Omicron CP TD12 - High-voltage bushings

CP TD12/15 User Manual
52 OMICRON
6 High-voltage bushings
High-voltage bushings are essential parts of power transformers, circuit breakers and of other power
apparatus. More than 37 % of all fire and explosion failures of high-voltage power transformer failures
are caused by defective bushings
[1]
. Although the price for a bushing is low compared to the cost of a
complete transformer, a bushing failure can damage a transformer completely. Regular capacitance and
DF measurements are highly recommended.
Testing and maintaining HV-bushings are essential for continued successful operation of transformers
and circuit breakers. Power outages may occur as the result of a bushing failure. The most frequently
used type of HV-bushings is the condenser and therefore, it is the only one which will be described in
this manual. 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 Figure 6-1).
Figure 6-1: Condenser bushing design
Condenser bushings may have:
• "Resin-Bonded Paper (RBP) insulation
• "Resin-Impregnated Paper (RIP) insulation
• "Oil-Impregnated Paper (OIP) insulation
• Resin-Impregnated Fibre (RIF) or Resin Impregnated Synthetics (RIS) insulation
1. CIGRE, "TB 642 - Transformer Reliability Survey," 2015
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: