Lightning And Surge Protection Of Photovoltaic .
Lightning and Surge Protection of Photovoltaic InstallationsTwo Case Histories: Vulcano and KythnosFrançois D. MartzloffNational Institute of Standards and TechnologyPartial reprint of NISTIR-89 - 4113SignificancePart 6: Textbooks, tutorials, and reviewsPart 7: Mitigation techniquesTwo large installations of photovoltaic (PV) system s located on Mediterranean islands were dam agedduring lightning storm s in 1986-88, even though the m anufacturers and installers had provided protectionhardware in the form of air term inals dispersed am ong the arrays, and surge-protective devices in thecircuits. The two sites were visited and the dam aged equipm ent that was still available on the site wasexam ined for analysis of the suspected lightning-related dam age. The evidence was insufficient toconclude that all the observed dam age was caused by the direct effect of lightning. A possible scenariom ay be that lightning-induced overvoltages in the circuits caused insulation breakdown at the edges of thephotovoltaic m odules, with subsequent dam age done by the dc current generated by the array. Othersurge protection considerations were also addressed, and suggestions were presented for furtherinvestigations.This partial reprint includes all the text and circuit diagram s. An extensive collections of photographsrecorded on the sites is not included. See the last page of this pdf file for inform ation on possible retrievalof the com plete docum ent from the National Technical Inform ation Service.
Lightning andSurge Protectionof PhotovoltaicInstallationsTwo Case Histories:Vdmo and KythnosFran oisD. MartzloffElectricity DivisionCenter for Electronicsand Electrical EngineeringPrepared for:Sandia National LaboratoriesAlbuquerque, NM 87185-5800June 1989U.S. DEPARTMENTOF COMMERCERobert A. M o s b s h e r , SecretaryNATIONAL INSTITUTE OF STANDARDSAND TECHNOLOGYRaymond 6. Kammer, Actlng Dlrsctor
Two installations of photovoltaic (PV) systemswere damaged during lightning storms. Thetwo sites were visited and the damaged equipment that was still available on the site wasexamined for analysis of the suspected lightning-related damage. The evidence, however,is insufficient to conclude that all the observeddamage was caused by the direct effect oflightning. A possible scenario may be thatlightning-induced overvoltages caused insulation breakdown at the edges of the photovoltaic modules, with subsequent damage doneby the dc current generated by the array. Othersurge protection considerations are alsoaddressed, and suggestions are presented forfurther investigations.1. IntroductionPhotovoltaic systems are inherently exposed todirect and indirect lightning effecfs. For highcapacity systems, the deployment of solar cellarrays requires a large area with commensurateexposure to direct lightning strikes at the localannual rate of ground strikes per unit area. Thepresence of a ground grid related to the PV system in an otherwise isolated area may act as acollector of lightning ground-current fromnearby strikes. For PV systems tied to a localpower grid, the exposure also includes surgescoming from the power grid and the possibledifferences in the ground potential of the acpower system and that of the dc array system.tIn the present development state of photovoltaic systems, occurrences of lightning strikeshave been rare, thus field experience is still limited. Nevertheless, justifiable concerns exist,both from the economic point of view of darnage versus cost of protection and from the lesstangible impact on the perceptions of reliabilityfor a technology still in the early stages ofcommercial utilization.The Sandia National Laboratories, sponsored bythe U.S. Department of Energy, are developinga Recommended Practice document for theelectrical design of photovoltaic systems. Aspart of that project, the National Institute ofStandards and Technology is contributing thelightning, surge protection, and groundingrecommendations for these systems, based onknown characteristicst of surge protectivedevices and on field experience. By this means,a review of the circumstances and effects oflightning in the few known or suspected casesof lightning damage to worldwide photovoltaicinstallations will contribute to more effectivedesign and application of future systems.In this report, two case histories are examined.These include the photovoltaic installations atVulcano Island (Italy) and at Kythnos Island(Greece). Following the description of thesetwo case studies, a discussion is presented,leading to firm conclusions when the evidenceis sufficient, and allowing conjectures when theevidence is less conclusive. Both should serveas an indication of the need for furtherinvestigations, laboratory work, or theoreticalstudy.Caain cornmerial devices are idcntifi in thin report in order to describe adequately the installation and expected pefiamance of the system.Such identification does not imply ncornmendation or endorsement by the National Institute of Standards and Technobgy, nor doer it imply thatthese devices are necessarily the best available for the purpose.1
2. Surge Protection at theVulcano Island Installationfield. This history makes that site an interestingcase study, considering the scarcity of documented lightning occurrences on photovoltaicsystems.2.1 BackgroundVulcano is one of the islands in the AeolianGroup in the Tyrrhenian Sea, north of Sicily.The photovoltaic system in this island wasdesigned by ENEL, the Italian national electricutility, as a research and demonstration facilityand was commissioned in 1984 (Photograph2-I).* ENEL has been operating this facility sincethe commissioning. The visit, arranged byDr. A. Previ of ENEL, took place in November1988.One case of damage attributed to lightning hasbeen reported, with damage to only one panel(Photograph 2-2). No other damage occurredin the system, not even to the protective varistors provided at each junction box in the array2.2 System ConfigurationThe Vulcano photovoltaic system (see Figure2-1) includes the following major components:the array (1); a storage battery (2); one selfcommutated, stand-alone inverter 0);one linecommutated inverter (4); a rectifier for chargingthe battery 0;and a static switch (6). Morecomplete system diagrams by ENEL are given inAppendix A [I].Photograph 2-3 shows the block diagram of thesystem provided on the control cubicle. Interface with the 20 kV ac grid of the island isobtained by the three-winding 150/150/20 000 Vtransformer which is an integral part of theJ -260 VDCLINEBATTERYCHARGERJL 7C(5)(4)ISELF'OMMUTATEDINVERTER(31STATIC6120 kV-GRIDFigure 2-1. Block diagram of the Vulcano photovoltaic system2Photognphs cited in this text are included in Appendix D, slating on page 37.xxvLOCALUSERS
Figure 2-2. Surge protective devices at system interfacesoutput circuit of the line-cornmutated inverter.A group of 40 local domestic users was originally supplied at 380 V by an existing substationconnected to the 20 kV grid. The 380 V bus ofthe substation was modified to allow powerflow from the output of the stand-alone inverter,through the static switch, to the local users.With this configuration, the system can operatein two modes: grid-connected, and stand-alone.In the grid-connected mode, the tie to the gridis obtained through the 150/150/20 000 V transformer, absorbing all of the plant output. Inthat mode, the storage battery is not in the circuit, and the local loads are supplied by the acgrid. In the stand-alone mode, the local loadsare supplied at 380 V directly from the selfcommutated inverter. In that mode, the storagebattery is connected to the dc bus and it caneither absorb power from the array or deliverpower to the inverter. The local loads can alsobe supplied, if necessary, from the island acgrid through a back-up transformer.Individual strings from the array can beswitched by dc contactors located in the controlroom, to be connected to the dc bus or discon-nected from the dc bus according to the chargestate of the battery. For maintenance purposes,a dc disconnect is located in terminal boxesnext to the respective strings of the array (Photograph 2-4). Mechanical interlock is providedbetween the contactor and the cover of the terminal box, which prevents accidental openingof the disconnect under load.2.3 Grounding PracticesA major design decision in a photovoltaic system is whether to ground or not to ground thedc side. In contrast to ac power systems, whichare grounded in most cases (by generallyaccepted practice or by mandate, depending onthe country), no general agreement has beenreached on grounding practices for photovoltaicsystems. Two reasons are generally cited for anungrounded system:(1) the possibility to continue operating withone ground fault on the system, and(2) some limitation of single L-G fault currentsand hence reduction of damage in case of afault, because two ground faults are thenrequired to produce a significant dc faultcurrent.3
In the Vulcano system, the dc system is notgrounded. A ground fault detection system isprovided (Figure 2-2), with alarm indication onthe control panel (Photograph 2-3) but no automatic trip nor remote indication of the faultcondition (the system is unattended). Experience with this system is described as satisfactoryafter an initial period of reported difficultiesassociated with insulation deficiencies in thepanels. (These were eventually corrected byfield or factory rework on the panels.)4While the dc system is not grounded, a groundgrid has been installed at the site, for safety,surge protection, and grounding of the ac side.In addition to a grid of ground cables runningalong the dc cables in the array (but outside ofthe plastic conduits containing the dc cables,see Photograph 2-5), ground rods (16 rods,each 2 m long) were driven into the earth.Considerable care was given to the implernentation of this ground grid. For instance, theintegrity and effectiveness of the grounding system for protection against step voltage andtouch voltage, in case of a ground fault on the20 kV system, were the subject of well-documented tests. Providing low impedance earthing was made easier by the volcanic nature ofthe soil, which resulted in the unusually lowvalue of 1.8 Q for the earth resistance. Thelower leg of each panel frame is bonded to theground grid (Photographs 2-6 and 2-7).Concerns frequently associated with groundingpraaices are corrosion of connections and leakageof the insulation from energized parts to groundAt this site, the ground grid was implementedwith direct-burial copper cables with welded connections (Photographs 2-5 and 2-7), an effectiveassurance against corrosion problems. Some coxrosion. problems occurred in the original metalboxes containing the module by-pass diodes(Photograph 2-8). The problems were correctedby improving the insulation to ground with abetter sealing of the metallic frames.The significance of a history of corrosion1 insulation backing is that these insulation problemsmay be a clue to a scenario other than that ofsimple direct lightning damage. One may speculate on a scenario involving a double groundfault that could have resulted in panel damage;this scenario will be presented in the discussionof the observations of Section 2.5.'2.4 Surge ProtectionOvervoltage protection for the Vulcano systemis provided at three interfaces, as sketched inFigure 2-2:(1) At the terminal box of each pair of strings(Photograph 2-9), between each of the two dclines and ground, by one 32-mm diametervaristor (4 total) rated 560 V dc (GE Cat. No.V420HE400). No further protective devices areprovided at the entrance of the dc cables to thepower conditioner house (the capacitor bank atthe input of the inverters can serve as overvoltage limiter for any impinging surge because thefront time is relatively long as a result of thecable impedance). The blocking diode for eachstring, located in the field terminal box, is protected by one 32-rnm diameter varistor (2 total),rated 560 V dc (GE Cat. No. V420HE400). Thisvaristor has a clamping voltage of 1200 V for a300 A peak surge current. The repetitive peakvoltage rating of the diode (IR Cat. No. SD7ONl2P) is 1200 V.(2) At the 380 V ac interface of the output ofthe inverters, by three varistors connected lineto-ground (Photographs 2-10 and 2-11). Theseare also 32-mm diameter varistors, with a 420 Vac rms rating (GE Cat. No. V420HE400). A fuserated 8 A, 500 V, 100 kA interrupting capacity isprovided in series with each varistor (Photograph 2-12). About 50 cm of leads are used toconnect the varistors to the 380 V terminals atthe base of each inverter cabinet. (In this case,this length is not significant because of the fronttime limitation discussed above.)
SUCCESSIVESTRIKINGDISTANCEJUMPS\POINTS OF HIGHFIELD, I . E .SHARP ANDPROTRUDINGGROUNDEDOBJECTSUNLIKELY POINTOF ORIGINFOR ASTREAMERFigure 2-3. Descending stepped leaderand rising streamer in a cloud-tagroundlightning strike(3) At the 20 kV interface with the island system, by "conventional" surge arresters installedat the potheads of the underground connection,and connected line to ground (Photograph2-13). The 20 kV overhead line stops about200 m from the control room, with the finalconnection to the plant made by undergroundcable (Photograph 2-14).2.5 Discussion2.5.1 Lightning damage reportThe damage caused to the PV panel by the presumed lightning strike is shown in Photograph2-2. (The photograph was supplied by Previ aspart of the background history; the damagedmodule was not available for examination.)This damage occurred during the commissioning period of the plant in the autumn of 1984;it was found early in the morning by the ENELstaff after a thunderstorm occurred during thenight. The glass and part of the cells weredescribed as "melted near the metallic frame ofthe rnod le. No failure of the blocking diodenor of the varistor of that string was found as aresult of that incident.2.5.2 Lightning damage scenariosThe damage to the module is located at thelower part of the array, as shown in Photograph2-2. Postulating a scenario of a direct strike tothe array, the point of attachment of the lightning would be the point of origin of the risingstreamer that meets the descending steppedleader (Figure 2-3).This position at the lower part of the array israther unexpected for the point of initiation ofthe streamer. A more likely point for streamerinitiation - and resulting termination of thestrike - would be the upper edge of the array,which is 2 m above grade level (Photograph2-15). Thus, there is some doubt on drawing aconclusion that the damage was the result of adirect strike terminating at the array.In view of the reported insulation problems thatoccurred during the initial period of operation,one might ask whether the damage to thatpanel might be the result of a leakage of dc current to the frame, rather than the simple directeffect of a lightning strike. This dc leakagemight be the consequence of a lightninginduced overvoltage stress that created a doublefault in one single event, or that created the second fault after the first had previously occurredbut remained uncorrected. The scenario couldunfold as follows:Assume that two independent ground faults, (A)and (B) have occurred on the system (Figure2-4). When the first, say (A), occurs, the faultdetection system indicates that event but noimmediate action is taken because of the unattended status of the system, and there is noground fault current resulting from that firstfault (except the insignificant current passing5
PV PANELthrough the detection circuit). A ground faultcurrent can exist only after the second faultoccurs, establishing the path through (A)and (B).Assume now that one of the two faults, say (B),involves a very low resistance. Then, even forsubstantial fault currents, little heat is generatedat fault (B). Assume further that (A) has a lowenough resistance to produce a "sufficientncurrent in the fault path, where "sufficient" isdefined as a level which, combined with thelow but finite resistance of fault (A), will createheat dissipation in (A), in contrast with thenegligible heat dissipation in (B).In this manner; we have the elements that couldcreate the observed effects, that is, an obviousfault with burning at (A), and a less obviousfault at (B), with a low resistance that may beeliminated during emergency maintenance workfollowing the occurrence of the incident. Thelikelihood of such a double fault is admittedlylow, but cannot be ruled out in view of thedesign of the ground fault detection systemwhich indicates faults locally only. This scenario, still associated with lightning, would notbe in contradiction with the observed low position of the damage since it does not require termination of the strike at that low point of thepanel. Furthermore, the low point on thesloped array is also a place where moisture ismore likely to accumulate and thus create agood candidate for a contributing cause in thescenario of two-stage insulation breakdown.6A variation on the theme of the double faultmight even be that the fault was entirely causedby long-term insulation breakdown, without the"coup de grace" administered by the lightningincident. However, the observation of a damaged module soon after a lightning storm wouldpoint to the lightning-induced overvoltagescenario.IFAULT (A1WITH MODERATERESISTANCEBETWEEN--DC CIRCUITAND FRAME7GROUNDEDFRAMEDC CABLE1IIFAULT (0) WITHLOW RESISTANCESOMEWHERE I N SYSTEMFigure 2-4. Scenario of double ground faultOne significant aspect of the failure mode is thereported shattering of the glass cover of thatmodule. The question is whether it could havebeen produced by the less violent action of a dcfault (glass breakage has been reported in theUnited States during dc ground faults), or couldbe explained only by the mechanical shockassociated with a lightning strike. The reportedmelting of the glass is also a clue that could beinvestigated further.If data were available on the failure modes ofhis type of module, some of the conjecturesproposed in this discussion might be replacedby more positive conclusions. The incentive forreaching such positive conclusions is not merelyone of intellectual curiosity. If overvoltagesinduced by indirect lightning are sufficient tocause insulation breakdown, then the provisionof lightning air terminals is irrelevant - and thusbecomes an unjustifiable cost - while improvingthe insulation levels in the modules would yieldbetter results for the added expense.
2.5.3 Insulation coordinationCoordination of the protective devices with thewithstand capability of the equipment to beprotected is sometimes overlooked in systemdesigns. At the Vulcano site, this coordinationwas presumed to have been incorporated in allthe system design and was not audited during avisit aimed primarily at a review of the lightningincident.However, given the concerns on the protectionafforded by the varistors, the coordination forone example of protection can be evaluated bya simple comparison: From their catalogdescription, the blocking diodes of the arraystrings have a repetitive peak voltage rating of1200 V (albeit not a perfect assessment of theirtransient withstand capability). Therefore, themaximum clamping voltage for the protectivevaristor should not exceed 1200 V. For a varistor rated 560 V dc, this maximum allowableclamping voltage of 1200 V corresponds to a300 A surge crest current. In other words, protection can be expected as long as surgecurrents do not exceed 300 A in that string.At first glance, this 300 A allowable level ofsurge current may appear low. However, whenpostulating a lightning-induced surge currentlevel in the wiring, one should not be influenced by the thousands of amperes of thedirect stroke, but rather consider the voltagerequired to drive the postulated current waveform along the inductance of the wiring: a highrate of current change means a high drivingvoltage
photovoltaic modules, with subsequent damage done by the dc current generated by the array. Other surge protection considerations were also addressed, and suggestions were presented for further invest igat ions. This partial reprint includes all the text and circuit diagrams. An extensive collections of photographs
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