surge protection considerations were also addressed, and suggestions were presented for further

investigations.

This partial reprint includes all the text and circuit diagrams. An extensive collections of photographs

recorded on the sites is not included. See the last page of this pdf file for information on possible retrie

of the complete document from the National Technical Information Service.

 
Lightning and 

Surge Protection 

of Photovoltaic 

Installations 

Two Case Histories: 

Vdmo and Kythnos 

Fran~ois D. Martzloff 

Electricity Division 

Center for Electronics 

and Electrical Engineering 

Prepared for: 

Sandia National Laboratories 

Albuquerque, NM 87185-5800 

June 1989 

U.S. DEPARTMENT OF COMMERCE 

Robert A. Mosbsher, Secretary 

NATIONAL INSTITUTE OF STANDARDS 

AND TECHNOLOGY 

Raymond 6. Kammer, Actlng Dlrsctor 
Two installations of photovoltaic (PV) systems 

were damaged during lightning storms. The 

two sites were visited and the damaged equipment that was still available on the site was 

examined for analysis of the suspected lightning-related damage. The evidence, however, 

is insufficient to conclude that all the observed 

damage was caused by the direct effect of 

lightning. A possible scenario may be that 

lightning-induced overvoltages caused insulation breakdown at the edges of the photovoltaic modules, with subsequent damage done 

by the dc current generated by the array. Other 

surge protection considerations are also 

addressed, and suggestions are presented for 

further investigations. 

1. Introduction 

Photovoltaic systems are inherently exposed to 

direct and indirect lightning effecfs. For highcapacity systems, the deployment of solar cell 

arrays requires a large area with commensurate 

exposure to direct lightning strikes at the local 

annual rate of ground strikes per unit area. The 

presence of a ground grid related to the PV system in an otherwise isolated area may act as a 

collector of lightning ground-current from 

nearby strikes. For PV systems tied to a local 

power grid, the exposure also includes surges 

coming from the power grid and the possible 

differences in the ground potential of the ac 

power system and that of the dc array system. 

In the present development state of photovoltaic systems, occurrences of lightning strikes 

have 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 less 

tangible impact on the perceptions of reliability 

for a technology still in the early stages of 

commercial utilization. 

The Sandia National Laboratories, sponsored by 

the U.S. Department of Energy, are developing 

a Recommended Practice document for the 

electrical design of photovoltaic systems. As 

part of that project, the National Institute of 

Standards and Technology is contributing the 

lightning, surge protection, and grounding 

recommendations for these systems, based on 

known characteristicst of surge protective 

devices and on field experience. By this means, 

a review of the circumstances and effects of 

lightning in the few known or suspected cases 

of lightning damage to worldwide photovoltaic 

installations will contribute to more effective 

design and application of future systems. 

In this report, two case histories are examined. 

These include the photovoltaic installations at 

Vulcano Island (Italy) and at Kythnos Island 

(Greece). Following the description of these 

two case studies, a discussion is presented, 

leading to firm conclusions when the evidence 

is sufficient, and allowing conjectures when the 

evidence is less conclusive. Both should serve 

as an indication of the need for further 

investigations, laboratory work, or theoretical 

study. 

t 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 that 

these devices are necessarily the best available for the purpose. 

1 
2. Surge Protection at the 

Vulcano Island Installation 

2.1 Background 

Vulcano is one of the islands in the Aeolian 

Group in the Tyrrhenian Sea, north of Sicily. 

The photovoltaic system in this island was 

designed by ENEL, the Italian national electric 

utility, as a research and demonstration facility 

and was commissioned in 1984 (Photograph 

2-I).* ENEL has been operating this facility since 

the commissioning. The visit, arranged by 

Dr. A. Previ of ENEL, took place in November 

1988. 

One case of damage attributed to lightning has 

been reported, with damage to only one panel 

(Photograph 2-2). No other damage occurred 

in the system, not even to the protective varistors provided at each junction box in the array 

260 VDC 

20 kV 

GRID 

field. This history makes that site an interesting 

case study, considering the scarcity of documented lightning occurrences on photovoltaic 

systems. 

2.2 System Configuration 

The Vulcano photovoltaic system (see Figure 

2-1) includes the following major components: 

the array (1); a storage battery (2); one self 

commutated, stand-alone inverter 0); one linecommutated inverter (4); a rectifier for charging 

the battery 0; and a static switch (6). More 

complete system diagrams by ENEL are given in 

Appendix A [I]. 

Photograph 2-3 shows the block diagram of the 

system provided on the control cubicle. Interface with the 20 kV ac grid of the island is 

obtained by the three-winding 150/150/20 000 V 

transformer which is an integral part of the 

Figure 2-1. Block diagram of the Vulcano photovoltaic system 

2 Photognphs cited in this text are included in Appendix D, slating on page 37. 

J- I 

LINE 

BATTERY 

CHARGER 

(4) (5) 

JL 

SELF 

7C 'OMMUTATED 

INVERTER 

(31 

STATIC 

- 

61 xx 

v 

- LOCAL 

USERS 
Figure 2-2. Surge protective devices at system interfaces 

output circuit of the line-cornmutated inverter. 

A group of 40 local domestic users was originally supplied at 380 V by an existing substation 

connected to the 20 kV grid. The 380 V bus of 

the substation was modified to allow power 

flow from the output of the stand-alone inverter, 

through the static switch, to the local users. 

With this configuration, the system can operate 

in two modes: grid-connected, and stand-alone. 

In the grid-connected mode, the tie to the grid 

is obtained through the 150/150/20 000 V transformer, absorbing all of the plant output. In 

that mode, the storage battery is not in the circuit, and the local loads are supplied by the ac 

grid. In the stand-alone mode, the local loads 

are supplied at 380 V directly from the selfcommutated inverter. In that mode, the storage 

battery is connected to the dc bus and it can 

either absorb power from the array or deliver 

power to the inverter. The local loads can also 

be supplied, if necessary, from the island ac 

grid through a back-up transformer. 

Individual strings from the array can be 

switched by dc contactors located in the control 

room, to be connected to the dc bus or disconnected from the dc bus according to the charge 

state of the battery. For maintenance purposes, 

a dc disconnect is located in terminal boxes 

next to the respective strings of the array (Photograph 2-4). Mechanical interlock is provided 

between the contactor and the cover of the terminal box, which prevents accidental opening 

of the disconnect under load. 

2.3 Grounding Practices 

A major design decision in a photovoltaic system is whether to ground or not to ground the 

dc side. In contrast to ac power systems, which 

are grounded in most cases (by generally 

accepted practice or by mandate, depending on 

the country), no general agreement has been 

reached on grounding practices for photovoltaic 

systems. Two reasons are generally cited for an 

ungrounded system: 

(1) the possibility to continue operating with 

one ground fault on the system, and 

(2) some limitation of single L-G fault currents 

and hence reduction of damage in case of a 

fault, because two ground faults are then 

required to produce a significant dc fault 

current. 3 
In the Vulcano system, the dc system is not 

grounded. A ground fault detection system is 

provided (Figure 2-2), with alarm indication on 

the control panel (Photograph 2-3) but no automatic trip nor remote indication of the fault 

condition (the system is unattended). Experience with this system is described as satisfactory 

after an initial period of reported difficulties 

associated with insulation deficiencies in the 

panels. (These were eventually corrected by 

field or factory rework on the panels.) 

While the dc system is not grounded, a ground 

grid has been installed at the site, for safety, 

surge protection, and grounding of the ac side. 

In addition to a grid of ground cables running 

along the dc cables in the array (but outside of 

the 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, the 

integrity and effectiveness of the grounding system for protection against step voltage and 

touch voltage, in case of a ground fault on the 

20 kV system, were the subject of well-documented tests. Providing low impedance earthing was made easier by the volcanic nature of 

the soil, which resulted in the unusually low 

value of 1.8 Q for the earth resistance. The 

lower leg of each panel frame is bonded to the 

ground grid (Photographs 2-6 and 2-7). 

Concerns frequently associated with grounding 

praaices are corrosion of connections and leakage 

of the insulation from energized parts to ground 

At this site, the ground grid was implemented 

with direct-burial copper cables with welded connections (Photographs 2-5 and 2-7), an effective 

assurance against corrosion problems. Some coxrosion. problems occurred in the original metal 

boxes containing the module by-pass diodes 

(Photograph 2-8). The problems were corrected 

by improving the insulation to ground with a 

4 better sealing of the metallic frames. 

The significance of a history of corrosion1 insulation backing is that these insulation problems 

may be a clue to a scenario other than that of 

simple direct lightning damage. One may speculate on a scenario involving a double ground 

fault that could have resulted in panel damage; 

this scenario will be presented in the discussion 

of the observations of Section 2.5. 

'2.4 Surge Protection 

Overvoltage protection for the Vulcano system 

is provided at three interfaces, as sketched in 

Figure 2-2: 

(1) At the terminal box of each pair of strings 

(Photograph 2-9), between each of the two dc 

lines and ground, by one 32-mm diameter 

varistor (4 total) rated 560 V dc (GE Cat. No. 

V420HE400). No further protective devices are 

provided at the entrance of the dc cables to the 

power conditioner house (the capacitor bank at 

the input of the inverters can serve as overvoltage limiter for any impinging surge because the 

front time is relatively long as a result of the 

cable impedance). The blocking diode for each 

string, located in the field terminal box, is protected by one 32-rnm diameter varistor (2 total), 

rated 560 V dc (GE Cat. No. V420HE400). This 

varistor has a clamping voltage of 1200 V for a 

300 A peak surge current. The repetitive peak 

voltage rating of the diode (IR Cat. No. SD 

7ONl2P) is 1200 V. 

(2) At the 380 V ac interface of the output of 

the inverters, by three varistors connected lineto-ground (Photographs 2-10 and 2-11). These 

are also 32-mm diameter varistors, with a 420 V 

ac rms rating (GE Cat. No. V420HE400). A fuse 

rated 8 A, 500 V, 100 kA interrupting capacity is 

provided in series with each varistor (Photograph 2-12). About 50 cm of leads are used to 

connect the varistors to the 380 V terminals at 

the base of each inverter cabinet. (In this case, 

this length is not significant because of the front 

time limitation discussed above.) 
SUCCESSIVE \ POINTS OF HIGH 

FIELD, I.E. 

STRIKING SHARP AND 

DISTANCE PROTRUDING 

JUMPS GROUNDED 

OBJECTS 

UNLIKELY POINT 

OF ORIGIN 

FOR A 

STREAMER 

Figure 2-3. Descending stepped leader 

and rising streamer in a cloud-taground 

lightning strike 

(3) At the 20 kV interface with the island system, by "conventional" surge arresters installed 

at the potheads of the underground connection, 

and connected line to ground (Photograph 

2-13). The 20 kV overhead line stops about 

200 m from the control room, with the final 

connection to the plant made by underground 

cable (Photograph 2-14). 

2.5 Discussion 

2.5.1 Lightning damage report 

The damage caused to the PV panel by the presumed lightning strike is shown in Photograph 

2-2. (The photograph was supplied by Previ as 

part of the background history; the damaged 

module 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 ENEL 

staff after a thunderstorm occurred during the 

night. The glass and part of the cells were 

described as "melted near the metallic frame of 

the rnod~le.~ No failure of the blocking diode 

nor of the varistor of that string was found as a 

result of that incident. 

2.5.2 Lightning damage scenarios 

The damage to the module is located at the 

lower part of the array, as shown in Photograph 

2-2. Postulating a scenario of a direct strike to 

the array, the point of attachment of the lightning would be the point of origin of the rising 

streamer that meets the descending stepped 

leader (Figure 2-3). 

This position at the lower part of the array is 

rather unexpected for the point of initiation of 

the streamer. A more likely point for streamer 

initiation - and resulting termination of the 

strike - would be the upper edge of the array, 

which is 2 m above grade level (Photograph 

2-15). Thus, there is some doubt on drawing a 

conclusion that the damage was the result of a 

direct strike terminating at the array. 

In view of the reported insulation problems that 

occurred during the initial period of operation, 

one might ask whether the damage to that 

panel might be the result of a leakage of dc current to the frame, rather than the simple direct 

effect of a lightning strike. This dc leakage 

might be the consequence of a lightning 

induced overvoltage stress that created a double 

fault in one single event, or that created the second fault after the first had previously occurred 

but remained uncorrected. The scenario could 

unfold as follows: 

Assume that two independent ground faults, (A) 

and (B) have occurred on the system (Figure 

2-4). When the first, say (A), occurs, the fault 

detection system indicates that event but no 

immediate action is taken because of the unattended status of the system, and there is no 

ground fault current resulting from that first 

fault (except the insignificant current passing 5 
through the detection circuit). A ground fault 

current can exist only after the second fault 

occurs, establishing the path through (A) 

and (B). 

Assume now that one of the two faults, say (B), 

involves a very low resistance. Then, even for 

substantial fault currents, little heat is generated 

at fault (B). Assume further that (A) has a low 

enough resistance to produce a "sufficientn current in the fault path, where "sufficient" is 

defined as a level which, combined with the 

low but finite resistance of fault (A), will create 

heat dissipation in (A), in contrast with the 

negligible heat dissipation in (B). 

In this manner; we have the elements that could 

create the observed effects, that is, an obvious 

fault with burning at (A), and a less obvious 

fault at (B), with a low resistance that may be 

eliminated during emergency maintenance work 

following the occurrence of the incident. The 

likelihood of such a double fault is admittedly 

low, but cannot be ruled out in view of the 

design of the ground fault detection system 

which indicates faults locally only. This scenario, still associated with lightning, would not 

be in contradiction with the observed low position of the damage since it does not require termination of the strike at that low point of the 

panel. Furthermore, the low point on the 

sloped array is also a place where moisture is 

more likely to accumulate and thus create a 

good candidate for a contributing cause in the 

scenario of two-stage insulation breakdown. 

A variation on the theme of the double fault 

might even be that the fault was entirely caused 

by long-term insulation breakdown, without the 

"coup de grace" administered by the lightning 

incident. However, the observation of a damaged module soon after a lightning storm would 

point to the lightning-induced overvoltage 

6 scenario. 

FAULT (A1 

WITH MODERATE 

RESISTANCE 

BETWEEN -- -- 7 

DC CIRCUIT 

AND FRAME 

GROUNDED 

FRAME 

I 

PV PANEL 

DC CABLE -1 

I 

I 

FAULT (0) WITH 

LOW RESISTANCE 

SOMEWHERE IN SYSTEM 

Figure 2-4. Scenario of double ground fault 

One significant aspect of the failure mode is the 

reported shattering of the glass cover of that 

module. The question is whether it could have 

been produced by the less violent action of a dc 

fault (glass breakage has been reported in the 

United States during dc ground faults), or could 

be explained only by the mechanical shock 

associated with a lightning strike. The reported 

melting of the glass is also a clue that could be 

investigated further. 

If data were available on the failure modes of 

 his type of module, some of the conjectures 

proposed in this discussion might be replaced 

by more positive conclusions. The incentive for 

reaching such positive conclusions is not merely 

one of intellectual curiosity. If overvoltages 

induced by indirect lightning are sufficient to 

cause insulation breakdown, then the provision 

of lightning air terminals is irrelevant - and thus 

becomes an unjustifiable cost - while improving 

the insulation levels in the modules would yield 

better results for the added expense. 
2.5.3 Insulation coordination 

Coordination of the protective devices with the 

withstand capability of the equipment to be 

protected is sometimes overlooked in system 

designs. At the Vulcano site, this coordination 

was presumed to have been incorporated in all 

the system design and was not audited during a 

visit aimed primarily at a review of the lightning 

incident. 

However, given the concerns on the protection 

afforded by the varistors, the coordination for 

one example of protection can be evaluated by 

a simple comparison: From their catalog 

description, the blocking diodes of the array 

strings have a repetitive peak voltage rating of 

1200 V (albeit not a perfect assessment of their 

transient withstand capability). Therefore, the 

maximum clamping voltage for the protective 

varistor should not exceed 1200 V. For a varistor rated 560 V dc, this maximum allowable 

clamping voltage of 1200 V corresponds to a 

300 A surge crest current. In other words, protection can be expected as long as surge 

currents do not exceed 300 A in that string. 

At first glance, this 300 A allowable level of 

surge current may appear low. However, when 

postulating a lightning-induced surge current 

level in the wiring, one should not be influenced by the thousands of amperes of the 

direct stroke, but rather consider the voltage 

required to drive the postulated current waveform along the inductance of the wiring: a high 

rate of current change means a high driving 

voltage. However, in this case, high driving 

voltage would not be possible because 

sparkover of the insulation would occur. Thus, 

the 300 A crest of an 8/20 ps postulated waveform appears an appropriate order of magnitude. In this example, therefore, insulation 

coordination was in fact achieved for voltage 

levels that might be induced in the wiring. 

2.6 Suggestions on the Design 

In his role of sponsor of the visit, Previ asked 

for comments on the surge protection provided 

at this site. Accepting for the moment the 

hypothesis that lightning was the cause for damage to the panel, the successful operation of the 

installation and survival of the electronics 

through one lightning occurrence are already a 

testimonial of the adequacy of the protection 

system. 

Taking a devil's advocate view in search of 

greater protection, a more conservative 

approach could have been to provide additional 

surge protection for the incoming dc cables at 

the interface with the inverter inputs, but experience so far has indicated survival without 

these additional protective devices. This observation, however, does not necessarily guarantee 

that another lightning strike scenario, with a different point of termination or higher amplitude, 

could not induce some damaging overvoltage 

along the cables between the array protections 

and the inverter input. 

A concern expressed by Previ was the failure 

mode of the varistors installed at the base of the 

electronic cabinets at the ac interface. These 

varistors can be expected, in case of failure, to 

be promptly isolated from the power source by 

operation of their series-connected fuses (that 

have ample interrupting capacity). Therefore, 

the generation of hot gases during the shortcircuit following failure of the varistor would be 

brief, Again, as an exercise in very conservative 

design, a further step could be applied to limit 

the consequences of a varistor failure by providing a partial metal shielding around the varistors 

to deflect any evolving gas away from the rest 

of the circuit. The 8 A rating of the fuses seems 

adequate to avoid premature aging of the fuses 

caused by repetitive surges I31, should such 

repetitive surges occur at that site. 7 
Previ also asked about the possibility of monitoring the condition of the varistor aging for the 

purpose of anticipating an impending failure. 

This question has been raised by many users, 

sensitized to the issue by competitive claims 

from advocates of silicon avalanche diodes. At 

this time, no easy method has been proposed 

for field measurements (especially in dc circuits 

where a clamp-on transformer is not 

suitable 141). 

Increasing concerns on the issue are likely to 

catalyze the development of such measurements. For the moment, the only technically 

simple but operationally difficult method would 

be to remove each varistor from the circuit and 

compare its present nominal voltage to its original nominal voltage. In existing installations, 

that information is not likely to be available. 

An intermediate solution for this installation 

would be to implement monitoring the varistors, albeit at a late stage of the project, and 

watch for trends, even though the initial value is 

not available. As a last resort, a surface temperature measurement on the varistor might give a 

warning of impending failure. 

This discussion of varistor failure scenarios 

should not be interpreted as an inference that 

the varistors are in fact in jeopardy. It is only 

an exercise in asking and answering conservative "what-if' questions. 

2.7 Specific Conclusions from the 

Vulcano Case 

The experience accumulated at the Vulcano site 

indicates no major problem of surge occurrences, with only one reported case of damage 

to one panel among several hundred. This one 

case of damage is not conclusively attributable 

to lightning. 

Furthermore, even if the damage were caused 

by lightning, then a partially satisfactory conclusion would be that sufficient protection 

could be provided for the electronic components in the power conditioning system, at least 

for that particular case. Power conditioning 

equipment is the most expensive part of the 

system and cannot be considered "expendable" 

in contrast with a few modules being lost with 

the rest of the system remaining operational. 

The ambiguity in attributing the damage to 

direct or indirect lightning might be resolved by 

further study of the failure modes of a panel (a 

module with its frame). One failure node to 

be investigated would be under simulated lightning strikes; the other failure mode would be 

under dc stress with surface contamination. 

Further discussion of this issue, from the technical as well as economic and intangible aspects, 

is offered in the general discussion of Section 5. 
3. Surge Protection at the 

Kythnos Island Installation 

3.1 Background 

Kythnos is one of the islands in the Cyclades 

Group, in the southern part of the Aegean Sea. 

The photovoltaic system on this island was 

designed and implemented in 1983 by Siemens. 

It is operated by the Greek Public Power Corporation. The visit, which was arranged by 

Dr. J. Chadjivassiliadis of Public Power 

Corporation, took place in November 1988. 

CONVERTERS 

VDC 

STORAGE 

BATTERY 

COMMUTATED 

INVERTER 

380 VAC - 

15 kV 

GRID 

Figure 31. Block diagram of the Kythnos 

photovoltaic system 

Several panel failures have occurred in 1986, 

1987, and 1988, which have been attributed to 

lightning. Lightning rods and surge arresters 

are provided at this site, making it an interesting subject of study, both for an explanation of 

the presumed direct strikes occurring in spite of 

the lightning rods, and for a study of the protection afforded by the surge protective devices 

installed in the circuits, as well as their failure 

modes. Further information on the history of 

panel damage is included in Appendix B. 

3.2 System Configuration 

The installation was designed and implemented 

by Siemens, as one of the experimental facilities 

coordinated by the European Economic Community (Photograph 3-1). The plant has a nominal output capacity of 100 kW. Figure 3-1 

shows a schematic of the system components. 

The modules are grouped in arrays formed by a 

series string of 20 modules, each producing a 

dc bus voltage of 160 V. Each of these 43 

arrays is terminated in a junction box in the 

field, where two or three strings are connected 

in parallel to bring the dc power to the power 

conversion cabin (Photograph 3-2). 

In the power conversion circuitry, the variable 

160 V dc is raised and regulated to 250 V by a 

dddc converter to match the battery voltage for 

optimum charging conditions and operation of 

the solar cells. The dc/dc conversion is performed by four units, each rated 25 kW. 

Depending on the instantaneous power transfer, one to four converters are in service. 

Conversion to ac power is performed by three 

inverters, each rated 50 kW. The output voltage 

of 380 V is stepped up to 15 kV for connection 

to the island power grid. Although the arrays 

and conversion equipment are located adjacent 

to the Diesel generating plant of the island, 

operation of the photovoltaic system can be 9 
Figure 3-2. Location of ground cables, air terminals, and modules 

presumed damaged by lightning 

Source: Kythnos records (Appendix 0) 

automatic, and does not require daily supervisory. Extensive monitoring and control of operating parameters is provided by a "Logistronic" 

control system and other controls incorporated 

in the design. 

3.3 Grounding Practices 

This site is located in the center part of the 

island next to the Diesel power plant, but with 

its ground grid isolated from that of the Diesel 

plant. This grid consists of several loops encircling each of the four groups of arrays. Part of 

the each loop follows the routing of the dc 

cables between the array junction boxes and 

the power conditioning cabin (Figure 3-2). 

The perimeter of the field is defined by stone 

walls, in keeping with the prevailing island 

practice for marking boundaries between pastures and cultivated fields. Consequently, there 

is no metal fence around the photovoltaic field, 

and thus no perimeter grounding cable. The 

10 conductors are made of 10-mm diameter galvanized steel, buried directly at the bottom of a 

trench, with the dc cables above the ground 

conductors. There are no driven ground rods 

added to this grid. The choice of galvanized 

steel probably reflects the German practice, 

where concerns over corrosion effects by 

buried copper seem to deprecate the use of 

copper. 

All the metal structures of the system, including 

the array supporting beams, junction boxes, 

lightning rods, and housings for the power conditioners and battery, are bonded to the ground 

grid. Connections are made using bolted connectors above ground (Photographs 3-3, 3-4, 

and 3-9, as well as under ground, with protection against corrosion being provided in accordance with the normal practices of the various 

manufacturers and contractors (these were not 

discussed during the visit). 

The dc system is not grounded, but includes a 

ground fault detection circuit with fault indication available only in the control cubicle of the 

system. The separation of the photovoltaic 

ground grid from the Diesel plant ground grid 
PV 

MODULE8 

FIELD JUNCTION BOX 

UNKROROUND 

OC CABLES 

1Ml V 

I 

I 

I 

L----------A 

--------------------- 

Figure 33. Overvoltage protection at system interfaces 

raises the question of a possible difference of 

ground potential between these two systems 

during a lightning mike. If instrumentation or 

telemetering equipment spans across the two 

systems, the difference in ground potentials 

might become a problem. However, no such 

problem was identified at that site. 

3.4 Surge Protection 

This installation presents an interesting case history because it includes both lightning rods 

(air terminals) in the array and surge arresters in 

the circuits. Damage to several panels, presumably as a result of lightning over a period 

involving three separate occurrences, raises 

questions on the effectiveness of the protection 

against direct strikes. Damage to the surge 

arresters also occurred in one of the field junction boxes, but no damage occurred on the 

power conversion units. Some damage 

occurred in the control circuits of the battery 

charger during the initial period, when they did 

not have surge arresters at their ac power input. 

After arresters were added to this ac input, no 

further damage events occurred but some 

upsets did still occur in the control system. 

3.4.1 Air terminals 

Air terminals (lightning rods) have been 

installed between rows of the array as shown 

on Photograph 3-6. The height of these air terminals is 10.5 m above grade level; the upper 

edge of the panels is 2 m above grade level, 

thus leaving a net elevation of the air terminals 

8.5 m above the upper edges of the panels. 

Considering a 45' cone of protection, one of 

the classical criteria, the panel upper edges 

would then be "protectedn within a radius of 

8.5 m from each air terminal. Those panels 

located beyond that radius would be left unprotected. Reviewing the location of panels 

involved in the damage (Figure 3-2) shows the 

following horizontal distance from the nearest 

air terminal: 11 
- Module E2/B4/1 - Location 10m 

- Module E2/B6/5 - Location I1 lorn 

- Module E3/B4/13 - Location 111 10m 

- Module E3/B4/19 - Location IV 12m 

- Module E3/B5/18 - Location V 8m 

- Module E4/B3/20 - Location VI 10m 

Thus, five of the six damaged modules were 

beyond the 45' cone, and the sixth was on the 

fringe of the cone. Some panels in the array, 

not impacted by lightning, are further away 

from an air terminal, the greatest distance in the 

field being 15 meters. Another interesting statistic is the distribution of the panels with respect 

to being within a protected area of 8.5 meters 

radius (approximately 75%) or outside the 

protected area (25%). 

Another protection criterion has been developed, that of the 'Yolling ball" [51, as discussed 

in section 3.5. According to that criterion, the 

protection radius would extend to 12 m so that 

all panels would have been expected to be in 

the protected zone. 

According to yet another definition of the cone 

of protection, sometimes cited by less conservative designers, a 2:l instead of a 1:l ratio of 

radius to height may be considered. In such a 

case, one would expect all of the panels to be 

protected" as the distance from the mast would 

increase to 17 m. 

It is not known whether such a 2:l cone, or the 

1:l (45') cone, or the rolling ball with a 30-m 

radius was used in the initial layout of the air 

terminals. The design has been described as 

Ynstalled according to VDE standardsn 

(VDE is the acronym for Verein Deutscher 

Electrotechniker) (see Appendix B). 

3.4.2 Overvoltage protection 

Overvoltage protection at the Kythnos installation is provided at four interfaces, numbered (1) 

through (4) in Figure 3-3: 

(1) At the junction boxes in field - There are 

several slightly different types of junction boxes 

in the field. Some include termination for two 

or for three strings, while some also contain 

additional circuitry for the data collection system. Photograph 3-7 shows a typical threecircuit box (undamaged). One surge arrester is 

connected between each of the floating dc lines 

(+) and (-) and a ground bus inside the box. In 

turn, this bus is bonded to the footing by a copper cable (in parallel with the inherent bonding 

between the metallic junction box and the 

I beam of the footing). 

These arresters appear similar to those for 

which the voltage response had been documented in a paper presented at the 1981 EMC 

Zii rich Symposium (Appendix C). From the 

voltage response characteristic reported in that 

paper, it appears that the surge arrester consisted of a silicon-carbide varistor with a series 

gap. The presence of a series gap is significant 

in discussing the upset events cited for the 

control circuits at this site. 

The string blocking diodes are mounted in the 

junction box and are protected by a metaloxide varistor connected in parallel with each 

.diode (Photograph 3-8). Photograph 3-9 shows 

another junction box with the additional data 

collection circuitry installed in the box cover. 

This particular box is the one where the 

lightning-suspected damage occurred, as shown 

in the close-up views of Photographs 3-10 and 

3-11. 

(2) At the power conversion units - The dc 

lines from the array are brought to the cabinets 

of the dc-dc converters where each of the four 

converter inputs is protected by two surge 

arresters (2a) (Photograph 3-12) connected 

between the (+) and (-) lines, and ground. This 

arrester is of the same type as that described for 

the array junction boxes. 
Similarily, the ac outputs of the inverters are 

protected against surges from the ac grid by 

four arresters (2b); one is connected between 

each line (a,b,c) and ground, and one between 

neutral and ground (N) (Photograph 3-13). 

While the grounding connection of the 

220/380 V system was not reviewed, presumably it follows the European practice of bonding 

to earth only at the secondary transformer, in 

this case the step-up transformer of the grid 

interface. This practice, different from that used 

in the United States, motivates and justifies the 

provision of the arrester between neutral and 

ground. 

(3) At the Logbtronk circuit power supply - 

The Ugistronicn circuit controlling the battery 

charger is powered from the 220 V ac line in 

the battery cabin. Thus, its power supply is 

exposed to surges that may occur on that supply. Initially, there was no protection on this ac 

supply; perhaps as a consequence, damage 

occurred three times in the early years of the 

system (Appendix B). Subsequently, two 

arresters were installed on the ac supply line 

ahead of the Logistronic input terminals (Photograph 3-14). After these ac arresters were 

installed, only upsets were recorded (four 

occurrences). This behavior is consistent with 

the voltage-limiting effect of the arresters but at 

the price of a steep voltage collapse when the 

gaps fire (Appendix C). This electromagnetic 

disturbance is a likely source of interference in 

nearby digital circuits. 

(4) At the ac grid interface - Protection 

against surges coming from the island ac power 

grid is provided by the three distribution-type 

arresters mounted on a cross-arm above the 

transformer (Photograph 3-15). No 'information 

was available on these arresters; they are likely 

to be of the conventional design using a silicon 

carbide varistor with a series gap. 

This type of arrester is perfectly adequate for 

protecting transformers against surges, but 

might not be sufficient for the electronic components on the 220/380 V side. For that reason, the secondary arresters described above 

are a good idea. However, gapless secondary 

arresters are now available that can offer a 

more comprehensive protection, including 

some degree of upset protection. 

3.4.3 Examination of the damaged modules 

3.4.3.1 Summary 

At the date of the visit, the three modules damaged in 1986 had been replaced in the array. 

These modules were still kept in storage at the 

site, so that it was possible to examine them 

closely. The two modules damaged in 1987 

and the one module damaged in 1988, however, were still in position in the array, as no 

spares were available. Detailed photographs 

and observations for each panel are given in 

the following paragraphs, in chronological 

order. 

At this site, the arrays are only one module 

high, so that the long edge of the module 

reaches from the highest to the lowest edge of 

the array. In all six failed modules, there is 

damage evident at one or both upper corners, 

along one or both long edges, and at the bottom of the module. The panel is completely 

separated from the frame in some cases, while 

in other cases, only partial separation occurred. 

One of the modules has severe bums marks on 

the top corner of the frame, while on the other 

modules the damage ranges from none to some 

readily visible burn marks. 

3.4.3.2 Detailed examinations 

MODULE - 307 0423 This module was in storage and had been at location E2 B4 1 ("I" on 

Figure 3-2), 10 m from the nearest air terminal. 

There are burn marks along both long edges, 13 
but not the complete length (Photograph 3-16). 

On the right side, the burns are mostly at the 

lower part of the edge, away from the most 

damaged corner (Photograph 3-17). On the left 

side, the burns are mostly in the upper part, 

with intriguing spots over some of the cells 

(Photograph 3-18). The top right comer shows 

some marks on the frame, with the most 

extensive damage at that corner (Photograph 

3- 19). 

MODULE - 303 0267 This module was in storage and had been at location E3 B5 18 ("V" on 

Figure 3-2), 8 m from the nearest air terminal. 

There are burn marks along both vertical edges, 

but not over the complete length (Photograph 

3-20). On the right side, the burns are mostly at 

the upper part of the module, with damage at 

both corners (Photographs 3-21 and 3-22). The 

top right corner (Photograph 3-21) shows heavy 

burn marks on the frame, while the top left corner (Photograph 3-23) shows light marks on the 

frame. It should be noted that this module, 

which has the heaviest burn marks on its frame 

among the six modules, is the only module that 

was located within the "cone of protection" of 

an air terminal. This remark will be discussed 

further in the next section. 

MODULE - 304 0294 This module was in storage and had been at location E3 B5 19 ("IV" on 

Figure 3-2), 12 m from the nearest air terminal. 

There are burn marks along all of the right side, 

and part of the left side (Photograph 3-24). 

Both top corners show damage (Photographs 

3-25 and 3-26). The top right corner (Photograph 3-25 shows light burn marks on the 

frame, while the top left corner (Photograph 3- 

26) hardly shows any burn marks on the frame. 

There is extensive separation of the panel from 

the frame along the right side (Photograph 

3-27) 

MODULE - 306 0417 This module is still in the 

14 array at location E2 B6 5 ("11" on Figure 3-2)) 

10 m from the nearest air terminal, and was 

found damaged on February 5, 1987. The 

bypass diode in the string allows the array to 

remain operational. The right edge shows 

burns (Photograph 3-28). Both right side corners show extensive destruction of panel material (Photographs 3-29 and 3-30), but the upper 

corner has no burn marks on the frame 

(Photograph 3-29). 

MODULE - 304 0300 This module is still in the 

array at location E3 B4 13 C'III" on Figure 3-2), 

10 m from the nearest air terminal, and was 

found damaged on February 5, 1987. The 

bypass diode in the string allows the array to 

remain operational. %ere is damage on three 

of the corners and some of the edges (Photographs 3-31, 3-32, and 3-33), but the heaviest 

damage is on the lower left corner (Photograph 

3-34). The two upper corners shows surface 

degradation on the frame, but these do not 

appear to be burn marks (Photographs 3-32 and 

3-33). 

MODULE - 310 0592 This module is still in the 

array at location E4 B3 20 ("VI" on Figure 3-2), 

10 m from the nearest air terminal, and was 

found damaged on February 25, 1988. The 

bypass diode in the string allows the array to 

remain operational. The damage is concentrated on the left edge of the module 

(Photographs 3-35 and 3-37). The panel is separated from the frame (Photograph 3-38). The 

apparent discoloration of the frame at the top 

left corner does not seem attributable to burns 

(Photograph 3-36). 

3.5 Discussion 

3.5.1 Effectiveness of air terminals 

Lightning protection of solar arrays by air terminals is still a subject of debate (effectiveness, 

shadow effects, cost, appearance). The observations made at the Kythnos site do not bring 
1 I I 1 I I 1 I 1 

0 20 40 60 80 100 120 140 160 

current. kA 

Figure 34. Relationship between potential 

current level of return stroke and striking 

distance 

(Source: Reference 161) 

Figure 35. Rolling ball criterion 

(Same: Reference 151) 

conclusive evidence for or against the effectiveness of correctly designed air terminals, 

although they tend to weaken the case for providing air terminals. 

The Kythnos experience involves points of 

(presumed) lightning termination that are at the 

edges of the zone of protection of several criteria, where this protection becomes more uncertain. Ironically, the most severe burn mark is 

found on the frame of the module that was 

closest to an air terminal, and within the zone 

of protection as detailed in paragraph 3.4.1. 

Thus, a brief review of the uncertainties of the 

zone-of-protection concepts will provide the 

necessary perspective on the issue. 

Indiscriminate application of the 45' cone of 

protection criterion to tall structures has led to 

contradictions. An example is occurrence of 

lightning strikes terminating on the side of tall 

buildings, within the cone of "protection", The 

original concept of a cone of protection is now 

generally replaced by the rolling ball criterion, 

based on the striking distance theory. According to this striking distance theory 161, the striking distance at the tip of the descending 

stepped leader increases with the amount of 

charge in the leader. Thus, the leaders having 

the highest potential current level have the 

longest striking distance (Figure 3-4). 

Conversely, leaders having the lowest potential 

current level have the shortest striking distance. 

The point of termination of a lightning strike 

can be anywhere within the striking distance 

from the last point of advance of the descending stepped leader. This fact can be represented by imagining a sphere with a radius 

equal to the striking distance, which is determined by the charge in the lower part of the 

leader. Any point at ground potential penetrating that sphere is a candidate for emitting a 

upward streamer that will complete the path for 

the return stroke. Thus, points at ground 

potential outside of the sphere are still "protectedn while the points inside the sphere are 

not. 

Considering now the configuration of a vertical 

mast on the ground plane (Figure 3-5), rolling a 

ball on the ground until it touches the tip of the 

mast defines the limiting condition when the 

descending leader will terminate at the tip of 

the mast, thus leaving other points below the 

sphere uninvolved. Figure 3-6 shows 15 
graphically the configuration for the 10.5 m 

masts used in Kythnos, with the upper edge of 

the panels at 2 m above the ground plane. 

Figure 3-6 shows the zone of protection as 

defined by the traditional 45" cone of protection, as well as that defined by a rolling ball of 

30 m radius, as specified in the Lightning Protection Code I51. Simple geometry shows a distance of 8.5 m from the mast for the 45' cone, 

while the graphical solution for the rolling ball 

shows a distance of 12 m from the mast. It 

should be emphasized that the selection of a 

30 m radius for the ball is somewhat arbitrary, 

in view of the data shown in Figure 3-4. From 

Figure 3-6, it is apparent that a pessimistic 

assumption would be a smaller radius for the 

rolling ball: such a smaller ball would roll closer 

to the mast and thus would reduce the 

"protected" distance from the mast. 

This observation needs to be combined with the 

statistical distribution of lightning current amplitudes as stated by Cianos & Pierce [21 to appreciate that the 30 m radius is only a pragmatic 

choice, not an absolute criterion. Therefore, 

observing points of presumed lightning termination at distances of 8.5 m to 12 m from the base 

of an air terminal is not startling, especially for 

low-current strokes. This observation shows 

how precarious the assurance of protection can 

be when only sparsely distributed air terminals 

are provided. In other words, increasing the 

degree of confidence that sufficient protection 

zones are established might require such a 

density of masts (or overhead wires) that the 

cost, appearance, and shadow effects would 

loom large in the overall trade-off. 

3.5.2 Lightning current path 

The resulting return stroke would then draw 

charges from the earth via the grounded structure, that is, the return current would come out 

of the grounding cable at the base of the column, and proceed by the shortest route toward 

the upper edge of the panel. This shortest path 

does not include the lower half of the panel 

45" CONE: PROTECTION TO 8.5m 

\ 

\ -- TOLLING BALL: PROTECTION 

2 4 6 8 10 12 

METERS 

Figure 36. Cone-of-protection and rolling-ball criteria 
edges, as it would require the lower panel 

brace plus the panel edge to become involved. 

While this path may still be somewhat involved, 

the major part of the current should only 

involve the upper half of the panel, a situation 

which is not reflected in the more or less even 

(or random) distribution of the damage 

observed on both upper and lower halves of 

the long edges of the modules. 

3.5.3 Direct versus consequential effects 

In the absence of definitive knowledge on the 

direct effect of a lightning current involving a 

module, only conjectures can be made on the 

failure mode of the panel. As discussed in the 

preceding paragraph, the presence of damage 

at the lower half of the panels is somewhat contradictory to the hypothesis of all the damage 

being done by the lightning current. This contradiction adds weight to the argument (also 

presented in the case of the Vulcano incident), 

that the observed damage may be the result of a 

dc fault current occurring after an initial insulation breakdown caused by an indirect lightning 

overvoltage induced in the dc circuit. The insulation breakdown would occur at the point of 

lowest withstand, not necessarily in the upper 

half of the panel, and the ensuing dc fault 

would proceed along the edge as the blowtorch effect associated with the high temperatures of the dc arc, lingering at the fault, would 

cause burning along the edges, similar to what 

was obsewed. 

On the other hand, the extent of the damage in 

the E2 B5 box (Photographs 3-10 and 3-11) 

appears to be greater than what could be 

expected from the dc current alone. Damage 

caused by the occurrence of a lightning surge 

current is a more likely scenario in this case. 

3.5.4 Mechanical effects 

The top glass cover plates of the damaged modules generally had several cracks, but do not 

have the frosty appearance associated with the 

tempered glass used in the Vulcano module. 

This difference may provide some clue about 

the sequence of the scenario, if it could be correlated with the mechanical characteristics of 

the glass. Damage to the glass during dc faults 

has been reported in the United States. However, no further detailed information is available in either case to pursue this line of 

thought. This subject could be part of a test 

program aimed at finding failure modes of PV 

modules related to dc faults and lightning (both 

direct and indirect). 

3.5.6 Integrity of the grounding system 

The grounding system has been implemented 

with galvanized steel conductors, in keeping 

with the standard German practice where concerns over cathodic corrosion have steered 

designers away from copper. In the salty environment of an island, questions may be raised 

on the long-term integrity of buried galvanized 

steel conductors. Even in the dry environment 

of the array footings, some signs of corrosion 

are apparent (Photograph 3-40). 

3.6 Specific Conclusions from the 

Kythnos Case 

The obsewed damage to the panels cannot be 

conclusively attributed to a direct lightning 

strike. The six reported incidents might involve 

a combination of effects, with one case involving a direct strike, and the others being an indirect effect. In other words, the evidence that 

might point to invalidating a particular scenario 

might not apply to the scenarios of other incidents. The surge-protective devices provided 

at the site performed well since no damage was 

inflicted to the electronics. Failure of one surge 

arrester in the performance of its protective 

duty can be viewed as the ultimate sacrifice of 

the device fulfilling its mission - but it raises 

the question of monitoring for failure of protective devices. 
4.1 To protect or not to protect ? 

The debate on whether to provide protection by 

air terminals or suffer the consequences of a 

direct strike is not settled by these case histories. In spite of the presence of air terminals at 

Kythnos, damage occurred. This damage my be 

a direct effect, or may be an indirect effect, or a 

combination of both. At Vulcano, with no air 

terminals, only one case of lightning-related 

damage has occurred, and this single case may 

be an indirect effect rather than a direct effect. 

Indirect effects are not eliminated by air terminals. A better argument could be made if a firm 

conclusion were reached on whether the damage was a direct or indirect effect. 

If the damage is attributed to direct effects, then 

the conclusion is that the air terminals, at the 

spacing and height used at Kythnos, were ineffective. However, precisely because air terminals were distributed perhaps too sparsely, the 

Kythnos case history does not invalidate protection if it' were ensured by appropriate air terminals with adequate height and density. 

If the damage is attributed to an indirect effect, 

then one would argue that the air terminals cannot serve any useful purpose - the counterargument being that the direct damage would 

have been even worse than what actually 

occurred. 

4.2 Grounding practices 

Differences in grounding practices leave many 

questions unanswered. On the materials 

aspects, there is the different approach of using 

copper or of using galvanized steel. On the circuitry aspects, there is the issue of grounding 

the dc circuit or leaving it floating (but with a 

ground fault detection scheme). This latter 

18 choice, however, raises questions on the implementation of a ground fault indication which is 

available only to local operators. That design 

may raise concerns in the context of long-term 

operation where immediate action on a ground 

fault may not be perceived as important. This 

postponing of action may then lead to the 

occurrence of a second fault caused by lightning or by further pollution of insulation, with 

damage to components at that time. 

4.3 Suggestions for further 

investigations 

The ambiguity on the interpretation of the 

reported damage gives added weight to the 

desirability of consolidating all available data on 

panel failure modes, and eventually performing 

lightning simulation tests, as well as insulation 

failure (tracking) tests. Evidence from the lightning damage incident that occured at the photovoltaic installation of the Sacramento Municipal 

Utility District (SMUD PV1) in California [7l 

should be compared to the damage observed at 

Vulcano and Kythnos. 

The conjectural scenario of a nearby lightning 

strike inducing sparkover at points of weak 

insulation, followed by damage caused by the 

dc current, could be more credible if knowledge were available on two parameters: 

(1) dielectric withstand of the insulation 

between the modules and their frame, under 

various conditions of contamination, and (2) 

levels of the overvoltages that could be induced 

in the circuits. The first parameter would 

require tests on the actual configurations, and 

might be impractical in view of the large number of possible configurations. The second 

parameter might be evaluated by theoretical 

analysis, such as that reported Stolte 181. 

The ambiguity in the post-mortem may be 

resolved by further study of the failure modes 

of a panel (module within its frame) under simulated lightning and under dc stress with 
surface contamination. The value of such tests 

would be to determine the need of further protection or design improvements in the panels to 

avoid damage, or to better understand the 

mechanism of the failure in order to settle the 

dilemma on the exact scenario leading to the 

observed damage. Ultimately, the knowledge 

would also provide the basis for an informed 

decision on the cost-effectiveness of air 

terminals. 

5. General Conclusions 

The two case histories presented in this report 

demonstrate that it is possible to provide protection for the power conversion electronics in 

the face of inescapable lightning strikes to the 

array field. In several instances, damage was 

limited to the modules; the surge protective 

devices performed their function with no damage to themselves. In one instance, damage 

was inflicted to the surge protective devices, but 

even while failing, they protected the expensive 

downstream circuitry. Depending on the point 

of view, achieving protection at the cost of a 

failed protective device may be considered successful, while an alternate view might be to 

expect proteaon with no sacrifice of the protective device. 

The observations made at these two sites, the 

evidence collected before the visits, and the 

preceding discussions lead to a set of conclusions, some still in the form of conjectures, 

some in the form of firm conclusions. Furthermore, implementation of the recommendations 

presented here may validate the conjectures and 

elevate them to the status of firm conclusions. 

A most important point to bear in mind, however, is that the unpredictability of lightning 

occurrences make it a risky business to draw 

sweeping conclusions based on only a few 

years of observation [61. 

Protection against lightning damage to the array 

modules is a more difficult and less clear-cut 

issue than operation and survival of protective 

devices incorporated in the circuit: 

- First, there is still some ambiguity in attributing all of the observed damage either to a 

direct effect of lightning, or to an indirect 

effect. 

- Second, there is no sufficient evidence and 

long-term data on the effects and costs of a 19 
presumed direct strike to rule out air 

termnals, although their cost-effectiveness 

appears questionable. 

5.1 Conjectural conclusions 

A likely scenario to explain the observed effect 

is a combination of lightning-induced overvoltages with low insulation withstand. This low 

withstand may be an inherent limitation of the 

photovoltaic module layout, or may be the 

result of pollution or moisture. 

The evidence at Vulcano tends to point away 

from a simple direct lightning strike because the 

reported damage was limited to the lower part 

of the array. However, no direct inspection of 

the failed module was possible in this case. 

The overvoltages associated with the one incident at Vulcano were successfully suppressed as 

no damage was inflicted to either the surge suppressor themselves, the first line of defense, or 

to the power conversion electronics, the potential victim equipment. However, since the 

amplitude of the lightning stroke in that incident is not known, the conclusion should not 

be that protection has been achieved for 

level of severity. 

The effectiveness of lightning rods appears 

questionable in view of the several incidents at 

Kythnos. However, a higher density of rods, or 

greater height, might have reduced the damage. 

Nevertheless, the scenario of possible damage 

by indirect effects leaves in doubt the justification for the expense and disadvantages of 

providing lightning rods. 

5.2 Firm conclusions 

The one obvious conclusion, not unexpected, is 

that lightning does represent a threat to photovoltaic arrays, either by direct damage or by 

2o indirect damage. 

Good evidence has been provided that surgeprotective devices with appropriate ratings 

(coordinated protection with the equipment to 

be protected, adequate surge current handling 

capability, and not excessively low clamping 

voltage for the systems voltage conditions) can 

protect the electronic equipment. 

The one case of failure of a surge protective 

device that occurred shows that with suitable 

failure mode (i.e., short-circuit), protection of 

the electronics can be obtained for the first incident. However, if the protective devices are 

associated with fuses, as in the case of Vulcano, 

failure of the protective device would result in 

blowing the fuse and, unless an indication of 

that situation were provided, the equipment 

would then be left unprotected for the next 

occurrence. 

5.3 Recommendations 

The ambiguity in attributing the damage to a 

direct lightning strike may be reduced if the 

suggestions proposed in this report for simulated lightning tests and study of failure modes 

were implemented: 

- Establish a common, world-wide data base 

summarizing all observations of documented 

or suspected lightning damage to panels. 

- Establish a common, world-wide data base 

summarizing all observations of damage to 

surge protective devices 

- Establish a common, world-wide data base 

summarizing all observations of documented 

dc insulation faults on panels. 

- Perform laboratory simulation of lightning 

attachment to panel frames and to module 

surfaces. 

In view of the prevalent practice, with apparent 

success, among European designers of not 

grounding the dc system, the quasi-axiomatic 
practice by U.S. designers of multiple-point 

grounding should be re-examined, and a 

dialogue initiated between the two parties. 

Operating procedures associated with the 

occurrence of the first fault in an isolated dc 

system should be reviewed and clearly defined. 

An intriguing although not crucial question is 

that of the nature (and thus cost) of the materials used for the ground grid. The Italian practice calls for copper, while the German practice 

applied in Kythnos calls for galvanized steel. 

The question of copper versus galvanized steel 

in this context should be re-examined by 

specialists of cathodic protection schemes. 

6. Acknowledgements 

The two site visits described in this report were 

made possible by the cooperation and hospitality of A. Previ and V. Messina of ENEL, and of 

J. Chadjivassiliadis and A. Grielas of Public 

Power. T.S. Key provided insights in reviewing 

the draft of this report. Funding for this project 

was provided by the Sandia National 

Laboratories for the U.S. Department of Energy. 
7. References 

[I] Previ, A., "The Vulcano Project," Int. J. 

Solar Energy, 1985, Vol3, pp 124-141. 

[21 Cianos, N. and Pierce, E.T. 'Agroundlightning environment for engineering 

usage." Stanford Research Institute, Menlo 

Park, Ca, 1972. 

[31 Martdoff, ED., "Matching Surge Protective 

Devices to their Environment, " IEEE 

Transactions, Vol IA-21, No.1, January1 

February 1985. 

[41 Shirakawa, S., E Endo, H. Kitajima, 

S. Kobayashi, and K. Kurita, Maintenance of 

Surge Arrester by a Portable Arrester Leakage 

Current Detector," IEEE Transactions, Vol 

PD-3, No. 3, July 1988. 

[51 NFPA 78 "Lightning Protection Code,," National 

Fire Protection Association, Boston, MA, 1986. 

[6l Golde, R.H., 'Lightning Protection," Chemical 

Publishing Company, New York, NY, 1975. 

[71 Collier, D.E., and Key, T.S., "Electrical Fault 

Protection for a Large Photovoltaic Plant 

Inverter," Proceedings, 20th IEEE Photovoltaic 

Specialist Conference, September 1988. 

181 Stolte, W.J., "Photovoltaic System Grounding 

and Fault Protection Guidelines,' Contractor 

Report SAND83-7025, National Technical 

Information Servcie, 1985. 
I. TITLE AND SUBTITLE 

BS-114A (REV. 24C) 

Lightning and Surge Protection of Photovoltaic Installations 

Two Case Histories: Vulcano and Kythnos 

Francois D. Martzlof f 

,. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions) 7. ContracdGrant No. 

2. Performing Organ. Report NoJ 3. Publication Date 

JUNE 1989 

U.S. O~PT. OF COMM. 

BIBLIOGRAPHIC DATA 

SHEET (See instructions) 

U.S. DEPARTWENT OF COMMERCE 

NAllOlUL INSTITUTE OF STANDARDS 

aurnaswna, uo zows 

'1. PUBLICATION OR 

REPORT NO. 

NISTIR 89-4113 

AND TECHNOLOQY I I. Type of Report & Period Coverec 

I 

I. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. Stote, ZIP) 

Sandia National Laboratories 

Albuquerque, NM 87185-5800 

0. SUPPLEMENTARY NOTES 

Document describes a computer program; SF-185. FlPS Software Summary, is attached. 

1. ABSTRACT (A 200-word or less factual summary of most significant inforinorion. If document includes a slgniflcont 

bibliography or literature survey, mention It here) 

Two installations of photovoltaic systems were damaged during lightning storms. 

The two sites were visited and the damaged equipment that was still available on 

the site was examined for analysis of the suspected occurrence. The evidence, 

however, is insufficient to conclude that all the observed damage was caused by 

the direct effect of a lightning flash. A possible scenario may be that lightninginduced overvoltages caused insulation breakdown at the edges of the photovoltaic 

modules, with subsequent damage done by the dc current of the array. Other surge 

protection considerations are also addressed, and suggestions presented for further 

investigations. 

2. KEY WORDS (Six to twelve entries; alphabetical order: capitalize only proper names; and separate key words by semicolons) 

lightning; photovoltaic; surge; varistors 

=Unlimited 

D For Official Distribution. Do Not Release to NTIS 

U Order From Superintendent of Documents. U.S. Government Printing Office, Washington. D.C. 

20402. 

EOrder From National Technical Information Service (NTIS), Springfield. VA. 22161 $14.95