What Are Transformer Bushing? Functions Of Transformer Bushing?

Bushings may be classified generally by design as follows:
a) Condenser type
1) Oil-impregnated paper insulation, with interspersed conducting (condenser) layers or oil impregnated paper insulation, continuously wound with interleaved lined paper layers
2) Resin-bonded paper insulation, with interspersed conducting (condenser layers)

b) Noncondenser type
1) Solid core or alternate layers of solid and liquid insulation
2) Solid mass of homogeneous insulating material (e.g., solid porcelain)
3) Gas filled

For outdoor bushings, the primary insulation is contained in a weatherproof housing, usually porcelain. The space between the primary insulation and the weathershed is generally filled with an insulating oil or compound (also, plastic and foam).

Some of the solid homogenous types may use oil to fill the space between the conductor and the inner wall of the weathershed. Bushings may also use gas such as SF6 as an insulating medium between the center conductor and outer weathershed.

Bushings may be further classified generally as being equipped or not equipped with a potential tap or power-factor test tap or electrode. Note Potential taps are sometimes also referred to as capacitance or voltage taps.)

The bushing, without a potential tap or power-factor tap, is a two-terminal device that is generally tested overall (center conductor to range) by the GST method. If the bushing is installed in an apparatus, such as a circuit breaker, the overall GST measurement will include all connected and energized insulating components between the conductor and ground.

A condenser bushing is essentially a series of concentric capacitors between the center conductor and the ground sleeve or mounting range. A conducting layer near the ground sleeve may be tapped and brought out to a tap terminal to provide a three-terminal specimen.

The tapped bushing is essentially a voltage divider and, in higher voltage designs, the tap potential may be utilized to supply a bushing potential device for relay and other purposes. In this design the potential tap also acts as a low-voltage power-factor test terminal for the main bushing insulation, C1.

Modern bushings rated above 69 kV are usually equipped with potential taps. (In some rare instances 69 kV bushings were equipped with potential taps.) Bushings rated 69 kV and below may be equipped with power factor taps.

In the power-factor tap design, the ground layer of the bushing core is tapped and terminated in a miniature bushing on the main bushing mounting range. The tap is connected to the grounded mounting range by a screw cap on the miniature bushing housing.

With the grounding cap removed, the tap terminal is available as a low-voltage terminal for a UST measurement on the main bushing insulation, C1, conductor to tapped layer.

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