TRANSFORMER BUSHINGS BASIC
INFORMATION
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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