4.2
High voltage bushings
Author: Douglas. M. Getson
Document updated: 2002-11-06
e-mail: douglas.m.getson@de.abb.com
A bushing is a device used to carry
current at high potential through a
grounded barrier, e.g. a transformer
tank. There are bushing designs to
meet various transformer interconnections such as air, oil and SF6 gas. ABB
also manufactures bushings with and
without condenser bodies. Stud or bulk
type bushings have no condenser and
use as its major insulation oil and
ceramic. Stud bushings are available
up to 35 kV system voltages where low
partial discharge are not required.
Condenser bushings facilitate electrical
stress control through insertion of floating equalizer (electrode) plates. The
condenser core decreases the field
gradient and distributes the field along
the length of the insulator, which provides for low partial discharge readings
well above nominal voltage readings.
Condenser bushing design
The condenser core is built up around a center
tube that may or may not be in the current carrying path depending on the design (see connection
systems). The condenser core is wound from kraft
paper and in some cases Nomex® for high temperature and current applications. The equalization plates are constructed of either metallic (aluminum) inserts or nonmetallic (ink) patches.
These plates are located coaxially so as to achieve
an optimal balance between external flashover and
internal puncture strength.
The condenser cores of today are either
impregnated with oil or resin for a totally dry
product.
High stress region
Grounded
mounting flange
Field grading
(condenser body)
No-field grading
(no condenser body)
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4.2
Oil-impregnated paper (OIP) bushings
Open-air insulation
OIP bushings remain the predominant technology
in 80% of today’s deliveries. The condenser core is
impregnated with transformer grade mineral oil
and placed inside an insulating envelope of oil
and porcelain (or composite), which keeps the
bushing oil from mixing with the transformer oil.
However, some bushing designs when applied
horizontally do allow for exchange of bushing and
transformer oil. OIP bushings maintain a low and
stable capacitive loss with low partial discharges
over their product life. ABB uses this construction
across various designs for meeting IEC (Type
GOB, GOE(K), GOM & GOH), CSA and IEEE
(Type AB, O Plus C & T) transformer standards.
For open air applications, the bushing is equipped
with an insulator made of either porcelain or silicone rubber whose sheds provide the necessary
creepage distance for withstanding voltages under
all conditions. ABB has traditionally used alternating long and short profile sheds, which have
the benefit of being self-cleaning.
Resin-impregnated paper (RIP) bushings
RIP bushings have a resin impregnated condenser
core that is in contact with the transformer oil.
This technology is used in combination with
either porcelain or silicon insulators for a
completely dry construction. Their lightweight
and compact construction provides greater
withstand to handling and severe service conditions. RIP is not to be confused with resinbonded technology that has been on the market
for many years. Resin-bonded condenser core
designs use a kraft paper that has been preimpregnated with resin which cures upon heating.
Most major bushing suppliers have discontinued
resin-bonded core manufacturing and have gone
to the more stable resin-impregnated technology.
The resin-impregnated cores are also wound with
kraft paper, but the resin is introduced during a
heating and vacuum process of the core. RIP
bushings provide for a low and stable tan delta (or
power factor) reading over the service life of the
product. ABB uses this construction across several product lines (Type GSA, PORIP and CORIP)
for meeting various industry standards and
applications.
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Silicone insulator with self-cleaning sheds
Silicone insulators have the added benefit of being
hydrophobic, a Greek term meaning "fears water".
This characteristic helps break up water films and
create separate droplets which reduces leakage current, prevents flashover and elevates the voltage
withstand capability during wet and highly contaminated conditions. In affect, the hydrophobicity
acts as a self cleaning property which extends service life and lowers substation maintenance cost.
4.2
Manufacturing
ABB has been manufacturing bushings since
1908. More than half a million units have been
put into service, and designs cover ratings between
600V-800kV and up to 50 kA. When manufacturing condenser bushings, the kraft paper is
wound on a heated central tube to lower the
moisture content of the paper. Depending on the
design and/or rating, the condenser may go
through an additional drying and impregnation
process (oil or resin) to further reduce moisture
levels. The different components are assembled
around the condenser and held in compression by
a spring mechanism. The springs also maintain
adequate pressure on the seals (Nitrile or in some
special cases Viton®) during the thermal contraction and expansion of its parts. The air inside the
assembly is then evacuated, and oil is introduced
while continuously checking seal integrity. The
drying, final evacuation and oil filling process
must be handled with care in order to avoid premature aging of the insulation. In a RIP bushing,
there is no final evacuation and oil filling.
Bushing assembly
Final evacuation and
oil filling
Testing
Finally, routine tests are performed which include,
one minute power frequency tests, continuous
partial discharge readings, tan ∂ or power factor
(capacitive losses) tests, as well as capacitance, and
tightness testing. Also, lightening impulse tests
(BIL) are performed on bushings having ratings
equal to or greater than 300 kV.
Test laboratory
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4.2
In-tank connection system
The unique, ABB tank connection system with
draw rods is offered on selected products (such as
Type GOE, AB, O Plus C and others) to allow
on-site installation without accessing the leads
from inside the transformer main tank. After
transformer testing, the lower part of the bushing’s connection system is left inside the transformer before sealing. During transformer field
installation, the upper part of the draw rod and
the bushing itself is lowered into its final position
on the transformer tank, guided by a guiding
cone. To ensure low through resistance, the necessary contact force is applied from the outside following a careful procedure to achieve sufficient
contact force. This design has been sucessfully
used by ABB for more than thirty years.
Type GOE optional draw rod transformer lead connection
(design can vary by product type)
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4.2
Bushing products
Type GOB, AB, O Plus C, GOE & GOM are
oil-impregnated condenser bushings for use in
transformers and oil-filled circuit breakers. They
are available for 15 kV through 800 kV ratings
meeting or exceeding applicable IEC, IEEE/
ANSI, and CSA standards. These designs have
power factor and partial discharge levels well
below industry requirements. The bushings are
suitable for new equipment as well as for direct
field replacement of existing power apparatus.
Type CORIP & GSA are resin-impregnated condenser bushings with composite insulator materials. These designs are perfect for coastal and
highly polluted environments. The environmentally friendly products perform according to the
latest seismic specifications. The bushings can be
stored and installed in any position. Being oil free,
easy to handle, water repellant, virtually maintenance free, light and safe, this is the ultimate
bushing for both new, replacement and mobile
power transformer applications. The bushings
meet or exceed the latest IEC and/or IEEE/ANSI
standards.
Type T & GOH are oil-impregnated condenser
bushings designed for both vertical and horizontal
mounting on high-current transformers. The
Type T bushing is the right choice for high temperature applications in bus ducts. It incorporates
high temperature materials, such as Nomex®
insulation and Viton® seals. The Type T combines the performance of a condenser or capacitance graded design bushing with the capability of
performing in open air up to 125 ºC. Our Type
GOH can also be applied in bus ducts by following the loading guidelines found in the technical
guide. They are available from 25 kV through
34.5 kV voltage ratings and 600 to 21,500
ampere current ratings.
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4.2
The Type A bushing is a stud-type design offering an economical and versatile bushing for transformer, regulator and generator applications. It
can be vertically or horizontally mounted and is
availabel in 600 to 23,000 ampere current rating
range and from 1.2 kV through 34.5 kV. The
simple design and rugged construction means that
the bushing requires little maintenance other than
periodic cleaning.
The Type LCRJ bushing is a robust stud-type
design suitable for vertical or horizontal mounting
on power transformers. Similar to the Type A,
this bushing requires little maintenance due to the
simple design. Type LCRJ is best suited when the
specification calls for a high current application
with low partial discharges in a non-condenser
design. Partial discharge is guaranteed to be less
than 50 microvolts at a test voltage of twice the
normal line-to-ground operating voltage. The
bushings are available from 15 kV through 25 kV
and from 2,900 to 23,000 ampere ratings.
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