technologies for the new millennium - phototvoltacis as a distributed resource

8/6/2019 Technologies for the New Millennium - Phototvoltacis as a Distributed Resource

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Technologies for the New M illennium:Photovoltaics a s a Distributed ResourceB. roposki,Member, IEEENationalRenewable Energy Laboratory1617Cole Blvd.

Golden, CO 80401

Abstract: As we enter the new millennium, photovoltaics (FW)is emerging as an important distributed resource. PV gives boththe benefits of a distributed resource and a clean power source.Because PV can be located at both residential and commerciallocations, it can be used to reduce peak demand when its outputis properly matched with load usage. It can also improve assetutilization by requiring less large capital generation spendingand delaying some equipment replacement. With the price ofsome grid-connected PV systems expected to reach $3/W in thenext 5 years, PV will become an economical option fordistributed power generation. One of the most importantaspects of establishing PV as a distributed resource isstandardizing the requirements for grid connection. IEEEStandards Coordinating Committee (SCC) 21 has recentlypublished IEEE Std 929 Recommend Practice for UtilityInterface of Photovoltaic Systems. This recommendedpractice details power quality, safety, and protectionrequirements for connection to the utility grid. This paperdescribes what types of PV systems are available, what thebenefits are for PV systems, and what the interconnectionissues and solutions are for using PV as a distributed resource.Keywords: Photovoltaics, Distributed Resource, Standards,Utility Interconnection, Grid-Connected, DistributedGeneration

I. INTRODUCTIONDeregulation of the electric utility industry is providingan opportunity for higher penetration and use ofdistributed resources (DR). Distributed resources aregeneration sources that can be located at or near loads.Distributed resources can provide benefits that bulkpower generation can not. PV systems are ideally suitedfor distributed resource applications. Photovoltaic (PV)systems produce DC electricity when sunlight shines onthe PV array, without any emissions. The DC power isconverted to AC power with an inverter and can be usedto power local loads or fed back to the utility. The fuelsource for photovoltaic systems -sunlight- is naturallydistributed. Distributed PV systems can be mounted oncurrently free real estate, such as building roofs. PVsystems are extremely modular and can be sized from100 watts to several megawatts. Although PV systemsca n only produce electricity during the day, they can bematched with energy storage devices such as batteries tomake a system that can meet the load continuously.

R. DeBlasio, Senior Member, IEEENational RenewableEnergy Laboratory1617Cole Blvd.Golden. CO 80401

11. TfPES OF PHOTOVOLTAIC SYSTEMSA. Grid-Connected PV SystemsGrid-connected PV systems consist of a PV array and aninverter cclnnected to an electrical utility power grid.When sunliight strikes the PV array, it generates DCelectricity. The electricity is then fed to an inverter,which conlverts the electricity into AC power thatmatches the voltage and fiequency of the electricalutility. All of the power produced by the system is fed tothe utility at the point of common coupling. Forresidential systems, this occurs at the point at which theelectric utility and the customer interface. Onsite loadsuse the power that is generated by the PV system duringthe day. Any excess power is fed back to the utility.During the night and periods of shortfall, the electricitycomes fiorn the utility to power the loads. All grid-connected I V ystems can act asdistributed resources.Grid-connected PV systems range fiom 100 watts toseveral megawatts. In the smaller power range, oneoption are AC modules, which are complete systems witha module and inverter in one package. AC modulepower can range between 100-300 watts. There areseveral benefits of AC modules. They allow users toinstall small PV systems that are easily expandable.They also add a level of redundancy in the case ofinverter failure for systems that have more than one ACmodule.Most residlential PV systems range b m 1-5 kW,depending on available area. PV systems in this sizerange usually consist of a PV array (20-100 modules)connected to one inverter properly sized to the array.Most of tliese systems are located on the roofs ofresidential homes.Systems thrit range in power levels fiom 10-100 kW aretypically customer-sited. Systems of this size areincreasingly becoming cost competitive with traditionalpower sources [1,2].Grid-connected PV systems larger than lOOkW aretypically utility-owned and are used as distributedresources, as well as bulk power generation. Examples

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of systems this sue used as distributed resources aresystems placed at the end of feeder lines to improvevoltage sags near customer loads.B. Stand-Alone PV SystemsStand-alone PV systems consist of a PV array, energystorage device (e.g. batteries), loads, and possibly aninverter, depending on load requirements. These systemsare designed for specific applications and are not usuallyconsidered when designing utility distributed resourceapplications. The benefit of this type of system is thatthey are usually cost-effective solutions for a particularproblem (e.g. light in remote locations where no utility ispresent). Because of the economic benefits, most PVsystems are of this ype.C. Hybrid PV SystemsHybrid PV systems contain at least one other generationsource (e.g. wind turbine, generator) that is not a utility.These types of systems are used where there is a need forcontinuous power and the PV array cannot alwaysproduce enough power to cover the load.

111.BENEFITS OF PHOTOVOLTAICSUsing PV as a distributed resource has several benefits.Because PV can be located at both residential andcommercial locations, it can be used to reduce peakdemand when its output is properly matched with loadusage. PV can shave peak demands on the utility grid.Demand for electricity often is highest just when the sunis at its brightest, during the hottest parts of the hottestdays.The use of PV systems can also improve asset utilizationby reducing required capital generation spending anddelaying some equipment replacement. It can also lessenthe need for new central generation equipment and newtransmission and distribution lines [3,4].PV is also a clean, renewable resource. The fuel supplyfor PV systems although variable is also free andunending. PV systems create no emissions whenproducing power. Avoided emissions is a very positiveaspect of PV power. A 10 kW PV system in Coloradoavoids 22,456 lbs. in CO2 emissions and 4 lbs. in NO,emissions [5 ] .Currently, the lowest documented price for grid-tied PVsystems in the United States as of 1998, was $5.07rW[6]. With the price of grid-connected PV systemsexpected to reach $3/W in the next 5 years [7],V willbecome an economical option for distributed powergeneration.

IV. PERFORMANCE AND RELIABILITYTerrestrial PV systems have been in use for over 20 years[SI. During this time, there have been markedimprovements in the performance and reliability of thePV systems and their components.Improvements have been made in the quality andreliability of PV modules due, in part, to the developmentof module qualification and safety testing. IEEE 1262Recommended Practice for Qualification of PVModules [9] and UL. 1703 Flat-Plate PhotovoltaicModules and Panels [lo] provide test procedures toevaluate module design performance, safety, andsusceptibility to known failure mechanisms. In thesetests, emphasis is placed on testing the module for knowndegradation mechanisms resulting from environmentalweathering, mechanical loading, corrosion, andshadowing. Some PV manufacturers are currentlyoffering up to 25-year warranties on their products.PV system inverters have also improved in theirreliability. Inverter reliability was a significant issuewith early system installation projects conducted by theSacramento Municipal Utility District (SMUD) and theU.S. Environmental Protection Agency (EPA) [111. Datafrom new installations show that inverter reliability isimproving through concerted efforts by the invertermanufacturers to improve their quality control and in-house testing [12].System maintenance issues fall into the area of systemreliability. Properly designed and installed PV systemsusually require little maintenance, unless a componentfails. Maintenance for stand-alone and hybrid systems isusually considerably greater than for grid-connectedsystems because of the use of batteries or othergeneration sources.

V. INTERCONNECTIONISSUESOne of the most important aspects of establishing PV as adistributed resource is standardizing the requirements forgrid connection. There are three important documentsthat cover the connection of PV systems to the utilitygrid:0 IEEE 929 Recommend Practice for Utility Interfaceof Photovoltaic Systems [1310 National Electrical Code (NEC) Article 690 SolarPhotovoltaic Systems [1410 UL 1741 Standard for Static Inverters and ChargeControllers for Use in Photovoltaic PowerSystems[151.

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IEEE Standards Coordinating Committee (SCC) 21recently revised and published IEEE 929. Thisrecommended practice details power quality, safety, andprotection requirements for connection to the utility grid.The power quality of the PV system interconnected to theutility is defined in the standard by normal operatingvoltage, fiequency, voltage flicker, harmonic distortion,and power factor. Line-commutated PV system invertersare designed to follow and operate at the utility linevoltage and frequency. The inverter of the PV systemwill cease to energize the utility lines whenever thevoltage deviates outside of 106 V to 132 V on a single-phase 120 V line. PV systems should have a fnedoperating frequency range of 59.3-60.5 Hz. Theallowable levels for both voltage flicker and harmonicdistortion are described in IEEE 519 [16]. The allowableamounts ensure that there are no adverse effects to otherequipment connected to the utility line. Total harmoniccurrent distortion shall be less than 5% of thefundamental frequency at rated output. Most invertersused in PV systems operate at unity power factor,although 0.85 lagging or leading is allowed in specialcases.IEEE 929 also makes recommendations for ensuring safeoperation of the PV system. These recommendationsinclude equipment protective functions and hardware forpersonnel safety. The following recommendations arefor 120V base electrical systems with PV systems ratedat 10 kW peak or less. The inverter should sense andrespond to voltage and frequency disturbances. Islandingis a condition where a portion of the utility system, whichcontains both load and generation, is isolated from theremainder of the utility system. Inverters in PV systemsare required to have some type of anti-islanding featurethat prevents an island condition fiom occurring.Inverters are also required to remain off-line for fiveminutes after any out-of-bounds condition. PV systemsshould not inject DC current greater than 0.5% of ratedinverter output current into the AC utility grid.Two other requirements for IEEE 929 come from theNational Electrical Code. These are grounding and theuses of a utility-interface disconnect switch.Article 690 Solar Photovoltaic Systems of the NationalElectrical Code covers safe installation practices for PVsystems. The NEC covers several areas, including:Circuit Requirements, Disconnecting Means, WiringMethods, Grounding, Marking, Connection to otherSources, and Storage Batteries. Excellent discussions onthe NEC are given in a Sandia Technical Report [17] andare covered in IEEE 1374 Guide for TerrestrialPhotovoltaic Power System Safety [181.Two UL documents are relevant to PV systems. UL1703 Flat-Plate Photovoltaic Photovoltaic Modules andPanels covers module safety, and UL 1741 Standard

for Static ihverters and Charge Controllers for Use inPhotovoltaic Power Systems covers the safety of PVinverters and charge controllers. UL 1741 covers thesafety of inverters and tests for anti-islanding capabilitiesand voltage and frequency trip points.Currently, .work is being conducted in IEEE SCC21 todevelop an interconnection document for all distributedpower resources (e.g. solar, wind, fuel cells, micro-turbines, and storage). This work is conducted in ProjectAuthorization Request (PAR) 1547 Standard forDistributed Resources Interconnected with ElectricPower Systems [191. This document provides a uniformstandard for interconnection of distributed resources withelectrical power systems. It provides requirements forthe performance, operation, testing, safetyconsiderations, and maintenance of the interconnection.This document is scheduled for completion by December2001.

VI. CONCLUSIONSPhotovoltaic power shows promise of becoming aneffective contributor for distributed resource applications.PV has a great number of benefits for use as a distributedresource including peak demand shaving and improvedasset utilization. PV also provides a clean and renewableenergy source. The performance and reliability of PVsystems has improved in the last 20 years, with the use ofqualification and safety standards. Interconnectionrequirements and installation codes have been developedfor PV systems that will allow for consistent deployment.

VII. ACKNOWLEDGEMENTSThe authors acknowledge the work of several researchersat the DOE National Laboratories: John Stevens (SandiaNational Laboratories) for work in utility interconnectionof PV system and chairing the working group for IEEE929; Ward Bower (Sandia National Laboratories) andJohn Wiles (Southwest Technology DevelopmentInstitute) foir work on the National Electrical Code and inPV system safety; Christy Herig (National RenewableEnergy Laboratory) for promoting the use ofphotovoltaics in utilities; Tom Basso (NationalRenewable Energy Laboratory) for facilitating the IEEESCC2 1 distributed power interconnection standarddevelopment activities as secretary to the IEEE P1547working group. This work was done under contract DE-AC36-99G010337.

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VIII. REFENECES [141 NFPA 70-1999, National Electrical Code (NEC).[l] Benner, J. and L. Kazmerski, Photovoltaics -Gaining Greater Visibility, IEEE Spectrum, September1999, pp. 35-42.[2] Herig, C., H. Thomas, R. Perez, and H. Wenger,Residential Customer-Sited Photovoltaics Markets1999, ASES 1999 Conference, June 1999.[3] Hoff, T., H. Wenger, and B. Farmer, DistributedGeneration: An Alternative to Electric UtilityInvestments in System Capacity, Energy Policy 24(2):137-147 (1996)[4] Hoff, T., and D. Shugar, The Value of Grid-SupportPhotovoltaics in Reducing Distribution System Losses,IEEE Transactions on Energy Conversion (1995),10(9):569-576[5] U.S. Environmental Protection Agency, GlobalWarming Web Site, Annual Emissions Calculator,http://199.223.18.23O/epa/rew/rew.ns~solar/index.htm1February 1999.[6] Osbom, D., Commercialization and businessDevelopment of Grid-Connected Photovoltaics atSMUD, Solar 98: Renewable Energy for the AmericasConference, Albuquerque, June 1998.[7] Thomas, H., B. Kroposki, P. McNutt, C. Witt, W.Bower, R. Bonn, and T. Hund, Progress in PhotovoltaicSystem and Component Improvements, 2nd WorldConference and Exhibition on Photovoltaic Solar EnergyConversion, Vienna, 1998, pp. 1930-1935.[SI Thomas, M., H. Post, and R. DeBlasio, PhotovoltaicSystems: An End-of-Millennium Review, Progress inPhotovoltaics: Research and Applications 7 , 1-19 ,1999.[9] IEEE Std. 1262 Recommended Practice forQualification of Photovoltaic Modules.[101UL Subject 1703, Flat-Plate Photovoltaic Modulesand Panels.[ll] Maish, C. Atcitty, S. Hester, D. Greenberg, D.Osbom, D. Collier, and M. Brine, Photovoltaic SystemReliability, 26 IEEE PV Specialist Conference,Anaheim, September 1997.[12] Stefanac,D. Process Quality in Manufacturing PVPower Conversion Products, 1998 PV Performance andReliability Workshop, Cocoa Beach, Florida, November1998.[13] IEEE Std 929 Recommended Practice for UtilityInterface of Photovoltaic Systems.

[151UL Subject 1741 Standard for Static Inverters andCharge Controllers for Use in Photovoltaic PowerSystems.[16] IEEE Std 519-1992 Recommended Practices andRequirements for Harmonic Control in Electrical PowerSystems.[171 Wiles, J. Sandia Report SAND96-2797,Photovoltaic Power Systems and The NationalElectrical Code : Suggested Practices, 1996.[18] IEEE Std 1374 Guide for terrestrial PhotovoltaicPower System Safety.[19] IEEE Project 1547 Standard for DistributedResources Interconnected with Electric Power Systems.http://grouper.ieee.org/groups/scc2 1 1547 March 2000.

E. IOGRAPHIESBenjamin Kroposki (S-90, M-93)s a senior engineer in the NationalCenter for Photovoltaics at the National Renewable Energy Laboratory(NREL) and Team Leader for the Engineering and Technology ValidationTeam. His expertise is in design and testing of photovoltaic systems, andhe has over 25 publications in the area of photovoltaics. Mr. Kroposkialso participates in the development of photovoltaic standards and codes,including IEEE, IEC, and NEC.Mr. Kroposki eceived his Bachelor andMasters degree in Electrical Engineering from Virginia Tech and is aregistered Professional Engineer.

Richard DeBlasio (M-65. SM-83) is a Principal Engineer and leadsthe D istributed Power Program and Photovoltaic System Performanceand StandardsDevelopment activities at the U. . National RenewableEnergy Laboratory (NREL). Before joining NREL (S EW in 1978,he was with the U.S. Atomic Energy Com mission in Washington,D.C., Underwriters Laboratories, and Stanford University. Mr.DeBlasio has focused primarily on system and component energyconversion and performance, design verification, reliability research,safety evaluation, and standards development and has over 30publications in the area of photovoltaics. He is an electrical enginee r,a senior member of the IEEE, chairs the IEEE Standards BoardCoordinating Committee 21 on Fue l Cells, Photovoltaics, DistributedPower, and Energy Storage, chairs the International ElectrotechnicalCommission Technical Committee 82 on Photovoltaic Systems, andserves on the Board of Directors of the Global Approval Program andthe PowerMark Corporation for component and system testing andcertification, and laboratory accreditation.

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technologies for the new millennium - phototvoltacis as a distributed resource

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