1064 1991 Stress Insulation

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HZEEStd1064-1991

IEEE Guide for Multifactor StressFunctional Testing ofElectrical Insulation Systems

SponsorMultifactor StressCommitteeoftheIEEE DielectricsandElectricalInsulationSociety

Approved June 12,1991EEEStandardsBoard

Abstract: A guide for developing test procedures for the functional test ing of insulation systemsused in long-life electrical equipment exposed t o more than one factor of influence in service ispresented. Included are descrpitions of technical problems and practical possibilities that may behelpful either for guidance o r as a check list. A sequence of action is recommended, and detail s ofthe procedures required for the specification of such tes ts ar e provided. The emphasis is on realis-tically modeling service aging in functional tests and making the t ests as simple and practical aspossible. Mechanisms of interaction between the factors influencing aging are reviewed.Keywords: Interaction, multifactor testing.

The Institute of Electrical and Electronics Engineers, Inc.345 East 47th Street, New York,NY 10017-2394,USA1991 by the Institute of Electrical and Electronics Engineers, Inc.All rights reserved. Published 1991Printed in the United States of America

ISBN 1-55937-149-8N o par t of this publicat ion may be reproduced in any formin an electronic retrieval system or otherwisewithout the prior writ ten permission of the publisher.

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IEEE Standards documents are developed within the TechnicalCommittees of the IEEE Societies and the Standards CoordinatingCommittees of the IEEE S tandards Board. Members of the committeesserve voluntarily and without compensation. They are not necessar-ily members of the Institute. The standards developed within IEEErepresent a consensus of the broad expertise on the subject within theInstitute as well a s those activities outside of IEEE th at haveexpressed an interest in participating in the development of thestandard.Use of an IEEE Standard is wholly voluntary. The existence of anIEEE S tandard does not imply tha t there are no other ways t o produce,test, measure, purchase, market, or provide other goods and servicesrelated to the scope of the IEEE Standard. Furthermore, th e viewpointexpressed a t the time a standard is approved and issued is subject tochange brought about through developments in th e state of the art andcomments received from users of the standard. Every IEEE Standardis subjected to review a t leas t every five years for revision o r reaffir-mation. When a document is more than five years old and has notbeen reaffirmed, it is reasonable to conclude that its contents, al-though still of some value, do not wholly reflect the present state of theart. Users are cautioned t o check to determine tha t they have the latestedition of any IEEE Standard.Comments for revision of IEEE Standards are welcome from anyinterested party, regardless of membership affiliation with IEEE.Suggestions for changes in documents should be in the form of a pro-posed change of text, together with appropriate supporting comments.Interpretations: Occasionally questions may arise regarding themeaning of portions of standards a s they rel ate to specific applica-tions. When the need for interpretations is brought to the attention ofIEEE, the Institute will initiate action to prepare appropriate re-sponses. Since IEEE Standards represent a consensus of all con-cerned interests, it is important to ensure that any interpretation hasalso received the concurrence of a balance of interests. For this reasonIEEE and the members of its technical committees are not able t oprovide an instant response t o interpretation requests except in thosecases where the matter has previously received formal consideration.Comments on s tandards and requests for interp retations should beaddressed to:

Secretary, IEEE Standards Board445 Hoes LaneP.O. Box 1331Piscataway, N J 08855-1331USA

IEEE S tandards documents are adopted by the Ins titut e of Electricaland Electronics Engineers without regard to whether their adoptionmay involve patents on articles, materials, o r processes. Such adop-tion does not assume any liability to any patent owner, nor does itassume any obligation whatever t o parties adopting the standardsdocuments.


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T his Foreword is n o t a p a r t of IEEE S t d 1064-1991, EEE Guide for Multifactor Stress Func t iona l Te s t ing of ElectricalInsula t ion Systems.)The Multifactor Stress Committee of the IEEE Dielectrics and Electrical Insulation Society was

charged with the development of a guide for endurance tests on electrical insulation where insula-tion life is influenced by more than the simple factor of temperature. Factors of influence o rstresses that operate on electrical insulation in equipment during normal operation can be identi-fied as thermal, electrical, environmental, and mechanical. Multifactor stress can be defined asthe simultaneous application of more than one of these stresses t o an electrical insulation system.The need t o comprehend the actual aging and failure mechanisms of the insulation in service andto simulate them appropriately in laboratory tes ts is leading to new approaches regarding methodsof aging, diagnostic procedures, and the final interpretation of the t est results. This document is aculmination of the committees efforts t o address this need by providing guidelines for standard-ized test procedures for the insulating materials and the methodology for functional tests of insu-lation used in long-life electrical and electronic equipment.

At the time tha t th is guide was approved, the Working Group had the following membership:.Flaherty,Jr., Chair

V K. AggarwalP. E. AlexanderA. I. BennetW. H. Bentley, Jr.E. . BoulterE. L. Brancato

L. E.Braswell, I11F. J. CampbellE . M. For tR. A. Fra ntzG. GelaT. J. LorenzH RosenW. T. StarrJ. A. T a n a k aC. de Tourreil

At the time t ha t this guide was approved, the Multifactor Stress Committee S-32-71f the IEEEDielectrics and Electrical Insulation Society had the following membership:L.E.Braswell,IU hair

V K. AggarwalP. E.AlexanderA. I. BennetW. H. Bentley, Jr.E. A. BoulterE. L. Brancato

F. J. CampbellR. J. Fla he r tyE.M. FortR. A. FrantzG. GelaT. J. LorenzH RosenW. T. StarrJ. A. TanakaC. de Tourreil

The final conditions for approval of this guide were met on June 12, 1991.This guide wasconditionally approved by the IEEE Standards Board on March 21, 1991 with the followingmembership:MarcoW.Migliaro, Chairman Donald C. Loughry,Vice ChairmanAndrew G Salem, Secretary

Dennis BodsonP a u l L. BorrillClyde CampJa me s M. Da lyDonald C. Fleckenste inJ a y F o r s te r *David F. Fra nkl inI n g r i d F r o m m

Thoma s L. H a n n a nDona ld N . He i rma nKenneth D. HendrixJ o h n W. orchBen C. JohnsonIvor N. nightJose ph L. Koepfinger*Irving KolodnyMichael A. Lawler

J o h n E. May, Jr.Lawrence V.McCallDonald T. Michael*Stig L. NilssonJoh n L . Ra nkineRonald H. Re ime rGary S.RobinsonTe r ra nc e R . Whi t t e more

*Me mbe r Eme r i tus


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h l h l t sSECTION PAGE1 General 51.1 Scope and Purpose ................................................................................... 5

1.2 Definitions 51.3 References ............................................................................................ 62 Guide for the Prepara tion of Multifactor Functional Tes ting Procedures.......................... 62.1 Diffe rent Complexity of Multifactor Functional T est s 62.1.1 Examples of Interaction Between Factors of Influence 82.1.1.1 Thermal Aging 82.1.1.2 Electrical Aging ................................................................. 92.1.1.3 Environmental Aging 92.1.1.4 Mechanical Aging .............................................................. 92.2 Preparation of Multifactor Test Procedures ..................................................... 10

2.2.2 Service Conditions to be Simulated 102.2.3 Interactions Between Factors of Influence ......................................... 102.2.4 Types of Test Procedures 112.2.5 Concerns Regarding the Specification of the Test Procedure 112.2.5.1 Aging Procedure 112.2.5.2 Acceleration of Tests .......................................................... 122.2.5.3 Diagnostic Factors and End-Point Criteria 122.2.5.4 Evaluation of Test Results .................................................... 122.2.5.5 Test Report 13

2.2.1 General Principles ........................................................................ 10

3 Bibliography 13FIGURESFig 1 Diagram for Choosing a Valid Test Procedure Consistent with Known orExperimentally Observed Service Relationships


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IEEE Guide for Multifactor StressFunctional Testing ofElectrical Insulation Systems1.General

1.1 Scope and Purpose. This document is in-tended for use a s a guide when developing tes tprocedures for the functional testing of insula-tion systems for use in long-life electricalequipment exposed to more than one factor ofinfluence in service. This document is ana l-ogous to IEC 792 Csll.This guide contains recommendations re-garding the sequence of actions and details ofthe procedures required for the specification ofsuch tests.

Simulation of service aging by one or sev-eral single-factor tests on separate specimensis not within the scope of this document.Concern for the reliability and adequateservice life of economically designed andmanufactured electrical equipment has in-creasingly motivated manufacturers andusers t o consider more advanced methods ofinsulation evaluation than either the simpleconventional test methods or the mere refer-ence to classification tables. The need to com-prehend the actual aging and failure mecha-nisms of the insulation in service and t o sim-ulate them appropriately in laboratory tests isleading t o new approaches regarding methodsof aging, diagnostic procedures, and the finalinterpretation of the test results. Since equip-ment insulation in service usually is sub-jected t o the actions of several factors of influ-ence, a multifactor tes t will, in many cases, beconsidered.This document contains descriptions oftechnical problems and practical possibilitiesthat may be helpful either for guidance or as acheck list. The emphasis of this documentparticularly concerns two matters: the realis-tic modeling of service aging in functional

tests and the concern tha t tes ts be a s simpleand practical as possible.This guide consists of two sections. The firstsection is a general section describing the pur-pose and scope, including definitions. Thesecond section contains an essay on multifac-t o r tests and details on preparation of multi-factor test procedures.1.2 Definitions. The following terms areintroduced in this document. Additionalterms are defined in IEEE Std 1-1986[U2interaction. Modification of the type or degreeof aging produced by the combination of two ormore factors of influence relative to the sum oftheir aging effects when acting individuallyon separate objects.NOTES: 1)Aging effects are understood to be any pri-mary changes in the insulation due to aging, e.g.,changes in chemical composition.2) Changes in physical properties usually are mea-sured and used to describe the degree of aging.However,they may depend on the aging effects in a very compli-cated manner. Therefore, even when interactions are ab-sent, the changes in physical properties may not be addi-tive as are the aging effects.3) The above definition differs from the accepted statis-tical definition of interactiondirect interaction. Interaction between simul-taneously-applied factors of influence that dif-fers from interaction occurring between se-quentially-applied factors of influence. Fac-tors producing direct interaction are not nec-essarily aging factors. (See 2.1.1.)indirect interaction. Interaction between si-multaneously-applied factors of influence thatremains essentially unchanged when thefactors are applied sequentially. Indirect in-teraction can only be caused by aging factors.

The numbers in brackets, when preceded by the letter 2The numbers in brackets correspond to those of theB,correspond to the bibliography in Section 3. references in Section 1.3.

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IEEE1064-1991synergism. The change in properties that isnot equal to the sum of the change in propertiesfrom two stresses. Synergisms may be seen ineither sequential o r simultaneous tests.Synergism may be positive or negative.

IEEE GUIDE FOR MULTIFACTOR STRESS FUNCTIONALnonaging stress. A stres s that does not causean irreversible change t o take place with time.

aging index. The process of aging is mea-sured by analytically determining some phys-ical o r chemical property of the material. Thechange observed in such a property is calledthe aging index.activation energy. A characteristic value re-lated t o the change in t he log of the rat e con-stant with absolute temperature (or otherstress). Equal acceleration by two stresses isnot obtained by equal increases of the stresses.Activation energy must be considered.end point. The definition in IEEE S td 1-1986Clis understood with the clarification that, formultifactor stress considerations, end po intrefers t o the reaching of a failure point for asingle sample of a system being tested.sp eci men . Defined for this guide as the insu-lation system in an apparatus o r test configu-ration modeling tha t apparatus that provides asingle datum point.fai lu re . In a multifactor tes t, it is that value ofa property t ha t is preselected as th e degree ofdegradation that is indicative of the end-of-life of the insulation system being studied.fa ct or of infl uen ce. A specific physical stressimposed by operation, environment, or tes t tha tinfluences the performance of an insulatingmaterial, insulation system, o r electricequipment.aging fa ct or . A factor of influence that causesan irreversible change (usually degradation)t o take place with time.diagnostic fa ct or . A variable or fixed stre ssthat can be applied periodically o r continu-ously during an accelerated test t o measure thedegree of aging without in itself influencingthe aging process.aging stress. A stress that causes an irre-versible change (usually degradation) t o takeplace with time.

1.3 ReferencesU1 IEEE Std 1-1986 IEEE Standard GeneralPrinciples for Temperature Limits in theRating of Electric Equipment and for theEvaluation of Electrical Insulation (ANSI).3[21 IEEE Std 98-1984 IEEE Standard for thePreparation of Test Procedures for theThermal Evaluation of Solid Electrical In-sulating Materials (ANSI).[31 IEEE Std 99-1980 IEEE RecommendedPractice for the Preparation of Test Proceduresfor the Thermal Evaluation of InsulationSystems for Electrical Equipment (ANSI).

2. Guide for thePreparationofMulti.lh r FunctionalTestingprocedures2.1Different Complexity of Multifacto r Func-tional Tests. All electrical insulations inservice are exposed to multifactor situations.The combination of, at least, temperature,environment, and electrical stress is alwayspresent. This may or may not give rise t ointeractive aging that can be defined in thefollowing manner. When the aging effectsproduced by the factors of influence differfrom the sum of the aging effects produced byeach factor in isolation, then interactive agingis said t o have taken place. The cause of thedepar ture from the combination of the separateaging effects shall be called interaction.In this context, the term aging effect isused to describe primary changes in t he insu-lation, e.g., changes in chemical compositionas a consequence of aging reactions or diffu-sion phenomena. Changes in physical proper-ties, which often are used to describe the degreeof aging, may depend on these primarychanges (aging effects) in a complicatedmanner. Therefore, the changes in physicalproperties may not be additive, as are the ag-ing effects (by definition), when interactionsare absent.

31EEE publications are available from the Institute ofElectrical and Electronics Engineers, Service Center, 445Hoes Lane, P.O. ox 1331, Fiscataway,NJ 08855-1331,USA 1-800-678-4333.

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TESTING OF ELECTRICAL INSULATION SYSTEMSThe aspect of factor interaction that is of in-terest in functional testing of insulations sys-tems i s the distinction between direct and in-direct interaction. This is borne out by the def-initions for direct and indirect interaction in1.2. In more physical terms, direct interaction

is a change, caused by the second factor, of therate of the degradation produced by the firstfactor. Direct interaction is caused by the sec-ond factor itself. In the case of indirect inter-action, however, it is not the presence of thesecond factor, but rather its irreversible effectson the st ate of the insulation, tha t is the cause ofthe modification of th e aging by th e first fac-tor. Such interaction can occur either due to achange of the intensity of the f irst factor or duet o a change of its aging effects at constant in-tensity.

Considerable experience already exists inseveral aspects of multifactor testing. I t is t o benoted that many kinds of electrical equipmentin reality experience multifactor stressing.However, where the thermal aging was domi-nant , the other factors were neglected.

E E E1064-1991When multifactor t esting is importa nt,

modeling of service situations may still beachieved by simplified procedures.When equipment in service experiencessimultaneously-acting factors, the aging canbe simulated by either simultaneous o r se-quential application of the factors in a func-tional test, depending on the kind of interac-tion between the factors.On the other hand, when equipment experi-ences sequentially-acting factors, the model-ing requires a sequential test irrespective ofhow aging by these factors would proceed ifthey had been acting simultaneously. For ex-ample, power electronic equipment operatingunder load for a period of time and periodi-cally shut down for an interval long enough toabsorb humidity and then returned t o serviceexperiences a periodic temperature cycle andhumidity cycle.Fig 1illustra tes the choices of tes t procedurethat may produce valid evaluations in differ-ent cases of factor combination and knowl-edge regarding interactions.

Fig 1DiagramforChoosingaValid TestProcedureConsistentwithKnownorExperimentally ObservedServiceRelationships

SEQUENTIAL SERVICE FACTORS SIMULTANEOUS SERVICE FACTORSInteraction Interaction Interaction Interaction Interaction InteractionABSENT UNKNOWN PRESENT ABSENT UNKNOWN PRESENT\\ A i N g . E T E c T\

SINGLE-FACTOR TESTS SEQUENTIAL SIMULTANEOUSSeparate batches MULTIFACTOR TESTS MULTIFACTOR TESTS

of specimen


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I1064-19912.1.1 Examples of Interaction BetweenFactors of Influence. A number of relatively

well-known mechanisms of interaction be-tween factors of influence are briefly reviewedbelow. This review is by no means exhaustive,but is merely a collection of typical examples.In actual service, any of these examples, aswell as other cases of interaction, may domi-nate the aging of an insulation system in aparticular type of equipment during its enti relife. It may, however, also happen that morethan one of these interactions exists at thesame time. A very important aspect, which isproven by the careful examination of servicefailures in quite different types of equipment,is the possibility of drastic changes of the ag-ing mechanism and of the main aging factorat some time during the life of the insulation.For example, gradual aging caused by onedominating factor (possibly with interactionfrom other factor s) may irreversibly change

the condition of the insulation in such a waythat degradation produced by a different factorbecomes dominant. This may result becausethe primary change intensifies either the sec-ond factor o r its action at constant level.Similar effects may also be produced by re-versible changes of the state of the insulationsystem, for example, the transformations thatoccur a t certain temperatures such a s melting,glass transition , etc.2.1.1.1 Thermal Aging. Thermal agingrefers t o that degradation of materials underoperating conditions that can be accelerated byincreasing the thermal stress. In principal, itmay involve changes in the material due t ocrosslinking, chain scission, phase segrega-tion, additive migration, etc. However, be-cause thermal aging generally refers to agingin air, it generally involves oxidative pro-cesses.When permanent, transitory, o r periodictemperature gradients result in mechanicalstresses, the degradation may be consideredunder the heading of either thermal or me-chanical aging depending on whether thedegradation is primarily due to the thermal ormechanical stress.2.1.1.1.1 Interaction w i th ElectricalStress. Electrical stress can contribute to fur-ther temperature increase but, more impor-tantly, can interact with normal thermal ag-ing by the generation of active species (e.g.,ozone) o r by field effects (e.g., treeing).

IEEE GUIDE FOR MULTIFACTORSTRESS FUNCTIONAL1) Increased temperature and modifiedthermal gradients due to dielectric

losses depend on voltage, frequency,and material properties. In extremecases, thermal instability may occur.This is a direct interaction. In test con-ditions, the average or highest tempera-ture increase should be taken into ac-count when determining the test tem-perature.

2) Electrical aging, in particular by par-tial discharges and t o some degree bytracking, also produces, in addition t oi t s immedia te ef fec ts, agents(chemically active species from the gasin which discharges occur, free radi-cals and ions in affected areas of theinsulation) that may significantly af-fect the rate processes. These interac-tions may be of the direct and/or indi-rect type, depending upon the life of theactive agents in the actual environ-ment.2.1.1.1.2 Interaction w i th Environ-mental Stress. Considering that a chemicalprocess is generally included in the mech-anism of thermal aging, it is evident that thecomposition of the environment of the insu-lation system will be important.For example, its oxygen content will have adirect action on oxidation. Also moisture canhave an important effect on thermal aging.When special environments are involved, o rwhen insulating fluids are part of the insula-

tion system, degradation products of these en-vironments or fluids may be chemically ac-tive. This is the case with agents that may bepresent in normal service o r due t o accidents(failure of cooling circuits, contamination bylubricants, etc.).When present, the effect of radiant energyshould be considered. The ultraviolet lightcomponent of daylight is capable of initiatingphotochemical reactions. In a nuclear envi-ronment, gamma and beta radiation must beconsidered.Depending on the case, the effects of the en-vironmental factors are either direct or indi-rect.

2.1.1.1.3 Interaction with MechanicalStress. The influence of mechanical agingcan be illustrated by the effect of a delamina-tion of a layered insulation structure on the

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TESTING O F ELECTRICAL INSULATION SYSTEMSheat transfer through this insulation. Thismay considerably increase the local tempera-ture with a resulting effect on rate processesand temperature gradients and their mechan-ical effects. Such effects ar e mainly indirect.2.1.1.1.4 Interactionwith Electrical andEnvironmentalStresses. It c n happen that thecombined action of electrical stress and envi-ronment play a role in thermal aging, for ex-ample, if an electrolytic or tracking process isinvolved.2.1.1.2 Electrical Aging. It is known thatall other factors of influence, including radia-tion, can affect electrical aging. In view of thediversity of the aging mechanisms and alsoin view of the possible interactions, some se-lected examples a re given here.2.1.1.2.1 Interaction with ThermalStress. This is probably the most extensivelystudied case of interaction between factors ofinfluence. It can have different aspects:

IEEE1064-1991voids when their shape is changed. This is anindirect interaction.2.1.1.2.4 Interaction with Thermal andMechanical Stress. In the same manner asstated above, thermomechanical stresses canbe of importance for the development of faultsin the insulation, with resulting partial dis-charges. This i s an indirect interaction.2.1.1.3 Environmental Aging. Environ-mental aging is, in most cases, correlatedwith rate processes a s discussed under 2.1.1.1.The increasing importance of radiation t othe aging processes of an insulation systemshould be recognized. Both direct and indirectinteraction may occur, depending on thenature of the radiation and its intensity.2.1.1.3.1 Interaction with ThermalStress. Temperature produces a decisive directinteraction with environmental aging. I t mayalso have an indirect effect by bringing anactive agent t o the place of reaction.

The formation of cracks for any reason maybring active products to internal regions. Thismight, for example, make hydrolysis a domi-nating reaction in this region by permittingmigration of water.2.1.1.3.2 Interaction with ElectricalStress. An electric field, even of low strength,can completely change the process of envi-ronmental aging, in particular by introduc-tion of electrolytic mechanisms. This is directinteraction.2.1.1.3.3 Interaction with MechanicalStress or Mechanical and Thermal Stresses.Mechanical (and thermomechanical) agingmay produce interfaces that may bring envi-ronmental agents (e.g., water) to new places.Such interactions are indirect.2.1.1.4 Mechanical Aging2.1.1.4.1 Interaction with Thermal,Electrical, and Environmental Stresses andTheir Combinations. Interaction with mech-anical aging applies particularly to the effectof temperature. Due t o differential expansion,mechanical o r thermomechanical) stressescan be induced by the action of temperaturealone. Thermal cycling may then produce asignificant mechanical aging.Thermal, environmental, and electricalaging may influence mechanical propertiesin ways that change the effects of mechanicalaging (e.g., embrittlement, plastification).Also, the presence of dust may produce abra-sion and wear.

Effe cts on the ca uses of electrical ag-ing. Temperature o r thermal aging candrastically change the partial dis-charge inception voltage and intensitythrough reversible o r irreversible pro-cesses (change of void configurationsas a consequence of different thermalexpansion of conductors and insula-tion, of gas pressure in a void, of sur-face conductivity, etc).Effe cts on the endurance properties o fthe insulation system. The degradationcaused by partial discharges of a givenintensity may change due t o tempera-ture-dependent material properties.

Whether these interactions are direct or in-direct depends on the case at hand.2.1.1.2.2 Interactionwith Environmen-tal Stress. It is well known that the nature andpressure of the surrounding gas is of greatimportance for partial discharges. Also, thedielectric properties of insulation systems aresensitive t o moisture, and this can sig-nificantly affect electrical aging. Electrolyticprocesses can be produced due t o particularatmospheric conditions. These interactionsare, in most cases, of the direct type.2.1.1.2.3 Interaction with MechanicalStress. The mechanical degradation thatleads t o a change of conformation may pro-duce partial discharges in new voids orchange the discharge intensity in existing

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IEEE1064-19912.2 Preparation of Multifactor Test Proce-dures. To establish the foundation upon whichthe evaluation procedure for the equipmentinsulation systems will be based, the usershould review the available evidence re-garding service experience, records of fail-ures, experience gained during the develop-ment of the insulation systems, common de-sign practice, etc. The user should base theevaluation procedure on available knowledgein such a way that the risk of unknown inter-actions invalidating the results is reasonablyreduced.Sequential procedures may be developed inwhich alternative consequential actions arelisted to cover each possible outcome of a pre-ceding action. A s another approach, the usermay instigate research or tests to establish ba-sic knowledge required for the specification ofa multifactor test procedure. Examples of sub-jects that are of particular concern are theidentification of the aging factors and theirnature. Such knowledge enables the develop-ment of technically sound tests that also arepractical and simple, a central concern of thisdo cum ent.2.2.1 General Principles. Multifactor func-tional t ests a re needed

When more than one aging factor cansubstantially affect the performance ofthe insulation system, changes in rele-vant properties are known o r suspectedt o be nonadditive, or the absence of fac-tor interaction is not confidentlyknownWhen direct interaction between atleast one aging factor and another fac-t o r of influence is known o r suspected toexist

Simultaneous service factors are most reli-ably simulated by a simultaneous test.Availability of aging facilities may, however,lead to the consideration of a sequential test asan alternative. This approach is valid on con-dition th at the interactions between the factorsare of the indirect type.Even in the case when separate single-factortests are considered t o be adequate whichmay be presumed but usually is not confi-dently known sequential test on the sametest objects may, in many cases, be preferableto a series of independent single factor tests for

IEEE GUIDE FOR MULTIFACTORSTRESSFUNCTIONALthe same factors of influence on differentbatches of test objects. While care must be ex-ercised t o ensure that sequential application ofthe accelerated factors does not produce failuremechanisms that do not occur in service, thismethod has the potential of reducing testing ef-fort while allowing indirect interactions to oc-cur as in service.The application of statistical methods maybe helpful in obtaining test economies whenpreparing screening test procedures t o estab-lish the absence or presence of and kind ofsignificant interactions.2.2.2 Service Conditions to be Simulated.The user should identify the aging factors andthe other factors of influence that a re known t oo r are supposed to affect the life of the insula-tion in service. The aging factors and mecha-nisms should be ranked according to their im-portance, and changes in this ranking duringthe life history of the insulation should benoted. The order of occurrence of the factors(simultaneous, sequential) should be estab-lished, as well as information regarding rela-tive operating times and factor stress levels inservice (rated, average, overload conditions,transients, stresses during rest periods, etc.).It is recommended that a set of referenceservice conditions be defined based on the list-ing above for purposes of selecting test agingconditions. The reference service conditionsshould specify the factors, their stress levels,their sequence, and their relative operatingtimes.To facilitate the specification of the simplestpossible test , it may be helpful to consider sepa-rately different service conditions that alwaysoccur a t different times, occur in a certain se-quence, or occur randomly. Such conditionsmay, for example, be: starting, normal loadchanges, different kinds of anomalous opera-tion (overload, transients), stops, prolongedrest, storage, and transportation. In somecases, cyclic variations such as day:and nightor season may be relevant. The actual combi-nations of external environmental factorswith load-dependent internally-generatedfactors may be identified relatively easilythrough such an approach. Expert knowledgemay then permit an estimation as t o which ofthe factors and combinations a re essential.2.2.3 Interactions Between Factorsof In-fluence. Knowledge of the actual physicalmechanisms of insulation deterioration is

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TESTING OF ELECTRICAL INSULATION SYSTEMSimportant t o a good and rational design offunctional tests. Knowledge regarding thepresence and kind of interaction between fac-tors may also permit simplifications of the ag-ing procedure t o be introduced relative to thescaled reproduction of the service situation.

Aging by simultaneous factors in servicecan be modeled by their simultaneous applica-tion in a test. This becomes a necessity in thecase of their direct interaction. If the interac-tion is known t o be indirect, then a sequentialapplication of these factors is a valid alterna-tive that may be preferred in order to reduce thetesting effort.Aging by the sequential action of factors inservice can best be simulated in a multifactortest by their sequential application. The userpreferably should identify the presence andkind of interaction between factors on the ba-sis of service experience and of evidence re-garding the actual mechanisms of insulationdegradation.NOTES: 1) Interaction may accelerate or decelerate therate of aging.2) Temperature and environment should not be disre-garded as possible interaction factors without a propercontrol or extensive experience, even in cases where theyare not known to produce direct interaction.3)A review of interactions between factors of influenceis given in 2.1.1.

2.2.4 Types of Test Procedures.The func-tional testing of insulation systems for use inlong-service-life equipment can, in the pre-sent state-of-the-art, only be of a comparativenature.Four possible cases can be distinguishedwhen the candidate insulation system is com-pared with a known reference system with adocumented service record:

IEEE1064-1991sequence of diagnostic factors should beas similar as possible, still retainingcorrect simulation of the reference ser-vice conditions.Upgrading to withstand higher agingstress levels. In this case, the diagnos-tic procedures should remain identical.The aging procedures will differ withregard t o the applied stress levels, butthe kinds, combinations, and se-quences of factors applied t o age thespecimens should be the same.Upgrading to withstand higher agingand nonaging s t ress leve ls . In thiscase, the aging procedures and the di-agnostic procedures will be different.

Qu alificatio n for identical service con-d i t i o n s . This is the simplest case. Allaging and diagnostic procedures willbe identical in this case for the candi-date and reference systems. Simul-taneous exposure in the same facilitiesis recommended to reduce dispersion ofresults.Upgra ding to withsta nd higher levels ononaging s t resses . The aging proce-dures for both systems will be identical.The diagnostic procedures will differwith regard t o the applied stress levelsand possibly also with regard to the di-agnostic factors. The combination and

2.2.5ConcernsRegarding the SpecificationoftheTestProcedm2.25.1 Aging Procedure2.2.5.1.1 General Principles.The ac-celerated aging test is frequently performed ina cyclic manner. Each cycle may contain thefollowing subcycles:1) Single-factor aging subcycles2) Multifactor aging subcycles with si-multaneous application of the respectivefactors3) Diagnostic subcycles

2.2.5.1.2 Single-Factor Aging Subcy-cles. In these subcycles, the insulation sys-tems specimens are exposed to only one factorat a time.

Aging factors in these subcycles are thosefactors from the reference conditions (see2.2.2) that act in service in a sequentialmanner and without any other simulta-neously-acting aging factor. If simultaneousfactors are known t o produce only indirectinteraction, then each of them may also beapplied in a single-factor aging subcycle.For each factor, one subcycle with the respec-tive factor intensified shall be performed. Toother factors, zero or a value producing negli-gible effects is given.2.2.5.1.3 Multifactor Aging Subcycles.In these subcycles, two o r more factors are ap-plied simultaneously to the test specimens. Ifpossible, the action of each factor should be in-creased to produce equal contributions t o theaging rate. When choosing the levels o r fre-quencies, available information regarding

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JEEE1064-1991the single-factor aging of the insulation sys-tem may be useful. Combined accelerationmay be significantly different as a conse-quence of interaction. Therefore, moderatevalues of the individual accelerations shouldbe selected.

2.2.5.1.4 Diagnostic Subcycles. Thesesubcycles often comprise a sequence of treat-ments with partly different purposes as, forexample:Potentially destructive treatments t oreveal degradation, e.g., by mechani-cal vibrations that may cause embrit-tled insulation t o develop cracksConditioning, e.g., by humidification,t o amplify the discerning capacity of asubsequent proof testPotentially-destructive diagnostic tests(proof tests), e.g., the brief application ofa specified voltageNondestructive o r destructive determi-nations of one o r more properties

Further considerations regarding the selec-tion of these treatments and of correspondingend points are given in 2.2.5.3.2.2.5.1.5 Cycle Lengths. Normally, theaccelerations and cycle lengths should be cho-sen so that one half of the test objects will failwith ten cycles of single-factor tests or at leastin such a single-factor test where the aging ef-fect is strongest). Then, when the multifactortests are carried out, more than half of the testsobjects would be expected to fail within ten ag-ing cycles.Additional diagnostic tests may be per-formed after the last planned cycle in order toincrease the information on the state of the in-sulation system at the end of the aging tests.Care should be taken that changes in rele-vant properties during any subcycle are notexcessive, because changes may produce un-expected mechanisms.

2.2.5.2 Acceleration of Tests. Fixed accel-erated values of level and/or frequency are se-lected for every aging factor and remain con-stant for the duration of the repetitive test cy-cles. Selection of the s tre ss acceleration valuesand thus the degree of single-factor intensifi-cation for each test subcycle are most conve-niently determined by the results of earliersingle-factor experiments.

IEEE GUIDE OR MULTIFACTOR STRESSFUNCTIONALFor each stress, approximately the same de-gree of acceleration should be recommended.See also 2.2.5.1.3.2.2.55 Diagnostic Factors and End-PointCriteria. The selection of the sequence of theindividual treatments comprising the diag-

nostic subcycle (See 2.2.5.1.4) should be guidedby available knowledge regarding the agingmechanisms in service. I t is recommended touse the nonaging factors and levels of the ref-erence service conditions mentioned in 2.2.2.In many applications, the sequence indicatedin 2.2.5.1.4 has been found to be satisfactory,i.e., potentially destructive treatments (con-ditioning treatments), proof tests, and/or de-termination of property values.

The diagnostic procedure must comprise thespecification of an end point. This may be thephysical failure during an aging subcycle ofthe t est object th at may be revealed immedi-ately o r during the next diagnostic subcycle. Itmay also be an event or condition, such asfailure due to a diagnostic treatment o r the at-tainment of a property limit. The end pointmay be characterized by a single condition(e.g., level of property A ) or by one of severalalternatives (level of A , or level b of B, etc),o r it may be given as a combined condition(level of A and level b of B, etc). Relevantend-point criteria may be found through fail-ure analysis, stress analysis, and/or appro-priate tests.

2.2.5.4 Evaluation of Test Results. Themost straightforward method of evaluating theresults of functional testing of the candidateand reference insulation systems is t o com-pare their test times to reach selected end-pointcriteria. If there is no statistically significantdifference, on a confidence level specified bythe uses, between the distributions of the timesto reach the end-point criteria for the candidateand reference test objects, then it may be con-cluded that they are equivalent in the test used.

Sometimes, however, there are no failures,or an insufficient number of failures, withinthe planned number of test cycles. Two possi-bilities then exist: either continue testing cy-cles until a sufficient number of failures occurfor statistical analysis, or carry out additionaldiagnostic tests designed to reveal the extent ofdeterioration in the insulation systems beingcompared in relation to the end-point criteriaselected.

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TESTING OF ELECTRICAL INSULATIONSYSTEMSIn general, presentation of the results in the

form of curves for selected properties of the in-sulation systems versus test duration can beinformative. These property determinationscan be made by either nondestructive or de-structive tests. When property changes mea-sured by nondestructive tests are selected ascriteria for insulation deterioration, they canbe compared for the candidate and referenceinsulation system after every cycle of func-tional testing using the same specimens eachtime. This procedure requires that the end-point criteria be appropriately related t o thechanges in properties.When destructive diagnostic tests are se-lected as the method of determining progres-sive deterioration of insulation system proper-ties during cyclic aging functional tests, itwill be necessary to provide a sufficient num-ber of test objects containing the candidate andreference insulation systems for proper statis-tical evaluation of the failure data. In thiscase, a planned number of test objects is re-moved periodically from functional testingaccording t o a schedule of aging cycles t o becompleted for each degree of aging for whichproperty data are desire. Normally, a group oftest objects will be removed only after exposureto all the subcycles of the la st aging cycle theyare scheduled to complete. Again, the testingresults, expressed in figures or graphs for eachsystem, may be compared to make the evalua-tion.

IEEE1064-1991For both nondestructive and destructive di-agnostic property tests , it is important t o pro-

vide measurements on unaged test objects sothat initial conditions may be recorded in thefigures and graphs.2.2.5.5 TestReport. The test report shouldinclude

1) Description of the insulation system2) Description of the service experienceconcerning the reference system, in-cluding interactions (when necessary)(3) Reference conditions

4) Aging factors and their levels in thetests, single-factor acceleration of eachfactor5) Test sequence6 ) Diagnostic treatments, tests and mea-surements7) End-point selected8 ) Aging curves(9) Times t o reach the end-points, individ-ual values and medians, statisticaltrea tmen t when feasible(10) Identification of the new system incomparison t o the reference system

3.Bibliography[Bll IEC 792: he multi-factor functionaltesting of electrical insulation systems.

1064 1991 Stress Insulation

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