OMICRON 57
High-voltage bushings
6 High-voltage bushings
6.1 Introduction
High-voltage bushings are essential parts of power transformers, circuit breakers and of other power
apparatus. More than 10% of all transformer failures are caused by defective bushings. Although the
price for a bushing is low compared to the costs of a complete transformer, a bushing failure can damage
a transformer completely. A regular capacitance and DF measurement is highly recommended.
6.2 Types of bushings
Testing and maintaining high-voltage bushings are essential for continued successful operation of
transformers and circuit breakers. Power outages may occur as the result of a bushing failure. High-
voltage bushings used on transformers and breakers exist in many forms, including:
6.2.1 Condenser
This type is most frequently used for high-voltage bushings and it is therefore the main one focused in
this guide. Cylindrical conducting layers are arranged coaxially with the conductor within the insulating
material. The length and diameter of the cylinders are designed to control the distribution of the electric
field in and over the outer surface of the bushing. The partial capacities are switched in series and the
voltage drops across the capacities is nearly equal to each other (see Figures 6-1 and 6-2).
Figure 6-1: Condenser bushing design
Tap electrode
grounded
layer/flange
Grounded
layer/flange
C
A
C
B
C
C
C
D
C
E
C
F
C
G
C
H
C
I
C
K
C
J
• Equal capacitances, C
A
through C
J
, procedure equal distribution
of voltage from the energized center conductor to the grounded
condenser layer and flange.
• The tap electrode is normally grounded in service except for
certain designs and bushings used with potential device.
Note:
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