Current Status Of The Potential Induced Degradation In .
Advances in Environmental and Geological Science and EngineeringCurrent Status of the Potential Induced Degradation in Czech RepublicPETR MASTNY, JAN MORAVEK, JIRI PITRON, MICHAL VRANABrno University of TechnologyDepartment of Electrical Power EngineeringTechnicka 12, 616 00 BrnoCZECH REPUBLICmastny@feec.vutbr.cz, utbr.cz, xvrana10@stud.feec.vutbr.czAbstract: - In connection with the development and the growing integration of renewable energy sources intoenergy systems, it is still important to acquire knowledge concerning the vice of their operationalcharacteristics. Article is focused on the operation of photovoltaic power plants in the conditions of the CzechRepublic. It describes the practical experience with in-field inspection and measurements.Key-Words: - Photovoltaic, Degradation, Testing, Reliability, Operational Diagnostics, PIDhybrid inverters can completely avoidthe electricity supply to the distributionsystem. Hybrid inverters that do notallow this function allow the supply ofelectricity to the grid – this situationexists where the battery is fully chargedand the consumption on site is ensured –that is similar like a grid-on systemo Utilization of equipment for optimizingconsumption and for minimization ofoverflows of produced electricity (forexample Wattrouter) is not an optimalsolution for unauthorized supply todistribution system. The reason is thatthe uncontrolled supply to the networkmay cause voltage and frequencyfluctuations, or may negatively affect theregulationofsmallcommunitytransformers. Island systemo Island photovoltaic system (grid-off) isnot connected to the electricity system.Their operation and connection does notrequire permission of the distributionsystem operator. Within the islandsystem can occur the frequencydeviations according to the generatedpower, which can have negative effecton rotation machines. This depends alsoon the quality of used inverters and theirfast response time.1 IntroductionIn the Czech Republic (CR), in spite of an importantincrease in the installed capacity of solarphotovoltaic (PV) systems legislative measures havebeen recently adopted that stopped this upwardtrend. Restrictions for connecting renewable energysources in the CR are mainly in terms of powersystem operation.Currently, it is possible to operate the followingtypes of photovoltaic systems: Grid-on systemo Traditionalphotovoltaicplantsconnected to the grid, which allowsoverflow of the electricity produced, if itis not utilized in place of consumption.o Among the traditional grid-on PVsystems also include the micro PVsystems, which usually consist of severalphotovoltaic panels located for exampleon the balcony. These systems areequipped of micro-inverter directly onthe panel. Even these miniature PVsystems working in the parallel operationwith the DS and therefore, in practice,the overflow may occur electricity to theDS although it may be at proper designof protective components. This type ofPV plant requires connection to afunctioning distribution system for itsoperation. Hybrid systemo Hybrid photovoltaic systems areequipped with battery, charge controllerand hybrid inverter. Selected types ofISBN: 978-1-61804-314-6The development of photovoltaic power plantsaccelerated by such support is over the useful needand possibilities of the power engineering in the78
Advances in Environmental and Geological Science and EngineeringCzech Republic and also it is over meetingobligations towards the EU.In accordance with the Directive of the EuropeanParliament and Council 2009/28/EC there weredefined the target values of the quota of renewablesources and the method of its calculation. Accordingto the mentioned directive there must be in CR by2020 covered the energy produced from renewablesources 13% of all energy consumption (heating andcooling energy, electricity, fuels for transport). It isthe fact that the distribution of fixed quota of RES inthe various types of energy is the responsibility ofevery state within EU. [1]In Fig. 1 is shown the progress of installedperformance of PV systems in CR for the period2007 to 2014. It is seen that between years 2009 to2011 was the increase of installed power of PVsystems very dynamic. The reason was the highstate support in the purchase prices of the electricityproduced from this energy source.Fig. 1 Development of PV installations in Czech Republic (2007 – 2014) [6]Currently, in Czech Republic are not supportednew installations of PV systems - this is valid forinstallation as from 2014. If the plant is not operatedfor business purposes, the law does not require alicense from the operator. For business electricityproduction in power engineering can mark eachactivity, which involves the supply of electricity tothe grid. This also applies to a situation where it iscollected green bonus, the purchase price or marketprice for electricity generated.Mainly because of high installed capacity of PVpower plants and also regarding decreasinginvestors’ efforts there were set new purchase pricesof electricity produced from such sources. Forillustration is in the Fig. 2 shown the developmentof purchase price between the years 2005 to 2014.Regarding the new measures there are by theoperators increasingly controlled technical andoperational parameters of photovoltaic power plants.For most PV power plants there is by a supplierguaranteed five-year warranty on mechanicalproperties of panels and 25 years on performanceISBN: 978-1-61804-314-6parameters. However, there are many possibilitieswhen PV panels do not match the declaredparameters, which have a negative impact on overallenergy and economic efficiency of operatedphotovoltaic power plant.The main causes of defects in the PV panels canbe divided into several categories: Differenttechnologicallevelofproduction regarding producers. Mechanical damage for example duringtransportation. Effect of adverse environmental impacts. Improper installation of the PV system.2 Potential Induced DegradationIn connection with efforts to increase the efficiencyof energy production from photovoltaic powerplants in the Czech Republic began at the end of theyear 2014 the discussion about the problem ofperformance degradation of PV modules79
Advances in Environmental and Geological Science and EngineeringFig. 2 Development of purchase prices between years 2005 to 2014 in CR [6]consequence the high voltage stress (PotentialInduced Degradation - PID). PID is a phenomenonthat’s been existing for several years. It is a problemoccurring on many photovoltaic panels resulting to adowngraded performance of solar panels. PID wasfirst recognized in 1970’s and from then on, a lot ofstudies have been geared towards assessing thecauses of this occurrence.Based on studies carried by several significantinstitutions (NREL, SolarEnergy, EERE, PI Berlin)have been defined the conditions and causes of PID:[7] Environmental factors.o Relative humidity.o Temperature. System factors.o At the system level, the most significantimpacts are the modules voltagepotential and sign, which depend on boththe modules position in the array and thesystem grounding topology. There arenumerousconsiderationsaffectingsystem and inverter classification, but forthe purposes of PID, the inverter can beclassified based on the voltageexperienced by the arrays. PID is mostoften associated with a negative voltagepotential to ground. Module factors.o The choice of glass, encapsulation, anddiffusion barriers has all been shown tohave an impact on PID. Cell factors.ISBN: 978-1-61804-314-6The domain pathway to cause PID is via thefront sheet as glass to the frame. Higher leakagecurrents can be caused by water entering the solarmodule causing the encapsulation material tobecome more conductive. So far the potentialdegradation mechanism is not monitored by thetypical PV tests listed in IEC 61215. [8] It is knownthat metal ions such as Na formed from the oxidesof the module glass can drift toward the cell if thecell is biased negatively. [9]But there is also the standard IEC 61646. Bothstandards have a similar testing sequence and theydiffer only on the requirements for powerdegradation. In the IEC 61646 standard is included afurther final investigation to evaluate the effect ofpower degradation due to light exposure (LID –Light Induced Degradation).As the actual progress of PID detection becamemore desirable, simple in-field methods which canadequately confirm the PID are tested.Measuring of the insulation resistivity by use ofhigh voltage, can be one of the ways that canindicate the PID. [4]Another way is using intelligent PV moduleanalyzers (such as TIGO), where is evaluated theactual power of each panel in the string, and isautomatically detected the decrease in the powerproduction. Disadvantage of this solution is theadditional investments on the installation.As a compromise can be considered the on-sitemeasurements of operational parameters, which, ifproperly done, can indicate multiple faults withinthe PV plant installation.Based on the available information, there havebeen several operational measurements at various80
Advances in Environmental and Geological Science and Engineeringof the irradiation lower than 5% during themeasurement.The output of the initial or periodic inspection isthe protocol, with the measured values andcompleted checklist. The model protocol andchecklist are the appendices of the ČSN EN 62446standard. The infra-red (IR) camera testing isrecommended as the additional method forinspection.PV installations in the country. The results of thesemeasurements and subsequent analyzes far indicatethat the incidence of PID in our conditions is verylow.Demonstrationofanalysisprocessmeasurement and discussion of the results isdiscussed in the next section of this paper.3 Operational MeasurementsIn order to detect the faults and defect in the PVplants installations, the proper inspection andvalidation of the available documentation should beperformed prior to the measurements.The following methods are generally used for PVpanel’s diagnostics: Infra-red camera – Suitable for detectingdefects on the cell level of the PV panel. Electroluminescence imaging – Used fordetecting the defects in the structure of thecell which are not detectable by visualinspection. Flash tester – Allows the I-V curvemeasurements under artificial and controlledcondition. Operational I-V characteristic measurement –Uses special analyzers to determine thepanel’s parameters.3.2 Description of the Inspection Performedon the 600 kWp PV PlantThe inspection has been performed due to lowerrevenues, which could have been caused by PID.The PV plant contained polycrystalline PV panelsconnected in strings of 17-18 panels. Neither of theDC poles has been grounded. During the inspectionhave been measured I-V curves of the individualstrings. In case of the suspicious strings with lowerpower have been measured individual panels inaffected string.For measuring of the I-V curves has been usedthe Metrel MI3108 Eurotest PV, that measured theI-V curves, temperature of the panel, and irradiation.Results of the measured parameters are shown inTable 1. It can be seen, that the panels parametersdo not match the nominal values. The Isc is higherthan it is intended to be. Also, the maximum poweris about 7% lower than its nominal value.3.1 Inspection According to the StandardČSN EN 62446Table 1 Nominal and measured parameters of thetested PV panel at STCThe standard ČSN EN 62446 [2] sets basicrequirements and a description of thedocumentation. It describes a procedure forinspection of the DC wiring, surge protectors, ACsystem, recommendation regarding the setting andwiring of the inverter. Furthermore, there arerequirements for safety, signs and labels. During theinitial inspection, in case of loss of documentationor periodic inspection, the check of the string wiringand proper polarity, measuring of the voltage atopen circuit and short circuit current should beperformed. Correct settings of the protectionsshould be verified.Operational measurements to check of themaximum power point tracker (MPPT) function,measuring the parameters in maximum power point,to compare string power with another or withexpected values can be used to analyze the real stateof the power plant. The measurements must beperformed with the stable ambient conditions duringthe measurements. The standard IEC 61215considers the condition stable, when there is changeISBN: p,nom7.98 A43.8 V7.63 A36.7 V280 WIsc,STCUoc,STCImpp,STCUmpp,STCPmpp,STC8.59 A43.1 V7.96 A33.0 V260 WFig. 3 I-V curves of the tested PV panel81
Advances in Environmental and Geological Science and EngineeringThe example of the measured I-V curve is shownin the Fig. 3. There is no mismatch on the curvecaused by the defects or shadowing of the panel.The results of measured power in maximumpower point (Pmpp) recalculated according ČSN EN62446 to the standard test condition (STC) valuesand the percentage difference compared to thenominal value of the panel which is 280 Wp areshown in the Table 2.Results of 1st string indicate no problem in thestring. The decrease of PSTC varies randomly andprobably is caused by the differences during themanufacturing.The 2nd string indicated lower power in stringmeasurements. During measuring the individualpanels has been found out, that the string containeddefect panel.The difference compared to its nominal value is75%. The bypass diodes of the panel have workingproperly therefore has been recommended to changethis defect panel.The 3rd string has been found out the globaldecrease of the STC power. This could be caused bydegradation of the panels or dust. Also has beendetected the loose connector, that didn’t provideadequate interconnection.The inspection also confirmed, that there areadditional defects, maintenance and design errors onthe panels. Example of defects and impropermounting can be seen on Fig. 4 and Fig. 5. The errorof design is in the parallel interconnection of threestrings on each inverter. During inspection havebeen detected parallel connections of strings withdifferent number of panels in series.Table 2 Results of measuring the PSTC of individual panels in selected stringsPanel 1Panel 2Panel 3Panel 4Panel 5Panel 6Panel 7Panel 8Panel 9Panel 10Panel 11Panel 12Panel 13Panel 14Panel 15Panel 16Panel 17ΣString 1PSTC (Wp) Δ P 71.2-3.14271.2-3.144 642.2Panel 1Panel 2Panel 3Panel 4Panel 5Panel 6Panel 7Panel 8Panel 9Panel 10Panel 11Panel 12Panel 13Panel 14Panel 15Panel 16Panel 17Σ-10-5String 2PSTC (Wp) Δ P .43268.0-4.29267.5-4.464 328.2Legend (%)0Fig. 4 Improper panel mountingISBN: 978-1-61804-314-65Panel 1Panel 2Panel 3Panel 4Panel 5Panel 6Panel 7Panel 8Panel 9Panel 10Panel 11Panel 12Panel 13Panel 14Panel 15Panel 16Panel 17ΣString 3PSTC (Wp) Δ P .75265.9-5.04261.1-6.754 405.510Fig. 5 Delamination in the structure of the panel82
Advances in Environmental and Geological Science and Engineering[4]4 ConclusionAccording to the actual state of PV power plants inCR, when new installations are no more installed,the main goal is to increase the operationalefficiency of the existing installations.The current research is focusing on degradationof the panels and their parameters validation, so thePV plants meet their designed values. Nevertheless,improper maintenance, low quality design andcomponents, can result into severe energy drops,that reduces the effectivity of entire power plant.The methods of detecting the faults and evaluationof the PV plants parameters are presented in thepaper. Additional research is connected withdetection of the PID, its proper detection andevaluation of the energy losses caused by the defectsand faults in the PV plant’s installation.Current results of operating measurement for PVinstallations in the Czech Republic show a very lowincidence of PID. We believe that this is mainly dueto the specific operating conditions in the CzechRepublic (environmental aspects) and also the duethe fact that most of PV systems are operatedmainly at voltages up to 1000 V DC.[5][6][7][8][9]AcknowledgmentsThe paper was prepared at Centre for Research andUtilization of Renewable Energy (CVVOZE).Authors gratefully acknowledge financial supportfrom the Ministry of Education, Youth and Sports ofthe Czech Republic under NPU I program (projectNo. LO1210) and from the Technology Agency ofthe Czech Republic (project No. TA04021196) andBUT Specific Research Program (project No.FEKT-S-14-2520).[10][11]References:[1] Ptacek, J., Modlitba, P., Spacek, T.,Possibilities of electricity production fromRES within conditions of CR regardingliabilities towards EU, Conference CKCIRED 2010, Czech republic 2010, pp. 1 – 9.[2] ČSN EN 62446. Grid connected photovoltaicsystems - Minimum requirements for systemdocumentation, commissioning tests andinspection, Geneva: IEC, 2010.[3] ČSN EN 60891. Photovoltaic devices –Procedures for temperature and irradiancecorrections to measured I-V characteristics,Geneva: IEC, 2009.ISBN: 978-1-61804-314-6[12][13]83Saly, V.; Vary, M.; Packa, J; Perny, M.Electrical Characterization of PV Modulesafter Three Years of Operation. InProceedings of the 16th InternationalScientific Conference Electric PowerEngineering 2015, VSB Ostrava, 2015, pp.484-487, ISBN 978-1-4673-6787-5.Bastidas, R., Petrone, J.D., Paja, R., C.A.,Spagnuolo, G., Photovoltaic modulesdiagnostic: An overview. IECON 2013 - 39thAnnual Conference of the IEEE IndustrialElectronics Society. pp. 96-101. DOI:10.1109/IECON.2013.6699117, ISBN 978-14799-0224-8.WWW pages of Energy Regulatory Office ofthe Czech Republic, www.eru.cz, cit. 22ndMay, 2015.WWW pages of Advanced w.advanced-energy.com,cit. 10th May, 2015.IEC 61215, Crystalline Silicon TerrestrialPhotovoltaic Modules - Design Qualificationand Type Approval, IEC, Geneva, 2005.Carlson, D. E., et al., Corrosion Effects inThin-Film Photovoltaic Modules, Progress inPhotovoltaics: Research and Applications,Vol. 11, No. 6, 2003, pp. 377- 386.Mastny, P., Machacek, J., System forMeasuring and Collecting Data - Results ofMeasuring on Combined System, 10thWSEAS Int. Conf. on Mathematical Methodsand Computational Techniques in ElectricalEngineering, Sofia, Bulgaria, 2008, pp. 192197.Milea, L., Otlu, O., Teodorescu, C., Muntean,V., Stoian, M., Photovoltaic Energetic System– Design and Implementation, 12th WSEASInternational Conference on Systems, Greece2008, pp. 372 – 375.Peeters, E., Van Bael, J. A Real Life Analysisof Small Scale Photovoltaic Installations,Proceedings of the 2nd IASME / WSEASInternational Conference on Energy &Environment (EE'07), Portoroz, Slovenia,May 15-17, 2007, pp. 158 – 162.Mastny, P., Radil, L., Mastna, Z. Possibilitiesof PV Panels Defects Identification andDetermination of its Effect on the Economyof Photovoltaic Power Plants Operation. InMathematical Models and Methods inModern Science. Puerto De La Cruz, Spain,WSEAS Press. 2011. p. 233 - 238. ISBN 9781-61804-055-8.
ČSN EN 62446 . The standard ČSN EN 62446 [2] sets basic requirements and a description of the documentation. It describes a procedure for inspection of the DC wiring, surge protectors, AC system, recommendation regarding the setting and wiring of the inverter. Furtherm
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