Oil Impregnated Paper (OIP) Bushing


OIP bushings are the most widely used bushing design in the high voltage power industry. OIP bushings are manufactured by winding untreated kraft paper with conduction inserts around a centre tube. Conductive inserts consist of aluminium layers placed at calculated and very precise axial and radial positions; these inserts grade (control/influence) the electric field. To give the bushing adequate dielectric strength, the condenser insulation is saturated (impregnated) with oil. The type of oil used is almost always mineral oil (the same oil that is used for power transformers and switchgear).

Condenser Core

Condenser Core

OIP Bushing Manufacturing Process

Before impregnating the condenser core with oil, the core is first treated to reduce its entrained moisture and gas levels; this stage ensures the core has the desired high dielectric breakdown strength. This stage may be carried out by placing the condenser core (or the whole bushing assembly) in an autoclave, or, by placing the bushing assembly under vacuum. If the impregnation stage is conducted correctly, no gaseous inclusions should be present in the condenser core materials. As a result, the inception of internal partial discharges occurs at much higher stress levels compared with RBP bushings. Most OIP bushings are designed to operate with radial stresses of approximately 45 kV/cm (114.3 kV/inch), but this value may be slightly higher depending upon the bushing’s design.

Once the afore mentioned stage is complete, the condenser core is encapsulated in a hollow cylindrical insulator. The upper air-side and lower oil-side insulators of OIP bushings are invariably made of porcelain.

OIP Bushing Construction and Components

A spring-loaded gas-filled expansion chamber is installed at the top of the bushing. The expansion chamber allows for oil volume fluctuations (expansion/contraction) due to varying temperature (a conservator tank on a power transformer performs a similar purpose). Note that temperature variations may be due to ambient temperature changes, but also due to loading of the transformer (more load means more heat).

An oil level gauge (prismatic or magnetic) is also installed at the head of the bushing. The size of the oil level gauge face is chosen to allow for easy visual inspection by personnel. Sealing is accomplished by oil-resistant rubber gaskets inserted into grooves; the gaskets form an oil tight seal containing the condenser assembly and insulating oil.

OIP Bushing Construction and Components

OIP Bushing Construction and Components

A clamping system imparts mechanical strength and support to the bushing. Springs (located in the expansion chamber) place the conductor of the bushing under constant tension. Constant tension of the central tube (conductor) produces a resultant compressive force on the insulators, flange and the gasket; this compressive force ensures the gaskets seal correctly even if the bushing’s dimensions change (e.g. due to temperature, ageing, or physical and/or electrical stresses placed on the bushing).

A mounting flange is installed near the base of the bushing; it is usually manufactured of corrosion-resistant aluminium alloy. The mounting flange serves two purposes:


  • To allow for easy mounting (installation) of the bushing.
  • To house a sealing gasket (a sealing gasket is installed between the lower mounting flange surface and the surface to which it is to be mounted).


The mounting flange is equipped with a test tap or voltage tap and may also have additional space for the installation of a ring type current transformer (CT).

The lower connecting end of the bushing is shielded by an epoxy resin coated aluminium shield; the shield helps control the excessive potential stresses inside the oil-filled enclosure.

Want to learn more about electrical bushings?

Then check out our Introduction to Electrical Bushings video course!

How Electrical Bushings Work

The below video is an extract from our Mechanical and Electrical Engineering Explained Online Video Course.

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