IEEE Std C57.19-100-2012 Guide for Application of Power Apparatus Bushings

IEEE Power and Energy Society

Sponsored by the Transformers Committee

IEEE 3 Park Avenue New York, NY 10016-5997 USA 22 February 2013

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IEEE Std C57.19.100

TM

-2012

(Revision of IEEE Std C57.19.100-1995)

IEEE Guide for Application of Power Apparatus Bushings Sponsor

Transformers Committee of the

IEEE Power and Energy Society Approved 5 December 2012

IEEE-SA Standards Board


Abstract: Guidance Abstract: Guidance on the use of outdoor power apparatus bushings is provided. The bushings are limited to those built in accordance with IEEE Std C57.19.00TM-1991. General information and recommendations for the application of power apparatus bushings when incorporated as part of power transformers, power circuit breakers, and isolated-phase bus are provided. Keywords: circuit Keywords: circuit breakers, IEEE C57.19.100TM, isolated-phase bus, power apparatus bushings, transformers

The Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA Copyright © 2013 by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved. Published Published 22 February 2013. Printed in the United United States of America. IEEE is a registered trademark in the U.S. Patent & Trademark Office, owned by The Institute of Electrical and Electronics Engineers, Incorporated. PDF: Print:

ISBN 978-0-7381-8132-5 ISBN 978-0-7381-8133-2

STD98087 STDPD98087

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Participants At the time this guide was submitted to the IEEE-SA Standards Board for approval, the Guide for Application of Power Apparatus Bushings Working Group had the following membership: Thomas Spitzer, Chair Jesse Patton, Vice Chair Carlo Arpino Ray Bartnikas Jeffrey Benach Gene Blackburn John Brafa Florian Costa John Crouse Larry Davis Arturo Del Rio Lonnie Elder

Fred Elliott Keith Ellis Mary Foster Charles Garner Joseph Garza John Graham Roger Hayes Chungduck Ko Reiner Krump Mario Locarno Van Nhi Nguyen

Leslie Recksiedler Randolph Rensi Devki Sharma Craig Steigemier John Stein Jane Vermer Eric Weatherbee Michael Williams Shibao Zhang Peter Zhao

The following members of the individual balloting committee voted on this guide. Balloters may have voted for approval, disapproval, or abstention. Mohamed Abdel Khalek Stephen Antosz Carlo Arpino Peter Balma Robert Barnett Barry Beaster Jeffrey Benach W. (Bill) J. Bergman Wallace Binder Thomas Blackburn W. Boettger John Brafa William Bush Arvind K. Chaudhary Bill Chiu Robert Christman Kurt Clemente Jerry Corkran John Crouse Willaim Darovny Gary Donner Fred Elliott Keith Ellis Gary Engmann James Fairris Jorge Fernandez Daher Patrick Fitzgerald Joseph Foldi Marcel Fortin Robert Ganser Charles Garner Saurabh Ghosh David Giegel David Gilmer Jalal Gohari Edwin Goodwin James Graham William Griesacker

Randall C. Groves Bal Gupta Charles Hand David Harris Roger Hayes Lee Herron Gary Heuston Gary Hoffman Philip Hopkinson John Kay Gael Kennedy Sheldon Kennedy Joseph L. Koepfinger Jim Kulchisky Saumen Kundu John Lackey Chung-Yiu Lam Hua Liu Albert Livshitz Thomas Lundquist Greg Luri Richard Marek J. Dennis Marlow Lee Matthews Andrew McNulty Georges Montillet Jerry Murphy Ryan Musgrove K. R. M. Nair Arthur Neubauer Michael S. Newman Joe Nims Ted Olsen Lorraine Padden Bansi Patel Shawn Patterson Jesse Patton Brian Penny

Paul Pillitteri Alvaro Portillo Lewis Powell Iulian Profir Reynaldo Ramos Jean-Christophe Riboud Johannes Rickmann John Roach Michael Roberts John Rossetti Marnie Roussell Thomas Rozek Daniel Sauer Bartien Sayogo Devki Sharma Gil Shultz James Smith Jeremy Smith Jerry Smith Brian Sparling Thomas Spitzer Gary Stoedter John Vergis Jane Verner Loren Wagenaar David Wallach Barry Ward Joe Watson Eric Weatherbee Peter Werelius Kenneth White Wael Youssef Jian Yu Matthew Zeedyk Shibao Zhang Peter Zhao Xi Zhu Waldemar Ziomek

vi Copyright © 2013 IEEE. All rights reserved.


When the IEEE-SA Standards Board approved this guide on 5 December 2012, it had the following membership: Richard H. Hulett, Chair John Kulick , Vice Chair Robert Grow, Past Chair Konstantinos Karachalios, Secretary Satish Aggarwal Masayuki Ariyoshi Peter Balma William Bartley Ted Burse Clint Chaplin Wael Diab Jean-Philippe Faure

Alexander Gelman Paul Houzé Jim Hughes Young Kyun Kim Joseph L. Koepfinger* David J. Law Thomas Lee Hung Ling

Oleg Logvinov Ted Olsen Gary Robinson Jon Walter Rosdahl Mike Seavey Yatin Trivedi Phil Winston Yu Yuan

*Member Emeritus

Also included are the following nonvoting IEEE-SA Standards Board liaisons: Richard DeBlasio, DOE Representative Michael Janezic, NIST Representative

Patrick Gibbons IEEE Standards Program Manager, Document Development Erin Spiewak IEEE Standards Program Manager, Technical Program Development

vii Copyright © 2013 IEEE. All rights reserved.


Introduction This introduction is not part of IEEE Std C57.19.100-2012, IEEE Guide for Application of Power Apparatus Bushings.

In August 1968, the ANSI C76 committee decided to separate ANSI C76.1 into three parts: The first (C76.1, currently IEEE Std C57.19.00-1991 a) part was to cover the general requirements and test procedures; the second (C76.2, currently IEEE Std C57.19.01TM-1991) was to cover explicit ratings and dimensions; and the third (C76.3) was to be an application guide. This document, IEEE Std C57.19.1002012, is the application guide. When the ANSI C76 committee was developing the first draft of the application guide, it was decided that the loading guide portion of the guide should be published for trial use before completion of the application guide. This would allow experience with its use and possible modifications prior to publication within the application guide. The trial-use loading guide was approved but not published before the disbanding of the ANSI C76 committee. The Working Group on Bushing Application Guide was established by the Bushing Subcommittee of the IEEE Transformers Committee to take over the development and completion of the application guide so that it could be submitted for IEEE Standards Board approval and publication. IEEE published the trial-use loading guide in July 1989 as IEEE Std C57.19.101-1989. It was upgraded to a full-use guide on June 18, 1992, and it was designated as IEEE Std C57.19.101-1992. The current guide, IEEE Std C57.19.100-2012, is the application guide in its entirety, which includes the loading guide (Clause 4), and hence, it supersedes IEEE Std C57.19.101-1992. This revision is intended to provide additional information to clarify the previous standard and include newer technologies for manufacture and operation of bushings currently in use. In the latest revision of this guide, the document was updated to current IEEE styles, references were updated, and general revisions were made. In addition, information on draw-lead and draw-rod bushings, the application of outdoor bushings indoors, bushing monitors, bushing repair, and additional guidance on power factor and capacitance testing were also added.

a

Information about references can be found in Clause 2.

viii Copyright © 2013 IEEE. All rights reserved.


Contents 1. Overview .................................................................................................................................................... 1 1.1 Scope ................................................................................................................................................... 1 1.2 Purpose ................................................................................................................................................ 1 2. Normative references.................................................................................................................................. 2 3. Definitions .................................................................................................................................................. 2 4. Thermal loading above nameplate rating for bushings applied on power transformers ............................. 3 4.1 General ................................................................................................................................................ 3 4.2 Temperature calculations for short-time loads above bushing rating .................................................. 5 4.3 Test procedures for derivation of mathematical model ....................................................................... 8 5. Special considerations for application of bushings to power transformers ............................................... 11 5.1 General .............................................................................................................................................. 11 5.2 Loading of bushings with transformer top oil temperature rises between 55 °C and 65 °C .............. 11 5.3 Application of bushings in transformers with conservator oil preservation systems ......................... 12 5.4 Draw-lead application........................................................................................................................ 12 5.5 Draw-rod application......................................................................................................................... 13 6. Thermal loading for bushings applied on circuit breakers ........................................................................ 13 7. Thermal loading for bushings used with isolated-phase bus .................................................................... 13 7.1 Concerns for bushings used in isolated-phase bus ............................................................................. 13 7.2 Thermal coordination between the bushings and the isolated-phase bus........................................... 14 8. Allowable line pull (cantilever loading) ................................................................................................... 14 8.1 General (transformers and circuit breakers) ...................................................................................... 14 8.2 Mounting angles greater than 20° ...................................................................................................... 14 8.3 Circuit breaker applications ............................................................................................................... 15 9. Application of bushings in unusual service conditions ............................................................................. 15 9.1 Contaminated environments .............................................................................................................. 15 9.2 High altitudes..................................................................................................................................... 18 9.3 Application of outdoor bushings in indoor locations......................................................................... 18 9.4 Bushing monitors............................................................................................................................... 18 10. Bushing maintenance practices .............................................................................................................. 18 10.1 Mechanical maintenance and inspection ......................................................................................... 18 10.2 Bushing repair.................................................................................................................................. 20 10.3 Routine and special tests .................................................................................................................. 21 10.4 Bushing storage ............................................................................................................................... 23 10.5 Bushing replacement ....................................................................................................................... 24 11. Checklist................................................................................................................................................. 25 Annex A (informative) Examples of calculation procedures to determine hot-spot temperatures for bushings applied on transformers ................................................................................................................. 26 Annex B (informative) Bibliography............................................................................................................ 30

ix Copyright © 2013 IEEE. All rights reserved.



IEEE Guide for Application of Power Apparatus Bushings IMPORTANT NOTICE: IEEE Standards documents are not intended to ensure safety, health, or environmental protection, or ensure against interference with or from other devices or networks. Implementers of IEEE Standards documents are responsible for determining and complying with all appropriate safety, security, environmental, health, and interference protection practices and all applicable laws and regulations. This IEEE document is made available for use subject to important notices and legal disclaimers. These notices and disclaimers appear in all publications containing this document and may be found under the heading “Important Notice” or “Important Notices and Disclaimers Concerning IEEE Documents.” They can also be obtained on request from IEEE or viewed at http://standards.ieee.org/IPR/disclaimers.html .

1. Overview

1.1 Scope Guidance on the use of outdoor power apparatus bushings is provided in this document. The bushings are limited to those built in accordance with IEEE Std C57.19.00 TM-1991 (not the latest revision). 1 The latest revision, IEEE Std C57.19.00 TM-2004, does not address the use of condenser bushings in oil circuit breakers and several voltage classes were dropped that are still in use.

1.2 Purpose The purpose of this guide is to present general information and recommendations for the application of power apparatus bushings when incorporated as part of power transformers, power circuit breakers, and isolated-phase bus. The loading model developed in this guide is based on oil-impregnated, paperinsulated, capacitance-graded bushings. Similar loading models could be developed for other bushing constructions.

1

Information about references can be found in Clause 2.

1 Copyright © 2013 IEEE. All rights reserved.


IEEE Std C57.19.100-2012 IEEE Guide for Application of Power Apparatus Bushings

2. Normative references The following referenced document is indispensable for the application of this document (i.e., it must be understood and used, so each referenced document is cited in text and its relationship to this document is explained). For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments or corrigenda) applies. IEEE Std 4 TM, IEEE Standard Techniques for High-Voltage Testing. 2,3 IEEE Std C37.010 TM, IEEE Application Guide for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis (including Supplement IEEE Std C37.010d). IEEE Std C37.017 TM, IEEE Standard for Bushings for High Voltage [over 1000V(ac)] Circuit Breakers and Gas-Insulated Switchgear. IEEE Std C37.23 TM, IEEE Standard for Metal-Enclosed Bus and Calculating Losses in Isolated-Phase Bus. IEEE Std C57.12.00TM, IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers. IEEE Std C57.19.00 TM-1991, IEEE General Requirements and Test Procedures for Outdoor Apparatus Bushings. IEEE Std C57.19.00 TM-2004, IEEE General Requirements and Test Procedures for Power Apparatus Bushings. IEEE Std C57.19.01 TM, IEEE Standard Performance Characteristics and Dimensions for Outdoor Apparatus Bushings. IEEE Std C57.91 TM, IEEE Guide for Loading Mineral-Oil-Immersed Transformers.

3. Definitions For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary Online should be consulted for terms not defined in this clause. 4 For definitions of terms used in this standard, also see IEEE Std C57.19.00-1991. draw-lead bushing: A bushing that will allow the use of a current-carrying draw-lead conductor drawn through the hollow tube and enabling its connection to the top terminal. draw-lead conductor: A cable or a solid conductor that has one end connected to the transformer or a reactor winding lead and the other end drawn through the bushing hollow tube and connected to the top terminal of the bushing. A solid conductor can b e either one piece or multiple pieces connected together. draw-rod: A non-current-carrying rod (current is carried by the bushing tube) that has one end connected to the transformer/reactor lead end terminal and the other end drawn through the bushing hollow tube and connected to the top end of the bushing. A draw-rod can be either one piece or multiple pieces connected. 2

This publication is available from The Institute of Electrical and Electronics Engineers ( http://standards.ieee.org/). The IEEE standards or products referred to in this clause are trademarks of The Institute of Electrical and Electronics Engineers, Inc. 4 IEEE Standards Dictionary Online subscription is available at: http://www.ieee.org/portal/innovate/products/standard/ standards_dictionary.html. 3




draw-rod bushing: A bushing that allows the use of a non-current-carrying rod drawn through the hollow tube and enabling a connection between the bushing’s inboard end terminal and the transformer or reactor winding lead.

4. Thermal loading above nameplate rating for bushings applied on power transformers

4.1 General The thermal loading capability of bushings varies with the way they are loaded, the way they are designed, and the ambient conditions in which they are applied.

4.1.1 Basis of rating and rationalization of thermal requirements rating Capacitance-graded, paper-insulated bushings that, at rated current, meet the requirements of IEEE Std C57.19.00-1991 and earlier versions of that standard may be applied in either 55 °C or 65 °C rise transformers. IEEE Std C57.19.00-1991 states that the temperature of the oil in which the lower end of the bushing is immersed shall not exceed 95 °C when averaged over a 24 h period. Refer to IEEE Std C57.91TM for loading requirements and operating oil temperatures of power transformers.

4.1.1.1 Operation above normal temperature 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 65 °C rise over ambient or a 105 °C total temperature because of the use of temperature index 105 insulating paper for the bushing condenser. If it should be determined that a transformer develops a top oil rise of 65 °C at rated current when operating in a 40 °C ambient, then the hottest-spot temperature of the bushing can be expected to exceed 105 °C. In addition, transformers can be expected to have bushing temperatures above 105 °C when loaded in accordance with IEEE Std C57.91. In each instance, the normal life expectancy of the bushing will be shortened by the higher operating temperatures. The loss-of-life of a bushing will, like transformers, be a function of the actual temperature and the time operating at that temperature. The severity of loss-of-life in a bushing can be minimized by installing bushings that have nameplate ratings greater than the transformer current ratings or by using bushings with special high-temperature insulation. An alternative is to operate the bushing with the higher inherent temperatures and accept a moderate degree of accelerated aging, as it is presently recognized for transformers.

4.1.1.2 Factors influencing bushing aging There are several factors that tend to decrease the severity of bushing aging. These are as follows: a)

The top oil rise of many transformers is significantly below 65 °C when the transformer is operated at nameplate loading. This is most likely to occur on forced oil-cooled (OFAF or ODAF) transformers.

b)

Bushings are totally sealed from the atmosphere at the time of manufacture, thus preserving their dielectric and thermal integrity. 3 Copyright © 2013 IEEE. All rights reserved.



c)

Bushing insulation is generally processed to a greater degree o f dryness than transformer insulation, thus providing a lower power factor, lower dielectric losses, and consequently prolonged life at any temperature.

d)

The end-of-life of cellulose insulation in transformers may be governed by its ability to withstand mechanical forces that are associated with fault currents through the transformers. Cellulose insulation in bushings is not subjected to similar forces.

e)

Although end-of-life of insulating materials is typically based on a given change in mechanical or chemical properties, no similar relationship for dielectric characteristics has been established. However, considering increased insulation power factor and capacitance as important criteria, welldried bushing cellulose material is probably equal in life expectancy to thermally upgraded (65 °C) transformer insulation.

f)

The use of bushings with current ratings greater than the transformer current ratings as described in 4.1.1.1 reduces the temperature rise inside the bushing at rated transformer current.

4.1.2 Overload concerns When a bushing is loaded above nameplate, it is exposed to the risks described in 4.1.2.1 through 4.1.2.5.

4.1.2.1 Pressure buildup When load current through a bushing exceeds the nameplate rating, internal pressures can develop that could cause one or more of the sealing gaskets to leak or fail. This pressure increase is caused by the expansion of the insulating oil within the bushing. The rate of oil expansion is normally considered to be approximately 0.0725% to 0.0755% per °C temperature increase for temperatures ranging from 0 °C to 100 °C.

4.1.2.2 Gasket seals Gasket materials will age according to the temperature adjacent to the gasket surface and the duration at that temperature. Usually gaskets will perform well at elevated temperatures; however, progressive changes in physical properties will occur when excessive temperatures are maintained for long durations. These changes could result in loss of seal and consequent loss of dielectric strength. Therefore, repeated occurrences at high temperature will require inspection for oil leaks and corrective actions where necessary.

4.1.2.3 Power factor and ca pacitance There are many reasons why insulation power factor and/or capacitance may increase over the life of a bushing. In fact, some slight increase of power factor can be tolerated. However, degradation of that portion of the insulation that operates at greatly elevated temperature could result in a substantial increase in power factor. An unusual increase in power factor may become an indicator of the detrimental mechanical and electrical effects of loading beyond nameplate rating. Bushings that have been loaded beyond nameplate rating should be tested more frequently.

4.1.2.4 Dielectric performance at elevated temperatures When bushing insulation is subjected to high electrical stress at above its normal operating temperatures, the insulation power factor increases due to increased dielectric loss. When the increase in dielectric loss exceeds the ability of the insulation to dissipate this increased loss, the temperature of the dielectric is




further increased. Under some extreme conditions, thermal runaway may occur. This risk should be considered when the guide is applied. Special capacitance-graded bushings built with insulation systems such as thermally upgraded paper or resin impregnated paper, rated higher than temperature index 105 insulation class, are sometimes used. These insulation systems may have higher power factors particularly at higher temperatures and may experience thermal runaway if loaded significantly beyond the nameplate rating. For specific information, the manufacturer should be contacted.

4.1.2.5 Stray magnetic flux Additional heating may occur in bushings placed in the magnetic field of the windings and leads. The heating can result from the eddy current flowing in the metallic portions of the bushing below the mounting flange. The magnetic flux will increase with the load current. Magnetic fields can create high eddy current losses in tanks, flanges, and bus enclosures during overload conditions, causing them to reach high temperatures. High temperatures of the part itself may not be of concern, but the heat may transfer to the bushing causing high-temperature bushing concerns.

4.2 Temperature calculations for short-time loads above bushing rating The hottest-spot temperature of a bushing is o f importance when it is loaded under various conditions. The five key elements that affect the bushing hottest spot are the bushing current, the ambient air temperature, the surrounding oil temperature, the air-end-connection temperature, and the oil-end-connection temperature. Easley and McNutt [B3] 5 gave an expression that contains each of these elements. Accurate information about the end-connection temperatures and coefficients is usually not available. Therefore, this guide uses a more conservative method that requires information only about the bushing current, the ambient air temperature, and the surrounding oil temperature to calculate the bushing hottestspot temperature. This method was developed from experimental data for bushings with no appreciable dielectric losses and no cooling ducts. Three constants are determined as described in 4.3.3. These constants are then used to estimate the steady-state and transient bushing hottest-spot temperatures. Mathematical models for bushings with appreciable dielectric losses and/or with cooling ducts may be developed in the future and could be used in the same manner.

4.2.1 Steady-state hottest-spot temperature calculations The steady-state temperature rise at the hottest spot of the conducto r for bottom connected bushings with no appreciable dielectric losses and no cooling ducts is estimated with Equation (1):

∆Θ HS = K1 I n + K 2 ∆Θ 0

(1)

where ∆ΘHS

is the steady-state bushing hottest-spot rise over ambient (°C)

∆Θ0

is the steady-state immersion oil rise over ambient (°C) (transformer top oil rise)

5

The numbers in brackets correspond to those of the bibliography in Annex B.




I n, K 1, and K 2

is the per unit load current based on bushing rating are constants that can be determined as described in 4.3

Typical values of K 1 range from 15 to 32. Typical values of K 2 range from 0.6 to 0.8. The exponent n generally ranges between 1.6 and 2.0, with 1.8 being the most common value. When a bushing is operated in the draw-lead mode, the thermal performance is dominated by an integral part of the transformer that is inserted through the tube of the bushing. This lead is not an integral part of the bushing, so the thermal performance cannot be directly related to a specific design of bushing that may also be operated in other transformers with different size draw leads. The temperature of the hottest spot of the conductor, when operated in the draw-lead mode, may be determined in the same manner, with I being the per unit load current of the draw lead.

4.2.2 Transient hottest-spot temperature calculations After changes in load current or ambient temperature occur, both the immersion oil temperature and bushing hottest-spot temperature will change with time from the initial to the final value in an exponential manner. Therefore, it is necessary to determine the initial and final transformer top oil temperature and the rate of change by the procedures established in IEEE Std C57.91. After the changed per unit current I , the transformer top oil rise ∆Θ0, and the transformer top oil time constant t o have been established, the transient response of the bushing may be determined using K 1, K 2, n, and the bushing time constant t . K 1, K 2, and n are the same constants and exponent used for the steady-state bushing calculations. The bushing time constant t o is the length of time required for the temperature change to reach 63.2% of the final temperature change.

4.2.2.1 Iterative method One method is to simulate the exponential rise by making a series of repeated calculations of the bushing hottest-spot temperature rise in successive time increments following steps A–F. where T ∆t t 1 t 2 τ τ o ∆ΘHS(t 1) ∆ΘHS (t 2)

is the elapsed time of the transient load (minutes) is the an arbitrary time increment to divide the elapsed time of the transient load into steps for calculation (minutes) is the initial time at start of an increment (minutes) is the time when transformer oil ∆Θ0 reaches practical equilibrium (minutes) is the bushing time constant (minutes) is the oil time constant of transformer (minutes) is the bushing hottest-spot temperature rise at time t 1 (°C) is the ultimate bushing hottest-spot temperature rise as calculated from the steadystate Equation (1)

For the new load (°C) ∆ΘHS (T ) is the bushing hottest-spot temperature rise at the end of the transient load period or ∆ΘHS (T ) = ∆ΘHS (t 1+Σ∆t) (°C) ∆Θ O(t 1) is the immersion oil temperature rise as determined for time t 1 (°C)




∆Θ O(t 2)

∆Θ O (t 1 + ∆t )

is the ultimate immersion oil temperature rise as determined from IEEE Std C57.91 for the new load conditions that apply during the transient load (°C) is the new immersion oil temperature rise at end of time increment t 1 + ∆t , (°C), as calculated in Equation (2):

∆θ0 ( t1 + ∆t ) = ∆θ0 ( t1 ) +  ∆θ0 ( t2 ) − ∆θ0 ( t1 )  1 − e−( ∆ / τ )  t

∆ΘHS (t 1+∆t )

0

(2)

is the new bushing hottest-spot temperature rise at end of time increment t 1 + ∆t , (°C), as calculated in Equation (3):

∆ΘHS ( t1 + ∆t ) = ∆ΘHS ( t1 ) +  ∆ΘHS ( t2 ) − ∆ΘHS ( t1 )  1 − e − ( ∆t / τ )  (3) 0

 Step A: Determine initial bushing hottest-spot temperature rise at start of first increment, ∆Θ HS (t 1) from Equation (1) for prior per unit load I and ∆Θ O (t 1).

 Step B: Determine new transf ormer immersion oil temperature rise at end of first increment, ∆Θ O (t 1+ ∆t ) from Equation (2).

 Step C: Determine the new ultimate bushing hottest-spot rise ∆ΘHS (t 2) for the conditions that apply from Equation (1) using ∆Θ O (t 1+ ∆t ) from step B.

 Step D: Calculate the new tr ansient bushing hottest-spot rise ∆ΘHS (t 1+∆t ) at the end of the time increment from Equation (3) using ∆ΘHS (t 1) and ∆ΘHS (t 2) from steps A and C.

 Step E: Use this new transient bushing hottest-spot rise ∆ΘHS (t 1+∆t ) as the new ∆ΘHS (t 1) for input to the subsequent incremental step.

 Step F: Repeat the incremental procedure of Steps A–E until the end of the transient load period (Σ∆t = T ). See the example in Figure 1.




Figure 1 —Bushing hottest-spot transient response 4.2.2.2 Single step method A simpler but less precise method is to make a single step calculation using Equation (4). This method yields a higher bushing hottest-spot temperature and therefore can be considered more conservative than the method in 4.2.2.1. ∆ΘHS (T ) = ∆Θ HS ( t1 ) +

{

K1 I

n

+ K 2 ∆Θ 0 ( t1 ) + ( ∆Θ 0 ( t2 ) − ∆Θ 0 ( t1 )) (1 − e− ( 

T / τ0 )

) − ∆Θ ( t )}{1− e

− (T / t )

HS

1

}

(4)

4.3 Test procedures for derivation of mathematical model When performance is to be determined by test, it is highly desirable that a uniform procedure be followed so that data may be accumulated on a consistent basis. These procedures are in no way to be construed as a mandatory design test for all bushings.




4.3.1 Procedure for performance testing of bottom-connected bushings This procedure applies to bushings that comply with Table 3 through Table 6 of IEEE Std C57.19.01 TM.

a)

Prepare the test unit by installing thermocouples on each terminal and in at least four locations not more than 762 mm (30 in) apart on the center conductor. The thermocouples may be attached directly to the outside of the conductor by removing portions of the insulation, or the thermocouples may make contact with the inside of a hollow conductor by means of a phosphor bronze thermocouple brush. The thermocouple leads may be threaded through the bottom end of a hollow center conductor, through a small hole in the top terminal, or brought out at some convenient location above the internal oil level.

b)

Install a pressure gage in such a way that the additional gas space of the gage and connections will not exceed 0.5% of the normal gas space. Seal the test unit with the gas chamber charged with the proper gas at the sealing pressure. If the thermocouple connections of item a) have disturbed the sealing characteristics of the test unit, then a duplicate unit may be prepared for pressure monitoring. If the test tank is of sufficient size to avoid proximity effects, then the pressure unit may be mounted adjacent to and connected in series with the test unit. As an alternative, the pressure unit may be tested separately.

c)

Mount the bushing on a suitable nonmagnetic metal plate that complies with the minimum size tab calculated as follows: Bushing mounting plate bolt circle (in) (mm) 6 to 9 1/4 152 to 235 13 ¼ to 15 ¾ 337 to 400 21 to 25 530 to 635

Cover plate size (square or round) (in) (mm) 18 457 24 610 36 914

Thickness (in) ¼ ½ 5/8

(mm) 6.4 13 16

d)

Attach oil-end terminal connectors suitable for the rated current.

e)

Attach air-end terminal connectors suitable for the rated current.

f)

Attach air-end bus at least 1 m (3 ft) long, projecting from the terminal connector in a horizontal plane. The cross section of the bus should be such that at rated current the temperature rise at a location 1 m (3 ft) from the bushing should be at least 30 °C above ambient.

g)

Attach thermocouples to the bus work connectors, mounting plate, and exterior of the bushing.

h)

Mount the bushing so that the oil level complies with either 5.4.1 of IEEE Std C57.19.00-1991 or the level required in the actual bushing application after the steady-state test tank oil temperature has been achieved.

i)

Heat and circulate the oil to maintain a minimum vertical temperature gradient over the bushing immersion depth without oil flow being directed at the test bushing.

j)

The ambient environment should be indoor air between 10 °C and 40 °C.

k)

Make load tests, as required, for obtaining the data necessary for a good statistical basis for a bushing mathematical model. Some suggested conditions are as follows: I Current (pu) 0.0

∆Θo Oil rise (°C) 55

0.7

55

1.0

55

1.25

55




∆Θo

I Current (pu) 0.0

Oil rise ( °C)

1.0

70

1.5

70

2.0

70

70

l)

Record temperatures at appropriate intervals until the thermal conditions become constant or until the measured temperatures do not increase by more than 1 °C for 2 h for bushings up through 900 kV basic insulation level (BIL) and not more than 1 °C for 4 h for bushings 1050 kV BIL and above.

m)

Report initial and final values of conductor hottest-spot rise, top and bottom terminal connector rises, and increase in pressure. Also report the bushing time and temperature readings.

4.3.2 Tests on draw-lead bushings When the thermal performance of a bushing with a specific transformer lead is to be determined by test, a procedure similar to the applicable portions of 4.3.1 may be followed.

4.3.3 Derivation of model constants Nominal values of the K 1, K 2, and n constants can be determined as follows: a)

Obtain a steady-state temperature profile at rated current with the bottom end immersed in hot oil by the procedure discussed in 4.3.1. This establishes ∆Θo( I = 1 pu) and ∆Θ HS( I = 1 pu).

b)

Reduce the current to zero and determine the steady-state temperature of the location that was the hottest spot at rated current. This establishes ∆Θo( I = 1 pu) and ∆Θ HS( I = 0 pu).

c)

The constants K 1 and K 2 can be calculated using Equation (5) and Equation (6):

d)

K 2

= ∆ΘHS ( I = 0 pu ) / ∆Θo ( I = 0 pu )

(5)

K1

= ∆Θ HS ( I = 1 pu ) – K 2 [ ∆Θo ( I = 1 pu )]

(6)

The exponent n can be calculated from additional tests using Equation (7): n

= [1 / ln ( I = Xpu ) ]ln{[ ∆Θ HS ( I = Xpu ) – K 2 ∆Θo ( I = Xpu )] / K1}

(7)

where X is the per unit value of the current. e)

The bushing time constant can be determined by analysis of the time–temperature curves from the tests.

Additional tests as recommended in item k) in 4.3.1 will confirm the nominal values of constants K 1, K 2, and n or give additional data to refine the estimates by graphical or statistical means.




5. Special considerations for application of bushings to power transformers

5.1 General The temperature limits of bushings applied to power transformers can be exceeded by the transfer of heat from transformer components and accessories. If the thermal coordination of these sources is not correct, then the bushing hottest-spot temperature may exceed 105 °C. The result may be accelerated aging. An additional concern is that the higher temperatures may deteriorate sealing gaskets. Potential sources of heat transferred to the bushing include the following: a)

Operation of bushings in transformers with top oil temperature rise greater than 55 °C

b)

Increased transfer of heat into the bushing from top oil in transformers with conservator oil preservation systems

c)

Improper thermal coordination of isolated-phase bus equipment (see Clause 7)

d)

Stray flux heating in the flange and other metallic bushing parts

5.2 Loading of bushings with transformer top oil temperature rises between 55 °C and 65 °C If a transformer has a top oil temperature rise greater than 55 °C but less than or equal to 65 °C, then a bushing with a higher nameplate current rating than the transformer current rating may be applied by using an appropriate derating factor. If the bushing thermal constants are known, then the derated current I d may be determined from the following: I d = dI r

(8)

where I d d I r K 1, K 2, and n

is the derated current at new transformer top oil temperature rise ∆Θo is the [(65 – K 2 ∆Θ o) / K 1]1/n is the bushing current rating are as defined in 4.2 and 4.3

If the bushing thermal constants are not known, then the curve in Figure 2, derived by setting K 1 = 21, K 2 = 0.8, and n = 1.6 in Equation (8), may be used to determine item d) in 5.1.




Figure 2 —Bushing current derating factor for transformer top oil temperature rises between 55 °C and 65 °C

5.3 Application of bushings in transformers with conservator oil preservation systems IEEE Std C57.19.00-1991 establishes bushing current ratings based on thermal tests run with the lower end of the bushing immersed to the minimum oil level, normally the bottom of the ground sleeve. When bushings are applied to transformers with conservator oil preservation systems, the bushing lower end is totally immersed in oil. If the transformer top oil temperature is higher than the bushing internal temperature, then additional heat from the transformer oil will transfer into the bushing reducing its current-carrying capability. Consult the bushing manufacturer for appropriate derating factors for these applications.

5.4 Draw-lead application The following guidelines can be used when bushings are used in a draw-lead application mode.

5.4.1 Bushing current rating The maximum continuous current rating of the bushing in the draw-lead mode is limited to the rating specified on the bushing nameplate. The maximum continuous current rating of the draw-lead cable is determined by the size and type of the cable supplied by the transformer manufacturer. Since the draw-lead cable is an integral part of the transformer, the current rating specified on the transformer nameplate




determines the rating of the draw-lead cable. When the bushing manufacturer supplies the draw-lead conductor as part of the bushing, the current is limited to the rating specified on the bushing nameplate.

5.4.2 Draw-lead cable/bushing loading The current-carrying capacity of the draw lead cable is determined by the size and type of the cable used by the transformer manufacturer. The current is limited to the rating specified on the transformer nameplate. When applying bushings in the draw-lead mode, the bushing manufacturer should be consulted for guidelines on draw-lead sizes, ratings, and loading. To minimize cable insulation loss of life during overloads, it would be preferable to choose a cable with current rating of at least 20% above the rated current of the transformer. If higher overloads are anticipated, then cables with even greater margin should be considered. Although specific guidelines should be obtained from bushing or transformer manufacturers, following general guidelines can be used for rating draw-lead cables with thermally upgraded 65 °C insulation.

 Maximum ambient 40 °C  Maximum cable hottest-spot temperature rise above ambient air at rated current 80 °C  Maximum cable hottest-spot temperature rise above ambient air under overload condition 100 °C The 80 °C rise limit agrees with the maximum winding hottest-spot temperature rise for 65 °C rise transformers as per IEEE Std C57.12.00. The 100 °C hottest-spot temperature rise limit is based on maximum oil temperature of 140 °C with maximum ambient air temperature of 40 °C. Operation at hottest-spot temperature above 140 °C may result in gassing of oil/paper insulation. Because it is not possible to detect the absolute hottest spot in the cable, it is prudent to keep temperatures below these maximums.

5.5 Draw-rod application In a draw-rod bushing, the load current is carried mainly by the bushing central tube. The thermal characteristics of such a bushing are essentially the same as a conventional bushing.

6. Thermal loading for bushings applied on circuit breakers Bushings applied on power circuit breakers will be subject to the requirements in IEEE Std C37.010 TM and IEEE Std C37.017 TM.

7. Thermal loading for bushings used with isolated-phase bus

7.1 Concerns for bushings used in isolated-phase bus Bushings used with isolated-phase bus meeting the requirements of IEEE Std C37.23 TM may be subjected 13 Copyright © 2013 IEEE. All rights reserved.



to conductor and enclosure temperatures that violate the conditions s pecified in 4.1 of IEEE Std C57.19.00-1991. Table 5 of IEEE Std C37.23-2003 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 equipment specification.

7.2 Thermal coordination between the bushings and the isolated-phase bus 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: a)

Increase cross-sectional area of the conductor or the connection between the bushing and the conductor.

b)

Use flexible cable or braids and silver-surfaced or tinned joints at the connection between the bushing and conductor.

c)

Increase the cross-sectional area and the diameter of the bus enclosure surrounding the bushing.

d)

Circulate forced air around the bushing or through the ventilated bus duct to keep the air temperature within the acceptable range.

As an alternative, bushings and gaskets suitable for high-temperature application can be considered, for instance bushings with aramid insulation, oil-filled bushings, or bushings with insulating materials other than oil-impregnated paper. Use of fluorocarbon or other high-temperature gasket materials may sometimes be necessary. Information on material temperature classification is covered in Table 3 of IEEE Std 1 TM-2000. Information on temperature rises of bus systems is covered in IEEE Std C37.23.

8. Allowable line pull (cantilever loading)

8.1 General (transformers and circuit breakers) The continuous cantilever loading (i.e., line pull, wind loading, and ice loading) applied to the bushing terminal should not exceed 50% of the test value, for the bushing ratings given in Table 8 of IEEE Std C57.19.01-2000. The cantilever loading applied to a bushing terminal as a result of continuous cantilever loading plus dynamic or short-time loading (i.e., short-circuit forces, seismic but not including seismic forces generated by the mass of the bushing itself) should not exceed 85% of the bushing test value given in Table 4 of IEEE Std C57.19.01-2000. Cantilever loading should not exceed allowable values for the equipment in which the bushing is installed.

8.2 Mounting angles greater than 20° When a bushing is mounted at an angle that exceeds 20° from the vertical recommended in IEEE Std C57.19.00-1991 under Unusual Service Conditions, it may be subjected to an excessive bending moment. If the bushing is not designed for such an application, then it may be subjected to excessive force that may result in leaks or damage to the bushing.




Standard bushings may not be acceptable for replacements on mobile transformers or substations. Always consult with the bushing manufacturer regarding the specific application before selecting a replacement.

8.3 Circuit breaker applications Condenser bushings applied on oil circuit breakers should be capable of withstanding the forces specified in Table 1 of IEEE Std C37.017-2010. If the bushing is not identified as being transformer breaker interchangeable (TBI), the manufacturer needs to be contacted to ensure that the bushing will withstand the forces of the breaker operation.

9. Application of bushings in unusual service conditions Usual service conditions are described in subclause 4.1 of IEEE Std. C57.19.00-1991.

9.1 Contaminated environments Standard bushing characteristics are specified for a standard clean environment. This promotes a common understanding between manufacturers and users of what bushing ratings mean. Proper application of bushings in environments different from the standard requires knowledge of how bushing performance changes from one environment to another. The purpose of this subclause is to highlight those issues that need to be considered in applying bushings in varied environments.

9.1.1 Types of environments Contaminated environments can be divided into the general types summarized in Table 1.

9.1.2 Types of contaminants

9.1.2.1 Natural deposits Natural deposits on bushings include such things as salts, dust, sand, and so on, left on the bushings as the result of natural action. They may be airborne, waterborne, or left behind after the melting of snow and ice.

9.1.2.2 Automotive/industrial effluents These are by-products put into the air as a result of industrial/commercial activity. They include particulates and gaseous materials that condense on bushing surfaces.




Table 1 —General types of contaminated environments Contamination level Light

Medium

Heavy

Extra heavy

Typical environments Areas without industries and with a low density of emission-producing residential heating systems. Areas with some industrial or residential density but subject to frequent winds and/or precipitation. Agricultural areas (exposure to wind-borne fertilizer spray or crop-burning residues can lead to higher contamination levels). Mountainous areas. These areas are not exposed to sea winds or located near the sea. Typical measured equivalent salt deposit density (ESDD) levels are 0.03 mg/cm 2 to 0.08 mg/cm2. Areas with industries not producing highly polluting smoke and/or with an average density of emission-producing residential heating systems. Areas with high industrial and/or residential density but subject to frequent winds and/or precipitation. Areas exposed to sea winds but not located directly on the coast. Typical measured ESDD levels are 0.08 mg/cm2 to 0.25 mg/cm2. Areas with high industrial density and large city suburbs with a high density of emission-producing residential heating systems. Areas close to the sea or exposed to strong sea winds. Typical measured ESDD levels are 0.25 mg/cm 2 to 0.6 mg/cm2. Small areas subject to industrial smoke-producing thick conductive deposits. Small coastal areas exposed to very strong and polluting sea winds. Typical measured ESDD levels are above 0.6 mg/cm 2.

9.1.2.3 Other deposits Other types of deposits such as agricultural residues can also occur as a result of specific types of activities in the vicinity of a bushing location.

9.1.3 Artificial contamination testing A design or production test method that fully duplicates an actual environment where a bushing will be applied is usually not practical. Therefore, artificial test methods have been developed that are intended to provide a realistic assessment of the characteristic being tested (see reports by the General Electric Company [B4] and the IEEE Working Group on Insulator Contamination [B11] for discussions of test methods). The three major categories of testing are discussed in 9.1.3.1 through 9.1.3.3.

9.1.3.1 Salt fog A bushing is energized at a constant test voltage and subjected to a salt fog of controlled salinity. Typical salinity values range from 2.5 g to 160 g of salt per cubic meter of fog solution. The fog is sprayed on the bushing through an array of nozzles with compressed air. The withstand salinity is the salinity at which there is a withstand in at least three of four 1 h test periods.

9.1.3.2 Wet-contamination Artificial contamination is applied to a bushing by a spray or flow-coating method. Three to five minutes later, before the contaminant has time to dry, a test voltage is applied to the bushing. The voltage is either raised until the bushing flashes over or raised to a test value and held constant until the bushing flashes over or the contaminant dries out and all scintillation activity stops. The contaminant is a mixture of water and kaolin or other nonconductive material with a controlled amount of salt added. A withstand value is sometimes determined by either three successful withstands without a flashover at a given test voltage or by statistical analysis of a number of trials. This method has an advantage over the other methods in simplicity, ease of use, and low test cost. 16 Copyright © 2013 IEEE. All rights reserved.



9.1.3.3 Clean fog A dry, artificially contaminated bushing is subjected to clean fog and test voltage. In one variation, the fog is applied to the bushing and then it is energized. In the other variation, the bushing is energized and then the fog is applied to it.

9.1.4 Natural contamination testing The primary way to identify the types of natural contaminant on a bushing is through chemical analysis and testing. This is especially important for cases of industrial pollutants when the identity of the polluting agent is not immediately known. In addition, special tests can be used to quantify the effect of the contaminants on the electrical bushing characteristics. The primary test for this purpose is the ESDD. This test is used to establish the conductivity of the watersoluble deposits on a bushing surface in terms of the density of a standard soluble salt deposited on a surface that would produce the same conductivity. A measured surface area on a bushing is washed in a known amount of water of very low conductivity. The resistivity of the wash water is then measured and the amount of sodium chloride (NaCl) needed to produce the same conductivity in the known quantity of wash water is calculated. The result is expressed as milligrams of NaCl per square centimeter of washed bushing surface area (mg/cm 2). Additional information on this method is contained in Appendix 1C of IEEE Std 4 TM-1995.

9.1.5 Countermeasures The user will need to evaluate the following and any other options available to determine their suitability to the situation: a)

Install extra creep distance bushings. The following minimum creep values based on the bushing nominal line-to-ground kV rating are recommended. These values may need to be adjusted for factors such as shape, number of sheds, and bushing inclination. Contamination

Creep distance

Light

28 mm/kV

Medium

35 mm/kV

Heavy

44 mm/kV

Extra heavy

54 mm/kV or greater

b)

Apply protective coatings. Protective coatings can be applied to the surface of the bushings to improve their dielectric performance. There are temporary coatings, such as silicone grease, that require periodic replacement and permanent coatings that are nonremovable. These coatings may cause power factor testing issues and they may become contaminated.

c)

Install conductive glaze bushings. Consult the manufacturer for specific application information.

d)

Install composite insulated bushings with nonceramic, contamination-resistant external insulation. Consult the manufacturer for specific application information.

e)

Periodic cleaning of bushing surfaces. Bushings with known contamination cycles can be cleaned periodically as part of a maintenance program.

f)

Eliminate air bushings. Installations can be designed to minimize the number of bushings exposed to atmospheric contamination.




9.2 High altitudes Refer to IEEE Std C57.19.00-2004 for altitude correction factors.

9.3 Application of outdoor bushings in indoor locations Bushings designed for outdoor application can also be applied indoors; however, the following should be considered: a)

The rating of bushings is assigned at an ambient air temperature not exceeding 40 °C, and the average temperature of the ambient air is not exceeding 30 °C in any 24 h period. These limits may be exceeded in the indoor locations. If it should be determined that the ambient air temperatures will exceed the allowable temperature, then the user may either select a bushing with higher rating or accept a moderate degree of accelerated aging.

b)

The environmental contamination should be reviewed. If the ventilation system allows the air from the outdoors to circulate freely indoors without any filtration, contaminants present outdoors will be indoors and will be deposited on the bushing. However, unlike outdoor bushings where the contaminants are washed away by rain, the contamination level on indoor bushings will keep increasing. The user may select bushings with higher creepage distance and institute maintenance programs to include bushing insulator cleaning.

9.4 Bushing monitors Bushing monitors are installed on bushing voltage/test taps to allow for on-line testing and monitoring of the bushing power factor. These allow for continuous monitoring or periodic testing of bushings without removing the bushing from service. Since the monitors replace the existing voltage/test tap cover, the user should ensure proper fit of the monitor to prevent moisture ingress into the test tap. A voltage is then brought from the monitor to an accessible area of the transformer to perform the monitoring. The user should consult with the manufacturer of the monitor to determine the voltage levels that are normally expected and the maximums that could be reached should the device fail. Operational guidelines can then be developed by the user to determine the level of safety requirements and experience of personnel who should have access to the devices.

10. Bushing maintenance practices The in-service maintenance frequency of bushings will normally vary according to circumstances and is generally combined with the inspection and maintenance of the associated equipment.

10.1 Mechanical maintenance and inspection

10.1.1 External porcelain Inspect the porcelain for damage and pollution deposits. At least the following guidelines should be considered during the examination. Small chips or breaks in the sheds are generally of no concern. The exposed unglazed surface may be painted with a suitable paint to improve the appearance. Large breaks or chips may reduce the creep distance and may require bushing replacement. 18 Copyright © 2013 IEEE. All rights reserved.



Small cracks in the sheds may be ground off to prevent further propagation. Large cracks may require bushing replacement. Any damage to the main porcelain body would be a cause for concern and may require bushing replacement. Bushings may be periodically cleaned by either hand-washing (deenergized installation) or by a suitable spray or jet method using low conductivity water. Silicone-based greases and coatings can be applied to increase the time interval between cleanings. However, this treatment prevents normal rainfall from cleaning the porcelain surfaces.

10.1.2 Terminals Inspect bushings for overheated connections when the unit is energized and loaded. Infrared cameras are sometimes used to detect overheated terminal connections. Loose connections should be tightened according to the bushing manufacturer’s recommendations.

10.1.3 Mounting hardware Inspect the mounting hardware for tightness.

10.1.4 Gaskets Gaskets that are part of the bushing normally do not require replacement. Be sure that replacement gaskets between the bushing flange and the associated equipment are the right thickness and suitable material. Gasket stop rings, if used, should be in place. Gaskets that are sensitive to ultraviolet radiation may deteriorate rapidly when exposed to combined sunlight, high humidity, and contamination. These materials should be avoided in these conditions. As an added precaution, gaskets in these conditions should be protected from exposure to sunlight.

10.1.5 Oil level Loss of oil threatens the integrity of a bushing; therefore, any bushing that shows an abnormal oil level should be investigated as soon as possible. Follow the manufacturer’s recommendations in correcting the cause of the abnormal oil level and in refilling the bushing. The associated apparatus should be checked to ensure that the lower end of the bushing is immersed in oil to the proper level. Special measures may be required to keep oil over internal insulation in bushings mounted at angles greater than 20° from vertical.

10.1.6 Bushing taps Inspect the bushing voltage and test taps for proper gaskets and grounding. The voltage tap compartments should be filled with insulating oil or compound when recommended by the bushing manufacturer.




10.2 Bushing repair Maintenance and repair recommendations vary widely from one manufacturer to another. In all cases, the user is advised to consult the manufacturer prior to attempting any bushing repairs. Extreme care should be taken when working on or around damaged bushings. Many bushings are constructed with high compressive forces and can be dangerous when components are damaged. If transporting the bushing is required, check with local regulations prior to shipment to determine oil polychlorinated biphenyl (PCB) level limitations. The following are common repair needs that may be encountered by bushing users and general recommendations for each: a)

Damaged porcelain: Small chips or breaks in porcelain are not detrimental to bushing performance, as modern electrical porcelain is a nonporous material. Larger chips or breaks generally will not hinder performance either but may have improved appearance by painting over the area with a suitable paint (bright-colored paint is often used for safety purposes). Cracks in the main porcelain body are cause for immediate replacement. Damages to sheds that significantly reduce creepage distances are also cause for bushing replacement or professional repair. Care should be taken when working with damaged porcelain, as broken edges are often extremely sharp.

b)

Damaged oil sight glasses: A cracked, chipped, or shattered oil sight glass often allows moisture to enter into the bushing and is cause for bushing replacement or professional repair. To minimize damage to the bushing core, it is recommended to wrap the affected area as soon as possible. Field repairs are not recommended.

c)

Malfunctioning oil-level gages: An oil-level gage that does not correctly indicate the oil level in the bushing can lead to future problems and is reason for repair. Many oil level gages can be replaced without disassembling the bushing. The mechanism that measures oil level, however, is internal to the bushing and cannot usually be replaced in the field. The manufacturer or a professional repair service should be contacted to determine if the gage design can be replaced in the field.

d)

Leaking seals: Loss of oil threatens the integrity of a bushing and may be cause for bushing replacement or professional repair. Gaskets that are bulging out between bushing sections, unevenly seated, cracked, overheated, have lost their elasticity, or appear in any way damaged, have most likely permanently ceased to protect the bushing and should be replaced, even if no leaks are in evidence, or at least the bushing should be closely monitored. Bushings that have been subjected to overload may have damaged gaskets. These should be examined closely and replaced if any signs of damage are present even if they are not currently leaking. Some leaks may be temporarily sealed using an oil-compatible sealant, but it is important to repair the seal as soon as possible. Professional repair services can often eliminate the leak by disassembling the bushing and replacing the gaskets. The manufacturer or a professional repair service should be contacted if a significant reduction in oil level has occurred.

e)

Leaking taps: Loss of oil into the bushing tap space can often be temporarily sealed with the provided tap cover. Some bushing taps can be replaced in the field by positioning the bushing horizontally and installing a suitable replacement. The manufacturer or a professional repair service should be contacted to determine if tap replacement is possible in the field.

f)

Damaged taps: Damage due to electrical activity at the bushing tap can cause high- or low power factor readings and is reason for repair. Some bushing taps can be replaced in the field by positioning the bushing horizontally and installing a suitable replacement. The manufacturer or a professional repair service should be contacted to determine if tap replacement is possible in the field.




g)

Power factor or capacitance abnormalities Significant or rapid changes to power factor or capacitance are often precursors to dielectric failure and may be cause for bushing replacement or professional repair. Investigation should follow manufacturer recommendations and the guidelines in 10.3.1. Baking the bushing core in an oven, a popular remedy in the past, will not result in a long-term improvement in power factor. Field repairs are not recommended.

h)

Overheated terminals: Excessive heat at bushing terminals is most often caused by loose connections or damaged threads. This situation can be corrected by tightening connections or repairing damaged threads, where needed. In these cases, consideration should be given to the duration the bushing was overheated and the impact on seals in proximity to the heat source.

10.3 Routine and special tests

10.3.1 Power factor and capacitance The bushing power factor and capacitance should be measured when a bushing is first installed and again one year after installation. After these initial measurements, the bushing power factor and capacitance should be measured at regular intervals (3 years to 5 years typically). If the bushing is installed in a transformer, then it is important to remember that the transformer winding can also influence the test results. Since the power factor can vary with temperature, test results should be converted to a common temperature base (usually 20 °C) using temperature-correction data that are available from manufacturers and various industry sources. The following procedure should be used: a)

Measure the bushing’s power factor

b)

Determine the bushing’s temperature

c)

Obtain the appropriate correction factor corresponding to the bushings temperature

d)

Multiply item a) and item c) —see example

Example A 115 kV oil-impregnated bushing:

 Calculated power factor = 0.42%  Bushing temperature = 30 °C  Multiplier from the temperature-correction table at 30 °C = 1.11  Corrected to 20 °C power factor = 0.42% × 1.11 = 0.47% These correction factors are average at best and therefore are subject to some error. The magnitude of the error is minimized if the tests are performed at temperatures near the reference temperature of 20 °C. If questionable power factors are recorded at a relatively high temperature, then the bushing should not be condemned until it has been allowed to cool down to near 20 °C and repeat tests have been performed. This also applies to bushings tested near freezing where a large (greater than 1.00) correction may cause the result to be unacceptably high; in this case, the equipment should be retested at a higher temperature. Bushings should not be tested when their temperatures are below freezing because moisture may have changed to ice, which has a significantly higher resistivity and therefore are undetected. In the case of bushings mounted in transformers, the bushing temperature is approximated by taking the average between the ambient and transformer top-oil temperatures.




Any bushing that exhibits a history of continued power factor increase should be further investigated and considered for removal from service. The bushing manufacturer should be consulted for guidance. If any bushing exhibits an increase in power factor over a period of time, then the rate of change of this increase should be monitored by more frequent tests. The decision on when to increase test frequency or remove from service should consider manufacturer recommendations, user and industry experience with service aged equipment, and bushing design, among other factors. Some bushing manufacturers consider an increase between 1.5 and 2 times the initial reading to be significant. Bushing capacitance should be measured with each power factor test and compared carefully with both nameplate and previous tests in assessing bushing condition. This is especially important for capacitance-graded bushings where an increase in capacitance of 5% or more over the initial/nameplate value is cause to investigate the suitability of the bushing for continued service. The manufacturer should be consulted for guidance on specific bushings. It is usually impossible to make absolute ungrounded specimen test (UST) measurements of the bushing core capacitance and power factor of resistance-graded bushings because of the influence of the resistive glaze on the surface of the bushing porcelain. Differences in the glaze can cause significant variations in measurements between different bushings of the same voltage class and type. In some instances, the measured UST power factor may even be negative. Standard practice during diagnostic testing of resistance-graded bushings is to record the measured UST values of capacitance and power factor for comparison with other measurements made on the same bushing. When there is evidence of a permanent increasing or decreasing trend in the measured values, the bushing manufacturer should be consulted for assistance in evaluation of the condition of the bushing. Most modern high-voltage bushings are equipped with voltage or test taps. Voltage (or capacitance) taps are used generally on bushings rated above 69 kV, and test (or power factor) taps are used generally on bushings rated at 69 kV and below. Routine bushing field tests should include power factor tests on the tap insulation. These measurements are effective for detecting problems such as deteriorated or contaminated oil, defective tap compartment seals, mechanical damage to the tap insulators, and the failure of the top terminal gaskets. Abnormal readings can alert the user to a potential failure hazard before the C1 insulation is affected. While the test tap (C2) insulation is not controlled during the manufacturing process, significant changes in either the C2 capacitance and/or power factor from an initial value may be indicative of a problem. The manufacturer should be contacted.

10.3.2 Gas in oil This test is not recommended as a routine test because it requires that the bushing be opened up and exposed to the outside atmosphere. This introduces the possibility of moisture entering the bushing while the bushing is open or after improper sealing of the opening. While this test is performed by some companies, the degree of expertise to perform and interpret it makes it impractical for most users. There are no IEEE values for DGAs in bushings. Transfor mer values are not applicable due to the differences in oil volumes and paper/oil ratios. IEC/TS 61464 [B7] may be used for a reference. The gas-in-oil test should only be used for diagnostic purposes on bushings that are suspect due to high power-factor measurements or other reasons. Gas-in-oil results should be compared with test results from other bushings and not with power transformer test results. The different mixture of materials in bushings and in transformers will give different results. Experts with experience in interpreting bushing gas-in-oil tests should be consulted if help is needed. A good noninvasive alternative to taking an oil sample is to perform the C2 test. This test energizes the oil in the vicinity of the energized tap lead near the bushing flange and thus includes any watts loss caused by contamination of that oil in the results. This contamination would be apparent as long as there was a sufficient amount in the tap lead area and if the tap housing area were clean and dry so as not to mask the contamination in the oil.




The bushing manufacturer should be consulted for assistance in taking samples and interpreting results. The bushing oil level should be checked and adjusted if needed after oil samples are taken.

10.3.3 Dielectric tests Bushing dielectric tests are sometimes performed in the field. Insulation dielectric strength generally depends on the level of insulation degradation. When dielectric tests are performed on service aged bushings, the following guidelines can help in determining the appropriate test levels: a)

Transformer bushings that will be removed from the transformer for testing should undergo 60 Hz tests at the 100% voltage test levels specified in IEEE Std C57.19.01. This will minimize any problems that may develop during the testing of the transformer after the bushing is reinstalled.

b)

Transformer bushings that will be tested while mounted in the transformer can only be tested at the lower of either the applicable bushing or transformer test levels. Test levels of 60 Hz should be limited to 1.5 times rated line-to-ground voltage or 85% of the withstand voltage level, whichever is lower. The voltage application should be limited to 1 min.

Partial discharges should be monitored during these tests to provide data for evaluating the condition of the bushing. IEEE Std C57.19.00-1991 gives additional information on partial discharge testing.

10.4 Bushing storage

10.4.1 New bushings It is recommended that the bushing manufacturer be consulted concerning bushing storage. They will provide instructions as to the proper storage of their bushing. Improper storage can result in damaging not only the bushing but also the equipment in which it is installed. New bushings should arrive from the manufacturer properly crated for short-term storage. If the bushings are to be used in a short time (6 months maximum), storage in the original crate should be adequate. If the bushings are kept at a construction location, then they should be positioned to be where they are protected from accidental breakage or vandalism before they can be installed. New bushings for stores or spares are possibly going to remain unused for long p eriods. Since most crates are not designed for long-term storage, arrangements should be made to protect them from weather. This can be done by applying a waterproof covering, plastic sheeting, exterior rated plywood, roofing material, etc., or by storing them indoors. They can also be uncrated and stored in metal racks either indoors or outdoors. Storage in this manner has the advantage that the bushings can be periodically tested. The disadvantage is the possibility of breakage as they are uncrated and then recrated for shipping. Since they are designed to be used in oil-filled equipment, storing them out of oil exposes them to air, moisture, and possibly ultraviolet rays. To ensure they are usable when needed, the manufacturer should be consulted for storage information. This is for both short- and long-term storage.

10.4.2 Used bushings Bushings manufactured before 1980 may be PCB contaminated. Consult with your environmental expert concerning the local and Environmental Protection Agency laws before attempting to store these older bushings of unknown contents. Newer bushings should be tagged to be non-PCB. If the tag is missing, then the manufacturer may be able to assist you in determining the PCB content.




The type of bushing and expected storage time should both be considered when determining storage options. Knowledge of bushing construction is helpful in developing storage plans. In all cases, the goal should be to cause no damage to the bushing and to keep it in good operating condition. If the bushings are subject to periodic testing, then the storage facility should be arranged such that this can be accomplished.

10.5 Bushing replacement Bushing replacement is a common practice, taking place thousands of times every year. The most critical item to remember when replacing existing bushings is that you do not change the electrical geometry inside the transformer. Any change to the electrical clearances or insulation structure inside the transformer may lead to a transformer failure. When specifying bushings, future replaceability should be a serious consideration. When replacing an existing bushing, a number of key items need to be addressed before a replacement bushing is selected. The following is a list of items to be considered:

 Bushing mounting flange configuration; bolt circle diameter and number of bolt holes.  Bushing length below the flange, this is normally noted on the nameplate as the “L” dimension.  How is the bushing connected to the transformer winding? Via draw lead cable or conductor or connected to the bottom of the bushing.

 Bottom terminal; threaded stud, two-hole spade, or bottom plate.  Height of bushing above the mounting flange.  Bushing current rating. A complete list is included in Clause 11when bushings must be ordered.

10.5.1 Replacing bottom-connected bushings If the replacement bottom connected bushing meets all the dimensional requirements below the mounting flange as the existing bushing, then the only other item to consider is the shielding. The key item to confirm is the thickness and diameter of the bottom plate. It must not be thicker than the existing bushing’s bottom plate nor larger in diameter. Bushing shields are related to both transformer and bushing design. Determination of shielding requirements will need input from both parties or from knowledgeable consultants.

10.5.2 Replacing draw lead connected bushings If the replacement draw lead bushing meets all the dimensional requirements below the mounting flange as the existing bushing, then the only other item to consider is the shielding. Bushing shields are related to both transformer and bushing design. Determination of shielding requirements will need input from bo th parties or from knowledgeable consultants.




11. Checklist The following is a checklist for ordering replacement bushings. Checklist for ordering replacement bushings

Bushing manufacturer Year built Bushing style Outline drawing (to be attached with order) Insulation class BIL V max L-g V withstand/1 min Rated current Conductor connection

— —

kV kV peak kV kV Amp Bottom connected or draw lead

Air side dimension Oil side dimension CT pocket lengtha Flange BCD/#Holes/ODa Minimum creep distance Arcing distance

inch inch inch inch inch

Insulator color Top terminal Bottom terminal Additional notes: a

BCD = bolt circle diameter; CT = current transformer; OD = outside diameter.




Annex A

(informative) Examples of calculation procedures to determine hot-spot temperatures for bushings applied on transformers

A.1 General information This annex contains examples showing the use of calculation procedures given in this guide. In general, the conditions to be evaluated will include a period during which the bushing and transformer have reached a steady-state condition followed by a peak load period that may or may not reach steady-state conditions. The load conditions and transformer parameters were obtained from actual operating data. The bushing parameters were obtained from published test data (Easley and McNutt [B3]). The parameters for the bushings and transformer used in the examples are as follows:

a)

Bushing coefficient K 1 = 29.07

b)

Bushing coefficient K 2 = 0.635

c)

Bushing time constant τ = 60 min

d)

Transformer oil time constant τo = 166 min

e)

Bushing exponent n = 2

A.2 Example 1 The equivalent load shape is 10 h at 0.64 pu followed by 14 h at 1.14 pu. The ultimate transformer oil temperature rises for the two load periods are 25.2 °C and 69.5 °C. The duration of the initial load and the peak load periods are relatively long compared to the time constants of both the transformer and the bushing. This means that essentially constant conditions will be achieved in both periods. The average ambient temperature during the 10 h period is 27 °C, and during the 14 h period, it is 33 °C. The rise above ambient of the hottest spot in the bushing can be calculated using Equation (1) for each of the two load periods.

A.2.1 Load of 0.64 pu for 10 h ∆ΘHS = 29.07 (0.64)2 + 0.635 × 25.2 = 27.9 °C Hottest-spot temperature = 27 °C + 27.9 °C = 54.9 °C




A.2.2 Load of 1.14 pu for 14 h ∆ΘHS = 29.07 × (1.14)2 + 0.635 × 69.5 = 81.9 °C Hottest-spot temperature = 33 °C + 81.9 °C = 114.9 °C

A.3 Example 2 The equivalent load shape is 1.22 pu for 11 h followed by 1.5 pu for 3 h. The ultimate transformer oil temperature rises for the two load periods are 78.2 °C and 115.7 °C, respectively. The average ambient temperature during the total of both load periods is 5.4 °C. The rise above ambient temperature of the hottest spot in the bushing can be calculated using Equation (1) for the 11 h period since it is long compared to the bushing and transformer constants.

A.3.1 Load of 1.22 pu for 11 h ∆ΘHS = 29.07 × (1.22)2 + 0.635 × 78.2 = 92.9 °C Hottest-spot temperature = 5.4 °C + 92.9 °C = 98.3 °C The conditions during the peak load of 1.5 pu for 3 h will not reach steady-state conditions. Therefore, it is necessary to use the procedures in either 4.2.2.1 or 4.2.2.2.

A.3.2 Load of 1.5 pu for 3 h The calculations in 4.2.2.1 are most easily performed using a digital computer or a programmable calculator. However, for the purpose of this example, manual calculations following the step-by-step procedure in 4.2.2.1 will be performed. Additional manual calculations will be made using Equation (4). Finally, the results of both methods obtained by u se of a digital computer will be tabulated. The time interval chosen for the calculations is 5 min. The calculations using procedures from 4.2.2.1 are as follows: a)

Step A1. The initial hottest-spot rise is 92.9248 °C as determined in A.3.1.

b)

Step B1. The oil temperature rise at the end of 5 min is:

∆θ0 ( 5 min ) = 78.2 + {115.7 − 78.2} {1 − e−5/166} = 79.312 °C Note that several significant figures are carried to improve the accuracy of the iterative calculations but are not to imply such a degree of accuracy in the final temperature rise. c)

Step C1. The ultimate hottest-spot rise based on conditions at 5 min is: ∆ΘHS = 29.07 × (1.5)2 + 0.635 × 79.312 = 115.771 °C

d)

Step D1. The hottest-spot temperature rise at 5 min is: ∆ΘHS(5 min ) = 92.925 + {115.771 – 92.925}{1 – e –5/60 } = 94.751 °C

e)

Step A2. The new hottest-spot rise is 94.751 °C.




f)

Step B2. The oil temperature rise at the end of 10 min is: ∆Θ0(10 min) = 79.312 + {115.7 – 79.312} {1 – e –5/166 } = 80.392 °C

g)

Step C2. The ultimate hottest-spot rise based on conditions at 10 min is: ∆ΘHS = 29.07 × (1.5)2 + 0.635 × 80.392 = 116.457 °C

h)

Step D2. The hottest-spot temperature rise at 10 min is: ∆ΘHS(10 min ) = 94.751 + {116.457 – 94.751} {1 – e –5/60 } = 96.487 °C

These steps can be repeated in the same manner until the entire 180 min time period has been covered. The calculations using Equation (4) from 4.2.2.2 for 180 min are as follows:

{

}

2 ∆θHS (180 min) = 92.9248 + 29.07 × (1 .5) + 0.635  78.2 + (115.7 − 78.2)× (1− e −180/166 )  − 92.9248  

×{1 − e−180/60 } =128.938 °C The computer output with complete results is shown in Table A.1.




Table A.1—Sample computer data

Input data

Results Elapsed time (min) 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 180 Final

Initial load current in pu = 1.22 Peak load current in pu = 1.5 Ultimate top oil temperature rise in °C = 115.7 K 1 = 29.07, K 2 = 0.635, n = 2 Duration of peak load in min = 180 Time increments in min = 5 Bushing time constant in min = 60 Transformer oil time constant in min = 166 Top oil Hottest-spot rise (°C) temperature rise Per 4.2.2.1 Per 4.2.2.2 (°C) 78.2 92.9248 92.9248 79.312 7 94.7515 80.392 3 96.4869 81.44 98.1368 82.456 5 99.7064 83.442 9 101.201 84.4 102.624 85.328 7 103.98 86.229 9 105.274 87.104 3 106.509 87.952 8 107.688 88.776 1 108.815 89.574 9 109.892 90.350 1 110.923 91.102 3 111.909 91.832 1 112.853 92.540 3 113.758 93.227 5 114.626 93.894 3 115.458 94.541 3 116.256 95.169 1 117.022 95.778 3 117.758 96.369 4 118.465 96.942 9 119.145 97.499 5 119.799 98.039 5 120.427 98.563 5 121.032 99.072 121.615 99.565 4 122.176 100.044 122.716 100.509 123.237 100.959 123.739 101.397 124.224 101.821 124.69 102.233 125.141 102.633 125.576 103.02 125.996 128.938 103 °C 126 °C 129 °C



IEEE Std C57.19-100-2012 Guide for Application of Power Apparatus Bushings

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