Development of High Current Draw Lead Transformer Bushings
Designed in Accordance with IEEE Standards
Keith P. Ellis
Haefely Trench
Nashville, TN 37138
Abstract Historically draw lead bushings, designed
in accordance with IEEE Standards were limited in
current carrying capacity to a maximum of 400
amperes through 69 kV and 800 amperes 115 kV
and above. For most medhirn powertransformersthe
high voltage bushings were applied ~“th draw lead
conductors. However, the low voltage side of these
transformers almost always required bottomcmnnectedbushings.This meant that if these bushing
ever had to be removed the transformer oil would be
drained and people had to enter the transformertank
to disconnect and reconnect the bushing. This
procedure was ve~ costly and time consuming, To
save these costs and provide a safer working
environment bushings with higher draw lead
conductorcapabilitywere required.
The second factor was the size of the central tube
of the bushing, through which the draw lead
conductor would pass. For bushings rated 69 kV
and below the inside diameter was usually 2.2
centimeters (0.875 inch). This small inside
diameter limited the size of the draw lead
conductor, limiting the current capacity.
The last factor was the draw lead conductor
itself. All draw lead conductors applied were
flexible cable(s). Flexible cable(s) when limited to
a specific diameter will only carry a small amount
of current. A solid draw lead conductor with the
same outside diameter as cable(s) would carry
substantially more current.
I. INTRODUCTION
II. OBJECTIVES
In reviewing the design of the historic draw lead
bushing it was found that there were three factors
that limited the current carrying capabfity. First
was the design of the top terminal. The top
terminal design required that the current be
transferred horn the draw lead conductor through
a threaded stud to the top terminal of the
bushing. The size of the draw lead stud and the
use of threads are the primary factor for the
Ihnited capacity.
Before actual development and design work got
underway a list of objectives were established.
These objectives were grouped into two areas,
performance objectives and engineering solutions.
The following is the list of petiorrnance
objectives:
1.
The bushing(s) must comply with the
requirements of IEEE C57. 19.00 –1991,
IEEE Standard General Requirements and
Test Procedures for Outdoor Power
Apparatus Bushings and IEEE C57. 19.011991, IEEE Standard Performance
Characteristics and Dimensions for Outdoor
Apparatus Bushings.
2.
The bushing(s) must be interchangeable
with existing bottom-connected bushings.
Including bottom-connected bushings
with current ratings above 1200 amperes,
which are not covered by the present
bushing standards.
0-7803-5515-6/99/$10.00 (c) 1999 IEEE
3.
Installation and application of the
bushing(s) must be similar to existing
bushing designs.
4,
The bushing(s) must be cost effective and
be able to compete in the marketplace
with bottom-connected bushings.
The following is a list of engineering solutions
that were identified as obstacles to developing a
h&h current draw lead bushings:
1.
High current transfer from the draw lead
conductor to the top terminal of the
bushing.
2.
Increasing the inside diameter of the
bushing central tube to allow a larger
diameter draw lead conductor and still
maintain the maximum outside diameter
of the bushing below the mounting flange
allowed by the standards.
3.
Meet these objectives with a proven oil
impregnated paper insulation system with
aluminum capacitance foil layers.
III. TECHNOLOGIES
With the objectives defined, the next task was to
rlsview the existing bushing technologies around
tlhe world. The search for existing technologies
was
focused
on bushings
designed
and
with
the
accordance
nnantiactured
in
Commission
Electrotechnical
International
Standard 137 for bushings. The search found that
most IEC bushing designs offered draw lead
currents higher than bushings designed in
accordance with IEEE Standards.
higher
draw
lead
ratings
were
These
alccomplkhed with the application of solid draw
lead conductors.
However,
IEC designed
bushings are rarely produced below 52 kV and a
bushing for lower voltage applications
are
required in the North American market. Many of
the IEC bushing designs, using the solid draw
lead conductor, still relied on threads for current
transfer to the top terminal, limiting the current
carrying capabtity. There were some designs that
did not use threads, which offered the potential
for higher current ratings.
One of these IEC designed bushings incorporated
the use of multiple spring contacts similar to the
ones applied on circuit breakers. This design was
evaluated closely and a similar design was
incorporated into the design of the new, high
current, draw lead bushing.
With many years of successfid experience around
the world applying solid draw lead conductors it
was determined that this concept would be
incorporated into the new, high current, draw
lead bushing. Together with the multiple spring
contacts, the solid draw lead conductor would
provide the high current capability we were
seeking.
IV. DESIGN
Using an existing,
high current,
bottomconnected bushing desig~ which was developed
in accordance with the IEEE Standards, the
design was modified. The modifications included
removing the solid fixed conductor and replacing
it with a hollow tube. The tube had nearly the
same outside diameter as the solid conductor,
which meant that the proven oil impregnated
paper condenser with aluminum foils would work
well within the limited space.
The new bushing would utilize the same
explosion resistant, fiberglass lower end insulator,
the same mounting flange and the same external
porcelain. The only external change was the head
of the bushing. The head design for the new
bushing had to incorporate the new multiple
spring contact assembly and accept the large
diameter of the solid draw lead conductor.
0-7803-5515-6/99/$10.00 (c) 1999 IEEE
The final design selected for the top terminal with
multiple spring contacts allowed for a 133°/0
loading margin above nameplate.
The first prototype was a 25 kV bushing with a
1,,500-ampere draw lead rating. This bushing had
the same physical dimensions as the 25 kV, 400
ampere draw lead bushing described in Table 2 of
IEEE C57.19.01 -1991
After successfid Type testing a 34.5 kV bushing
with a 1,500-ampere draw lead rating was
designed and Type tested. This bushing had the
same physical dimensions as the 34.5 kV, 400
ampere draw lead bushing described in Table 2 of
IIEEE C57.19.01 – 1991.
Next a 34.5 kV bushing with a 3,000-ampere
rating was developed and Type tested. Since
3,000-ampere bushings are not covered by the
present IEEE Standards
the bushing was
designed to be interchangeable with the most
common 2,000 and 3,000-ampere
bushings
presently in use. This interchangeability was
accomplished by designing the mounting flange
to accept all common-mounting configurations.
V. LOADABILITY
Once the prototype bushings had pasted all the
electrical and mechanical Type Tests a series of
temperature tests were preformed to determine
the Ioadability of the draw lead conductors.
These tests were conducted with flexible cable(s)
and removable solid conductors.
In addition to tests preformed in accordance with
current
IEEE
Standards,
tests were also
preformed in accordance with IEC Standards.
Table 1 is offered to outline some of the
diierences between the two bushing standards.
Ambknt temperature
I -30 to +40”C I -40 to +40”C
+30”C
24 hour average ambient for I
+30°c
thermal analyse;
I
+55°C
Maximum temperature of I
+60”C
eonduetorin eon&t with oil
Maximum temperature of
I eonduetorin eontaet with class
A insulationduringoverload
Permissibletemperature rise of
eonduetorin eontaet with class
A insulation
I
]
I
I
I
+150°c
I
I
Io”c
I
I
15°c
Table 1. Comparisonbetween IEEEand IECbushing standards
Another dillerence between these two standards
is the transformer oil level during the test. The
IEC Standard places the oil level, inside the
bushing’s central tube, at l/3rd of the arcing
distance of the bushing above the mounting
flange. The IEEE
Standard
requires
the
transformer oil level to be at the minimum
allowable oil level of the specific bushing. (Below
the mounting flange)
The tests verified that the IEEE test requirements
were more demanding on the thermal capabtity
of the conductor
since the hottest
spot
temperature on the conductor always occurred in
the air space above the oil in the central tube of
the bushing.
Temperature tests in accordance with the IEEE
Standards verified the loadability for each class of
bushing
and a recommended
draw
lead
conductor-loading guide was developed for the
difllerent bushing ratings.
0-7803-5515-6/99/$10.00 (c) 1999 IEEE
In table 2 the recommended conductor loading
fbr 25 and 34.5 kV class high current draw lead
bushings are listed. It is important to bear in mind
that these results are for a specific bushing design
and bushings of comparable design will have
difllerent recommendations.
DRAWLEAD
CONDUCTOR
TYPE
- (Copper
CABLE
- CABLE
CABLE
- CABLE CABLE
CROSS
SECTION
CABLE
285
[)
CABLE
SOLID
SOLID
k
nun=
50
70
95
150
185
350
960
1645
MAXIMUMLOADWITH
TEMPERATURE
INDEX105INSULATION
Amps
Anlpshnm’
155
3.10
215
3.07
*EC
- -Jn
Au>
.4 ./7
I
385
..-.
4XJ
1
2.57
- .-
I
L.LJ
I
,cAll
04U
2.45
a
“CA
lx)
*C
-%A
I
L.14
1.56
1.83
1500
3000
Table 2. Draw lead conductorloading
for 25 and 34.5 kV bushings
“~able 2 is based on having the draw lead
conductor insulated with index 105 insulation. An
additional loading capability can be obtained
when the draw lead conductor is insulated with
index 155 insulation. Tests have proven that 20°/0
a~dditional loading can occur before the bushing’s
index 105 insulation is subjected to a total hottest
of 105°C. This provides
spot temperature
increased loadability of 20°/0 over nameplate
v~ues. Solid conductors that are in direct contact
with the central tube of the bushing do not
require insulation and provide this increased
capability.
in addition to temperature testing the 25 and 34.5
kV class bushings temperature tests were also
conducted on higher voltage bushings with the
r-e
technology. The results have proven that
bushings as large as 500 kV can be provided with
draw lead conductor ratings of over 1600
amperes. This is more than twice present
capabilities of bushings design in accordance with
present IEEE bushing standards.
VI. APPLICATION
The solid draw lead conductor use in the bushing
is designed with a split, located 1.2 centimeters
(% inch) below the gasket surface of the
mounting flange. The split allows the conductor
to taken apart when transporting the transformer
with the bushings removed. This split has led to
calling the conductor a “Split-Conductor”.
Application
of the
“Split-Conductor”
on
bushings, 25 kV through 69 kV, for new or remanufactured transformers requires some special
consideration at the time the transformer is
designed. The inside diameter of the mounting
flange does not allow enough space to assemble
or disassemble the “Split-Conductor”.
For this
reason the transformer design must allow the
“Split-Conductor”
to be lifted vertically 15.24
centimeters (6 inches). This additional vertical lift
capability is not required when the bushing is
applied as a replacement bushing on an existing
transformer.
The only other application consideration is the
design of the mounting flange shipping cover.
The shipping cover will require a supporting bar
welded to the center of the underside. This
supporting bar must have matching threaded
holes to allow the inboard section of the “SplitConductor” to be secured during transport of the
transformer. This application consideration would
apply to all voltage class bushings, which are
equipped with a “Split-Conductor”. In addition
some provisions should be considered to lift the
shipping cover when the bushings are installed.
0-7803-5515-6/99/$10.00 (c) 1999 IEEE
VII. INSTALLATION
IX. ACKNOWLEDGEMENTS
Installing high current draw lead bushings for the
first the is exactly the same as installing a
lmttom-cormected bushing. The bushing is fixed
to the mounting flange and the transformer’s
winding leads are connected to the bottom
terminal at the end of the “Split-Conductor”.
The
author
gratefidly
acknowledges
the
contribution of the following individuals in the
development of these new bushings, for without
their expertise in all aspects of bushing desig~
these bushing may never have been developed:
Clnce the bushing has been installed, future
removal or replacement is accomplished the same
m any other draw lead bushing. Just remove the
top terminal, support the draw lead conductor
and slip the bushing out of the transformer tank.
13raining the transformer oil is not required and
no person has to enter the transformer tank to
disconnect the bushing.
Dr. Volker Karius
Haefely Trench AG
Basel Switzerland
Heinz-J(irgen Jeske
Haefely Trench AG
Basel, Switzerland
JoachimSchmid
HaefelyTrenchAG
Basel, Switzerland
Peter D. Zhao
HaefelyTrench
Scarborough,Ontario
Rolf Fluri
Haefely Trench AG
Basel, Switzerland
Christian Schiegel
Haefely Trench AG
Basel, Switzerland
X. BIOGRAPHY
VIII. SUMMARY
After more than three years of dedicated work by
a number of talented and experienced Engineers a
high current, low voltage, draw lead bushing,
designed in accordance with IEEE Standards, has
become a reality. This effort was borne out of the
needs of the Electric Power Industry to lower
costs and improve worker safety. AU of the stated
objectives have been met.
13ushings, designed in accordance with IEEE
fltmdmds,
draw
lead
with high current,
conductors are now available through 345 kV.
Higher current, draw lead conductor bushings are
being considered but conductor weight may make
them impractical.
Actual
field experience
has shown
that
transformers delivered with high current, draw
lead bushings can substantially reduce installation
time. The potential saving, if the bushing must be
replaced in field, has not yet been seen due to
lack of field problems. It has been estimated that
these savings will be substantial.
Keith P. Ellis was born in Vallejo, California, on October’
17, 1948.He graduatedfrom Mare Island Navel Shipyard,
Vallejo, California with a journeyman certificate in
machine technology. Attended University of Californirq
majoring in Mechanical Engineering. After serving in the
US Navy during the Vietnam War, he joined RTEIASE4
Waukesha, WI. While with RTE/ASEA he worked in the
engineering and marketing. In 1977 he was promoted to
field sales for RTE and RTIYASEA in upstate New York
where he achieved the position of Senior Sales Engineer.
In 1986 he returned to ASEA Electric, Waukesha, WI as
Manager of Transformer Components, Marketing and
Sales. In 1989 he was transferred to ABB Power T & D
Company where he held the position of OEM Sales
Manager for the Components Division. In 1992 he joined
Haefely Trench, as Manager of Marketing and Sales for
North America. In 1995 he was promoted to the position
of Product Manager, Bushings. He is a Member of
IEEE/PES and takes particular interest in component
applications to power transformers with special interest in
high voltage bushings and on-load tap changers.
0-7803-5515-6/99/$10.00 (c) 1999 IEEE