Bushing Selection
“Making the Right Choice”
By: Keith P Ellis
Trench Limited
Abstract: -- Bushings are a critical component for liquid filled power transformers and reactors. Bushing
selection is a complex number of decisions which can affect the performance and life of liquid filled power
transformers and reactors. Understanding the requirements and limitations placed on modern capacitance
graded bushing is the means to making the right selection. Knowing the specific requirements detailed in
IEEE bushing standards; C57.19.00 and C57.19.01 is the first step in the selection process. Making the
wrong selection or missing a key piece of information can lead to costly errors that can lead to the failure of
the bushing and even the power transformer or reactor. It is known that many reported bushing failures are
the direct result of selection errors. Not making these errors will improve the overall performance of the
substation or power plant.
Key Terms: -- Bushings, capacitance graded, IEEE, Power Transformers and Reactors.
I.
INTRODUCTION
With over 100 years of real life experience selecting bushings for liquid filled transformers and reactors it is
clear that poor selections are still being made today. Bushings are critical apparatus for these key
components of substations and power plants. This makes selecting the right bushings for new transformers
and reactors or bushing replacements an important procedure.
The ensuing sections of this paper will detail the required information needed to reach the right choices in
bushing selection. This will include required information regarding the liquid filled power transformer or
reactor design, the bushing’s performance capability as defined in the IEEE Standards, the substation or
power plant design and layout, geographic location of the site and the general environmental conditions at
the site.
If the selection points to a special or unique bushing requirement the paper will review the options that are
available to minimize the impact the selection may have on the project.
II. REQUIRED INFORMATION
Bushings are listed by the System Voltage Class that they are applied on, however, a bushing is a singlephase apparatus and selection must be based on the bushing’s line-to-ground rating. Figure 1, lists the system
voltages and the corresponding bushing line-to-ground ratings.
As indicated in figure 1, a 115 kV class bushing may be applied on a 138 kV class system, a 138 kV class
bushing may be applied on a 161 kV class system and a 161 kV class bushing may be applied on a 230 kV
class system. The key piece of information is the line to ground voltage rating for the specific bushing
design.
1
Figure 1
Bushing line to ground ratings
System Voltage,
kV
25
34.5
69
115
138
138
161
161
230
345
500
765
Line to Ground
Voltage, kV
16
22
44
88
88
102
102
146
146
220
318
485
When IEEE C57.19.01 was revised in 2000, it eliminated 15, 25, 46 115 and 161 kV class bushings. Figure 1
indicates a 88 kV, 138 kV class bushing can be applied on 115 kV class systems and that a 138 kV class
bushing or a 230 kV class bushing can be substituted for 161 kV class bushings on new power transformers
or reactor as long as the line to ground voltage is equal to or greater than the system line to ground voltage.
The next piece of required information is the transformer’s winding’s Basic Insulation Level, BIL. When
selecting bushings the rule is that the bushing’s BIL must be equal to or greater than the winding’s BIL.
IEEE C57.19.01 – 2000 lists 7 standard bushing BIL ratings as listed in Figure 2 in the appendix.
Voltage Class, kV
Figure 2
Standard Bushing BIL Ratings
34.5
69
138
230
345
500
765
B I L, kV
200
1675
2050
350
650
900
1175
The 7 standard BIL ratings should cover 100% of the most common power transformer and reactor
applications. Should there be a situation where a higher BIL rating is required; there are additional BIL
levels available in the industry. For 345 kV system voltage the industry offers 1300 kV BIL, for 500 kV
class, 1800 kV is available and for 765 kV systems BIL levels above 2050 kV are available. Note that these
special designs carry longer lead times and may increase the cost of the transformer or reactor.
The next piece of required information is the transformer’s maximum rated through current for each
winding. Figure 3 is a helpful table for load currents for varying transformer ratings from 3.125 MVA though
700 MVA, though 500kV class.
Modern oil impregnated paper (OIP) bushings utilize Kraft paper with index 105 insulation and are rated for
a total average operating temperature of 95°C. When operating a bushing at rated current in conjunction with
a 65°C average winding rise rated transformer, the hottest-spot temperature of the bushing is limited to a
75°C rise over ambient or a 105°C total temperature. If the transformer develops a top oil rise of 65° C at
rated current when operating in a 40°C ambient, the hottest-spot temperature of the bushing can be expected
to exceed 105°C, which is acceptable as long as the daily average does not exceed 30°C. In this situation,
transformer can be expected to have bushing temperatures above 105°C when loaded in accordance with
IEEE Standards with loss of life.
Given that all modern power transformers are designed with uprated Kraft paper IEEE standard C57.19.00 –
2004 revised the requirement for bushing that all new designs must be suitable for operation at rated current
in a 65°C average winding rise rated transformer. This only applies to new bushing designed after the
publication of the revised document in 2004.
The best way to avoid exposing the bushing to overloads and loss-of-life is to select a bushing that has a
nameplate rating at least 15% above the maximum rated through current of each transformer winding.
2
Figure 3
Specific requirements:
Bushing standards:
The specifications for the power transformer or reactor must include the latest IEEE bushing standards.
These standards are covered be the C57 group of standards and guides under the IEEE Power and Energy
Society, Transformers Committee. There are no other IEEE bushing standards that pertain to liquid filled
power transformers and reactors. These standards are listed in the Reference section of this paper.
Bushing brand preference:
Indicate all brands of bushings that are approved. If your specification list one or two brands and states “or
approved equal”, the transformer or reactor manufacturer bidding on your requirement may not take the time
to confirm what other brands are approved. This could have a negative impact on their competitive offer if
their preferred approved brand is not listed.
3
Preferred bushing technology:
There are five base technologies available for bushing today:
1.
2.
3.
4.
5.
Oil Impregnated Paper (OIP), the most common technology used today applied to the highest voltages.
Resin Impregnated Paper (RIP), an established technology developed in the 1950’s, limited to 69 kV
class.
Epoxy Resin Impregnated Paper (ERIP), an upgrade of the RIP technology rapidly gaining wide
acceptance around the world available though 765 kV.
Resin bonded Paper (RBP), the original capacitance graded bushing technology developed in 1908. Not
recommended for power transformers today because of high allowable partial discharge levels.
Bulk bushings, the first bushing technology from 1883. Basically a conductor though a porcelain
insulator. High partial discharge levels and no IEEE standards covering performance and dimensions.
The specifications should indicate the acceptable technology or technologies and/or those that are not
acceptable.
Required top terminal plating:
The specification should indicate the required top terminal plating for the bushing; Silver, Tin or no plating.
Silver is the number one specified plating requirement and is considered standard by bushing manufacturers.
Tin plating is widely used in Canada and in certain industrial applications. No plating is more uncommon.
Preferred bushing connection method to the power transformer windings:
A specific requirement that has been growing for the past 15 years is the transformer purchaser specifying
how the bushing is connected to the transformer’s winding leads. Historically draw lead connections to the
transformer’s winding lead were limited to 400 ampere though 69 kV class bushings and 800 amperes above.
Today, bushing manufactures have developed bushings that can achieve high current draw lead rating. The
most popular design utilizes a removable copper conductor that permits draw lead rating up to 3,000 amperes
using multi-spring contacts in the top terminal. Another design connects the top terminal with the bottom
terminal using a steel draw rod and another design uses the steel draw rod with the multi-spring contacts at
the bottom of the bushing.
In order to take advantage of these new, high current draw lead bushings the transformer specification must
indicate that draw lead bushings are required. There is a useful table of available draw lead bushing ratings
by voltage class in figure 4 in the appendix.
III. UNUSUAL SERVICE CONDITIONS
Unusual service conditions are listed in detailed in the IEEE transformer standard, C57.12.00 and the
Bushing standards, C57.19.00.
Ambient temperatures greater than a maximum of 40°C with a 24 hour average of 30°C.
Ambient temperatures lower than -30°C.
Altitudes greater than 3,000 feet (1,000 meters)
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Unusual environmental conditions: Damaging fumes or vapors, excessive abrasive or semi-conducting dust,
explosive mixtures of dust or gases, steam, salt spray or wet conditions.
Bushings mounted at angles greater than 30° from vertical.
Seismic considerations: Low seismic zone or moderate seismic zone or high seismic zone
Bushings applied inside isolated-phase bus duct:
Bushings used with isolated-phase bus meeting the requirements of IEEE C37.23 may be subjected to
conductor and enclosure temperatures that exceed the conditions specified in IEEE C57.19.00. Table 1 of
IEEE C37.23 lists the temperature limits of isolated-phase bus conductors, enclosures, insulation, and
terminations. After selecting the temperature rise rating of the conductor and enclosure, the user should
identify this unusual service condition in the transformer specification.
Any of the above unusual services conditions that are not taken under considerations during the bushing
selection process could have disadvantageous consequences for the bushing and or the power transformer or
reactor. In the following section mitigations options for these unusual service conditions will be reviewed.
IV. MITIGATIONS CONSIDERATIONS
Ambient temperatures greater than a maximum of 40°C with a 24 hour average of 30°C:
As indicated in section II, bushings designed before 2004, where designed for operation in power
transformers with a 55°C raise insulation. In situations where the ambient temperatures are greater than the
maximum of 40°C with a 24 hour average of 30°C closer assessment of the bushing’s rating is required.
A recommended selection guideline would be to select a bushing with a current rating that has a current
rating that is equal to the percentage increase in average ambient or the increase in the maximum
temperature. A more accurate current rating can be obtained taking into Account the load profile and the
thermal constants for bushings as explained in IEEE C57.19.100.
However, the bushing rating selected should be one of the standard ratings listed in C57.19.01 – 2000 if
possible. They are 1,200, 2,000 and 3,000 amperes. At 34.5 kV there is a 5,000 ampere model listed.
Bushing manufacturers offer a wide range of current ratings and will likely have exactly what is required.
Ambient temperatures lower than -30°C:
There are two issues for bushings operating at low ambient temperatures; they are gaskets and oil viscosity.
Gaskets can become brittle, leading to failure and if the bushing’s oil becomes too cold it will not properly
circulate the heat generated causing hotspots in the insulation system.
In the power transformer or reactor specifications make it a requirement that the bushings must be capable of
operations at the stated possible low ambient. The bushing manufacture, if they are not using gasket and oil
suitable for operation in an ambient below -30°C, they will have use oil which is required in the Canadian
CSA bushing standards as well as gaskets.
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Altitudes greater than 3,000 feet (1,000 meters):
Over the past ten year bushing manufacturers that produce IEEE Standard bushing have standardize their
production utilizing insulator rated for 10,000 feet (3,300 meters). However, it is always recommended that
if the site for the substation or power plant is above 3,000 feet (1,000 meters) that the actual elevation be
included in the specification.
Unusual environmental conditions:
Increasing the creepage of the porcelain outdoor insulator has been the general practice and continues to be a
viable method for mitigating the unusual environmental conditions listed in the standards.
In 2000, when the revised IEEE Standard, C57.19.01 was published it listed two standard creepage levels;
the light level of 28 mm/kV and the heavy level of 44 mm/kV. Bushing manufacturers producing IEEE
standard bushings adopted the heavy level of 44 mm/kV as standard for their new design. Today almost all
bushing designs listed in IEEE Standard C57.19.01 are offered with 44 mm/kV creepage level.
Today a proven technology is now gaining wide acceptance around the world as an alterative to porcelain.
With over 25 years of outstanding field performance, insulators utilizing an explosion resistant, reinforced
fiberglass tube with silicone rubber sheds molded over the tube are an excellent alterative to porcelain. The
author refers to these insulators as SRI, (silicone rubber insulator).
It has long been known that SRI provides excellent pollution abatement because of the inherent properties of
silicone rubber. There have been many studies and papers on this subject which are too many to reference in
this paper. The key word is; Hydrophobic, (hí΄dro-fō΄bik), “If the surface of a high voltage insulator is
Hydrophobic, the leakage currents as well as the surface discharge activities will be reduced”. Silicone
rubber has inherently better hydrophobic properties than porcelain. All the benefits of SRI for pollution
abatement have been available on OIP Instrument transformers and Bushings since 1985.
Bushings mounted at angles greater than 30° from vertical.
Depending on the bushing manufacture and design most bushing can be operated at greater angles than 30°
from vertical. However, some designs may require special considerations before mounting at angles greater
than 30° from vertical.
These include the oil level indication method. Bushing with prismatic glass sight glass may loose oil level
indication if the angle too great. This can be overcome by mounting the bushing with the glass facing 45° to
the down angle.
Mobile substation transformers may require special bushing designs. These include special high strength
porcelain and high strength bushing mountings flanges to prevent breakage at extreme angles when
transporting the substation over the road.
Mobile substations manufacturers are experienced at applying bushings at extreme angles and work with
their customers to insure that the right bushings have been selected.
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A comment from the author is offered to purchaser of mobile substations; consider non-IEEE standard
bushing for you mobile substations. IEC bushings have no dimensional requirements and therefore can be
produced with CT pockets lengths that meet the needs of the number of CTs required, allowing the
transformer to be smaller gaining more MVA for the weight limitations required. IEC bushings are produced
in North America insuring quick accessibility to spare and replacements bushings.
Seismic considerations:
In February 2006 the IEEE 693 Working Group announced that the test protocol for transformer bushings in
the latest revisions of the document, IEEE Recommended Practice for Seismic Design of Substations is not
valid. The Working Group discovered that bushing designs that had passed the existing test protocol
experienced failures during moderate earthquakes.
The Working Group, in cooperation with a Utility consortium has been working on developing a new test
protocol for bushings. Testing is on-going with excellent results. It is expected that the 693 Working Group
will finish their work in 2010.
Bushings applied inside isolated-phase bus duct:
In order to ensure proper thermal coordination between the bushing and the bus, steps should be taken to
reduce the temperature of the bus conductor, the surrounding medium, and the bus duct. Such steps could
include the following:
Increase cross-sectional area of the conductor or the connection between the bushing and the conductor. Use
flexible cable or braids and silver-surfaced or tinned joints at the connection between the bushing and
conductor. Increase the cross-section and the diameter of the bus enclosure surrounding the bushing. Install
forced air around the bushing or through the ventilated bus duct to keep the air temperature within the
acceptable range.
Select bushing designs with gaskets suitable for high-temperature application. Consider capacitive graded
bushings with high temperature insulation such as Mylar, Nomex™ or bushing utilizing Epoxy-resin
impregnated paper technology.
The listed mitigation options are not inclusive as there are other options that may be considered based on the
type and severity of the unusual condition.
V. CONCLUSIONS
It is evident that understanding the bushing selection process is an important segment of the power
transformer or reactor specification development. The benefit includes increasing the reliability of the
substation or power plant and reducing spare bushing inventories that support the power system.
By having detailed requirements regarding the bushings required for the new power transformer or reactor
the risk of receiving bushings that do not meet the requirements specified in the IEEE bushing standards will
be minimized.
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VI. REFERANCES
1. IEEE Std. 693™ - 2007, IEEE Recommended Practice for Seismic Design of Substations
2. IEEE Std. C57.12.00™, IEEE Standard for Standard General Requirements for Liquid-Immersed
Distribution, Power, and Regulating Transformers
3. IEEE Std. C57.19.00™ – 2004, IEEE Standard General Requirements and Test Procedures for Power
Apparatus Bushings
4. IEEE Std. C57.19.01 – 2000, IEEE Standard Performance Characteristics and Dimensions for Outdoor
Apparatus Bushings
5. IEEE Std. C57.19.100™ - 1995, IEEE Guide for Application of Power Apparatus Bushings
6. IEEE Std. C37.2™ - 2003, IEEE Standard for Metal-Enclosed Bus
7. Conference paper, Twenty Years of Field Experience: OIP Instrument Transformers and Bushings
Supplied with Silicone Rubber Insulators (SRI), By Keith P. Ellis in Rio de Janeiro at the 2007 INMR
Conference.
8. Doble Conference Paper, 2003, Development of Transformer Bushings with Non-ceramic Insulators, A
Viable Alternative to Porcelain, by Peter D. Zhao and Keith P. Ellis
9. IEEE Conference paper 0-7803-5515-6/1999© Development of High Current Draw Lead Transformer
Bushings Designed in Accordance with IEEE Standards by Keith P. Ellis
VII. BIOGRAPHY
Keith P. Ellis is responsible for the development, promotion, sales and technical support for bushings for the
Trench Bushing Group including HSP, serving as Bushing Product Manager and OEM Sales Manager,
Americas. Before joining Trench Keith was Sales Manager for the ABB Power T & D Company’s
Components Division. This position was also held under the company names of Westinghouse/ABB and
ASEA Electric. Before assuming the Sales and Marketing responsibilities of transformer components, Keith
was Senior Sales Representative for RTE and RTE-ASEA in Upstate New York. Keith began his career in
the power transformer industry with RTE/ASEA as an Application Engineer.
Keith graduated from Mare Island Naval Shipyard with a journeyman certificate in Machine Technology.
Attended the University of California, where he majored in Mechanical Engineering. After serving with
distinctions in the US Navy during the Vietnam War he continued his education at the University of
Wisconsin, Milwaukee. He is a member of IEEE, PES, Transformers Committee and Working Group
Chairman for C57.19.00. He takes particular interest in component applications to power transformers with
special interest in high voltage bushings and on-load tap changers. Keith enjoys RV vacations and is
restoring a 1926 Model T Fordor.
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VIII. APENDX
Amps
400
800
1400
1500
1600
2000
3000
kV Class
BIL
Figure 4
Available Draw Lead Bushing Ratings
25 34.5 46 69 115 138 161 230 345 345 500 765
150 200 250 350 550 650 750 900 1175 1300 1675 2050
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Other rating may be available from Draw-rod bushing designs
9