bushings, it’s crucial to consider factors such as the transformer’s voltage class, environmental conditions, and specific application requirements.

Manufacturers provide detailed information on the materials and classifications of bushings in their product specifications and guidelines.

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