Transformer Bushings: What You Should Know About

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

For more information about transformer active and auxiliary parts read my other article here.

Why Do Transformers Have Bushings?

Transformer bushings
Transformer bushing from the inside of the transformer

The bushings on the transformers are used to carry the lines out of and into the transformer, either the HV (High Voltage) or the LV (Low Voltage) lines, and to provide an insulation between the transformer body and the live wires, to prevent any possible short circuit.

The bushings of transformers allow the live wire, either HV or LV, to supply the power out of the transformer.

Transformer primary side gets power by the source wires, while the secondary provides power to the load by transformer output wires, these wires are energized, may be medium, low or high voltage.

To prevent any possible contact between the energized wires and the transformer tank bushings are the solution.

The primary and secondary wires enter the transformer tank through the bushing. Of course, the bushing of each side should be suitable for the voltage level.

Transformer Bushing Principle

11KV transformer bushings
11KV transformer bushings

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

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

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

Can Bushings Cause Transformer Oil Leakage?

Bushings can cause a reasonable amount of oil leakage through the transformer end point.

The reason of this leakage is the atmospheric conditions, aging or miss installing of bushing.

Rough atmospheric condition, like cold and heat, cause rusting that leads to metal loss of the bushing pockets, and causes oil seal cracks that eventually leads to the leakage.

We had a severe oil leakage due to miss installing and aged oil seal. One of our team members used an old cracked oil sealing, after energizing the transformer we recognized a severe oil leakage from the bushing, You can watch this video of the transformer leak here before we repair it again, Its on my YouTube channel.

Generally, another solution to the leakage in power transmission system is dealt with the “Drill and Inject” technique but in this case some extra steps are taken before applying the technique.

A precision clamp of Aluminum cut using the machines is utilized using already existing bolts and nuts.

What this clamp does is to provide some extra volume to the inner filled mineral oil and pressure, working as a cavity itself and when it gets filled the pressure stabilizes and leakage stops because it is clamped around the leakage bushing pockets at the leakage points.

The precision-aluminum machine-cut-clamp is filled from outside to maintain the inner pressure and the mineral-oil level to meet its Thermodynamic properties.

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

Bushings and insulators are important components of the power system.

However the insulators are commonly used in OHTL to carry the conductors and isolate them from the tower body.

Hereunder some differences between insulators and bushings.



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 di-electric. 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 Has?

All transformer types and ratings have bushings. However, the number of bushings depends on some factors like:

  • Number of phases.
  • Connection type at low and high voltage sides.

Single phase transformers

In the case of the single-phase transformers, there are 4 bushings on it. Two bushings for the low voltage and the others are for the high voltage side.

Three phase transformers

In the case of the three-phase transformer, there can be 6-7 bushings on the basis of type of connection.

  • The Delta-Delta (Dd) connection: This type of connection has 6 bushings, 3 for the primary and 3 for the secondary.
  • The Wye-Wye (Yy) connection: Just like delta-delta connection wye-wye connection also has 6 bushings.
  • The Delta-Wye with neutral (Dyn) connection: This connection has 7 bushings, 3 primary and 3 secondary connections with a neutral connection at the secondary adding up-to 7 connections and a bushing per connection is needed.
  • The Wye with neutral -Delta (YNd) connection: This connection also has 7 bushings but 4 at the primary end and 3 at the secondary. 3 connections at the primary end are for the primary coil and the 4th connection is for the neutral wire that also need a bushing.

Transformer Bushings Material And Classifications?

Transformer bushings are made of insulating materials. Generally, bushings are made of Porcelain, polymers or glass and other composite materials used for insulation.

There are different categories on which we can divide these bushings such as:

  • The construction
  • The inner insulation of the bushings
  • The insulating material at the ends

Bushing construction types

This type differentiates the bushings on the basis of their construction.
There are 2 sub-categories of this category:

  • The Capacitance graded
  • The Solid type

The capacitance graded

Starting from top to down the capacitance graded bushing has a magnetic oil gauge that has an oil reservoir inside it for the insulation between wire and the insulating material.

An upper insulator, a capacitance graded core after which it is named, a voltage top receptacle, a mounting flange, a CT pocket that has a current transformer (CT) inside, a lower end insulator and lastly a lower end shield.

The solid type

The simplest bushing used in the transformers is the “Solid Type” of bushings. The construction of this type from top to bottom consists of a central conductor that is threaded on both ends with the lead (Pb), a cap, a sealing gasket, the mounting flange, a cushion washer, a spring washer, and a clamping nut at the bottom end.

The inner insulation of the bushings type

The sub-categories of inner insulation type of bushings are:

  • Gas-insulated Bushings: These bushings have Sulphur Hexafluoride (SF6) pumped in them with pressure. This pumped gas insulates the flange of insulating material from the conductor inside the bushings.
  • Resin Bonded Bushings: Typically known as the impregnated paper insulating bushing, these bushings have capacitance graded core that is fabricated with a resin impregnated paper. There are simpler resin bonded bushings where the paper is coated with resin and then fabricated on the capacitance graded core.
  • Cast Insulation Bushings: Constructed using the cast insulation material, these bushing can be either of the capacitance-grade or the solid type bushings.
  • Oil Impregnated Bushings: The most widely used bushings are the oil impregnated bushings. These bushings have mineral oil that has di-electric characteristics used along with the craft-paper that results in a composite material, enhancing a great di-electric capability.
  • Oil-insulated Bushings: This type of bushing has mineral oil filled between the insulating material and the conductor passing through the insulator.
  • Air Filled Bushings: This type of insulation has air filled between the insulating material and conductor.

Insulating materials at the end type

The insulating material at the ends is mandatory as the both ends need to be insulated from another material found to prevent all kinds the fatalities. Some sub-categories of this type are:

  • Air to Oil
  • Air to Air

Air to Oil: These bushing has one end at the transformer that is filled with the air and insulator that is filled with the oil.

Fluid to Fluid: Some subcategories of fluid-to-fluid bushings are:

  • Air-to-SF6
  • Oil to Oil
  • SF6 to Oil

Depending on Power and types of connection

Capacitance graded Bushings: from 25kV to 1500kV systems.
Solid type: from 25kV to 90kV systems.

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