IEC 62979 Edition 1 0 201 7 08 INTERNATIONAL STANDARD Photovoltaic modules – Bypass diode – Thermal runaway test IE C 6 2 9 7 9 2 0 1 7 0 8 (e n ) ® Copyright International Electrotechnical Commission[.]
I E C 62 9 ® I N TE RN ATI ON AL S TAN D ARD IEC 62979:201 7-08(en) P h oto vol tai c m od u l es – B ypas s d i od e – Th erm al ru n away te s t Copyright International Electrotechnical Commission Edition 201 7-08 T H I S P U B L I C AT I O N I S C O P YRI G H T P RO T E C T E D C o p yri g h t © I E C , G e n e v a , S wi tz e 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 I EC copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or your local I EC 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 Ab ou t th e I E C The I nternational Electrotechnical Commission (I EC) is the leading global organization that prepares and publishes I nternational Standards for all electrical, electronic and related technologies Ab o u t I E C p u b l i ca 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 Catal og u e - webstore i ec ch /catal og u e 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 I E C pu bl i cati on s s earch - www i ec ch /search pu b 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variety of criteria (reference number, text, technical committee,…) It also gives information on projects, replaced and withdrawn publications E l ectroped i a - www el ectroped i a org The world's leading online dictionary of electronic and electrical terms containing 20 000 terms and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 G l os sary - s td i ec ch /g l oss ary 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 Pu bl i s h ed - webstore i ec ch /j u stpu bl i sh 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 stom er S ervi ce C en tre - webstore i ec ch /csc 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 62 9 ® Edition 201 7-08 I N TE RN ATI ON AL S TAN D ARD P h oto vol tai c m od u l es – B ypas s d i od e – Th erm al ru n away te s t INTERNATIONAL ELECTROTECHNICAL COMMISSION ICS 27.1 60 ISBN 978-2-8322-4587-3 Warn i n g ! M ake s u re th a t you ob tai n ed th i s p u b l i cati on from an au th ori zed d i stri b u tor ® Registered trademark of the International Electrotechnical Commission Copyright International Electrotechnical Commission –2– I EC 62979: 201 © I EC 201 CONTENTS FOREWORD I NTRODUCTI ON Scope Norm ative references Terms and definitions Thermal runaway test Diode thermal runaway Test conditions Preparation of test specimen 4 Test equipment Test procedure Pass or fail criteria Test report Figure Figure Figure Figure Figure Figure – I llustration of how therm al runaway occurs – Circuit for m easurem ent of Tlead and forward voltage – Circuit for flowing a forward current to the bypass diode – Circuit for appl ying a reverse bias voltage to the bypass diode – The typical pattern of therm al runaway 1 – The pattern of non-therm al runaway 1 Copyright International Electrotechnical Commission I EC 62979: 201 © I EC 201 –3– INTERNATI ONAL ELECTROTECHNI CAL COMMISSI ON P H O T O VO L T AI C M O D U L E S – B YP AS S D I O D E – T H E RM AL RU N AW AY T E S T FOREWORD ) The I nternati on al Electrotechni cal Comm ission (I EC) is a worl d wid e organization for stan dardization com prisin g all n ation al el ectrotechnical comm ittees (I EC National Comm ittees) The object of I EC is to prom ote internati onal co-operation on all questions concerni ng stand ardi zati on in the el ectrical an d electronic fields To this end and in additi on to other acti vities, I EC publish es I nternational Stan dards, Techn ical Specifications, Technical Reports, Publicl y Avail abl e Specificati ons (PAS) an d Gu ides (h ereafter referred to as “I EC Publication(s)”) Thei r preparation is entrusted to technical comm ittees; any I EC National Comm ittee interested in the subj ect dealt with m ay partici pate in this preparatory work I nternational, governm ental an d n on governm ental organ izations l iaising with th e I EC also participate i n this preparation I EC collaborates closel y with the I ntern ational Organi zation for Stand ardization (I SO) in accordance with ditions determ ined by agreem ent between th e two organi zati ons 2) The form al decisions or ag reem ents of I EC on tech nical m atters express, as n early as possible, an i nternati onal consensus of opi nion on the rel evant subjects since each technical com m ittee has representati on from all interested I EC N ational Com m ittees 3) I EC Publications have the form of recomm endations for intern ational use an d are accepted by I EC National Com m ittees in that sense While all reasonable efforts are m ade to ensure that the tech nical content of I EC Publications is accu rate, I EC cann ot be h eld responsi ble for th e way in which th ey are used or for an y m isinterpretation by an y en d u ser 4) I n order to prom ote intern ational u niform ity, I EC National Com m ittees und ertake to apply I EC Publications transparentl y to the m axim um extent possible i n their national an d regi on al publicati ons Any divergence between an y I EC Publication and the correspondi ng national or regi on al publicati on sh all be clearl y in dicated in the latter 5) I EC itself d oes n ot provi de an y attestation of conform ity I n dependent certificati on bodies provi de conform ity assessm ent services and, in som e areas, access to I EC m arks of conform ity I EC is not responsi ble for any services carri ed out by ind ependent certification bodi es 6) All users shou ld ensure that th ey have the l atest editi on of thi s publicati on 7) No liability shall attach to I EC or its directors, em ployees, servants or ag ents inclu din g in divi dual experts an d m em bers of its technical com m ittees and I EC Nati on al Com m ittees for any person al i njury, property d am age or other dam age of any nature whatsoever, wheth er di rect or indirect, or for costs (includ i ng leg al fees) and expenses arisi ng out of the publ ication, use of, or relian ce upon, this I EC Publicati on or any other I EC Publications 8) Attention is drawn to th e N orm ative references cited in th is publ ication Use of the referenced publ ications is indispensable for the correct applicati on of this publication 9) Attention is drawn to the possibility that som e of the elem ents of this I EC Publication m ay be the su bject of patent rig hts I EC shall not be held responsibl e for identifyi ng any or all such patent ri ghts I nternational Standard I EC 62979 has been prepared by I EC technical committee 82: Solar photovoltaic energ y systems The text of this I nternational Standard is based on the following docum ents: FDI S Report on votin g 82/1 269/FDI S 82/1 31 /RVD Full information on the voting for the approval of this I nternational Standard can be found in the report on voting indicated in the above table This docum ent has been drafted in accordance with the I SO/I EC Directives, Part Copyright International Electrotechnical Commission –4– I EC 62979: 201 © I EC 201 The comm ittee has decided that the contents of this docum ent will rem ain unchanged until the stability date indicated on the I EC website under "http://webstore iec.ch" in the data related to the specific document At this date, the docum ent will be • • • • reconfirmed, withdrawn, replaced by a revised edition, or amended A bilingual version of this publication may be issued at a later date Copyright International Electrotechnical Commission I EC 62979: 201 © I EC 201 –5– INTRODUCTION During the norm al operation of PV m odules the bypass diodes are reverse biased When the PV m odule is partiall y shaded (for exam ple by utility poles, buildings, or leaves), some of the cells in the PV module m ay not be able to produce the current being produced by the other cells in the series string The shaded cells are then driven into reverse bias so the bypass diode of the shaded cell-string becomes forward bias protecting the shaded cells Under these circum stances, the tem perature of the bypass diode increases due to the forward current flowing through the diode I t is in this condition that the diodes are tested in accordance with I EC 61 21 5-2: 201 6, 4.1 : Bypass diode thermal test When the shade is rem oved, operating conditions return to norm al and the bypass diode is again reversed biased Som e of the diodes utilized as bypass diodes in PV m odules have characteristics where the reverse bias leakage current increases with the diode tem perature So if the diode is alread y at an elevated temperature when reverse biased, there will be a substantial leakage current and the diode j unction tem perature can increase considerabl y The worst case occurs when this heating exceeds the cooling capability of the junction box in which the diode is installed As a result of this increasing tem perature and leakage current, the diode can break down These phenom ena are called “thermal runaway” The thermal design of the bypass diode in the j unction box shall be verified to ensure that thermal runaway does not occur Copyright International Electrotechnical Commission –6– I EC 62979: 201 © I EC 201 P H O T O VO L T AI C M O D U L E S – B YP AS S D I O D E – T H E RM AL RU N AW AY T E S T S cop e This document provides a method for evaluating whether a bypass diode as m ounted in the module is susceptible to therm al runaway or if there is sufficient cooling for it to survive the transition from forward bias operation to reverse bias operation without overheating This test methodology is particularl y suited for testing of Schottky barrier diodes, which have the characteristic of increasing leakage current as a function of reverse bias voltage at high tem perature, m aking them m ore susceptible to thermal runaway The test specim ens which em ploy P/N diodes as bypass diodes are exempted from the thermal runaway test required herein, because the capability of P/N diodes to withstand the reverse bias is sufficiently high N o rm a t i ve re fe re n c e s The following docum ents are referred to in the text in such a way that som e or all of their content constitutes requirements of this docum ent For dated references, onl y the edition cited applies For undated references, the latest edition of the referenced docum ent (including an y am endm ents) applies I EC TS 61 836, Solar photovoltaic energy systems – Terms, definitions and symbols T e rm s a n d d e fi n i t i o n s For the purposes of this document, the term s and definitions given in I EC TS 61 836 as well as the following appl y ISO and I EC m aintain 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 • • re v e rs e c u rre n t current flowing in the opposite direction to the polarity of the bypass diode re v e rs e b i a s v o l t a g e voltage applied to the opposite direction to the polarity of the bypass diode 3 T l ead tem perature of the lead-wire of the bypass diode measured by thermocouple Copyright International Electrotechnical Commission I EC 62979: 201 © I EC 201 4 –7– T h e rm a l ru n a w a y t e s t D i o d e t h e rm a l ru n a w a y Some of the diodes utilized as bypass diodes in PV m odules have characteristics where the reverse bias leakage current increases with the diode temperature So if the diode is alread y at an elevated tem perature when reverse biased, there m ay be a substantial reverse current and the diode j unction temperature can increase considerabl y The worst case occurs when this heating exceeds the cooling capability of the junction box in which the diode is installed As a result of this increasing tem perature and leakage current, the diode can break down These phenomena are called “therm al runaway” The therm al design of the bypass diode in the j unction box shall be verified to ensure that thermal runaway does not occur How the thermal runaway does or does not occur is illustrated simpl y in Figure The curve R indicates the relation of the power inj ected by the reverse bias voltage versus the junction tem perature As shown, the power injected will rapidl y increase at the higher j unction temperature The cooling capability of the junction box is indicated by the curve “H eat dissipation” and the critical tem perature TC is the crossing point of the curve R and the curve “Heat dissipation” Therm al runaway d oes not occur when TF < TC TF1 : Equili bri um temperatu re at IF1 Power inj ected or dissipated (W) F1 : I njected power by forward current IF1 versus Tj TF2 : Equili bri um temperatu re at IF2 F2: I njected power by forward current IF2 versus Tj TC : Critical Heat d issipation: Coolin g capability of junction box versus Tj u nd er certai n environm ental tem peratu re temperature R: I njected power by reverse bias voltage Vr versus Tj J unction tem perature (° C) IEC F i g u re – I l l u s t t i o n o f h o w t h e rm a l ru n a w a y o c c u rs I f the reverse bias voltage is applied at a junction temperature higher than the critical tem perature TC , the injected power will be m ore than the cooling capability and the junction tem perature will keep increasing until the diode undergoes thermal runaway Copyright International Electrotechnical Commission –8– I EC 62979: 201 © I EC 201 On the other hand, if the reverse bias voltage is applied at a junction tem perature lower than the critical temperature TC , the injected power will be less than the cooling capability and the junction temperature will graduall y decrease toward the environm ental temperature The curves F1 and F2 show the relationship of the power inj ected by the forward current IF1 and IF2 versus the junction tem perature The crossing points of these curves and the cooling capability “H eat dissipation” show the equilibrium tem perature when the forward current is applied The equilibrium temperature TF1 corresponding to the curve F1 is higher than TC and the therm al runaway m ay occur when the diode is reverse biased The equilibrium temperature TF2 corresponding to the curve F2 is lower than TC and the thermal runaway will not occur when the diode is reverse biased Test d i ti on s The test conditions under which the therm al runaway test should be perform ed are as follows: a) I nitial module tem perature: (90 ± 2) °C Modules that carry a label that says “For use in open rack mount onl y” may be tested at a reduced tem perature of (75 ± 2) °C As the occurrence of thermal runaway is related to the temperature at the instance of the reverse bias voltage application, the therm al runaway test is to be perform ed under the highest environm ental temperature the m odule could encounter during the norm al operation The module tem perature may be m easured by Tlead b) Specified forward current: , 25 × “Short circuit current ( ISC ) at STC” of the PV module for the bypass diode to be tested c) Specified reverse bias voltage: Open circuit voltage ( VOC ) at STC of the cell string of the module protected by the bypass diode to be tested Preparati on of test speci m en The test specimen should be the actual module or the special sample having the sam e construction of the actual m odule I n order to perform the test by using a reasonabl e sized heat chamber, special samples m ay be used The special sam ple m eans the junction box including bypass diodes bonded by an adhesive onto a suitable glass-substrate lam inated with the back-sheet in order to simulate the actual m odule Because the occurrence of therm al runaway depends upon cooling of the bypass diode, the test shall be performed with the diode m ounted in the same way as in the actual module The special sam ple m ay be provided by the m odul e or junction box m anufacturer I n case an actual m odule is used, the cell strings should be electricall y disconnected from the bypass diodes The test specim en shall be provided with the connection cables for the test m odule I n order to measure Tlead and voltage of each bypass diode, connections of the lead-wires and thermocouples are required to be provided wi th the test specimen as shown in Figure Thermocouple should be mounted on the cathode lead as close as possible to the diode bod y Copyright International Electrotechnical Commission I EC 62979: 201 © I EC 201 –9– Care should be taken to m inim ize an y alteration of the properties of the diode or its heat transfer path Therm ocouple +V1 –V1 Therm ocouple +V2 –V2 +I Therm ocouple n –Vn +Vn –I J-Box IEC Figure – Circuit for measurement of Tlead and forward voltage 4.4 Test equipment a) Cham ber for heating the module to a temperature of (90 ± 2) °C b) Means for monitoring the temperature of the cham ber to an accuracy of ± 2,0 °C and repeatability of ± 0,5 °C c) Thermocouples and means for recording the Tl ead of the test specim en to an accuracy of ± °C Comm onl y used T-type thermocouple (copper-constantan) with soldering joint is perm issible for this test, however it has a limitation since the highest m easurable tem perature is at 200 °C to 250 °C, which would be above the observed tem perature TC (critical temperature) When a therm al runaway occurs, the tem perature will likel y go up beyond the therm ocouple limitation, but by m easuring the reverse current flowing through the diode the therm al runaway phenomena will still be caught d) Means for appl ying the forward current specified in b) Means for monitoring the forward current through the m odule and the forward voltage of the diode selected for the test, throughout the test e) Means for applying the reverse bias voltage specified in c) to the bypass diode with capability of suppl ying the current equal to , 25 x ISC of the test m odule under the specified reverse voltage Means for measuring the leakage current and the reverse voltage of the bypass diode f) Means for making the swift switching (wi thin m s) from forward current injection to reverse bias voltage application as illustrated in the test circuit of Figure and Figure The equipm ent shall be designed so that harmful voltage peaks are avoided Copyright International Electrotechnical Commission – 10 – I EC 62979: 201 © I EC 201 High tem perature ch am ber J-box ❷ Measurem ent of diod e Vf and Vr Measurem ent of tem peratu re ❶ Voltage source, Vr If Current source, I f ❶ Forward cu rrent test circuit ❷ Reverse bias test circuit IEC Figure – Circuit for flowing a forward current to the bypass diode High temperatu re chamber J-box Ir Measurem ent of tem peratu re Measurem ent of diod e Vf and Vr ❷ ❶ Voltage source, Vr Current source, I f ❶ Forward cu rrent test circuit ❷ Reverse bi as test circuit IEC Figure – Circuit for applying a reverse bias voltage to the bypass diode 4.5 Test procedure a) To obtain initial characteristics of bypass diode and to m ake sure that bypass diode functions correctly, measure the reverse characteristic including reverse current at specified reverse voltage in c) at room temperature (25 ± 5) °C b) For the selection of the bypass diode to be tested, appl y the specified forward current (4 b) to all the bypass diodes in series in the test specimen at (25 ± 5) °C Select the bypass diode which shows the highest tem perature I n case that the diodes are mounted som ewhere else – like in the laminate and so on, the bypass diode having the highest tem perature should be tested c) After putting the test specim en(s) with necessary m easuring and m onitoring equipm ent into the test cham ber, heat them to the initial m odule temperature specified in 4.2 a) The consideration should be taken to minim ize the effect of the air flow to the test specim en in the chamber Copyright International Electrotechnical Commission I EC 62979: 201 © I EC 201 – 11 – d) Appl y the specified forward current (4 b) to the bypass diodes for at least 40 m in and until the range of Tl ead change during m in becomes within 0,3 °C Shut-off the forward current Within ms after that appl y the reverse bias voltage specified in c) to the bypass diode to be tested Continue to observe the reverse current and tem perature of the reverse biased bypass diode I n m ost of the cases the reverse current and the Tlead are expected either to rise (as indicated in the Figure 5) or to decrease (as indicated in the Figure 6) soon without staying stead y I n borderline cases som e time is needed to see the final direction of the change I n such a case, the test should be continued for at least I f the reverse current increases up to current limit of the reverse bias power suppl y, it is regarded as therm al runaway occurred and the reverse bias application m ay be stopped immediately Tl ead (° C) I R (A) Tl ead Reverse current (I R) Application of reverse bias voltage 40 m in or m ore Tim e Several seconds IEC Fi gu re – Th e typi cal pattern of th erm al ru n away Tl ead (° C) I R (A) Tl ead Application of reverse bias voltage 40 m in or m ore Tim e Reverse current (I R) Several seconds IEC Fi gu re – Th e pattern of n on -th erm al ru n away e) Remove the test specimen from the cham ber f) I n order to check the diode performance after test, allow the sam ple to cool and m easure the reverse characteristics of the bypass diode including reverse current at specified reverse voltage (4 c)) at room tem perature (25 ± 5) °C Copyright International Electrotechnical Commission – 12 – I EC 62979: 201 © I EC 201 Then com pare the reverse current measured after test with the initial measurement taken during step 4.5 a) Pass or fai l cri teri a a) If Tlead and the reverse current decrease during the reverse bias voltage portion of the test (second paragraph of step 4.5 d)), and if the reverse current measured after the test during step 4.5 f) is not more than times of the initial value measured during step 4.5 a), the bypass diode is considered to have passed the test b) If Tlead and the reverse current increase during the reverse bias vol tage portion of the test (second paragraph of step 4.5 d)), or if the reverse current measured after the test during step 4.5 f) is more than times of the initial value measured during step 4.5 a), the bypass diode is considered to have failed the test Test report The report shall contain the inform ation necessary to reproduce test results and the details of the sample tested, specificall y, m ake note of the following: a) b) c) d) e) f) g) h) i) a title; nam e and address of the test laboratory and location where the tests were carried out; unique identification of the report and of each page; name and address of client, where appropriate; detail specification, description and identification of the test specim en; date of receipt of test item and date(s) of test, where appropriate; identification of test method used and test instrum ents and other equipment used; reference to sam pling procedure, where relevant; the values of the specified test conditions with an y deviations from, additions to, or exclusions from, the test method and an y other information relevant to a specific test, m easurem ents, exam inations and derived results supported by tables, graphs, sketches and photographs as appropriate including: – The initial module temperature used for the test – Specified forward current (4 b)) – Specified reverse bias voltage (4 c)) – Forward current inj ected for bypass diode – The Tlead measured after appl ying the forward current until Tl ead stabilises – The duration for which the forward current was applied – The switching time until appl ying reverse bias voltage from the shut-off of forward current – Reverse bias voltage applied for bypass diode – The application tim e of specified reverse bias voltage – Records of the Tlead after the application of reverse bias voltage – Records of the leakage current of the diode after the application of reverse bias voltage – Photos and a description of the specimens tested – Diode characteristics measured before and after the therm al runaway test; j) a statem ent of the estimated uncertainty of the test results (where relevant); k) a signature and title, or equivalent identification of the person(s) accepting responsibility for the content of the certificate or report, and the date of issue; Copyright International Electrotechnical Commission I EC 62979: 201 © I EC 201 – 13 – l) where relevant, a statem ent to the effect that the results relate onl y to the item s tested; m) a statem ent that the certificate or report should not be reproduced except in full, without the written approval of the laboratory _ Copyright International Electrotechnical Commission 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 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