Applying Removable Conductor Bushings to
Existing and New Transformers
Keith P. Ellis
Trench Limited
Old Hickory, TN 37138
USA
ABTRACT
In the last decade of the last millennium a new generation of transformer bushings were introduced. These
new bushings, designed in accordance with IEEE Standards, offered substantially higher draw-lead
ratings then were previously available. This was accomplished with the use of a removable conductor
replacing the flexible cable historically used. A key feature offered by these new bushings was that they
are fully interchangeable with existing bushings. However, there where a number of applications issues
that need to be addressed when applying these new bushings to existing and new transformers.
INTRODUCTION
This paper provides a general overview of available removable conductor bushings, with ratings, voltage
classes and standard mounting flange configurations. It will review mounting flange considerations,
connection terminals and in-board end length. It will cover selecting removable conductor bushings for
existing transformers as well as for new transformers. It will outline special considerations required when
applying these new bushings to new transformers.
AVAILABLE BUSHINGS
The following table will list the removable conductor bushings that are presently available including
common mounting flange arrangement:
kV CLASS
25
25
25
34.5
34.5
34.5
46
46
46
69
69
115
138
161
230
REMOVABLE CONDUCTOR RATING
1,500 AMPS
3,000 AMPS
3,000 AMPS
1,500 AMPS
3,000 AMPS
3,000 AMPS
1,400 AMPS
2,000 AMPS
2,000 AMPS
1,400 AMPS
2,000 AMPS
1,400 AMPS
1,400 AMPS
1,400 AMPS
1,600 AMPS
FLANGE CONFIGURATION
7 ¼” – 4-HOLES
8 ¼” – 4-HOLES
9 ¼” – 6-HOLES
7 ¼” – 4-HOLES
8 ¼” – 4-HOLES
9 ¼” – 6-HOLES
8 ¼” – 4-HOLES
8 ¼” – 4-HOLES
9 ¼” – 6-HOLES
9 ¼” – 6-HOLES
9 ¼” – 6-HOLES
13 ¼” – 6-HOLES
14 ¼” – 6-HOLES
15 ¾” – 8-HOLES
21” – 12-HOLES
MOUNTING FLANGES
For existing transformers the mounting flange configuration has already been determined, making it
necessary to select a replacement bushing with the same mounting flange configuration. If a bushing with
the exact mounting flange configuration is not available then a flange adapter must be used.
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©2002 Doble Engineering Company
All Rights Reserved
As a guide the following table will list the more common mounting flange configurations by voltage class
presently in service today:
kV Class
FLANGE
6 ¼” – 4
7 ¼” – 4
8 ¼” – 4
9 ¼” – 6
10 – 6
10 ¼” – 6
10.875” – 6
13 ¼” – 6
14 ¼” – 6
15 ¾ “ – 8
21” – 12
15
X
X
25
34.5
46
X
X
X
X
X
X
X
X
69
115
138
161
230
X
X
X
X
X
X
X
X
For new transformers the mounting flange configuration is generally not an issue. However, from the
transformer manufacturer's standpoint it is always best to standardize on as few mounting flange designs
as possible.
CONNECTION TERMINALS
Bushings have been supplied with a wide variety of connection terminals for termination of the
transformer winding leads to the bushing. For bushings above 69 kV the most common connection
terminal is the breaker plate as defined in IEEE C57.19.01, figure 2, 3 or 4.
For bushings 69 kV and below the two most common terminals are threaded studs and 2-hole spades.
However, the figure 2, breaker plate has been found on certain 46 and 69 kV bushings produced over the
years.
A new connection terminal has been adopted in the latest revision of IEEE C57.19.01 which is a 4-hole
spade terminal. This new connection terminal is now being applied on many new transformers.
When selecting a removable conductor bushing for an existing transformer, the same connection terminal
that is on the existing bushing must be specified.
The selection of connection terminals for new transformers is generally based on what is available for the
specific bushing. However, the new removable conductor bushings offer an endless variety of connection
terminals allowing transformer manufacturers to standardize on one or two terminals for most of their
transformers.
IN-BOARD END LENGTHS
The length below the bushing’s mounting flange is a function of the voltage class and the specified
current transformer space (CT pocket). For voltage classes through 69 kV there are a wide variety of
lengths in the field. For these bushings with the most common CT pocket lengths being 10 inch, 16.5 inch
and 21 inch. However there have been other lengths supplied over the years.
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For bushings above 69 kV the most common CT pocket lengths are 23 inches for 115, 138 and 161 kV
class bushings and 26.75 inch for 230 kV class bushings (formally 196 kV class). As with the lower
voltage classes, other CT pocket lengths may be encountered. In the future the 230 kV class bushings will
also be supplied with a 23 inch CT pocket.
It is important that when selecting a replacement bushing that the new bushing has the same in-board end
dimension as the existing bushing. This is always true for 46 kV and above bushings that are on the
primary of the transformer. The primary bushings generally dictate the height of the transformer tank and
the internal clearances allowing for more clearances than are normally required for the secondary
bushings. For this reason the replacement secondary bushing can be slightly longer than the existing
bushing. A shorter secondary replacement bushing may not be used because the winding lead length may
not be long enough to reach the connection terminal of the bushing. (It is always advisable to confirm
internal clearances with the original transformer manufacturer)
REPLACING EXISTING BUSHINGS
What ever the reason is to replace an existing bushing the cost of replacing the bushing far outweighs the
cost of the bushing. This is particularly true when the existing bushing is bottom connected. To replace a
bottom connected bushing the transformer’s oil must be drained, either partially or entirely, the
transformer tank must be purged with dry air and someone must enter the transformer tank and disconnect
the bushing. Once the bushing is replaced the transformer oil is returned, either directly or under vacuum.
All of this takes time and adds risk to the personnel as well as the transformer.
It has been estimated that a simple bottom connected bushing replacement could cost the transformer
owner more than $25,000.00 in actual out of pocket expense. In addition, indirect costs for loss of
revenue and additional loading on other transformers could add thousands more to the cost.
If the bushing to be replaced is a draw-lead or removable conductor bushing these costs are greatly
reduced. In most cases little or no oil needs to be removed from the transformer. No one needs to enter the
transformer tank and the time to complete the change out can be reduced from days to hours. This means
the total cost to replace a draw-lead or removable conductor bushing is as much as 80% less than a
bottom connected bushing.
For this reason, when a bottom connected bushing needs to be replaced, many transformer owners are
opting to replace all the bottom-connected bushings at the same time. The high costs to replace the
bushing are there whether replacing one bushing or four bushings. The only added expense is the small
additional cost of the bushings and the extra time to replace the additional bushings.
SELECTING FOR EXISTING TRANSFORMERS
There are a number of considerations that are taken into account when bushings are selected for existing
transformers. These will include the actual load current from the transformer winding, the required
overload and the available bushings. When selecting a replacement removable conductor bushing the
same considerations are taken into account. With this information the right removable conductor bushing
can be selected for the specific application.
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The following actual removable conductor selection examples are offered:
Example 1
The transformer had a maximum rating of 28 MVA at 65°C. The low voltage winding was rated
13.8 kV with a rated winding current of 1,171 amperes. The existing bushing was rated 2,000
amperes. The mounting flange had a 7 ¼” bolt circle with 4 holes. The in-board end length was
29.5” with an 1 ½” – 12 thread connection terminal.
The direct replacement removable conductor bushing selected was rated 1,500 amperes, it had the
same mounting flange configuration and a slightly longer in-board end length. The rating of the
new removable conductor bushing was well above the actual through current and the small
additional in-board end length was determined not to be factor. Even though only one existing
bushing had to be replaced the transformer owner elected to replace all 4 low voltage bushings at
the same time.
Example 2
The transformer had a maximum rating of 56 MVA at 65°C. The low voltage winding was rated
12.47 kV with a rated winding current of 2,593 amperes. The existing bushing was rated 3,000
amperes. The mounting flange had a 9 ¼” bolt circle with 6 holes. The in-board end length was
34.312” with a 2-hole spade connection terminal.
The direct replacement removable conductor bushing selected was also rated 3,000 amperes, it
had the same mounting flange configuration and the same in-board end length. The new
removable conductor bushing was a direct replacement for the existing bushing. In this example
the transformer owner elected to replace only the two bushings damaged by an animal on the
cover of the transformer.
Example 3
The transformer had a maximum rating of 450 MVA at 65°C. The high voltage winding was
rated 230 kV with the lowest de-energized tap at 218.5 kV with a rated winding current of 1,189
amperes. The existing bushing was rated 1,600 amperes. The mounting flange had a 21” bolt
circle with 12 holes. The in-board end length was 59.5” with an IEEE figure 3, breaker plate
connection terminal.
The direct replacement removable conductor bushing selected was also rated 1,600 amperes, it
had the same mounting flange configuration, the same in-board end length and the same IEEE
figure 3 breaker plate connection terminal at the in-board end of the removable conductor. The
new removable conductor bushing was a direct replacement for the existing bushing. In this
example the transformer owner elected to replace all three high voltage bushings.
In examples 1 and 2 the transformers were converted from bottom connected bushings on the secondary
to “all” draw-lead bushings on the transformer. In example 3 only the high voltage bushings were
converted to draw-lead with the removable conductor bushing concept.
INSTALLATION ON EXISTING TRANSFORMERS
The installation procedures for removable conductor bushings on existing transformers are exactly the
same as the procedures to install the old style bottom connected bushings. No special tools are required.
And the joint or split used on new transformer applications is not required on removable conductors
bushings below 115 kV when installed on exiting transformers.
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SELECTING FOR NEW TRANSFORMERS
There are a number of factors that are taken into consideration when selecting a bushing for a new
transformer. The first one, and most important, is the customer’s specification. The specification may
specify a specific brand or style of bushing. It may specify a specific overload requirement for the
transformer and or a specific location for the bushings. (I.e. sidewall mounted) The other major
consideration is the transformer’s voltages and maximum through current. When considering selection of
the new removable conductor bushings exactly the same considerations are taken into account as with the
old style of bushing.
The following examples of bushing selection for a new transformer are offered:
Example 1
The transformer has a maximum rating of 25 MVA at 65°C. The low voltage is 12.47 kV with
rated winding current of 1,158 amperes. The customer specified an overload requirement of
125%, bring the potential maximum through current to 1,448 amperes. If an old style bushing
were to be selected it would be a 2,000 ampere bushing. However, when selecting the new
removable conductor bushing a 1,500 ampere model is selected. This bushing meets all the
overload requirements while providing all the benefits of the removable conductor.
Example 2
The transformer has a maximum rating of 40 MVA at 65°C. The low voltage is 12.47 kV with
rated winding current of 1,852 amperes. The customer specified an overload requirement of
125%, bring the potential maximum through current to 2,315 amperes. If an old style bushing
were to be selected it would be a 3,000 ampere bushing. However, when selecting the new
removable conductor bushing a 3,000 ampere model is selected. This bushing meets all the
overload requirements while providing all the benefits of the removable conductor.
Example 3
The transformer has a maximum rating of 200 MVA at 65°C. The low voltage is 115 kV with
rated winding current of 1,057 amperes. The customer specified an overload requirement of
125%, bring the potential maximum through current to 1,322 amperes. If an old style bushing
were to be selected it would be a 1,600 ampere bushing. However, when selecting the new
removable conductor bushing a 1,400 ampere model is selected. This bushing meets all the
overload requirements while providing all the benefits of the removable conductor.
From these examples it is easy to see that most new transformers can be supplied with either draw-lead
cable bushings or the new removable conductor bushings.
APPLYING TO NEW TRANSFORMERS
Application of removable conductor bushings through 69 kV, for new or re-manufactured transformers,
requires special consideration at the time the transformer is designed. The removable conductor has a
joint or split located ½” below the gasket surface of the bushing mounting flange. This split allows the
conductor to be separated just below the mounting flange for shipment of the transformer. The inside
diameter of the mounting flange does not allow enough space to assemble or disassemble the removable
conductor when it’s below the flange. For this reason the transformer design must allow the removable
conductor to be lifted vertically 6” while still connected to the transformer winding leads. This additional
vertical lift capability is not required when these bushings are applied as replacement bushings on existing
transformers.
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A special installation tool has been developed to support the removable conductor during field assembly
and to prevent any hardware from falling into the transformer tank. This tool is not mandatory but it will
make installation easier and safer for the transformer.
The only other consideration is the design of the mounting flange shipping cover. The shipping cover will
require a supporting bar welded to the center underside of the shipping cover. This supporting bar must
have matching threaded holes to allow the inboard section of the removable conductor to be secured
during transport of the transformer. In addition, some provisions should be considered to lift the shipping
cover when the bushings are installed. The removable conductor for high voltage bushings can be very
heavy.
When removable conductor bushings are selected for high voltage application additional shields or
barriers may be required depending on the applied voltage and/or mounting conditions.
CONCLUSION
No bushing is immune to failure, therefore it is always the best choice to specify and use a bushing which
is easy and simple to replace. And the easiest and simplest bushing to replace today is the proven
removable conductor bushing.
Keith P. Ellis graduated from Mare Island Navel Shipyard, Vallejo, California with a journeyman certificate in
machine technology. Attended University of California, majoring in Mechanical Engineering. After serving in the
US Navy during the Vietnam War, he joined RTE/ASEA, Waukesha, WI. While with RTE/ASEA he worked in
engineering and marketing. In 1977 he was promoted to field sales for RTE and RTE/ASEA 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
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.
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