Basics of Medium-Voltage Wiring for PV Power Plant AC Collection Systems

Basics ofMedium-Voltage Wiringfor PV Power Plant AC Collection SystemsBy Dan Simpson, PE Basics ofMedium-Voltage Wiringfor PV Power Plant AC Collection Systemssolarprofessional.com|SOLARPRO59Theaccollectionsysteminautility-interactivePV system includes all of the wiring and components from the inverter output circuit(s) to the intercon-nectionpointwiththeutility.Commercial-scale PVplantsaregenerallyinterconnectedtothe utilityviabuildingwiringsystemsatcommonbuildingutili-zationvoltages,suchas208Vacor480Vac.However,asthe rated capacity of these interactive systems increases, so does the physical footprint of the PV power plant and the distances betweenwiringpoints.Basedonthesitelayout,thedesired PVgeneratingcapacityandotherproject-specifcvariables, system designers working on commercial-scale projects may fnd a practical need to use medium-voltage (MV) wiring for ac collection systems. In this article, I provide an introduction to some common components used in MV circuits in PV systems and discuss basic design considerations for their application. I cover MV components for use in ac collection systems, including dis-tribution transformers, overhead and underground feeders, pad-mountedswitchgear,andmetal-enclosedandmetal-clad switchgear. I also provide example single-line diagrams (seep.71)showingthreerepresentativeusesofMVwiring methodsandcomponentsincommercial-andutility-scale PV systems. SYSTEM VOLTAGE CLASSES AND WORKER SAFETY IusethetermMV heretodescribeelectricalsystemcompo-nentsratedbetween5kVand38kV.Tisdefnitioncorre-sponds to common US utility-distribution voltages. In practice, voltage class defnitions are somewhat tricky topindown.Teyvaryfromindustrytoindustryandfrom onesetofcodesorproductstandardstoanother.Onone hand,TeAuthoritativeDictionaryofIEEEStandardsTerms (IEEE 100) defnes medium voltage as a class of nominal sys-tem voltages greater than 1,000 V but less than 100,000 V. On the other, Article 490 of the National Electrical Code defnes highvoltageasmorethan600volts,nominal.Whilethe NECdoesnotincludeastand-alonedefnitionofmedium voltage, Article 328 details the Code requirements related to medium-voltage or Type MV cable. As described in NEC Sec-tions328.2and328.10,MVcableisratedat2001voltsor higher and is permitted for use on power systems rated up to and including 35,000 volts, nominal. While the exact defnition of MV may vary by context, the implications for worker safety are indisputable. Te voltage and potential fault energy levels of MV wiring systems pose signifcantsafetyhazards.Inadditiontoelectricshockand arc-fashburnhazards,bluntforceandprojectileinjuries are also possible due to arc-blast hazard. While it is beyond thescopeofthisarticletogointothedetailsofMVelec-trical safety, personnel must always be trained to recognize theparticularhazardsassociatedwiththespecifcclassofCourtesy Patrick ByrdMedium-voltage wiring is typically used for ac collection systems in utility-scale PV power plants. Depending upon project capacity and site complexity, it can also be a useful design option in commercial-scale PV applications. 60SOLARPRO| December/January 2013systemvoltagetheywillbeworkingwithandunderstand how to mitigate those hazards through safe work practices. Workers must have the proper tools and PPE, and be trained in their use. Inadditiontoapplicablecity,stateandcountyrequire-ments, work in PV power plants may fall under one or both of the following OSHA standards: CFR 1910.269Electrical Power Generation, Transmis- sion and Distribution CFR 1910 Subpart SElectricalGENERAL DESIGN CONSIDERATIONSTe design goals for a MV ac collection system are essentially thesameasthoseforotherpowerdistributionsystemsor any electrical system, for that matter. According to Eaton Cor-porations Consulting Application Guide (see Resources), Te bestdistributionsystemisonethatwill,cost-efectivelyand safely,supplyadequateelectricservicetobothpresentand futureprobableloads.TeonlydistinctionisthattheMV wiring in autility-interactive PV application does notsupply loads,butratherisdesignedtosupplyadequateserviceto interconnected electric power production sources. Te Consulting Application Guide goes on to list seven elec-trical distribution design goals, the most important of which is a safe installation that does not present any electrical haz-ards to people or equipment. Te other six design goals can be summarized as follows: Minimize initial investment Maximize service continuity Maximize fexibility and expandability Maximize electrical efciency and minimizeoperating costs Minimize maintenance costs Maximize power qualityTeseobjectivesarenotonlylaudable,butalsofamil-iartoanyonewhospendstimeoptimizingPVpowerplant designsaccordingtolevelizedcostofenergy,initialcostor someothermetric.Basedonthesegeneralgoals,anengi-neercancreatealistoftangibleprojectrequirements,and thenproceedtodevelopaMVaccollectionsystemdesign thatmeetsthoserequirements.Oneadditionaldesigncon-sideration that I fnd often applies to PV systems is the need to identify a design that can be constructed within the allot-ted project time frame. Intheinterestofsafety,mostPVprojectsarerequired bylawtocomplywiththerequirementsfoundinthelocally adoptedversionoftheNationalElectricalCode.Tefollow-ing is a partial list of NEC content pertinent to the design and construction of MV ac collection systems. Article110:RequirementsforElectricalInstallations, Part III Over 600 Volts, Nominal Article 240: Overcurrent Protection, Part IXOvercurrent Protection Over 600 Volts, NominalMedi um-Vol t ageWi ri ngCourtesy Eaton Corporation and Cooper Power SystemsEaton 1.5 MW Power Xpert Solarutility-scale inverter Inverter output tolow-voltage windingsof pad-mounted transformerCooper Power Systems1.5 MVA loop-feed,pad-mounted transformer 12.47kV loop-feedac collection circuitto next inverter skid12.47 kV collection circuit to plant MV switchgear15 kV metal-clad main plant switchgearPole riser12.47 kV plant main output feeder circuitFusedcutout12.47 kV overhead plant output feederCollection system componentsThis SketchUp diagram shows the major MV components and wiring methods used in a PV power plants ac collection system. CONT I NUE DONPA GE 6 262SOLARPRO| December/January 2013 Article 300: Wiring Methods and Materials,Part II Requirements for Over 600 Volts, Nominal Section 310.60: Conductors for General Wiring,Conductors Rated 2,001 to 35,000 Volts Article 314: Outlet, Device, Pull and Junction Boxes,Part IV Systems over 600 Volts, Nominal Article 328: Medium Voltage Cable, Type MV Article 399: Outdoor Overhead Conductors over600 Volts Article 490: Equipment, Over 600 Volts, NominalDesignerscanidentifyotherNECsectionsthatmayapply specifcally to MV wiring by referring to the entries under the term Over 600 volts in the index for the NEC. WhenreviewingNECrequirements,rememberthatthe Code outlines minimum requirements for an electrical system that is free of hazards. According to Section 90.1(B), NEC com-pliance is no guarantee of adequacy. As explained in Section 90.1(B), a hazard-free installation may not be efcient, conve-nient, or adequate for good service.DonWagner,vicepresidentofengineeringatDelta DiversifedEnterprises,afull-serviceelectricalcontractor servingtheWesternUS,providesarelevantexample.He pointsoutthatsimplymeetingCode-minimumworking clearancerequirementsforMVequipmentdoesnotneces-sarilyensureconstructability.Wagneradvises:Toprovide sufcient space for installation and maintenance, it is often necessarytoprovidemoreworkingspacethanisrequired in the NEC.DISTRIBUTION TRANSFORMERS A step-up transformer is required in a MV ac collection system tomatchthesourcevoltage(theinverteroutputvoltage)to the 3-phase distribution voltage. Ideally, the ac output voltage isinitsnativeform,meaningdirectfromthesemiconductor switching devices prior to any isolation transformer. Note that manyinvertersinthe2013CentralInverterSpecifcations: 600 Vdc Models (pp. 8490) are matched to common 3-phase utilizationvoltagessuchas208Vac,240Vac,480Vacor600 Vacgenerallybymeansofaninternalorexternalisolation transformersoldwiththeinverter.Meanwhile,theacoutput voltagesforproductsinthe2013CentralInverterSpecifca-tions:1,000VdcModels(pp.4652)areinthesamegeneral voltage range (300 Vac to 700 Vac), but do not necessarily match commonutilizationvoltages.Tatisbecausethelatterare native inverter ac output voltages. CONT I NUE DONPA GE 6 4

Medi um-Vol t ageWi ri ngBacked by extensive research and years of experience, ERITECH brand of grounding products deliver proven performance for demanding solar applications.ERITECH brand of Solar Bonding Lugs: Offer unique lay-in feature (pre-mount the lug before the wire) Provide easy installation; completed with one tool Accept #14 solid to #6 stranded AWG copper grounding conductors Offer a direct burial version Are available with #8 hardware, #10 hardware or no hardware UL 467 ListedERICO also has standard and made-to-order custom braids for solar applications. Visit www.erico.comUL is a registered trademark of Underwriters Laboratories, Inc.ERICO: Your Source for Solar Grounding SolutionsES1059AM12NA-01.indd 1 8/8/12 8:13 AM64SOLARPRO| December/January 2013Pad-mounted, 3-phase distribution transformershere-afterreferredtosimplyasPMtransformersaretypically usedintheMVaccollectionsystemofaPVpowerplant. Tistypeoftransformeriscommonplace.Itisusedwher-ever3-phasebuildingloadsareservedbyanunderground MV distribution system. Transformer location often varies based on system capac-ity.Incommercial-scalePVsystemsthatareinterconnected usingasingleinvertertypicallyinthe100kWto500kW rangeatransformerisusuallyinstalledateachinverter location.Onlargercommercial-andutility-scalesystems, integratorsnormallyinstallonetofourinvertersonacom-mon skid or within an enclosure along with a MV transformer. Teseintegratedinverterblocksoftencarrynameplaterat-ings in the 500 kW to 2 MW range. Whenthesepackagedsystemsarespecifed,theinverter manufacturer usually provides a step-up transformer for each block of inverters that matches the rated power output of the inverterskid.Onebeneftofthesepackagedsolutionsisthat thelow-voltagewiringbetweentheinverteroutputsandthe low-voltagebushingsonthetransformerisfactoryinstalled, which reduces construction labor costs. Another beneft is that thespecifedstep-uptransformershouldworkoptimallywith theinvertersasasystem.Sinceensuringproperperformance is critical, I recommend having the inverter manufacturer pro-vide the MV step-up transformer as part of its inverter package whenever possible.CONT I NUE DONPA GE 6 6 Dead-front MV terminationThe electrical connec-tions to the high-voltage bushings inside this dis-tribution trans-former are made with 15 kV class fused load-break elbow connectors. Separable connec-tors like these are common in under-ground distribution systems. Courtesy Blue Oak EnergyMedi um-Vol t ageWi ri ng66SOLARPRO| December/January 2013While it is beyond the scope of this article to detail all of the considerations applicable to the selection and specifcation of aMVdistributiontransformerforaMVaccollectionsystem, some of the main criteria include: Power rating Primary and secondary voltage ratings Winding confguration Dielectric fuid class High-voltage connection type Number of high-voltage bushings per phase Optional accessoriesPower rating. Te unit of measurement used to identify the powerratingofatransformeristhekilovolt-ampere(kVA). TePMtransformersusedinaPVplantsMVaccollection systemarecommonlyratedat500kVA,750kVA,1,000kVA, 1,500 kVA, 2,000 kVA or 2,500 kVA. According to Kleber Facchini, solar inverter product man-ageratEaton,Transformersareusuallysizedatthesame power rating as the inverters. However, there are other consid-erations. He continues: Be sure to let the transformer supplier know that the end use is an inverter application, as it is impor-tant to understand the inverter harmonics. Site temperature is anotherimportantconsideration,sincethesupplierneedsto size the transformer so that the transformer winding hot spot doesnotexceedANSIlimits.Whiletransformersareusually designed to operate 24 hours per day and 7 days per week.Voltage ratings. By defnition, the primary winding of a trans-former is the winding that power is applied to, and the second-arywindingisthewindingthatvoltageisinducedon.Inthe case of a MV ac collection system, the inverter applies power to the low-voltage side of the transformer, and MV is induced on the high-voltage side of the transformer. However, in practice, it is conventional to refer to the high-voltage windings as the pri-maryandthelow-voltagewindingsasthesecondary.Bothon theequipmentitselfandinelectricalschematics,low-voltage connections to the transformer are often labeled X1, X2, X3 and so on, whereas high-voltage connections are labeled H1, H2, H3 and so on. Te ratio of the number of primary winding turns to sec-ondarywindingturnsisknownastheturnsratio,whichis proportionaltotheratiooftheprimaryvoltagetothesec-ondaryvoltage.AtransformerusedinaMVaccollection systemisreferredtoasastep-uptransformer,indicating thatinthisapplicationtherearemoreturnsinthehigh-voltagewindingthaninthelow-voltagewinding,result-ingin,forexample,a1:10or1:30turnsratio.Notethatin aconventionalpowerdistributionapplicationinwhicha MVutility-distributionnetworksupplies3-phasepowerto acommercialfacilitythesamedevicecouldbeusedasa step-down transformer. StandardPMtransformersaremanufacturedwithturns ratiosmatchedtocommondistributionandutilizationvolt-age combinations, such as 13.8 kV to 480 V. While these stan-dardwindingconfgurationsmayworkinsomePVpower plant applications, native inverter output voltages do not nec-essarilymatchcommonutilizationvoltages.Terefore,cus-tom turns ratios are often specifed for PM transformers used in utility-scale MV collection systems. Windingconguration.Whilethesystemengineermay haveadesignpreferencewithregardtotransformerwind-ingconfguration,inpracticethechoiceofinverteronthe onehand,andtheutilityinterconnectionrequirementson theother,signifcantlyimpactthisdecision.Terefore,it isimportanttocoordinatecarefullywithboththeinverter manufacturer and the AHJ when specifying transformers for a MV collection system.ChrisTompsonisthesolarbusinessunitmanagerfor Eaton. He notes that three types of MV transformers are used insolarapplications,dependingupontheinvertermanufac-turersrequirements.Tompsonelaborates:Someinverter manufacturers use the transformer like an inductor to provide flteringandthereforerequireaspecialPWM-capabletrans-former, one that is designed to operate with a pulsed inverter. Ten there are multi-winding transformers, which have mul-tiple isolated low-voltage windings and are used for inverters that cannot be connected in parallel. As an example, a 3 MW multi-windingtransformermighthavetwoseparate1.5MW low-voltage windings, each of which would be dedicated to a separate 1.5 MW inverter. Te third type of transformer used insolarapplicationsisthesimplestsolution,withonelow-voltage winding and one medium-voltage winding.Transformermanufacturerscanprovidebothlow-and high-voltage windings in either a delta or a wye confguration. InPVapplications,itiscommontouseawyeconfguration for low-voltage windingseither grounded or ungrounded, as directed by the inverter manufacturerand a delta confgura-tion for high-voltage windings. Dielectricuid.Liquid-typePMtransformersareoften used in PV power plant applications. Tese are also referred toasliquid-flledorliquid-immersiontransformersbecause the steel enclosure is flled with a dielectric fuid, a fuid that is not conductive under normal circumstances. Te primary andsecondarywindingsareinstalledaroundacommon coreandimmersedwithinthisfuid.Inadditiontoelectri-cally insulating the internal components, the dielectric fuid helps keep them cool. As waste heat is generated in the core and windings, the dielectric fuid circulates via natural con-vection within the transformer tank, facilitating heat trans-fer to the environment.Engineersspecifyingliquid-typePMtransformersfor MVcollectionsystemstypicallyhavetwobasicdielectric fuid options: mineral oil or vegetable oil.CONT I NUE DONPA GE 6 8 Medi um-Vol t ageWi ri ng68SOLARPRO| December/January 2013Mineraloilhasbeenusedasadielectricfuidformany decades and is a proven and cost-efective option. Vegetable-based dielectric fuids are less fammable than mineral oil and more environmentally friendly. Vegetable oil not only is biode-gradable, simplifying disposal, but also is made from a renew-ableresource.Examplesofvegetable-baseddielectricfuids include Envirotemp FR3, which Cargill recently acquired from Cooper Power Systems, and BIOTEMP, which ABB developed. Bothofthesevegetable-baseddielectricfuidsareavailable from major transformer manufacturers.AdamPetersonisanapplicationsengineeratCooper PowerSystemswhospecializesinsolarenergyapplications. According to Peterson, seed oilbased Envirotemp FR3 dielec-tric fuid is broadly specifed in renewable energy generation projectsforseveralreasons:Ithasbeenspecifcallyformu-latedtobebiodegradableandnontoxic;ithasanextremely highfashpoint;andbecauseofitsthermalcharacteristics, thetransformercanbedesign-optimizedtothesolarload profle in terms of cost, footprint and insulation life. High-voltageconnectiontype.Engineersefectivelydeter-mine the type of high-voltage connection based on the trans-former construction type they specify. A live-front transformer useselectricallyexposedhigh-voltageconnections,suchas porcelain bushings with eyebolts or spade terminals. Alterna-tively, a dead-front transformer provides electrically insulated andshieldedhigh-voltageconnectionpoints.Applications engineerstypicallyrecommenddead-fronttransformersand connectorsforPVpowerplants.Dead-frontconstruction providesanadditionallevelofsafetyincomparisontolive-front construction, and the vast majority of PM transformers employ it.Teepoxybushingsinthehigh-voltagetransformercom-partmentinFigure1aretypicaldead-fronttransformerter-minationpoints.Tisiswhereinstallersmakeconnections totheMVaccollectionssystemwiring.Intheoppositecom-partment,inverteroutputconductorscanbeconnectedto the low-voltage bushings of a PM transformer using standard compression-type spade connectors with NEMA hole patterns. Number of high-voltage bushings per phase. Depending upon theapplication,PMtransformerscanbespecifedwitheither one or two high-voltage bushings per phase. A radial feed trans-former has one bushing per phase; a loop-feed transformer has two high-voltage bushings per phase. Te transformer in Figure 1 is a loop-feed transformer.CONT I NUE DONPA GE 7 0Medi um-Vol t ageWi ri ngCourtesy Cooper Power SystemsFigure 1The major features inside a 3-phase, pad-mounted compartmental distribution transformer are labeled above. The two sets of high-voltage bushings indicate that this is a loop-feed transformer.Bay-O-NetfusingLoad-break switchDripshieldHigh-voltage bushingSill(suitable for skidding, rolling and jacking)ParkingstandFive-positiontap changerGround pad andstrap for X0Liquid levelgaugeNameplate (laser-scribed anodized aluminum)Low-voltage bushing supportLow-voltage bushing (low-voltage molded epoxy bushings with NEMA spades)Removablecabinet walls70SOLARPRO| December/January 2013According to Peterson at Cooper Power Systems, loop-feed transformerconfgurationsarepopularinPVpowerplant applicationsbecausetheyallowseveraltransformerstobe paralleledtogetheronacommonMVcollectioncircuit.How-ever,increasingthecapacityconnectedtothecollectioncir-cuitmeansthattheengineermustaccountforhigherdesign currents. Peterson cautions, Whenever multiple transformers are paralleled on a single collectioncircuit, the engineer must consider the efect this has on the required ratings for the trans-formerbushingsandcableterminationsforexample,600A bushings may be required in lieu of 200 A bushings based on the total connected kVA. Optionalaccessories.Irecommendthatengineersspecify-ing PM transformers for collection circuits include the follow-ing items, which are typically available from the manufacturer asoptionalaccessories:tapchanger,overcurrentprotection, load-breakrated switch and surge protection.Tapchanger:AsexplainedintheCooperPowerSystems documentS210-12-1,Tree-PhasePad-MountedCompart-mental Type Installation and Maintenance Instructions (see Resources),Transformersequippedwithatapchangercan be changed from one operating voltage to another. To safely usethetapchanger,aworkerfrstneedstode-energizeand groundthetransformer.Anelectriciancanthenoperatethe tap-changerhandlewithahotstick,aninsulatedpolethat electric utility workers use as a standard service tool. Ahot-stickoperablefve-positiontap-changerswitchis showninsidethehigh-voltagetransformercompartmentin Figure 1 (p. 68). Te available operating voltages are indicated totherightoftheswitch.Basedonthelocationofthelock screw, the tap changer in this photo is set to position C, which inthiscasecorrespondsto24,320volts.Ifthistransformer were used in a PV application, the four additional tap settingsrangingfrom25,570to23,070voltswouldprovidetheplant operator with some fexibility for dealing with inverter-inputvoltage tolerance issues that might arise. Overcurrentprotection:Ifmultipletransformersarecon-nected in parallel on a common circuit, then overcurrent pro-tection is likely required. Since a variety of schemes can provide the necessary protection, I recommend consulting an applica-tions engineer to get advice for your specifc project. InaPMtransformer,Bay-O-Netfuseassembliesareused. TreeBay-O-Netfusehousingsarevisibleabovethedrip shieldinFigure1.Ahotstickcanbeusedaccordingtothe transformermanufacturersinstructionstosafelyinstallor remove these fuse assemblies.Medi um-Vol t ageWi ri ngsolarprofessional.com|SOLARPRO71EXAMPLE 1: MV Wiring Used forLong-Distance Interconnection This is a single-line diagram of a 300 kW PV sys-tem installed at an agricultural site. The array and inverter are located approximately 2,000 feet from the 3-phase 240 Vac premises wiring. Because of the long distance and the low interconnec-tion voltage, MV wiring is used in the inverter output circuit. The inverter produces a 480 Vac output, which is stepped up to 12.47 kV via a pad-mounted distribution transformer. A 12.47 kV feeder is run underground via MV cable to the interconnect point, where it is stepped down via a second pad-mounted distribution transformer to the interconnect voltage of 240 Vac. EXAMPLE 2: Simple MV AC Collection System This is a single-line diagram for a 2 MW PV sys-tem at a university campus. A pad-mounted transformer is installed adjacent to each of the six inverters to step the inverter output voltage up to 12.47 kV. Two MV collection circuits are used, each with three loop-feed transformers. The feeder circuits are collected at a lineup of 12.47 kV metal-enclosed switchgear that provides fused disconnecting means for the collection circuits and energy metering for the PV power plant. A single PV plant output feeder is routed from the switch-gear to the point of common coupling, which in this case is the campuss primary metered 12.47 kV distribution system.EXAMPLE 3: Utility-Scale MVAC Collection System This is a theoretical example of a MV collec-tion system for a 20 MW utility-scale PV power plant. Twenty 1 MW inverter skids are distrib-uted throughout the array eld, each with a MV distribution transformer that steps the inverter output voltage up to 34.5 kV. Three ac collec-tion circuits are routed back to the MV metal- clad switchgear. A single output feeder is routed to a substation transformer that steps the inter-connection voltage up to 69 kV. {InverterPad-mounteddistributiontransformer used to step up invertervoltage to 12.47 kVPad-mounted distribution transformer used to step voltage down to 3-phase 240 Vac 12.47 kV feeder (2,000 ft)Interconnectinto premisesbuilding wiringPV arrayM15 kV metal-enclosed switchgearInverterPV arrayInverterPV arrayInverterPV arrayInverterPV arrayInverterPV arrayInverterPV arrayInterconnect intocampus distributionsystem at 12.47 kV12.47 kV loop-feedpad-mountedtransformerM38 kV metal-clad switchgearInverterPV arrayInverterPV arrayInverterPV arrayInverterPV arrayInverterPV arrayInverterPV arrayInterconnectinto 69 kVsubstation34.5 kV loop-feedpad-mountedtransformerInverterPV arrayMV circuit 3MV circuit 1(similar tocircuit 3)MV circuit 2(similar tocircuit 3)34.5 kV to 69 kVsubstation transformer1 MW inverter skid(typical of 20)Three Examples of MV Wiring in PV Power Systems72SOLARPRO| December/January 2013Load-breakratedswitch:Ifaload-breakratedswitchisincludedinaPM transformer,thenitispossibletousea hotsticktomanuallyopenorclosethe connectionbetweentheloop-feedcon-ductorsandtheprimarytransformer windings.Tisoptionisusefulwhen-everserviceneedstobeperformedon thelow-voltagesideofthedevice.Te PMtransformerinFigure1includesa two-position load-breakrated switch.Surge protection: Te purpose of surge arresters in a PM transformer application istoprotectagainstovervoltagesurges due to lightning or other transients. Con-sultaproductapplicationsengineerfor helpselectingthepropersurgearrester for your application.Otherconsiderations.Whilesome basic requirements can be self-evident, liketransformercapacityandvoltage ratings,more-nuanceddesigndeci-sionsrequirecarefulconsultationwith applicationsengineers.Forexample, Petersonpointsout,Transformer impedance needs to be matched with that of the associated inverters, as specifed by the inverter manufacturer. He con-tinues,Tereareharmonicandshieldingrequirementsto consider, as well as gauge package options to specify.MV WIRING MVwiringisnecessarytocarryPVplantpowerfromthe PMtransformerstothepointofcommoncoupling,where the power production equipment connects to the utility dis-tributionnetwork.TeMVwiringcomponentsutilizedin PVplantsareessentiallythesameasthosefoundinpower companydistributionsystemsandinindustrialpowersys-tems.TetwobasiccategoriesofMVwiringareoverhead and underground.Overheadwiring.Uninsulatedconductorsattachedto woodorsteelpolesviaporcelaininsulatorsarerepresen-tativeofoverheadMVwiringmethods.Installersplace anchoredguywiresinspecifclocationsasrequiredtoof-setlateralforcesonthestructures.Whereoverheadwiring transitions to underground wiring, a MV cable riser extends upthepoleandterminatesthere.FusedcutoutsattheMV cableterminationpointprovideovercurrentprotection.In addition,installersoftenplacesurgearrestorsattheover-head-to-undergroundtransitionpoint.Figure2identifes thecomponentsusedtotransitionbetweenunderground and overhead MV wiring systems.InMVcollectionsystems,overheadwiringmethodsare less common than underground methods. Tis is due in part to the fact that pole-mounted overhead wiring is problematic whencrossinganarrayfeld,sinceshadingcanadversely impactPVperformance.Nevertheless,insomeinstances overhead wiring methods provide an opportunity for cost sav-ings.OneexampleiswhenaPVplantsmainoutputfeeder needstorunforsomedistancetoaremoteinterconnection point.Overheadwiringwilllikelyprovemorecostefective than underground wiring in this situation. Undergroundwiring.TypeMVcable,inoneofitsmany forms,isusedinundergroundMVwiringapplications.Type MVcableisavailableinbothsingle-conductor(single-core) and three-conductor (three-core) variations. As shown in Figure 3 (p. 74), six distinct layers, each per-formingauniquefunction,areusedintheconstructionofa single-conductorTypeMVcable.Movingfromtheinsideto the outside, these layers are: conductor, strand shield, insula-tion, insulation shield, metallic shield and cable jacket. Conductor: Tis layer can be made of copper or aluminum andcarriescurrent.Aluminumconductorsarecommonly used in utility-distribution applications, whereas copper con-ductors are commonly used in industrial applications. While either material is acceptable in PV plant installations, alumi-num is generally more cost efective.Strandshield:Madeofasemiconductingmaterial,this layer separates the conductor and the insulation. Its function is to shield the insulation from air pockets between the con-ductor and insulator. Without the strand shield, this air would ionize and cause partial discharges that could deteriorate the insulation and lead to cable failure. Insulation: Tis layer contains the voltage within the cable. CommonMVcableinsulationmaterialsCONT I NUE DONPA GE 7 4 Medi um-Vol t ageWi ri ngCourtesy Taylor RyMarFigure 2The major components used at the transition between underground and overhead MV wiring methods are identifed here.Overhead lineExpulsion fuseFused cutoutCableterminationMV cableSurge arrester74SOLARPRO| December/January 2013includewater-treeretardant,cross-linkedpolyethylene (TR-XLPE) and ethylene propylene rubber (EPR). MV cables are typically available with two basic insulation thickness options: 100% level or the thicker 133% level. According to the Engineer-ingHandbookpublishedbyelectricalcablemanufacturerthe Okonite Company (see Resources), 100% level cables are gener-ally intended for applications in which ground faults are cleared within1minuteorless;133%levelcablesshouldbeapplied whenfaultclearingtimesinthe1minuteto1hourrangeare expected, or when increased insulation strength is desired.Insulationshield:Madeofasemiconductormaterial,the functionofthislayerissimilartothatofthestrandshield. Te insulation shield protects the insulationfrom air pockets between the metallic shield and insulator material. Metallicshield:Tislayerservesseveralpurposes.Itcon-fnes the cables electric feld, equalizes electrical stress within the cable, limits radio interference and reduces shock hazard. Various types are available, including tape shield, wire shield and concentric neutral shield. Cable jacket: Te jacket layer provides mechanical protec-tion for the cable. PVC is commonly used as a jacket material, butsomeTypeMVcableisavailablewithPVC-jacketedalu-minum armor for enhanced mechanical protection. MVcableterminations.MVcablesmustbeproperlytermi-nated for reliable service. In addition to facilitating the cables electrical connection, MV cable terminations perform several otherimportantfunctions,suchasrelievingvoltagestress that would otherwise build up at the insulation shield termi-nationpoint;sealingtheterminationagainstmoistureand environmentalcontaminates;andpreventingelectricaltree-ing or tracking at the termination point.Various materials and methods are available for MV cable terminations, including separable insulated connectors, cold shrink,heatshrink,tapeandporcelain.Separableinsulated connectors,alsoknownaselbows,aretypicallyusedwhere separable dead-front construction is desired. Common sepa-rable elbows include 200 A load-break and 600 A dead-break connectors. Load-break elbow connectors with a 200 A rating canberemovedwithahotstick,whereas600Adead-break elbowsareboltedconnectors.Wherenon-elbowtypecon-nectorsareused,molded-rubbercold-shrinkterminations aregenerallypreferredbecausetheyrequirelesstechnical skill to install than tape terminations and they do not require atorch.PorcelainbushingsarecommonlyusedwhereMV cables transition to overhead wiring.ProperlyterminatingMVcablesrequiresspecialized skills and tools. It is essential that installers receive product-specifctrainingandfollowthemanufacturersinstructions. SinceMVcableterminationmethodsandproceduresvary by manufacturer, the best resource for installers is always the specifc product manufacturer.MV SWITCHGEARIn a PV plant with a MV collection system, it is often necessary to use MV switchgear to collect the various MV feeder circuits. In addition, the MV switchgear often serves as the demarcation point between the PV power plant and the utility distribution system.Teswitchgearservesmultiplepurposes.Itprovides Medi um-Vol t ageWi ri ngCourtesy Taylor RyMarInsulationPVC cablejacketConductorStrand shieldMetallic shieldInsulation shieldConductorStrandshieldInsulationInsulation shieldMetallicshieldPVC cablejacketFigure 3The six layers that make up a single-conductor Type MV cable are shown here in cross-section and side views.solarprofessional.com|SOLARPRO75both disconnecting means and overcurrent protection for the MV feeders that make up the collection system. In addition, it provides a location for the utility and the facility owner to mea-sure the total energy output of the PV plant. In some cases, the switchgearalsoprovidestheutilitywithremotesupervisory and control capabilities. Te concepts that govern the use of MV switchgear are simi-lar to those governing the use of equivalent 600 Vrated devices. MV switchgear is specifed according to basic parameters such asvoltageclass,continuouscurrentrating,momentarycur-rentrating,interruptingratingandenclosuretype(indooror outdoor).ToproperlyuseMVswitchgearinanaccollection system,theengineermustconsiderspecifcPVpowerplant parameters and information about the interconnecting utility grid, such as the available fault current. MVswitchgeardifersconsiderablyfrom600Vrated devicesinitsconstruction.InPVapplications,threediferent types of MV switchgear construction are common: metal clad, metalenclosedandpadmounted.Inmanycases,thedesign engineer tasked with specifying the MV switchgear can choose between MV circuit breakers or MV fused switches to provide the required disconnecting and overcurrent protection means.Metal-cladandmetal-enclosedswitchgear.Historically, metal-clad and metal-enclosed switchgear are used primarily in large commercial, institutional and industrial applications. However, in PV plants with MV ac collection circuits, a single lineupofmetal-cladormetal-enclosedequipmentisoften used as the plants primary switchgear. Metal-cladswitchgearisbuilttoANSIStandardC37.20.2 andusesdraw-outcircuit-breakerdevices.Metal-enclosed switchgearisbuilttoANSIStandardC37.20.3andcan accommodatedraw-outcircuitbreakers,fxed-mountedcir-cuit breakers or load-break switches and fuses. By the nature of its construction, metal-clad switchgear ofers superior fault isolationpropertiescomparedtothemetal-enclosedtype; however, it costs signifcantly more. ScottBrady,adistrictapplicationsengineerforEaton, describesthediferencesinconstructionbetweenmetal-clad and metal-enclosed switchgear: Metal-clad switchgear is con-structedusingdraw-outovercurrentdevicesonlyandinsu-lated primary bus connections; it is available with voltages up to 38 kV, high fault-current ratings and automatic shutters that close when draw-out devices are removed. In contrast, metal-enclosedswitchgearcanusefxed-mounteddevices,suchas vacuumbreakersorfuses;livepartsarenotindividuallyiso-lated; and bus connections can be uninsulated. Metal-enclosed switchgear is voltage range limited to 15 kV with vacuum break-ers or to 38 kV with fused switches. Since the breakers or fuses are front accessible only, shutters are not required.WhilethetypeofMVswitchgearusedinPVapplications varies according to system size, Brady notes that the intercon-nection voltage and utility requirements also drives this choice. For the main interconnect switchgear, says Brady, if the dis-tributionvoltageis15kVandbelow,metal-enclosedswitch andbreakerswitchgeartypicallyprovidesthebestvaluefor the project. Tis type of switchgear incorporates a switch and fxed breaker in a single structure and can be provided with the typeofmeteringcompartmentsrequiredbyWesternutilities. Te switch portion of the switchgear provides a visible means ofdisconnect,whichutilitiessometimesrequire,whilethe vacuumcircuitbreakerincorporatesadjustable3-phaseover-current protection, remote operation if required, and arc-fash reductionsafetyfeaturesnotavailablewithafuse.Ifthedis-tribution voltage is above 15 kV, then metal-clad switchgear is typically required to meet utility interconnection requirements. Metal-enclosed fused switches do not provide the relay protec-tion required to coordinate with the utility protective devices, and remote operation is complex with switches versus stored-energy vacuum circuit breakers.Pad-mounted switchgear. Electric utilities commonly use pad-mountedswitchgearintheirdistributionsystemsthroughout cities and neighborhoods. It has the advantage of being much morecompactthanmetal-cladormetal-enclosedswitchgear, withthetrade-ofofprovidingfewerfeatures.Unlikemetal-clad or metal-enclosed switchgear, pad-mounted switchgear is typically limited to three distribution circuits. Fused switching meansaregenerallyforcircuitprotectionandisolation;how-ever,otherdesignoptionsareavailable.Pad-mountedswitch-gear is typically hot-stick operated and is available in live-front and dead-front construction.TesimplestformofaPVplantMVaccollectionsystem involves one or more collection feeder circuits extending from loop-feedtransformersdirectlybacktotheplantsprimary Shawn SchreinerLive-front MV terminationThe MV bus (top left) inside this GE substation is insulated using porcelain bushings. The MV cables bolted to this bus are terminated with compression-type spade connectors and molded-rubber cold-shrink seal-ing assemblies.76SOLARPRO| December/January 2013MVswitchgear.However,inlargerPVplants,pad-mounted switchgear is sometimes used in the array feld to further sec-tionalizeMVfeedercollectioncircuits.Inthisscenario,MV ac collection circuits extend from the plants main switchgear to one or more pad-mounted switch locations, at which point theyseparateintoseveralsmallercollectionfeedercircuits. When used this way, pad-mounted switchgear can reduce the number of main ac collection feeder circuits and therefore cut downonthenumberofdistributioncubiclesrequiredinthe main MV switchgear. Teuseofpad-mountedswitchgearinstrategicloca-tionsinthearrayfeldofersreliabilityandmaintenance beneftsbyallowingthemainaccollectioncircuitstobe brokenintosmallerradialcircuitsthatcanbeindividually fusedandswitched.Tispracticealsoallowsfortheuseof smaller feeder conductors downstream from the switch loca-tions. Disadvantages associated with the use of pad-mounted switchgearincludeincreasedaccollectionsystemcomplex-ity, higher cost for the pad-mounted switches and additional space requirements for the switches. MVcircuitbreakers.MVcircuitbreakersystemsconsistof two parts: the circuit breaker and one or more relays. Te cir-cuitbreakercomponentisessentiallyasetofcontactsthat openorclosethecircuit.Teprimarydiferencebetween thevariousMVcircuitbreakertechnologiesavailableisthe mediumusedtoextinguishthearcthatdevelopswhenthe breakerisoperatedunderload.Dependingonthedesignof theMVcircuitbreaker,theinsulationmediumcouldbeair, oil,SF6gasorvacuum.Modernvacuumcircuitbreakersare the most common choice for new installations.MVcircuitbreakersareavailableforeitherdraw-out mountingorfxedmounting.Adraw-outbreakerconsistsof two parts: the base, which is bolted to the MV cubicle frame, andtheactualbreakeritself.Tistwo-partconstruction allows the breaker to be racked in or racked out of the cubicle formaintenanceorreplacement.Inadditiontobeingeasier toservicethanfxedbreakers,draw-outbreakershavethe advantageofprovidingvisualconfrmationwheneveracir-cuit is disconnected from the switchgear.Sincemostcircuitbreakersdonotcontainanyinternal operational logic, a separate relay is required to operate the breaker.Tisrelaysensesthecircuit-faultconditionand sends a signal to the MV breaker that causes it to open. His-torically,relayswereelectromechanicaldevicesthateach performedonlyoneprotectivefunction.Ifasinglecircuit breakerrequiredmultiplecircuit-protectionfeatures,that alsonecessitatedseveralelectromechanicalrelays.Inmod-erncircuitbreakers,solid-staterelays,whichprovidemul-tiplecircuit-protectionfunctionsinasinglepackage,have replaced electromechanical relays. Eachofthecircuit-protectionfunctionsavailablein asolid-staterelayhasauniqueANSIdevicenumber.For example,SchweitzerEngineeringLaboratoriesSEL-351, whichiscommonlyusedasafeeder-protectionrelayinutil-ityandindustrialelectricalsystems,includesthefollowing circuit-protectionfunctions:overcurrent(ANSIdevicenum-bers50and51),undervoltage(27),overvoltage(59)and frequency(81).Terelayssensevoltageandcurrentusing instrumenttransformers.Ifcircuitbreakerswitchingis required during a power outage, then battery-backup systems can provide control power to the relays. Inadditiontofacilitatingmanymodesofcircuitprotec-tion,solid-staterelaysalsooferfexibilitywithregardto selectivecoordination,whichreferstotheabilitytolocalize theefectsofanoutage.Te2008cycleofrevisionsadded thisconceptofcoordinationtotheNEC;seethedefnitionin Article100.AsdescribedintheexplanatorytextintheNEC Handbook, Te main goal of selective coordination is to iso-latethefaultedportionoftheelectricalcircuitquicklywhile at the same time maintaining power to the remainder of the electrical system.MVswitchesandfuses.TeswitchesusedinMVgear forload-breakswitchingoperationsareclassifedasload- interrupterswitches.Teseswitchesaregangoperatedand canbefusedornonfused.TeyareCONT I NUE DONPA GE 7 8 Medi um-Vol t ageWi ri ngMetal-enclosed switchgearTwo separate 15 kV rated, 1,200 A metal-enclosed switchgear assemblies, manufac-tured by Myers Power Products, are delivered to PG&Es 15 MWac solar farm in Five Points, CA. Blue Oak Energy pro-vided engineering and construction management services on the project to the EPC contractor, SOLON USA.Courtesy Blue Oak Energy78SOLARPRO| December/January 2013generally provided with viewing windows that allow operators to verify switch blade position without exposure to live parts.MVfusesutilizedinMVswitchgeararebroadlyclassi-fed as either the expulsion type or the current-limiting type. Expulsionfusesareventedandusehotgasestofacilitate the mechanical interruption of the circuit. Current-limiting fusesaresealedanddonotexpelheatedgasduringopera-tion.Asthenameimplies,current-limitingfuseslimitthe magnitudeofthefaultcurrent,anddosowithinaquarter ofacycleprovidedthattheimpedanceofthefaultislow enough.Expulsionfusesaregenerallyavailablewithhigher voltageratingsthancurrent-limitingfuses,whilecurrent-limitingfusesaregenerallyavailablewithhigherinterrupt-ing ratings than expulsion fuses.Circuit breakers vs. fused switches. MV circuit breakers and MV fusedswitcheseachhavetheiruniqueadvantages.Teadvan-tages of MV fused switches include lower cost, simplicity and the possibilityofprovidingcurrent-limitingcircuitprotection.Te advantages of MV circuit breakers include higher available con-tinuouscurrentratings,theabilitytorelativelyeasilyincorpo-rate remote switching, and the benefts associated with the use ofsolid-staterelays,whichincludemultiplecircuit-protection functions and selective coordination.CONT I NUE DONPA GE 8 0Medi um-Vol t ageWi ri ngMV cable pullWorkers at PG&Es Five Points solar farm use an electric winch to pull MV cables into one of the sites metal-enclosed switchgear assemblies. The distribution volt-age around the 100-acre site is 12.47 kV.Courtesy Blue Oak Energy80SOLARPRO| December/January 2013PLANNING FOR SUCCESS Giventheconsiderableexpense and long lead times associated with PMtransformersandMVswitch-gear,itiscriticaltoensurethatthe productsspecifedmeeteveryones criteria.Tisincludestheowners specifcations,thetechnicalneeds of the project, the inverter manufac-turersguidelines,thetermsofthe utilityinterconnectionagreement and the AHJs requirements. BryanGrogan,seniorproject superintendent for Delta Diversifed Enterprises,notes,Efectivecoor-dinationwiththeinterconnecting utilitycompanywillreducecostly delaysandequipmentchanges. Heexplains:Tiscommunication needstohappenatvariousstages oftheprojectincludingdesign, construction and acceptance phases.During the design phase, it is important to verify the utility companys requirements for meteringanddistributionequipment,aseachutilitytypically hasitsownspecifcrequirements. Youneedtounderstand what is important to that utility and to make sure this is incor-porated early in the design process.To smooth the fnal utility acceptanceprocess,itisadvisabletosubmitequipmentshop drawingsforapprovalbeforeorderinganyequipmentandto request a courtesy inspection of the metering equipment when it arrives on-site.Until there is greater equipment standardization, the most efectivewayofdealingwithlongproductleadtimesisto determinetheapplicationrequirementsassoonaspossible and order the equipment as early as possible. Grogan warns: Te long lead times required for MV switchgear often prove challenging during construction. TeselogisticalchallengesarenotlimitedtoPMtrans-formersandMVswitchgear.JefSchilling,Phoenixdistrict ofce sales manager for Okonite, observes, Renewable proj-ectsgoatlightningspeedcomparedtoindustrialorutility constructionprojects,whichmakescomponentdeliverya critical issue. Strategic design decisions can ameliorate some of these issues. Schilling encourages designers to specify cable sizes that are commonly stocked (2 AWG, 1/0 AWG, 4/0 AWG, 350-kcmil,500-kcmil,750-kcmiland1,000-kcmil),andnotes that15kVcablesarecommonlyavailablewith133%level insulation,whereas35kVcableistypicallyavailablewith 100% level insulation. When product lead times do not ft within the project con-struction schedule, the specifying engineer can consider mak-ingsubstitutions.AccordingtoSchilling:Whilealuminum conductor cables with a tape shield are commonlyrequestedforasolarproj-ect,theyarenotacommonlystocked itemandmaynotbeavailableintime to meet the projects schedule.In these instances,higher-costcoppercables with tape shield or aluminum conduc-tors with concentric neutral, which are commonlyusedbyelectricutilities, canbeconsideredasasubstituteto meet scheduling requirements.Schillingcontinues:Whiledirect-burial jacketed cable is commonly used insolarfeldapplications,armored cableandjacketedcableinstalledin conduitareotheroptionstoconsider. Cable installed in conduit has the ben-eftofbeingrelativelyeasytoreplace shouldafaultoccur.However,there arecostandreliabilitytrade-ofsasso-ciated with each approach.ForGroganatDeltaDiversifedEnterprises,thehigher up-front costs associated with cable in conduit are not neces-sarilyadealbreaker.Heexplains:Wegenerallyprefercable installed in conduit as opposed to direct-bury installations. Te use of conduit eliminates many of the concerns associated with direct-bury cable. Tis approach can even cost less than direct-bury cable, in some cases, if it can eliminate the need for sifting or importing trench bedding material to the site.Medi um-Vol t ageWi ri ngg C O N T A C TDan Simpson / Taylor RyMar / Tempe, AZ /[email protected] / tr-corp.comManufacturersCooper Power Systems / 877.277.4636 / cooperindustries.comEaton / 855.386.7657 / eaton.comMyers Power Products / 866.696.9377 / myerspowerproducts.comThe Okonite Company / 201.825.0300 / okonite.comSchneider Electric / 888.778.2733 / schneider-electric.comSchweitzer Engineering Laboratories / 509.332.7990 / selinc.comResourcesConsulting Application Guide, Eaton Corp., 15th edition, June 2012Engineering Handbook: Engineering Data for Copper and Aluminum Conductor Electrical Cables, The Okonite Company, 2010 Three-Phase Pad-Mounted Compartmental Type Installation and Maintenance Instructions, Cooper Power Systems, Service Information S210-12-1, August 2012Draw-out breakerA Square D metal-clad draw-out vacuum circuit breaker from Schnei-der Electric is shown here. Typical breakers in this equipment class weigh in at 350480 pounds and are rated for up to 15 kV and 1,2003,000 A.Courtesy Schneider Electric