TALKING POINTExplaining PV plantmonitoring toIEC 61724-1Words: Clive Lee, Technical Sales & Services Manager, Kipp & ZonenReliable measurement of solar irradiance, and other environmentalparameters, is crucial to accurately calculating the performance ratio (PR) ofa solar energy plant. IEC 61724, published in March 2017, is the latest andmost comprehensive international standard to address these issues. Butwhat does it mean?IEC 61274-1 Photovoltaic systemperformance - Part 1: Monitoring also servesas the basis of two standards for1PES SOLARperformance analysis that rely upon the datacollected, IEC TS 61724-2 and IEC TS61724-3. Part 1 outlines equipment,methods, and terminology for theperformance monitoring and analysis of solarenergy PV plant systems; from irradiance
TALKING POINTinput to AC power output. It is applicable tofixed angle, single-axis tracking and dual-axistracking conventional PV modules and toconcentrator (CPV) systems. It does notcurrently cover bifacial installations.For Classes C and B, it is acceptable toestimate some measurements from othernearby and reliable data sources, but theseare rarely available. For Class A allparameters must be monitored at the site.The types of sensors required for eachmonitoring system (station) depend largelyupon the application. Measurement ofDiffuse Normal Irradiance (DNI) and DiffuseHorizontal Irradiance (DHI) is included in thestandard for specific purposes, but here wewill concentrate on the measurement ofGlobal Horizontal Irradiance (GHI) andPlane-of-Array irradiance (POA) and otherkey meteorological (weather) parameters.Communication and ConnectionThe industry standard for datacommunication within PV plants is now2-wire RS-485 with Modbus RTU protocol,using individually addressable inverters andmonitoring equipment on a small number ofdata bus loops. Distributed sensor power is12 or 24 VDC.Recent inverters, hubs and gateways oftenhave no analog inputs. There may be no fielddata logging, all the devices connect back tothe site supervisory control and dataacquisition (SCADA) system for storage,analysis and visualization by the plantmonitoring software and for remote access.Where a digital output is not available from adevice, a 4-20 mA signal is usually preferredIEC 61724-1 PV plant performance monitoring classificationsfor use with the analog inputs of low-costindustrial data loggers.Class C Monitoring System P OA irradiance – any type of thermopilepyranometer, PV reference device orphotodiode sensor; to be recalibrated asper the manufacturer’s requirements. A mbient air temperature – for an indicationof the effect on module efficiency,although it is much better to monitor themodule temperature directly.The efficiency of monocrystalline andpolycrystalline silicon cells typically drops byaround 0.5% per C temperature rise,compared to the standard test condition of25 C. At 45 C in the sunshine the outputcould be 10% lower. This is not a fault; it is acharacteristic that needs to be measuredaccurately and taken account of in PRcalculations.This type of monitoring system is largelyintended for commercial rooftopinstallations to give an idea of the PVinstallation efficiency where there are nosignificant performance-related financialand contractual implications.Class C irradiance sensor with compliant module temperature sensorWWW.PESSOLAR.COM2
TALKING POINTClass B Monitoring System G HI and POA irradiance – ISO 9060:2018Spectrally Flat Class B pyranometers or PV‘reference’ devices with measurementuncertainty 8 % for hourly irradiancetotals (photodiode sensors are notacceptable); to be recalibrated every2 years.Heating is required if condensation and/orfrozen precipitation, including dew, will affectthe irradiance measurement on more than14 days per year. Ventilation of pyranometersis optional, but in many locations (particularlywhere cleaning is infrequent) soiling can be asignificant issue. External ventilation unitsblow filtered clean air over the pyranometerdome to reduce the deposition of dirt, dustand other soiling contaminants and reducethe frequency of cleaning that is needed.They also include the option of heating theairflow to prevent dew forming on the dome,evaporate rain droplets and melt frost. P V module temperature – 6 sensors persystem, distributed along the strings, fortemperature correction of PR. As thenumber of modules in a string increases theloading along the string and thetemperature of the cells varies. There arestrict IEC guidelines regarding sensorposition, thermal conductivity of the fixingand overall accuracy of the measurement. A mbient air temperature – for historicaltrending analysis, the sensor must have aPyranometer with ventilation and heating unit3PES SOLARPyranometer measuring plane of array (POA) irradianceventilated solar radiation shield to preventbuild-up of static air. W ind speed – affects surface cooling of themodules. R ainfall – to correlate with powergeneration loss due to water on themodules.Class B is not likely to be popular becauseutility-scale PV plants often have significantcontractual under-performance penalties.The uncertainty of the hourly irradiancetotals will be around 6-8% and this is notgood enough for meaningful PR calculationsthat will satisfy all stakeholders. It is unlikelythat stakeholders will see a necessity toinstall 6 module temperature sensors permonitoring system.
TALKING POINTClass A monitoring system: GHI and POA pyranometers, weather station, soiling monitorsClass A Monitoring Systemtrending analysis. G HI and POA irradiance – ISO 9060:2018Spectrally Flat Class A pyranometers or PVreference devices with measurementuncertainty 3 % for hourly irradiancetotals (these genuinely ‘reference’ cellstypically cost more than an equivalentpyranometer); to be recalibratedevery year. R ainfall – to correlate with powergeneration loss due to water onthe modules.Heating is required if condensation and/orfrozen precipitation will affect the irradiancemeasurement on more than 7 days per year,ventilation of pyranometers is mandatory. Inpractice this means that heating andventilation are always required. See thecomments for Class B regarding theadvantages of an external ventilation unit toreduce dome soiling.An all-in-one weather station with a singlemounting and power/data connection is aconvenient way to monitor all themeteorological parameters other than solarirradiance. Typically, these also monitor airpressure and relative humidity and calculatethe dew-point, which is useful to correlatewith reduced yield. The wind speed anddirection sensor are ultrasonic with nomoving parts. P V module temperature – 6 sensors persystem, distributed along the strings, fortemperature correction of PR; as describedfor Class B. A mbient air temperature – for historicaltrending analysis, the sensor must have aventilated solar radiation shield. W ind speed – affects surface cooling ofthe modules. Wind direction – for historical S now – to correlate with power generationloss if this is an issue at the location. S oiling ratio – if the power loss is 2 %, toinform when module cleaning is advisable.Unlike a tipping bucket rain-gauge, a radarprecipitation sensor does not need regularcleaning and outputs data in real time, tocorrelate with the power generated, and itcan detect whether the precipitation is rain(liquid) or snow (solid).A soiling monitor that can be mountedalongside PV modules or within an array getsdirty at the same rate as the surroundingmodules. An optical soiling measurementtechnology monitors the loss of lighttransmitted through the glass and, for anytype of module, the effect on powergeneration can be calculated from themanufacturer specifications. This type ofsoiling monitor requires no maintenance andis only cleaned at the same time as themodules around it.Different areas of a large PV plant get dirty atdifferent rates and multiple monitors canprovide a soiling map of the plant. Bycomparing the opportunity cost of lostenergy production with the cost of cleaningthe O&M staff can decide when, and where,to clean modules on a site-specific basis.The uncertainty of the hourly irradiancetotals will be around 3 % (daily total typicallywithin 2 %) which is very suitable for reliableplant PR calculations. Class A monitoringrequires that the irradiance sensors arecleaned at least once per week andrecalibrated every year and the airtemperature and module temperaturesensors are to be recalibrated every 2 years.There is a cost associated with this.Therefore, it is possible that a PV plant mightbe specified with Class A monitoringequipment but that the O&M procedures arenot compliant.WWW.PESSOLAR.COM4
TALKING POINTNumber of Plant Monitoring SystemsIEC 61724-1 refers to the designed AC powercapacity of the plant for the recommendedminimum number of monitoring systems, onthe basis that all the PV modules are installedat the same orientation and that there is noredundancy or back-up monitoring.Where sites have varying module angles, forexample over hillsides or in valleys, thereshould be more POA pyranometers andsoiling monitors; some orientations could beEast and some West.ISO 9060 – The Standard for PyranometerPerformanceISO 9060 ‘Solar energy - Specification andclassification of instruments for measuringhemispherical solar and direct solar radiation’defines what is a pyranometer, and what is apyrheliometer for measuring DNI. IEC61724-1 refers to ISO 9060:1990 because itwas developed and published before thestandard was updated in November 2018,which ‘de facto’ replaces the earlier edition.The 1990 version specifies pyranometerperformance in order of improvingperformance as:Second Class First Class Secondary StandardRadiation from 350 to 1500 nm is the criticalwavelength range for photo-voltaic materialsIEC 61724-1 minimum number of monitoring systems by plant capacityand the standard requires that pyranometersrespond evenly over this range and are‘spectrally flat’. For this reason, it does notallow ‘photoelectric’ sensors, such as siliconcells and photodiodes, because of theirrestricted and uneven spectral response.The most noticeable change in ISO9060:2018 is the classification names:Class C Class B Class AThe standard now allows photoelectricsensors. If a pyranometer meets the 1990Kipp & Zonen pyranometer ISO 9060:1990 and 2018 classifications5PES SOLARcriteria for Secondary Standard, it is nowtermed Spectrally Flat. If the 95 % responsetime is less than 0.5 second, it is FastResponse. Fast Response Class C is typicalfor the analog output of a photodiode sensorwithout amplification.Radiometers with RS-485 Modbus RTU serialdata communication usually update values inthe output registers every second andtherefore are not ‘fast response’. This is alsotrue of most amplified analog pyranometeroutputs due to noisereduction filters.
TALKING POINT‘By comparing the opportunity cost of lost energyproduction with the cost of cleaning the O&M staff candecide when, and where, to clean modules on asite-specific basis.’Different module orientations require extra POA pyranometersISO 9060:2018 states that the newclassifications are ‘roughly corresponding’ tothe classes from 1990 and there are somesignificant differences in the way thatspecifications are defined. However, ingeneral, a pyranometer that met the 1990Secondary Standard requirement shouldmeet the 2018 Spectrally Flat Class Arequirements.Note that Class A pyranometers must beindividually tested for temperature responseand directional response (cosine error) todemonstrate that it meets the requirementsof the standard. This means that if aSecondary Standard pyranometer did nothave these individual tests carried out, and itmet the new limits, it cannot be ‘updated’ toClass A without the additional testing beingcarried out.Many people involved in the solar energysector have heard about this standardbut do not appreciate what it covers.We strongly recommend purchase of thedocument from the IEC Webstore ppzonen.comThis article is an excerpt from‘Solar Energy International Standards’,a whitepaper produced by Kipp & Zonento explain the standards most frequentlyused in solar energy: ISO 9060, IEC61274-1, ISO 9846/9847 and ISO/IEC17025. You can download it atwww.kippzonen.com/standardsWWW.PESSOLAR.COM6
Sep 16, 2018 · IEC 61724-1 Words: Niketan Arawade, Research Analyst, Adroit Market Research IEC 61274-1 Photovoltaic system performance - Part 1: Monitoring also serves as the basis of two standards for performance analysis that rely upon the data collected, IEC TS 61724-2 and IEC TS 61724-3. Pa