Iec ts 61463 2016

I E C TS 61 463 ® TE C H N I C AL S P E C I F I C ATI ON IEC TS 61 463:201 6-07(en) B u s h i n g s – S ei s m i c q u al i fi cati on Copyright International Electrotechnical Commission Edition 2.0 201 6-07 TH I S P U B L I C ATI O N I S C O P YRI G H T P RO TE C T E D C o p yri g h t © I E C , G e n e va , S w i tze rl a n d All rights reserved Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either IEC or IEC's member National Committee in the country of the requester If you have any questions about IEC copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or your local IEC member National Committee for further information IEC Central Office 3, rue de Varembé CH-1 21 Geneva 20 Switzerland Tel.: +41 22 91 02 1 Fax: +41 22 91 03 00 info@iec.ch www.iec.ch Abou t th e I E C The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes International Standards for all electrical, electronic and related technologies Ab o u t I E C p u b l i c a ti o n s The technical content of IEC publications is kept under constant review by the IEC Please make sure that you have the latest edition, a corrigenda or an amendment might have been published I E C C atal og u e - webs tore i ec ch /catal og u e E l ectroped i a - www el ectroped i a org The stand-alone application for consulting the entire bibliographical information on IEC International Standards, Technical Specifications, Technical Reports and other documents Available for PC, Mac OS, Android Tablets and iPad The world's leading online dictionary of electronic and electrical terms containing 20 000 terms and definitions in English and French, with equivalent terms in additional languages Also known as the International Electrotechnical Vocabulary (IEV) online I E C pu bl i cati on s s earch - www i ec ch /s earch pu b I E C G l os s ary - s td i ec ch /g l os s ary The advanced search enables to find IEC publications by a variety of criteria (reference number, text, technical committee,…) It also gives information on projects, replaced and withdrawn publications 65 000 electrotechnical terminology entries in English and French extracted from the Terms and Definitions clause of IEC publications issued since 2002 Some entries have been collected from earlier publications of IEC TC 37, 77, 86 and CISPR I E C J u st P u bl i s h ed - webs tore i ec ch /j u s u bl i s h ed Stay up to date on all new IEC publications Just Published details all new publications released Available online and also once a month by email Copyright International Electrotechnical Commission I E C C u s to m er S ervi ce C en tre - webs tore i ec ch /cs c If you wish to give us your feedback on this publication or need further assistance, please contact the Customer Service Centre: csc@iec.ch I E C TS 61 463 ® Edition 2.0 201 6-07 TE C H N I C AL S P E C I F I C ATI ON B u s h i n g s – S ei s m i c q u al i fi cati on INTERNATIONAL ELECTROTECHNICAL COMMISSION ICS 29.080.20 ISBN 978-2-8322-351 8-8 Warn i n g ! M ake s u re th at you obtai n ed th i s pu bl i cati on from an au th ori zed d i s tri bu tor ® Registered trademark of the International Electrotechnical Commission Copyright International Electrotechnical Commission –2– I EC TS 61 463: 201 © I EC 201 CONTENTS FOREWORD I N TRODU CTI ON Scope N ormative references Terms and definitions Symbols and abbreviated terms Methods of seismic qualification Severities At the ground At the bushing flange Qualification by static calculation 1 Qualification by dynamic analysis General Modal analysis using the time-history method Modal analysis using the RRS Qualification by vibration test General General Mounting External load Measurements Standard frequency range Test methods Testing Test on complete apparatus Test on the bushing mounted on a simulating support Test on the bushing alone Evaluation of the seismic qualification 1 Combination of stresses Cantilever test Acceptance criteria 1 N ecessary exchange of information 20 1 I nformation supplied by the apparatus manufacturer 20 1 I nformation supplied by the bushing manufacturer 20 Annex A (informative) Flow chart for seismic qualification 23 Annex B (informative) N atural frequency and damping determination: Free oscillation test 24 B Free oscillation test 24 B Sine sweep frequency search 25 Annex C (informative) Static calculation method – Additional considerations 26 C General 26 C Effect of the first bending mode 26 C Determination of Sc 26 C Value of a bg 26 C Typical seismic response of cantilever type structures 27 Copyright International Electrotechnical Commission I EC TS 61 463: 201 © I EC 201 –3– C Superelevation factor K 29 Annex D (informative) Qualification by static calculation – Example on transformer bushing 33 D Seismic ground motion 33 D Critical part of the bushing 33 D Static calculation 33 D General 33 D Seismic load 34 D 3 Wind load 35 D Terminal load 35 D Guaranteed bending strength 36 Annex E (informative) Center clamped bushings 37 Bibliography 40 Figure – Example of model of the transformer system Figure – RRS for ground mounted equipment – ZPA = 0, g [1 ] [2] Figure – Response factor R 21 Figure – Test with simulating support according to 22 Figure – Determination of the severity 22 Figure A – Flow chart for seismic qualification 23 Figure B – Typical case of free oscillations 24 Figure B – Case of free oscillations with beats 25 Figure C – Single degree of freedom system 27 Figure C – Structure at the flange of a bushing with cemented porcelain [5] [7] 28 Figure C – Spring stiffness C in function of cemented part geometry [5] [7] 29 Figure C – Superelevation factor due to the existence of transformer body and foundation [5] 30 Figure D – Critical part of the bushing 33 Figure D – Forces affecting the bushing 34 Figure D – Porcelain diameters 35 Figure E – Failure process [6] 37 Figure E – Failure process, flow chart [5] [6] 38 Figure E – Stress profile during the opening process [6] 38 Figure E – Relation between compression and tensile stress in the bottom edge of the porcelain due to the opening process [6] 39 Table – Ground acceleration levels Table – Dynamic parameters obtained from experience on bushings with porcelain insulators ( f0 = natural frequency, d = damping) Table – Dynamic parameters obtained from experience on bushings with composite insulators ( f0 = natural frequency, d = damping) Table – Example of qualification level: AG5: ZPA = 0, g Table – Response factor R 21 Table C – Examples of typical seismic responses 31 Copyright International Electrotechnical Commission –4– I EC TS 61 463: 201 © I EC 201 I NTERNATI ONAL ELECTROTECHNI CAL COMMI SSI ON _ B U S H I N G S – S E I S M I C Q U AL I F I C AT I O N FOREWORD ) The I ntern ati on al El ectrotechni cal Comm i ssi on (I EC) i s a worl d wi d e organi zati on for stan d ard i zati on com pri si ng al l nati onal el ectrotech ni cal commi ttees (I EC N ati on al Commi ttees) Th e obj ect of I EC i s to prom ote i ntern ati on al co-operati on on al l q u esti ons concerni ng stand ard i zation in the el ectri cal and el ectroni c fi el d s To th i s end and i n ad d i ti on to other acti vi ti es, I EC pu bl i shes I nternati onal Stand ard s, Techni cal Speci fi cati ons, Techni cal Reports, Pu bl i cl y Avai l abl e Speci fi cati on s (PAS) and Gu i d es (hereafter referred to as “I EC Pu bl icati on(s)”) Thei r preparati on i s entru sted to techni cal commi ttees; any I EC N ati on al Commi ttee i nterested i n the su bj ect d eal t wi th may parti ci pate i n th i s preparatory work I n ternati onal , g overnmental and non govern mental organ izati ons l i si ng wi th the I EC al so parti ci pate i n thi s preparation I EC col l aborates cl osel y wi th th e I nternational Organi zati on for Stand ard i zati on (I SO) i n accord ance with cond i ti ons d etermi ned by ag reement between the two organ i zati ons 2) The formal d ecisi ons or ag reements of I EC on techn ical matters express, as nearl y as possi bl e, an i ntern ati on al sensu s of opi n i on on the rel evan t su bj ects si n ce each techni cal commi ttee has representati on from al l i nterested I EC N ati onal Com mi ttees 3) I EC Pu bl i cati ons h ave the form of recomm end ati ons for i nternati onal u se an d are accepted by I EC N ati onal Com mi ttees i n th at sense Wh i l e al l reason abl e efforts are mad e to ensu re that the techn i cal content of I EC Pu bl i cati ons i s accu rate, I EC can not be hel d respon si ble for the way i n wh i ch th ey are u sed or for an y mi si nterpretati on by any end u ser 4) I n ord er to promote i ntern ati onal u ni form ity, I EC N ati on al Commi ttees u n d ertake to appl y I EC Pu bl i cati on s transparen tl y to the maxi mu m extent possi bl e i n thei r nati onal and regi onal pu bl i cati ons Any d i vergence between any I EC Pu bl i cation and the correspond i ng nati onal or regi on al pu bl i cati on shal l be cl earl y i nd i cated i n the l atter 5) I EC i tsel f d oes n ot provi d e an y attestation of conformi ty I nd epen d ent certi fi cati on bod i es provi d e conform ity assessm ent servi ces an d , i n some areas, access to I EC marks of form ity I EC i s not responsi bl e for an y servi ces carried ou t by i nd epend ent certi fi cati on bod i es 6) Al l u sers shou l d ensu re that they have th e l atest ed i ti on of th i s pu bl icati on 7) N o l iabi l i ty shal l attach to I EC or i ts d i rectors, empl oyees, servants or agen ts i ncl u d i ng i nd i vi d u al experts and members of i ts techni cal commi ttees and I EC N ati onal Com mi ttees for any personal in j u ry, property d am age or other d amage of any n atu re wh atsoever, whether d i rect or i nd i rect, or for costs (i ncl u d i ng l egal fees) and expenses ari si ng ou t of the pu bl i cati on, u se of, or rel iance u pon, th i s I EC Pu bl i cation or any other I EC Pu bl i cati ons 8) Attenti on i s d rawn to the N ormati ve references ci ted i n thi s pu bl i cati on U se of the referenced pu bl i cati ons i s i nd i spensabl e for the correct appli cati on of thi s pu bl i cati on 9) Attention i s d rawn to the possi bi l i ty th at som e of the el ements of th is I EC Pu bl i cati on may be the su bj ect of patent rig hts I EC shal l not be hel d responsi bl e for i d enti fyi ng an y or al l su ch patent ri gh ts The main task of I EC technical committees is to prepare I nternational Standards I n exceptional circumstances, a technical committee may propose the publication of a Technical Specification when • • the required support cannot be obtained for the publication of an I nternational Standard, despite repeated efforts, or The subject is still under technical development or where, for any other reason, there is the future but no immediate possibility of an agreement on an I nternational Standard Technical specifications are subject to review within three years of publication to decide whether they can be transformed into I nternational Standards I EC 61 463, which is a Technical Specification, has been prepared by subcommittee 36A: I nsulated bushings, of I EC technical committee 36: I nsulators Copyright International Electrotechnical Commission I EC TS 61 463: 201 © I EC 201 –5– The text of this document is based on the following documents: En q u i ry d raft Report on voti ng 36A/1 78/DTS 36A/1 79/RVC Full information on the voting for the approval of this document can be found in the report on voting indicated in the above table This publication has been drafted in accordance with the I SO/I EC Directives, Part The committee has decided that the contents of this publication will remain unchanged until the stability date indicated on the I EC website under "http: //webstore iec ch" in the data related to the specific publication At this date, the publication will be • • • • • transformed into an I nternational standard, reconfirmed, withdrawn, replaced by a revised edition, or amended A bilingual version of this publication may be issued at a later date I M P O RTAN T th a t it – Th e co n ta i n s u n d e rs t a n d i n g c o l o u r p ri n t e r Copyright International Electrotechnical Commission of 'col ou r i n s i d e' c o l o u rs i ts wh i ch co n te n ts l ogo a re U s e rs on th e c o ve r p a g e c o n s i d e re d sh ou l d to t h e re fo re o f th i s be p ri n t p u b l i ca ti o n u s e fu l th i s fo r i n d i ca te s th e d ocu m en t c o rre c t u sing a –6– I EC TS 61 463: 201 © I EC 201 I NTRODUCTI ON As it is not always possible to define accurately the seismic severity at the bushing flange level, I EC TS 61 463, which is a Technical Specification, presents three alternative methods of qualification The three methods are equally acceptable I f the required response spectrum (RRS) at the bushing flange is not known, a severity (in terms of acceleration values) based on standard response spectra at the ground level may be used to carry out qualification through one of the three methods described in this document When the environmental characteristics are not sufficiently known, qualification by static calculation is acceptable Where high safety reliability of equipment is required for a specific environment, precise data are used, therefore qualification by dynamic analysis or vibration test is recommended The choice between vibration testing and dynamic analysis depends mainly on the capacity of the test facility for the mass and volume of the specimen, and, also if non-linearities are expected When qualification by dynamic analysis is foreseen, it is recommended that the numerical model be adjusted by using vibration data (see Clause 5) This document was prepared with the intention of being applicable to bushings whatever their construction material and their internal configuration The information contained, originally directed to porcelain bushings, has been partially updated to include also composite bushings Copyright International Electrotechnical Commission I EC TS 61 463: 201 © I EC 201 –7– B U S H I N G S – S E I S M I C Q U AL I F I C AT I O N Scope I EC TS 61 463, which is a Technical Specification, is applicable to alternating current and direct current bushings for highest voltages above 52 kV (or with resonance frequencies placed inside the seismic response spectrum), mounted on transformers, other apparatus or buildings For bushings with highest voltages less than or equal to 52 kV (or with resonance frequencies placed outside from the seismic response spectrum), due to their characteristics, seismic qualification is not used as far as construction practice and seismic construction practice comply with the state of the art This document presents acceptable seismic qualification methods and requirements to demonstrate that a bushing can maintain its mechanical properties, insulate and carry current during and after an earthquake The seismic qualification of a bushing is only performed upon request N o rm a t i v e re fe re n c e s The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document For dated references, only the edition cited applies For undated references, the latest edition of the referenced document (including any amendments) applies I EC 60068-2-47, Environmental testing – Part 2-47: Test – Mounting of specimens for vibration, impact and similar dynamic tests I EC 60068-2-57, Environmental testing – Part 2-57: Tests – Test Ff: Vibration – Time-history and sine-beat method IEC 60068-3-3: 991 , Environmental testing – Part 3-3: Guidance – Seismic test methods for equipments IEC 601 37, Insulated bushings for alternating voltages above 000 V I EC 61 462, Composite hollow insulators – Pressurized and unpressurized insulators for use in electrical equipment with rated voltage greater than 000 V – Definitions, test methods, acceptance criteria and design recommendations IEC 621 55 , Hollow pressurized and unpressurized ceramic and glass insulators for use in electrical equipment with rated voltages greater than 000 V IEC 6221 , Polymeric insulators for indoor and outdoor use – General definitions, test methods and acceptance criteria ISO 2041 , Mechanical vibration, shock and condition monitoring – Vocabulary T e rm s a n d d e fi n i t i o n s For the purposes of this document, the terms and definitions given in I EC 60068-3-3, I EC 601 37, I EC 61 462, I SO 2041 and the following apply Copyright International Electrotechnical Commission –8– I EC TS 61 463: 201 © I EC 201 I SO and I EC maintain terminological databases for use in standardization at the following addresses: • • I EC Electropedia: available at http: //www electropedia org/ I SO Online browsing platform: available at http: //www iso org/obp 3.1 critical cross-section section of the bushing that is most likely to fail during an earthquake 3.2 response spectrum plot of the maximum response to a defined input motion of a family of single-degree-offreedom bodies at a specified damping ratio [SOU RCE: I EC 60068-2-57: 201 3, 8, modified — The words "as a function of their natural frequencies and at a specified damping ratio" has been replaced by "at a specified damping ratio" ] 3.3 rigid equipment equipment whose natural frequency is greater than 33 H z is considered rigid for the purpose of this technical specification 3.4 standard frequency range predominant frequencies of a typical earthquake N ote to entry: Th i s range i s gen eral l y between 0, H z and 33 H z N ote to entry: Thi s range i s su ffi ci ent to d etermi n e the cri ti cal freq u enci es of the eq u i pment and for its testi ng I n certai n cases the test freq u ency rang e may be extend ed or red u ced d epend ent on the cri ti cal freq u enci es present, bu t thi s shal l be j u sti fi ed 3.5 zero period acceleration high frequency asymptotic value of acceleration of a response spectrum (above the cut-off frequency of 33 H z) N ote to entry: Thi s accel erati on correspond s to the maxi mu m accel eration of the ti me hi story u sed to d eri ve the spectru m a bg af ag d dp f0 Symbols and abbreviated terms equivalent maximum acceleration to the centre of gravity of the bushing during the seismic event maximum acceleration of the bushing flange maximum acceleration of the ground resulting from the motion of a given earthquake N OTE a g is eq u al to the zero period accel erati on (ZPA) of Fi gu re damping of the bushing distance between the centre of gravity of the part of the bushing which is under consideration and the critical cross-section first natural frequency of the bushing Copyright International Electrotechnical Commission – 28 – I EC TS 61 463: 201 © I EC 201 The actual seismic acceleration at the middle of the structure is, however, for cases of distributed mass, clamped and elastic base, not equal to the acceleration of the response spectrum but lower, typically in the range 0, to 0, of that acceleration level, depending on the shape of the lowest eigenmode For the acceleration at the tip of the structure, a value of about , times the response acceleration is appropriate (see the examples given in Table C1 ) The model of concentrated mass and elastic base needs a definition of the spring stiffness (rotational spring constant) (Table C ) This value however generally can not be easily estimated from the dimensions and weight of the bushing An experimental formula is reported below The lower portion of a bushing with cemented flange is composed by the porcelain, by the cement and by the metal flange (Figure C 2) M t h θ ød IEC Figure C.2 – Structure at the flange of a bushing with cemented porcelain [5] [7] This system is not rigid at it could appear, but the cement allows a certain elasticity if the bushing is subjected to a moment M As a consequence it is possible to define a spring stiffness C due to cementation (see for reference Figure C 2), by means of the following experimental formula: C = 66,7 × d × h t where C d h t is is is is the the the the spring stiffness, in MN m/rad; external diameter of the porcelain, in m; height of the metallic flange, in m; thickness of the cement, in m For a visual check, a diagram can be built with C as ordinate and the ratio dh /t as abscissa (Figure C 3) When this elastic phenomenon is not considered, generally the calculated natural frequency of the bushing shows a value slightly higher than real frequency Validity of this equation has been confirmed by a campaign of measurements carried out in J apan [5] [7] Copyright International Electrotechnical Commission C = 66, × dh / t (M N m/rad ) I EC TS 61 463: 201 © I EC 201 – 29 – 000 000 00 10 0, 1 10 00 dh / t (m ) IEC N OTE The ci rcl es are real cases Figure C.3 – Spring stiffness C in function of cemented part geometry [5] [7] C.6 Superelevation factor K The amplification factor at the top of the transformer body, were the bushing is fixed, depends on the existence of the transformer body itself and of its foundation (ref to and Figure 4) When f1 is the natural frequency of bushing-bushing turret system, f0 is the natural frequency of soil-foundation-transformer body system, the relation between the ratio f1 / f0 and the amplification or superelevation factor is presented as shown in Figure C These data come from J apanese measurements carried out on different transformers [5] [7] According to these results, the superelevation factor given by the system transformer body and foundation is of the order of , to 2, as stated in Copyright International Electrotechnical Commission Su perelevati on factor ( K) – 30 – I EC TS 61 463: 201 © I EC 201 2, 2, 550 kV transformers 300 kV transformers ,8 ,6 ,4 ,2 0, 0, 0, 0, 1 ,5 2, f1 / f0 IEC Figure C.4 – Superelevation factor due to the existence of transformer body and foundation [5] Copyright International Electrotechnical Commission – E x a m p l e s o f t yp i c a l s e i s m i c re s p o n s e s C h a c t e ri s t i c T y p e o f s t ru c t u re G e n e ri c E xa m p l e C o n c e n t t e d m ass, D i s t ri b u t e d e l a s ti c b a s e Centre of gravity hei g ht [E I EI , 53] 2, m 32, e l a s ti c b a s e = mp MNmm2 x % 2% Spri ng sti ffness C 4, M N m/rad mp dp dp Bend i n g sti ffness mass, / (CH ) 250 kg mp Dampi ng rati o D i s t ri b u t e d H Total mass dp m ass, base mp 5m H cl am ped dp Total hei g ht I EC TS 61 463: 201 © I EC 201 Tab l e C EI – 31 – EI C Ms Ms M ode N atu ral freq u ency ( fi ) [H z] 2π ( C mp x dp2 ( M ode H H mp ( 24, K × SA1 K × SA 24, Centre of g ravity accel erati on ( dp = H/2), [m /s ] 8, EI K × SA Fl ang e response spectru m = grou n d response spectru m × K (wh ere K = , 5) [m/s ] E xa m p l e 0,56 SA ξ1 , f1 SA ξ1 , f1 right International Electrotechnical Commission ) ) 3, 6, Grou nd response spectru m (spectral response accel eration, see Fi gu re 2, wi th fi and x %) a bg E xa m p l e 0, 53 ) K × SA1 M od e 3, 51 E xa m p l e 50, EI H Hm ( p 6, SA ξ , f2 24, K × SA2 2, Ms IEC IEC 0, 62 ) K × SA2 M od e 208 EI H H mp ( SA ξ1 , f1 7, K × SA1 4, 65 E xam p l e 0, 73 ) K × SA1 IEC M od e 2,544 H EI H mp E xa m p l e 36, 6, S A (ξ , f2 ) 24, K × SA2 7, 7, 0, 46 K × SA2 3, 45 C h a c t e ri s t i c T y p e o f s t ru c t u re G e n e ri c E xa m p l e C o n c e n t t e d m ass, D i s t ri b u t e d e l a s ti c b a s e _ Top of the beam accel erati on [m/s ] Ben d i ng moment at the base (cri ti cal crosssecti on ), Ms [N m ] 0, K SA m p H N OTE Formu l ae obtai ned by ri gorou s mathemati cal an al ysi s N OTE K cl am ped _ ,6 75 938 0, 45 K × SA1 K SA1 m p H m ass, D i s t ri b u t e d base 38, 0, 87 68 344 0, 04 [E I K × SA2 6, 53 , 51 K SA2 m p H 875 0, mass, / (CH ) = e l a s ti c b a s e , 53] K × SA1 36, 0, 76 K SA1 m p H 75 938 0, 005 K × SA2 5, K SA2 m p H 234 = Su perel evati on factor N OTE f1 an d f2 for the el asti c beam wi th el asti c base are, g eneral l y speaki n g, l ower than the correspon d i ng natu ral freq u enci es of the el asti c beam wi th cl amped base: th e hi gher the rati o EI /(CH ) i s, th e l ower are th e two freq u enci es f1 an d f2 (h ig h-base flexibi l ity m eans l ow valu es of C) The l owest val u e of f1 , correspond i ng to C = 0, i s zero: th e beam i s ri gi d l y rotati ng abou t its base; the lowest val u e of f2 i s obtai ned by su bsti tu ti ng 2, 55 wi th 2, 45 (abou t 70 % of th e cl amped base n atu ral freq u ency) N OTE SA = Spectral accel erati on [m/s ] N OTE Ei gen val u e eq u ati on of the mod el wi th d i stri bu ted mass and el asti c base i s the fol lowi ng : CH = – 32 – EI + cos λ H cosh λ H λ H ( sin λ H cosh λ H − sinh λ H cos λ H ) Wi th λ = ω2 ρ A EI ρ : mass density A : cross area fN = 1 ( λN H )2 H 2π EI HmP Fol l owin g the exampl e i n thi s tabl e, i t i s obtai ned etc right International Electrotechnical Commission EI/CH = , 53, that gi ves λ H = , 42, λ H = 3, 998, λ H = 7, 1 3, and conseq u ently f1 = 0.208 EI H Hm p f2 = 2, 544 EI H Hm p , I EC TS 61 463: 201 © I EC 201 When as sol u ti ons of thi s eq u ati on i t i s pu t λN H wi th N = , 2, 3… natu ral freq u enci es are expressed as foll ows I EC TS 61 463: 201 © I EC 201 – 33 – Annex D (informative) Qualification by static calculation – Example on transformer bushing D.1 Seismic ground motion I n all calculations of earthquakes affecting bushings, the vertical acceleration shall be applied downwards in the direction of the acceleration due to gravity This gives the greatest load on the bushing: m/s 2, m/s – horizontal ground acceleration, a gh , (ZPA): – vertical ground acceleration, a gv : D.2 Critical part of the bushing When a cantilever test is performed or during an earthquake, the most critical part of the bushing is at the insulator base The two major critical factors are the risk of oil leakage (see Figure D ) and the bending stress at the insulator base For this reason, the bending moments are calculated at the insulator base Leakage IEC Figure D.1 – Critical part of the bushing D.3 D.3.1 Static calculation General The transformer tank is very heavy compared to the bushings, but finite element (FEM) analysis shows that the transformer tank cannot be considered as rigid The ground acceleration is amplified through the transformer tank to the transformer tank cover with an amplification factor K Without background information, the amplification factor K is assumed to be , (see 2) I f the bushing is mounted on a turret, this can be considered rigid Therefore, the transformer tank cover and the turret are subjected to the same acceleration: – horizontal acceleration at the transformer tank cover/turret – vertical acceleration at the transformer tank cover/turret (K × a gh = K× ZPA): 7, m/s a gv ): 3, 75 m/s (K × The acceleration of the transformer tank cover will be amplified to the bushing with the response factor R The response factor depends on the natural frequency and the damping of the bushing mounted on the transformer tank cover The value of the response factor R is taken from Figure Copyright International Electrotechnical Commission – 34 – I EC TS 61 463: 201 © I EC 201 I f the response factor R cannot be estimated, the conservative value of the response factor at a certain value of damping is used For a bushing mounted on a transformer tank cover, a damping ratio of % can be assumed: – natural frequency for bushing mounted on the transformer tank cover (H z): – damping ratio for bushing mounted on the transformer tank cover: – response factor R , taken from Figure (conservative value): unknown 5% 2, The response is then multiplied by a coefficient, Sc , which takes into account both multifrequency excitation and multimode response The conservative value of the coefficient is , The acceleration of the transformer tank cover, the response factor of the bushing mounted on the transformer tank cover, the static coefficient and the air side mass, m p, of the bushing give rise to a force that affects the bushing at the air side centre of gravity (see D ) I f the bushing is mounted at angles to the vertical plane, both the vertical and the horizontal parts of the earthquake will affect the bushing D.3.2 Seismic load Fti p dti p dh Fh dp Fw Fb Fa Fv α IEC Figure D.2 – Forces affecting the bushing I n these seismic calculations, the vertical acceleration is applied downwards, in the same direction as the acceleration due to gravity This produces the greatest load on the bushing The air side mass of the bushing, m p , is the mass of all the parts of the bushing above the bushing flange Copyright International Electrotechnical Commission I EC TS 61 463: 201 © I EC 201 – 35 – is the distance from the critical part of the bushing flange to the air side centre of gravity (see Figure D 2): dp – – – – – – – – air side mass, m p : dp : mounting angle to the vertical plane, α : horizontal force, Fh , vertical force, Fv , (mp compressive force, Fa , bending force, Fb , bending moment due to the seismic event and gravity, Mbs , D.3.3 (mp × K × a gh × R × Sc ): × K × a gv × R × Sc + m p × g): (– Fh ( Fh × sin α + Fv × cos α ): × cos α + Fv × sin α ): ( Fb × dp ): 63 kg 590 mm 20° 772 N 51 N 81 N 21 84 N , 29 kN m Wind load Wind loads are considered as static loads As a combination of the extreme values of all electrical and environmental service loads would lead to unrealistic conservatism, a wind pressure of 70 Pa acting at the same time as an earthquake should be assumed The resulting wind force ( Fw ) affects the bushing in its air side centre of gravity (see Figure D 3): De Di IEC Figure D.3 – Porcelain diameters – – – – wind pressure, p : outer diameter of the porcelain sheds, D e , see Figure D 3: outer diameter of the porcelain core, D i , see Figure D 3: distance from the critical cross-section to the top of the bushing, dh , see Figure D 2: ( p × ( D e + D i )/2 × – wind force, Fw , see Figure D 2, – bending moment due to the wind, Mbw , ( Fw × D.3.4 70 Pa 280 mm 50 mm dh ): dp ): 205 mm 8, N 0, 01 kN · m Terminal load The tip load at an earthquake event is equal to 70 % of the cantilever operating load specified for the bushing according to 1 : – cantilever operating load, taken from I EC 601 37 ( Ur = 70 kV, Ir = 250 A, Class I ), Fop : – tip load at the terminal, Fti p , see Figure D 2, – distance from the critical cross-section to the terminal, dti p , see Figure D 2: – bending moment due to the tip load, Mbti p , Copyright International Electrotechnical Commission ( Fop ( Fti p 0, 7): 800 N 560 N × dti p ): 325 mm 0, 74 kN m × – 36 – D.4 I EC TS 61 463: 201 © I EC 201 Gu aranteed bendin g strength The bushing must withstand a cantilever test load in accordance with I EC 601 37 without leakage or damage The bending moment occurring during this test should be compared with the total bending moment occurring at the critical cross-section due to the seismic, wind, terminal loads and the effect of gravity: – cantilever withstand load, Ftest : – bending moment occurring under cantilever test: – total bending moment occurring during the seismic event: ( Mbs + ( Ftest × dtip ) Mbw + Mbti p ) 600 N 2 kN m 2, 04 kN m Result of qualification: The bending strength is greater than the stress during the specified seismic event The bushing is therefore qualified As an alternative, it can be compared the mechanical stress (moment due to cantilever) at which the insulator mounted on the bushing has been subjected during its type tests with the one above found, which must be lower Copyright International Electrotechnical Commission I EC TS 61 463: 201 © I EC 201 – 37 – Annex E (informative) Center clamped bushings The external insulator of a traditional oil-to-air bushing for transformer can be of four types: composite, porcelain cemented to the flange, porcelain mechanically clamped to the flange and porcelain center clamped Figure E shows the typical structure of a center clamped bushing The porcelain is compressed by the axial clamping force of the springs placed in the bushing head that act on the pre-tensioned central tube or rod of the bushing The lower end of the insulator is not fixed to the flange, as for the other types, and generally a gasket is inserted between the lower end of the porcelain and the flange When a lateral increasing force is applied, the bending moment increases up to a value that exceeds the opposite resistance moment given by the axial compression force of the springs, and the porcelain starts to uplift from its end fitting When this opening process is only small, in case of an oil impregnated paper bushing, some oil starts to exit and when the bending force decreases the bushing recloses without damages (the nonmetallic gasket normally placed between porcelain and end fitting protects in this case from ruptures), but when the opening process is more evident the bushing remains damaged (gasket extrusion or even worse porcelain breaking) The failure process of a center clamped bushing is typical of this type of bushing and can be schematized in the following steps (make reference to Figure E and to the following flow chart of Figure E 2), depending on the lateral force entity: d) e) f) g) opening starts and in presence of oil there is a slight spilling; slippage of porcelain and extrusion of the gasket; increase of tensile stress at the lower end of porcelain; cracking of the edge of the porcelain Spri ngs Cl ampi ng force Tensi l e stress (bottom of porcel n) Sei sm i c force Sei smi c force Fai l u re pattern Fl an ge Oi l l ea kege Compressi on stress (si d e of porcel ain ) Gasket IEC Figure E.1 – Failure process [6] Copyright International Electrotechnical Commission – 38 – I EC TS 61 463: 201 © I EC 201 Cen ter cl amped bu shi ng I npu t earthq u ake wave Porcel n -fl ang e j oi nt opens Cl ampi ng force Cl ampi n g force concen trated i n the porcel n ed ge i n tact wi th gasket Thi ckness of gasket Ri gi d i ty of sl eeve-fl ange Tensi l e stress takes pl ace i n th e edge of the porcel ain d u e to concen trated l oad i ng M axi mu m tensi l e stress exceed s the li m i t val u e Breakd own starts from the l ower ed ge of porcel n IEC Figure E.2 – Failure process, flow chart [5] [6] The profile of the stresses, compression stress ( σ c) and tensile stress ( σ t), during the opening phase of the insulator is qualitatively indicated in Figure E Porcel n Porcel n Gasket Sl eeve flange σC σT Gasket Sl eeve fl ange IEC Figure E.3 – Stress profile during the opening process [6] Following to some experimental and FEM studies, it has been found a dependence between the compression stress and the tensile stress in the edge of the porcelain part due to the concentrated loding A diagram is shown in Figure E [6] Copyright International Electrotechnical Commission Compressi on stress σ c (N /m m ) I EC TS 61 463: 201 © I EC 201 – 39 – 280 240 200 60 20 σ t/ σ c = 0, 45 Test d ata 80 40 FEM an al ysi s 30 40 50 60 70 80 90 00 1 20 Tensi l e stress σ t (N /mm ) IEC Figure E.4 – Relation between compression and tensile stress in the bottom edge of the porcelain due to the opening process [6] This relation can be useful because is possible to measure the compression stress with strain gauges, and calculate consequently the tensile stress Special consideration shall be paid for the seismic evaluation of this kind of bushings, because due to their structure they show typical non-linear phenomena, to be taken into account during the vibrational tests The following three types of non-linear phenomena are observed ) N atural frequency shifts to low frequencies when increasing the input level As a consequence, when a sinusoidal wave as sine-beat with natural frequency is used, an accurate searching of maximum response will be necessary in order to calibrate the exciting frequency 2) Vibration mode changes from simply bending to a rotation + bending The rotation is consequence of the opening phenomenon of the porcelain This leads to a behavior no more axisymmetric 3) The response of the bushing shows non-linear characteristic when the input level is increased Copyright International Electrotechnical Commission – 40 – I EC TS 61 463: 201 © I EC 201 Bibliography [1 ] [2] I EC 60721 -2-6, Classification of environmental conditions – Part 2-6: Environmental conditions appearing in nature – Earthquake vibration and shock I EC 60068-2-6: 2007, Environmental testing – Part 2-6: Tests – Test Fc: Vibration (sinusoidal) [3] I EC 62271 -207: 201 2, High-voltage switchgear and controlgear – Part 207: Seismic [4] I EEE Std 693-2005, IEEE Recommended Practice for Seismic Design of Substations [5] J EAG 5003-201 0, Guideline for Seismic Design for Electric Facilities in Substations [6] Denki Kyodo Kenkyu (Electric Cooperative Research Report), Vol 34-3 (J apan): [7] [8] qualification for gas-insulated switchgear assemblies for rated voltages above 52 kV (Japan) Seismic Design for Substation Equipment Denki Kyodo Kenkyu (Electric Cooperative Research Report), Vol 38-2 (J apan): Seismic Design for Transformer Bushings I EC 62231 , Composite station post insulators for substations with a c voltages greater than 000 V up to 245 kV – Definitions, test methods and acceptance criteria _ Copyright International Electrotechnical Commission Copyright International Electrotechnical Commission I N TE RN ATI O N AL E LE CTRO TE CH N I CAL CO M M I S S I O N 3, ru e d e Va re m bé P O B ox CH -1 1 G e n e va S wi tze rl a n d Te l : + 41 F a x: + 22 91 02 1 22 91 03 00 i n fo @i e c ch www i e c ch Copyright International Electrotechnical Commission