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Dimensional Standards for Bushings Applied to Liquid Filled Power Transformers and Reactors

and why they are Important to your Electric Power System

Keith P. Ellis, Member, IEEE/PES

Abstract:--This paper will outline the history of IEEE

Standard Performance Characteristics and Dimensions for Outdoor Apparatus Bushing, C57.19.01-2000 and how the input from US utility groups brought these standards to their present form. This will include how these efforts to standardize bushing ratings and dimensions are having an impact on increasing the reliability of electrical systems in the United States. It will include comparisons between the requirements of the IEC Bushing Standard 60137 Insulated Bushings for Alternating Voltages Above 1000 V, used in Latin America, and the IEEE C57.19 bushing standards. It will offer scenarios that can be considered for harmonizing existing requirements for IEC bushings and the IEEE dimensional bushings. Recommendations will be offered to insure that your new power transformers or reactors will conform to your requirement for bushing interchangeability. In the conclusions, the impact of adding dimensional requirements to your bushing requirements will discussed in detail.

Index Terms--Bushings, Dimensions, IEC, IEEE, Power Transformers, Reactors.

I. INTRODUCTION ITH nearly 50 years of real life experience using ANSI/IEEE bushing interchangeable standards, utilities in the United States have found that following these

standards have reduced their spare bushing inventories, shortened the lead times for replacement bushings and lowered the overall cost of supporting the reliability of their liquid filled power transformers and reactors.

Those Utilities around the world that utilize the IEC

bushing standards, which do not provide dimensional requirements, have struggled with the issue of bushing interchangeability. Each bushing supplier has their own unique dimensional profiles for their bushings, preventing interchangeability with other brands. If the Utility adopted a unique dimensional standard for their bushings they soon found the lead-times for those bushings increasing and in many cases, the cost escalating.

Keith P. Ellis is the Bushing Product Manager, Americas for the Trench Bushing Group (e-mail: Keithcota@aol.com) 978-1-4244-2218-0/08/$25.00 ©IEEE

The only other alternative the Utility had was to purchase spare bushings with each new liquid filled power transformer or reactor. This action increased the purchase price of new equipment, increased their physical inventory of spare bushings, increased warehousing space and increased maintenance requirements for this inventory. After all of this extra effort many Utilities discovered that when the need for a specific spare bushing arose, locating the correct bushing style was a challenge and many times, when located, the spare bushing was not suitable for service.

II. HISTORY OF DIMENSIONAL STANDARDS

The first bushing dimensional standard was developed by the American National Standards Instituted (ANSI) in the late 1950s and added these requirements to ANSI C76.1 – 1943. In 1968 the ANSI C76.1 Committee decided to divide the bushing standard into three parts. The ratings and dimensions standard became ANSI 76.2. In 1977 ANSI C76.2 – 1977 / IEEE Std. 24 added duel ratings (Example 1200/1600 amps) for transformer (1200 A) and circuit breakers (1600 A), By 1991 the bushing standards were purely IEEE Standards with the Ratings and Dimensions standards becoming IEEE Std. C57.19.01 – 1991.

In 1991 the IEEE, PES, Transformers Committee, Bushing Subcommittee began work to revise the 1991 document. When this work began feedback from EEI (Edison Electrical Institute), OEMs (original equipment manufacturers) and other users indicated that a greater degree of standardization was needs in bushing ratings. With this mandate from the Industry, the PC57.19.01 Working Group began a nine year effort to meet the goals set by this feedback. The result of this work is IEEE Standard Performance Characteristics and Dimensions for Outdoor Apparatus Bushing, C57.19.01 – 2000.

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The most significant and sometimes most controversial change to C57.19.01 was the reduction of the “standard” bushing voltage class ratings. 15, 25, 46, 115 and 161 kV classes were removed. This left 34.5, 69, 138, 230, 345, 500 and 765 kV classes. The most controversial of these reductions was removing the 25 and 115 kV classes. The Working Group agreed to include the electrical characteristics of those bushing deleted from the revised standard in the annex of C57.19.01 – 2000. The table A.1 in Annex A lists electrical ratings for those bushing classes removed from the standard.

CT pocket lengths were standardized with two lengths, 533 mm for bushings though 69 kV class and 584 mm for all bushings above 69 kV class This action greatly reduced the number of bushing designs,

Another major change to this document was removing bushings for oil circuit breakers. In 1993 the last new oil circuit breaker was produced in the United States. Since standards are for new equipment only, there was no longer a requirement to include this type of bushing application in C57.19.01.

III. BUSHING STANDARDS COMPARISON

It is clear that totally adopting all IEEE bushing standards may not be a practical approach to adding “standardization” to your bushing requirements. However, before incorporating the dimensional requirements of the IEEE bushing standards into your liquid filled power transformers and reactors specifications it is important to compare the IEEE bushing group of standards to the IEC bushing standard you now use. The following tables will highlight the similarities and differences for your consideration:

TABLE I: MAJOR DIMENSIONS CONTROLLED Dimensions IEEE IEC

Mounting Flange Yes No

Length Below the Flange Yes No

CT Pocket Length Yes No

Top Terminal Yes No

Bottom Terminal Yes No

Voltage Tap Yes Yes

Test Tap No No

The dimensions controlled by IEEE C57.19.01 are the critical dimensions in providing bushing interchangeability.

As you review the Routine, Design and Special tests required for each standard you will note that they are very close regarding the required tests.

TABLE II: ROUTINE TESTS REQUIREMENTS

Test IEEE IEC

Internal Pressure Yes Yes

Capacitance Yes Yes

Power Factor Yes Yes

Tao Withstand (Test or Voltage Yes Yes

Dry Power-Frequency Withstand Yes Yes

Partial Discharge Measurement Yes Yes

Tightness Test of the Flange No Yes

Dry Lightning Impulse > 850 kV BIL No Yes

Verify Nameplate Markings Yes No

TABLE III: DESIGN TESTS REQUIREMENTS

Test IEEE IEC

Internal Pressure Yes Yes

Draw-Lead Cap Pressure Yes No

Cantilever Yes Yes

Capacitance Yes Yes

Power Factor Yes Yes

Tap Withstand (test or Voltage) Yes No

Full Wave Lightning Impulse Yes Yes

Wet Withstand Voltage 230 kV & Below Yes Yes

Wet Switching Impulse Voltage > 300 kV Yes Yes

Temperature Yes Yes

Thermal Stability No Yes

Verify Nameplate Markings Yes No

TABLE IV: SPECIAL TEST REQUIREMENTS Test IEEE IEC

Thermal Stability Yes No

Front-of-Wave Impulse Yes No

Seismic Withstand Yes Yes

Artificial Pollution No Yes Special Tests are agreed on between the purchaser and manufacturer at time of order.

Optional bushing accessories are not listed as available within the IEEE standards. The bushing manufacturers do offer options such as different top terminal plating, none, silver or tin. Within the IEC standards a few optional accessories are available. Besides the accessories listed below items such as arcing horns are offered for IEC bushings. For the most part, manufacturers of either IEEE or IEC bushings can offer accessories to meet most customer requirements.

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TABLE V: ACCESSORIES

Standard IEEE IEC

Test Tap 72 kV and Below Yes Yes

Test Tap Above 72 kV No Yes

Voltage Tap Above 72 kV Yes No

Optional

Voltage Tap Above 72 kV No Yes An option that is very common for all bushings is the altitude rating and the creep distance on the outdoor insulator.

All bushings are designed to operate at rated voltage up to 1,000 meters. For operation above 1,000 meters the bushings design must electrically de-rated or be modified by increasing the arcing distance from live part (Metal top part of the bushing) and ground (The bushing’s mounting flange). The actual arcing distance required to increase the altitude rating is subject to the specific characteristics of a particular design. Today, most IEEE bushing manufacturers in North America are standardizing on ratings up to 3,000 meters for new designs.

The creep level is arrived at differently between the IEEE and IEC standards. IEC levels are based on the highest system voltage (Um), while IEEE levels are based on the nominal line to ground voltage of the bushing. The result of using either method is very small, usually only a few mm in favor of the IEC method. Today many new IEEE bushing designs being developed are with creep levels that comply with the Heavy level of creep listed in C57.19.100, IEEE Guide for Application of Power Apparatus Bushings.

If high pollution levels are a concern consider selecting a Silicone Rubber Insulator (SRI) instead of porcelain. SRI is proven to perform much better in heavy pollution then porcelain.

Table VI provides the formulas for calculating both IEEE and IEC standard creepage levels for bushings.

TABLE VI: CREEP DISTANCES

Contamination Level IEEE IEC

Light 28 mm/kV 16 mm/kV

Medium 35 mm/kV 20 mm/kV

Heavy 44 mm/kV 25 mm/kV

Extra Heavy 54 mm/kV 31 mm/kV

Basic insulation levels are one of the reasons bushing interchangeability with IEC designed bushings can be frustrating. The following table may show why this is a possible issue:

TABLE VII: VOLTAGE CLASSES & BIL RATINGS

kV Class (Um) kV BIL IEEE IEC 3.6 kV 40 No Yes 7.2 kV 60 No Yes 12 kV 75 No Yes 15 kV (IEEE) 110 Yes No 17.5 kV 95 No Yes 24 kV 125 No Yes 25 kV (IEEE) 150 Yes No 34.5 kV (IEEE) 200 Yes No 36 kV 170 No Yes 46 kV (IEEE) 250 Yes No 52 kV 250 No Yes 69 kV (IEEE) 350 Yes No 72.5 kV 325 Yes No 100 kV 380 No Yes 450 No Yes 115 kV (IEEE) 550 Yes No 123 kV 450 No Yes 550 No Yes 138 kV (IEEE) 650 Yes No 145 kV 450 No Yes 550 No Yes 650 No Yes 161 kV (IEEE) 750 Yes No 170 kV 550 No Yes 650 No Yes 750 No Yes 230 kV (IEEE) 900 Yes No 245 kV 650 No Yes 750 No Yes 850 No Yes 950 No Yes 1050 Yes Yes 300 kV 850 No Yes 950 No Yes 300/362 kV 950 No Yes 1050 No Yes 345 kV (IEEE) 1175 Yes No 362/420 kV 1050 No Yes 1175 No Yes 420/550 kV 1175 No Yes 1300 No Yes 1425 No Yes 500 kV (IEEE) 1675 Yes No 550 kV 1300 No Yes 1425 No Yes 1550 No Yes 765 kV (IEEE) 2050 Yes No 800 kV 1425 No Yes 1550 No Yes 1800 No Yes 1950 No Yes 2100 No Yes 2400 No Yes

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The following rules of thumb that are used in applying bushings to liquid filled power transformers and reactors may assist in reducing the number of different bushing design that will be required for each specific utility system: 1. The bushing’s BIL must be equal to or greater than the

BIL of the winding. 2. Higher BIL bushings will not necessarily increase the

altitude capability of a specific bushing. 3. Higher creep levels will not allow bushings to operation at

higher altitudes. 4. For the best pollution abatement performance, select

bushings with SRI. 5. For the highest Seismic requirements always specify

bushings with SRI.

The information contained in this section is not intended to promote the use of either IEC or IEEE standards for your bushings but rather to allow you to evaluate applying IEEE dimensional standards to IEC standard bushings.

IV. HARMONIZING IEC AND IEEE BUSHING STANDARDS

In section III it was made clear that, for the most part, IEC

and IEEE standards are fairly compatible with each other. The largest difference is in the large number of available BIL levels for IEC standard bushings. This wide range of BIL levels is not readily available on IEEE standard bushings. Fortunately, the IEEE standard BIL ratings are generally equal to the highest BIL level for a given IEC voltage class.

When selecting an IEEE Standard bushing rating for your system you need to select a bushing with a maximum line to ground (phase-to-earth) rating that will meet or exceed your maximum system voltage (Um) as defined in IEC 60137, clause 4. Once this is determined you can select an IEEE standard bushing rating from Table VIII.

Table VIII contains two ratings that are not listed in IEEE C57.19.01 – 2000. These are the 138 kV Class bushing with a 102 kV line to ground rating (located in Annex A) and a 1050 kV BIL, 230 kV bushing with a 156 kV line to ground rating. These ratings are available from most IEEE standard bushing manufacturers in North America.

TABLE VIII: IEEE VOLTAGE CLASSES & BIL RATINGS Voltage Class & BIL

Max. Line to Ground

Rating kV

Max. System

Voltage kV 1 min. dry

rms kV

34.5 kV - 200 kV BIL 22 38 80

69 kV - 350 kV BIL 44 76 160

138 kV - 650 kV BIL 88 152 310

138 kV - 650 kV BIL 102* 176 310

230 kV - 900 kV BIL 146 252 425

230 kV - 1050 kV BIL 156* 270 460

345 kV - 1175 kV BIL 220 381 520

500 kV - 1675 kV BIL 318 550 750

765 kV - 2050 kV BIL 485 840 920

* These rating are available upon request

IEC 60137 does not standardize on nor recommend current ratings for bushing. IEC does provide current rating limits for the cantilever test requirements and most IEC bushing manufacturers have designed bushings with current ratings inline with those listed in IEC 60137 for the cantilever test. .

IEEE C57.19.01 – 2000 has specific standard current ratings and therefore, all the IEEE standard bushing manufacturers have designed their standard bushings with those current ratings found in Table IX:

TABLE IX: IEEE STANDARD CURRENT RATINGS

Voltage Class

400 A

800 A

1200 A

2000 A

3000 A

5000 A

34.5 kV X X X X X

69 kV X X X X

138 kV X X X X

230 kV X X X X

345 kV X X X X

500 kV X X X X

765 kV X X X X

Current ratings of 400 and 800 amps are for draw lead cable applications. These rating can be higher if the liquid filled power transformer or reactor manufacturer insulates the cable with Class F, high temperature insulation. Bushings with current ratings of 1200, 2000 and 3000 amps can be supplied with removable draw lead conductors for ease of installation and removal. Consult with the bushing manufacturer for available deigns.

Bushings that require current ratings higher then those listed above are available from all IEEE bushing manufacturers with a high degree of interchangeability between brands.

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V. BENEFITS OF ACHIEVING INTERCHANGEABILILTY

Utilities in the United States have been enjoying the benefits of bushing standardization for more than 40 years. In the early days the immediate benefit was bushing interchangeability between liquid filled power transformers and reactors and bulk oil circuit breakers. By specifying a Transformer/Breaker Interchangeable (TBI) bushing they could apply the same spare bushing inventory to support liquid filled power transformers and reactors as well as their bulk oil circuit breakers.

With bushing standardization the overall spare bushing inventory is reduced. In the United States the bushing inventory incurs a larger tax burden from accruing bushing costs in years 2 to 5. This real cost was a major reason C57.19.01 – 2000 reduced the number of standard bushings being offered from 56 to 21 designs while increasing the current ratings from 1200 amperes to 3,000 amperes across the board and adding a 5,000 ampere design at 34.5 kV. If your utility does not experience this type of taxation there are still other tangible savings from standardization.

With fewer spare bushings in inventory the costs of warehousing these bushings is reduced. These savings extend to lowering the expense of periodic inventory testing as well as the size of the warehouse itself.

The real benefits come when the spare bushing is required. With a standardized bushings inventory, that is well maintained, the outage time for your liquid filled power transformers or reactors is assured to be as short as possible. Compare this with your own experience trying to locate a serviceable spare for many of your IEC standard bushings. Dimensionally interchangeable bushings increases system reliability and minimize customer irritation.

IEEE standard bushings have the shortest standard lead-times in the World. Manufacturers of IEEE standard bushings can generally ship these bushings in eight weeks or less on average. Many maintain small revolving inventories that can ship in less then one week, even at 500 kV. This is certainly not the case with IEC standard bushings. Considering that these bushings have thousands of possible dimensional the one you may require could take six months or more to deliver.

Additional benefits include lower liquid filled power transformer and reactor costs. (Standard bushings designs cost much less then special bushing designs) The cost of spare bushings is reduced as well when replacing the spare bushing once it is used to replace an old bushing.

It is important to note that the above cost information applies when comparing standard IEEE standard designed bushings with special IEEE standard designed bushings. IEEE standard bushings have specific dimensional requirements and mandated features. These requirements dictates a higher cost then a typical IEC standard bushing, which does not mandate those specific dimensions or required features. This difference could be between 10% and 30% per bushing. When you consider that bushings represent less then 5% of the value of the liquid filled power transformers or reactors, adding 10 to 30% to price of each bushing will hardly be noticed in a competitive market when purchasing new liquid filled power transformers or reactors. VI. ACHIEVING BUSHING INTERCHANGEABILITY

The easiest way to achieve bushing interchangeability

would be to just specify IEEE C57.19.01 – 2000 for the dimensional requirements for bushings in your liquid filled power transformers or reactors specification. This would certainly accomplish this objective. However, before you do this you need to consider some of the minor items that could present issues for your operation.

The first item would be the bushing’s top terminal stud. The IEEE standards specify the use of a top terminal stud that utilizes threads. In most of the rest of the World threads are not used for the bushing top terminal stud, they use a smooth terminal stud. This difference may present a logistic issue in obtaining line terminal in your region that utilizes threads. Most bushing manufacturers that produce IEEE standard bushings can easily supply the IEEE standard bushing with a smooth top terminal stud.

The next item would be the color of the outdoor insulator. Since the late 1960s the United States government has required that insulators used by Utilities be sky gray. This was the results of beatification efforts inspired by Lady Bird Johnson, wife of then President Lyndon B. Johnson. Since then all IEEE standard bushings have been delivered with sky gray porcelain as the standard offering. Brown porcelain has always been available but at a small added cost and a long lead-time. Those Utilities in Latin America and Mexico that have adopted IEEE standard dimensional requirements have accepted sky gray porcelain or sky gray SRI for their new liquid filled power transformers and reactors.

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Since there is not a great deal of difference between the between IEC and IEEE bushing testing this area should not present an issue. However, one item under tests that should be discussed is the lightning impulse testing. IEC requires that all bushings with a Basic Insulation Level (BIL) greater then 850 kV must receive a routine impulse test. This is not a requirement within IEEE. This issue has been discussed by IEEE, PES, Transformers Committee, Bushing Subcommittee and the consciences of the Group was that since the bushings would receive the impulse tests on the new liquid filled power transformer or reactor, that adding this test to the routine tests were not needed. In addition, the frequency of actual impulse test failures of the bushings at transformer OEM’s was extremely rare. The conclusion was that the added cost of 100% impulse testing at the bushing factory could not be justified by the extremely low rate of failures during transformer factory testing.

However, this author offers the following commentary on this issue for your consideration: A. At voltage levels above 345 kV class, given the high cost

of these bushings and potential delays caused by a test failure at the transformer OEM, it is recommended that the bushings receive a routine impulse test by the bushing manufacturer.

B. Bushings purchased as a spare do not receive a factory impulse test. It is recommended that when purchasing spare bushings with BIL ratings above 850 kV BIL, that they receive a routine impulse test at the bushing factory.

C. Bushings below 900 kV BIL have an inherently high reliability record, therefore paying the high cost of a routine impulse test is not justified.

One item that we have not discussed yet is bushing

technologies. Today, there are two major bushing technologies being utilized around the World. The oldest of these is Oil Impregnated Paper (OIP) and the newer Epoxy-Resin Impregnated Paper (ERIP). Both of these technologies are available in IEEE standard dimensions according to C57.19.01 – 2000. This will allow each Utility to select the technology that best meets their needs.

The following scenarios for specifying dimensional bushing requirements for your liquid filled power transformers and reactors specification are offered: A. All bushings shall be capacitance graded and shall be

designed to incorporate the dimensional requirements as stated in IEEE C57.19.01 – 2000 for the specific voltage class specified. The outdoor insulator shall be sky gray. The top terminal of the bushing shall be a smooth copper alloy stud of amble metric diameter to carry the full rated current as stated on the bushing’s nameplate. The top terminal shall be silver plated. All bushings rated above 345 kV class shall be subjected to a routine impulse test by the bushing manufacturer.

B. Another scenario that could be considered is to make it

clear that it is desired that the electrical performance of the bushing shall comply with the requirements of IEC 60137 and then state the requirements in scenario number 1. To address the 1050 kV BIL rating for the 230 kV class IEEE bushing you could state that for your 245 kV system a BIL rating of 1050 kV is required at 156 kV line-to-ground.

C. Many Utilities in the United States are very specific

regarding the bushings they will accept on their new liquid filled power transformers and reactors by listing the specific catalog numbers of each bushing class and brand they have approved. Table X is an example of what is found in some specifications.

TABLE X: TYPICAL US UTILITY BUSHING SPECIFICATION

Nominal System Voltage

kV BIL kV

Current Rating Amps ABB Trench

34.5 200 1200 034W0412AP 200-H015-21-AG3-01-AYS

3000 034W3000BC 200-F030-21-AG3-01-AYS

69 350 1200 069W0412AN 350-H014-21-AG3-01-AYS

2000 069W2000UH 350-F020-21-AG3-01-AYS

138 650 800 138W0800AA 650-H014-23-AG3-01-A

3000 138W3000UA 650-F030-23-AG3-01-AEP

500 1675 2000 550W2000UT 1675-F020-23-AG3-01-AFP

Each Utility must evaluate if other requirements are

required to meet the needs of their electric power system.

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X. BIOGRAPHEY

VII. CONCLUSIONS

Experience has proven that standardizing the dimensions and features of bushings for liquid filled power transformers and reactors has a positive impact on the performance of electric power systems. It reduces overall operating expenses, decreases system outage time and improves customer satisfaction. Dealing with fewer bushing styles increases reliability as employees rapidly gain a high degree expertise in dealing with the fewer styles of bushings with standard dimensions and features.

Keith P. Ellis is a 16 year member of IEEE, PES, Transformers Committee, past Working Group Chairman for C57.19.00, active in all Bushing Subcommittee Tasks Forces and Working Groups and is a Working Group member of IEEE 693, Seismic Substations Standards. He graduated from Mare Island Navel 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 joined RTE-ASEA’s engineering group at the onset of this new joint venture. While with RTE-ASEA, he also worked as an application engineer in Marketing and was then promoted to field sales for RTE and RTE-ASEA in Upstate New York. After nine successful years in field sales he returned to Waukesha to start up the OEM Components sales operation for ASEA, as Sales and Marketing Manager. When the Waukesha transformer operation was sold he was promoted to Sales Manager for the new ABB Power T & D Company’s Components Division. He was then hired by Trench Limited to develop a line of IEEE/ANSI Standard bushings for the US market. Once the new bushings went into production in Canada he was promoted to Bushing Product Manager, Americas, for the worldwide Trench Bushing Group. He takes particular interest in component applications to power transformers with special interest in high voltage bushings and on-load tap changers.

Implementing bushing standardization within a utility will

require support from Management and it is hoped that this paper will provide the support to accomplish this cost saving program for the utilities in Latin America.

VIII. REFERENCES

Standards [1] IEEE Standard General Requirements and Test Procedure for Outdoor

Power Apparatus Bushings, IEEE Standard C57.19.00 – 2004 [2] IEEE Standard Performance Characteristics and Dimensions for

Outdoor Apparatus Bushings, IEEE Standard C57.19.01 – 2000

[3] IEEE Guide for Application of Power Apparatus Bushings, IEEE

Standard C57.19.100 – 1995 R2000 [4] International Electrotechnical Commission, Insulated Bushings for

Alternating Voltages Above 1000 V, IEC Standard 60137

Papers Presented at Conferences [5] Revision of IEEE Standard C57.19.01, Doble Paper, 2003 BIIT4, by

Mark Rivers, Doble Engineering Company

IX. ACKNOWLEDGEMENTS

I wish to thank my employer, Trench Ltd for supporting my efforts in writing this paper.