IEC TS 61 724 2 Edition 1 0 201 6 1 0 TECHNICAL SPECIFICATION Photovoltaic system performance – Part 2 Capacity evaluation method IE C T S 6 1 7 2 4 2 2 0 1 6 1 0 (e n ) ® THIS PUBLICATION IS COPYRIGH[.]
I E C TS 61 4-2 ® TE C H N I C AL S P E C I F I C ATI ON Ph otovol tai c s ys tem perform an ce – IEC TS 61 724-2:201 6-1 0(en) Part : C apaci ty eval u ati on m eth od Edition 201 6-1 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variety of criteria (reference number, text, technical committee,…) It also gives information on projects, replaced and withdrawn publications 65 000 electrotechnical terminology entries in English and French extracted from the Terms and Definitions clause of IEC publications issued since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 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 4-2 ® Edition 201 6-1 TE C H N I C AL S P E C I F I C ATI ON Ph otovol tai c s ys tem perform an ce – P art : C apaci ty eval u ati on m eth od INTERNATIONAL ELECTROTECHNICAL COMMISSION ICS 27.1 60 ISBN 978-2-83223-664-2 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 –2– I EC TS 61 724-2: 201 © I EC 201 CONTENTS FOREWORD I NTRODUCTI ON Scope Normative references Terms and definitions Test scope, schedule and duration Equipment and measurements 1 Procedure Documentation of the performance targets under “unconstrained” and “constrained” operation 1 General Definition of test boundary to align with intended system boundary Definition of the reference conditions for “unconstrained” operation Definition of the performance target under “unconstrained” and “constrained” operation Definition of the temperature dependence of the plant output under “unconstrained” operation 6 Definition of irradiance dependence Definition of the performance target under “constrained” operation Uncertainty definition Measurement of data 2.1 General 2.2 Data checks for each data stream 2.3 Shading of irradiance sensor 6 2.4 Calibration accuracy 6 2.5 Using data from multiple sensors 6 2.6 Unconstrained operation and constrained operation when the output limit of the inverter is reached Calculation of correction factor 3.1 General 3.2 Measure inputs 3.3 Verify data quality 3.4 Calculate the correction factor for each measurement point 3.5 Correct measured power output 3.6 Average all values of corrected power 3.7 Analyse discrepancies Comparison of measured power with the performance target Uncertainty analysis Test procedure documentation 20 Test report 21 Annex A (informative) Example of model for module temperature calculations 22 A General 22 A Example heat transfer model to calculate expected cell operating temperature 22 Annex B (informative) Example of model for system power 25 B General 25 I EC TS 61 724-2: 201 © I EC 201 –3– Example model 25 B Annex C (informative) Inconsistent array orientation 26 Bibliography 27 Table – Data validation and filtering criteria Table – Example guide for seasonal minimum stable irradiance requirements for flatplate applications Table A.1 – Empirically determined coefficients used to predict module temperature 23 Table A.2 – Hellmann coefficient, α for correction of wind speed according to measured height, if values in Table A are used 23 , –4– I EC TS 61 724-2: 201 © I EC 201 I NTERNATIONAL ELECTROTECHNI CAL COMMI SSI ON P H O T O VO L T AI C S YS T E M P E RF O RM AN C E – P a rt : C a p a c i t y e v a l u a t i o n m e th o d FOREWORD ) The I ntern ati onal El ectrotechnical Commi ssi on (I EC) is a worl d wid e org anizati on for standard izati on comprisi ng all nati onal electrotech nical committees (I EC N ational Comm ittees) The object of I EC i s to promote i nternati on al co-operati on on al l q u esti ons cernin g standard izati on i n the el ectrical and el ectronic fi el ds To this end an d in ad di ti on to other acti vi ti es, I EC pu blish es I nternati onal Stan d ards, Technical Speci fi cati ons, Technical 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that they h ave the l atest edi ti on of this pu blicati on 7) N o li abili ty shal l attach to I EC or i ts di rectors, empl oyees, servan ts or agents in clu di ng i nd ivi du al experts and members of i ts technical commi ttees and I EC N ati onal Comm ittees for any personal i nju ry, property d amage or other d amage of any natu re whatsoever, wh eth er di rect or i nd i rect, or for costs (i nclud i ng l egal fees) an d expenses arising ou t of the pu bli cati on, use of, or rel iance u pon, this I EC Pu bl ication or an y oth er I EC Pu blicati ons 8) Attention is d rawn to the N orm ative references cited i n this pu bl icati on U se of the referenced pu blicati ons is i ndi spensabl e for the correct appli cati on of this publicati on 9) Attention is d rawn to th e possibili ty that some of the el ements of thi s I EC Pu bl icati on may be the su bj ect of patent ri ghts I EC sh al l not be held responsi bl e for i d en ti fyi ng any or all su ch patent ri ghts The main task of I EC technical committees is to prepare International 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 International 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 TS 61 724-2, which is a technical specification, has been prepared by I EC technical committee 82: Solar photovoltaic energy systems I EC TS 61 724-2: 201 © I EC 201 –5– The text of this technical specification is based on the following documents: En q ui ry d raft Report on voti ng 82/1 01 /DTS 82/1 59/RVC Full information on the voting for the approval of this technical specification can be found in the report on voting indicated in the above table This document has been drafted in accordance with the ISO/I EC Directives, Part A list of all parts in the I EC 61 724 series, published under the general title Photovoltaic system performance , can be found on the I EC website 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 International standard, reconfirmed, withdrawn, replaced by a revised edition, or amended A bilingual version of this publication may be issued at a later date –6– I EC TS 61 724-2: 201 © I EC 201 I NTRODUCTION The performance of a PV system is dependent on the weather, seasonal effects, and other intermittent issues, so measurement of the performance of a PV system is expected to give variable results I EC 62446-1 , Photovoltaic (PV) systems – Requirements for testing, documentation and maintenance – Part Grid connected – Documentation, commissioning tests and inspection , describes a procedure for ensuring that the plant is constructed correctly, but does not attempt to verify that the output of the plant meets the design specifications I EC 61 724-1 , Photovoltaic system performance – Part 1: Monitoring , defines the performance data that may be collected, but does not define how to analyze that data in comparison to predicted performance ASTM E2848-1 Standard test method for reporting photovoltaic non-concentrator system performance describes a method for determining the power output of a photovoltaic system based on a regression I EC TS 61 724-3 Photovoltaic system performance – Part 3: Energy evaluation method describes a one-year test that evaluates performance over the full range of operating conditions and is the preferred method for evaluating system performance However, it is essential that plant performance can also be quantified with a shorter test, even if there can be higher uncertainty associated with that test This document is designed to complete an evaluation in a short time as a complement to I EC TS 61 724-3 As a capacity test, it measures power (not energy) at a specified set of reference conditions (which can differ from standard test conditions that have been designed to facilitate indoor measurements) The method in I EC TS 61 724-2 is a non-regression-based method for determining power output This method uses the design parameters of the plant to quantify a correction factor for comparing the plant’s measured performance to the performance targeted under reference conditions In other words, the measured performance, adjusted by the correction factor, is then compared with the target plant performance to identify whether the plant operates above or below expectations at the target reference conditions Multiple aspects of PV system quality are dependent on both the weather and the system's quality, so it is essential to have a clear understanding of the system being tested For example, the module temperature is primarily a function of irradiance, ambient temperature, and wind speed, all of which are weather effects that can be difficult to simulate precisely However, the module-mounting configuration also affects the module temperature, and the mounting is an aspect of the system that is being tested This document presents a process for test development and clarifies how measurement choices can affect the outcome of the test so that users can benefit from streamlined test design with consistent definitions, while still allowing flexibility in the application of the test so as to accommodate as many unique installations as possible I t is to be noted that when the output of a PV system exceeds the capability of the inverter, the output of the system is defined more by the inverter operation than by the PV modules I n this case, the measurement of the capacity of the plant to generate electricity is complicated by the need to differentiate situations in which the inverter is saturated and when the output of the PV system reflects the module performance For PV plants with high DC-to-AC power ratios, the operation of the plant can reflect the capability of the inverters for the majority of the day, with the capability of the DC array only being measurable for a short time in the morning and in the evening In this case, it can be necessary to disconnect parts of the DC array to reduce the DC-to-AC power ratio during the measurement period I EC TS 61 724-2 is applicable to times when the system is fully available Methods presented in this document can be used in place of ASTM E2848-1 to determine photovoltaic system performance _ U nd er preparati on Stag e at tim e of pu bl ication: I EC/FDI S 61 724-1 : 201 I EC TS 61 724-2: 201 © I EC 201 –7– P H O T O VO L T AI C S YS T E M P E RF O RM AN C E – P a rt : C a p a c i t y e v a l u a t i o n m e th o d Scope This part of I EC 61 724 defines a procedure for measuring and analyzing the power production of a specific photovoltaic system with the goal of evaluating the quality of the PV system performance The test is intended to be applied during a relatively short time period (a few relatively sunny days) I n this procedure, actual photovoltaic system power produced is measured and compared to the power expected for the observed weather based on the design parameters of the system The expected power under reference and measured conditions are typically derived from the design parameters that were used to derive the performance target for the plant as agreed to prior to the commencement of the test For cases when a power model was not developed during the plant design, a simple model that increases transparency is presented in the annexes as a possible approach The intent of this document is to specify a framework procedure for comparing the measured power produced against the expected power from a PV system on relatively sunny days This test procedure is intended for application to grid-connected photovoltaic systems that include at least one inverter and the associated hardware The performance of the system is quantified both during times when the inverters are maximum-power-point tracking and during times when the system power is limited by the output capability of the inverter or interconnection limit, reducing the system output relative to what it would have been with an inverter with generation freely following irradiance, if this condition is relevant This procedure can be applied to any PV system, including concentrator photovoltaic systems, using the irradiance (direct or global) that is relevant to the performance of the system This test procedure was designed and drafted with a primary goal of facilitating the documentation of a performance target, but it can also be used to verify a model, track performance (e g , degradation) of a system over the course of multiple years, or to document system quality for any other purpose The terminology has not been generalized to apply to all of these situations, but the intent is to create a methodology that can be used whenever the goal is to verify system performance at a specific reference condition chosen to be a frequently observed condition A more complete evaluation of plant performance can be accomplished by using the complementary Technical Specification I EC TS 61 724-3, Photovoltaic system performance – Part 3: Energy evaluation method 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 –8– I EC TS 61 724-2: 201 © I EC 201 I EC 61 724-1 , Photovoltaic system performance – Part 1: Monitoring I EC TS 61 836, Solar photovoltaic energy systems – Terms, definitions and symbols I SO/I EC Guide 98-1 , Uncertainty of measurement – Part 1: Introduction to the expression of uncertainty in measurement ASME, Performance Test Code 19.1 Terms and definitions For the purposes of this document, the terms and definitions given in IEC 61 724-1 , I EC TS 61 836, the ASME Performance Test Code 9.1 and the following and definitions apply 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 constrained operation operation of a plant in a condition when all inverters are limited by the capability of the inverters (otherwise referred to as inverter saturation) rather than by the output from the PV array, as is observed for a system with high DC rating relative to the AC rating and when the irradiance is high 3.2 correction factor ratio of the power expected for the reference conditions to the power expected for the measured conditions 3.3 curtailed operation output of the inverter(s) is limited due to external reasons such as inability of the local grid to receive the power or contractual agreement 3.4 expected power power generation of a PV system that is expected for actual weather data collected at the site during operation of the system based on the design parameters of the system 3.5 measured power electric power that is generated by the PV system N ote to entry: See also to defi ne the l ocati on of measu rement 3.6 model simulation model used to calculate the predicted or expected PV power generation based on the design parameters of the system _ U nd er preparati on Stag e at tim e of pu bl ication: I EC/FDI S 61 724-1 : 201 – 16 – I EC TS 61 724-2: 201 © I EC 201 The number of data points identified as meeting the criteria in Table will affect the uncertainty of the test As a guide to determining an adequate, yet reasonable, number of data points, Table may be used The larger number of data points during the summer reflects the ease of collecting more data on longer days and is expected to result in a higher accuracy measurement, depending on the local weather Locations that seldom experience clear, sunny days may require longer data collection times or reduction of the targeted number of data points, resulting in higher test uncertainty For CPV applications, Table is not directly relevant For CPV, after filtering for stable conditions, the data collected should include at least 30 data points (assuming averages) or at least 7,5 h of filtered data if averages for a different time period are used For systems with high DC-to-AC power ratios, the number of data points acquired for “unconstrained” operation may be an insufficient sample size I f the test cannot be completed because of this, or if there is concern that the characterization only during early morning and late afternoon will cause bias in the results, the definition of the system boundary and the TRCs should also direct that a fraction of the PV strings will be temporarily disconnected to reduce the DC-to-AC power ratio Table – Example guide for seasonal minimum stable irradiance requirements for flat-plate applications Season (northern hemi sphere) Dates M inimum POA irradiance (W/m ) Requ ired nu mber of 5min average data points Wi nter 22/1 to 21 /1 450 20 Spri ng 22/1 to 23/3 550 30 Su mmer 24/3 to 21 /9 650 60 Au tu mn 22/9 to 22/1 550 40 The data may also be screened according to normal function of the system Time periods for which tracker malfunction or system soiling would affect the results of the test may be omitted or included depending on the purpose of application of the test These inclusions or exclusions should be reported as part of the test report (see Clause 8, item 8)) 6.2.3 Shading of irradiance sensor Because of the sensitivity of the test to the irradiance data, special attention shall be given to the irradiance data Specifically, irradiance data that may result from accidental shading of a sensor or sensor malfunction should be removed before taking the average of the data from the remaining sensors The use of multiple sensors at each weather station is especially helpful for identifying issues with shading of some sensors Additionally, if an irradiance sensor is not correctly oriented (e g if mounted on a tracker and the tracker stops), the data from this sensor should be rejected 6.2.4 Calibration accuracy All sensors shall have accurate calibrations to provide a test result with low uncertainty consistent with the requirements described in I EC 61 724-1 for the desired class of measurement 6.2.5 6.2.5.1 Using data from multiple sensors General I n the case where multiple sensors have been used, if data inspection identifies errors in the output of a sensor, that data should be discarded before taking the average of the data pool This action should be done only with mutual consent of the parties I EC TS 61 724-2: 201 © I EC 201 6.2.5.2 – 17 – Multiple irradiance sensors The irradiance used as input to the power model should be the average of the available measurements, except where a measurement is determined to be erroneous, in which case the input to the model should be the average of the remaining measurements, as described previously 6.2.5.3 Multiple ambient temperature sensors The ambient temperature used as input to the model should be the average of the available measurements, except where a measurement is determined to be erroneous, in which case the input to the model should be the average/median of the remaining measurements 6.2.5.4 Multiple PV module temperature sensors Any PV module temperature used as input to the model should be the average of the available measurements, except where a measurement is determined to be erroneous, in which case the input to the model should be the average/median of the remaining measurements 6.2.6 Unconstrained operation and constrained operation when the output limit of the inverter is reached As described in 6.1 , data shall be flagged depending on whether all inverters were maximumpower-point tracking or all inverters limited the output because their output capabilities were reached All other data are discarded I f the inverters limit the output in different ways depending on the operating conditions, then the data shall be binned to identify those that are all under the operating condition of interest 6.3 6.3.1 Calculation of correction factor General The correction factor is calculated to adjust the measured power to the conditions used for the performance target Subclauses 6.3.2 to 6.3 provide a step-by-step procedure 6.3.2 Measure inputs Measure all variable inputs, including meteorological data and plant-specific parameters necessary to define the measurement conditions 6.3.3 Verify data quality As necessary, validate the measured variable input data as per 6.3.4 Calculate the correction factor for each measurement point Input measured meteorological data into the system’s model and calculate the correction factor needed to translate the measured data to the temperature, wind and irradiance conditions specified by the TRC for all points measured during “unconstrained” stable operation Calculate the correction factor for each point using the power model and Equation (1 ): CF = PPred targ / PPredmeas (1 ) – 18 – I EC TS 61 724-2: 201 © I EC 201 where CF is the correction factor; PPred targ is the power predicted at the target conditions; PPred meas is the power predicted at the measured conditions Both predicted powers are taken from the model agreed to by the parties See the annexes for an example model 6.3.5 Correct measured power output Correct the measured power by the correction factor for all points measured during “unconstrained” stable operation as calculated from the power model that describes the plant using Equation (2): Pcorr = Pmeas CF 6.3.6 (2) Average all values of corrected power Taking care to consider only the data that were included after data filtering (see 2.2), average all corrected power output values taken under “unconstrained” operating conditions, and separately average all power values measured during constrained operation 6.3.7 Analyse discrepancies I f an individual averaged corrected power deviates from the average by more than %, then a root cause diagnosis should be completed for the data point to see if any outlier situation was in effect and not caught by the data filtering I f the averaged power values deviate from the performance target values significantly (as established by the parties to the test), then a root cause diagnosis should be completed The test report shall comment on whether the test should still be considered valid 6.4 Comparison of measured power with the performance target The average measured corrected power (see 3) and performance target can be compared either as a simple difference, percent difference, or ratio calculation Difference calculation: Pcorr – PTarget (3) [ Pcorr – PTarget ] · 00 / PTarget (4) Percent difference calculation: Ratio (performance index for power): Pcorr / PTarget (5) ( Pcorr · 00) / PTarget (6) Ratio (units of %):