2012 international conference on lightning … › nlsi_lhm › lplargepvarray.pdf2012 international...

2012 International Conference on Lightning Protection (ICLP), Vienna, Austria

Lightning Protection Practice for Large-ExtendedPhotovoltaic Installations

Dr. Nikolaos KokkinosResearch and Development Department

Elemko SAAthens, Greece

[email protected]

Dr. Nicholas ChristofidesDepartment of Electrical Engineering.

Frederick UniversityNicosia, Cyprus

[email protected]

Dr. Charalambos CharalambousDepartment of Electrical and Computer Engineering.

University of CyprusNicosia, Cyprus

[email protected]

Abstract— This paper aims to analyse the lightning protectionsystem (LPS) of an isolated large and extended Photovoltaic (PV)installation park. The area where the PV plant operates ischaracterised by the high ground flash density (25thunderstorm days per year) and the extremely high soilresistivity value (i.e. pure rock with a resistivity of more than2000m). The paper includes the LPS system design afterexperimental testing results, which were performed in thelaboratory. It also includes solutions to some difficult overcomingproblems that were faced during the application of the lightningprotection design.

Keywords-Photovoltaic park, Lightning protection system

I. INTRODUCTION

In a country like Greece where the sun is shining for mostof the year round, the number of photovoltaic (PV) installationshas been significantly increasing during the last years.Nowadays, the interest and investment in large scale PV parksin the MWp range is becoming very common. The knowledgehowever of a proper lightning protection system (LPS) designand installation, including surge protection, for such large andextended structure areas (with long cabling loops) is still underresearch. This is the reason for the development of the newCENELEC document; TS 50539-12: 2009 [1] describingapplication principles of surge protection in PV installations.The investments in such large scale PV parks are considerableand it is merely common sense that investors should choose toadopt a LPS for their systems. When compared to the incomelosses incurred due to a failure or damage resulting from alightning strike, not to mention the technical and practicaldifficulties associated with the repairs or componentreplacements, the cost of a LPS system is negligible. It istherefore advisable, not to say self-evident, that a LPS isnecessary.

The main objective of this on-going work is to address theissues necessary to form a global framework for the lightningprotection system (LPS) design of isolated large and extendedphotovoltaic installations - PV parks. In particular, this paperdescribes the preliminary work on LPS system designs withparticular emphasis on experimental testing that is performed atELEMKO’S H.V laboratory in Greece. This work aims inframing proposals and solutions to overcome challenges andproblems that may rise during the installation of lightningprotection designs.

II. SITE SURVEY

The particular PV park under study is installed on amountain peak, flat area, occupying a total surface of around115,000m2. In total it contains 180 DC/AC inverters of 11kWnominal power, operating at 800VDC and 7,300 solar panels of270Wp nominal power with dimensions of 2m x 1m each. ThePV park is connected to the 21kV medium voltage (MV)distribution system via 8 substations (MV/LV). The soil wasrocky (>2000Ωm) and the support structure of the PV panelswas a combination of concrete reinforced bases embedded insoil and aluminium supports above soil.

III. INITIAL LIGHTNING PROTECTION SYSTEMDESIGN CONSIDERATIONS

Due to the high resistivity of the soil, which was notpromising for an effective earthing system and in conjunctionwith the high ground flash density of the area, the design of thelightning and surge protection was considered of high priority.Due to the extensive area coverage of the PV park, the designof the external LPS considered both possible cases, the one foran isolated application as well as the one for a non-isolatedapplication design, as per IEC 62305 – 3 [2].

Fig. 1 and Fig. 2 illustrate the 2 cases considered whendesigning the LPS. In Fig. 1, for the non-isolated case, the airterminals for every 2 consecutive rows are connected to one toone electrode installed between the 2 rows. In this scenario, itis advised that the distance between the PV frame of each rowand the earth electrode does not exceed 3m. Fig. 2, illustratesthe isolated case, where the air-terminal of each PV row isconnected to a separate electrode directly and not through thePV framework. Each PV frame is also bonded on the sameearth electrode but with independent bonding conductors.

≤ 3m≤ 3m

Figure 1. Design of Type B earth electrode using non-isolated LPS. The air-terminals of 2 consecutive PV rows are connected to 1 electrode which is

installed between them

≤ 3m

Figure 2. Design of Type B earth electrode using isolated LPS. Each isolatedair terminal is connected to a separate electrode directly and not through the

metal PV framework

For the isolated LPS it was necessary to provide an earthelectrode behind each PV row so as to earth the isolated airterminals directly. For the non-isolated case however, an earthelectrode for every 2 consecutive PV rows was necessary,combining the earthing of the air-terminals.

For PV system installations in open fields, in order tominimize the cost and at the same time increase the efficiencyand life time of the earthing system (by avoidingelectrochemical corrosion) it is very important that during thedesign the type of foundation used for the PV façade –framework is taken into consideration.

In this particular project, the foundation material wasreinforced concrete buried into the soil, therefore copper coatedand solid copper earth electrodes were used. Fig. 3 and Table 1,depict information about the selection criteria of the materialsused for the earthing system, depending as mentioned above,on the type of the foundation used for the PV framework orstructures.

In addition, the PV park under study was designed utilizingsmall 11kW inverters and not central inverters. Therefore, theDC cable loop was small and there was no need to use T1+T2SPDs on the DC side of the inverter even for the non-isolatedLPS case. Fig. 4, illustrates the installation of T2 only typeSPDs on the DC side resulting from the fact that the lightningloop formation is limited or non-existent.

1 2 3 411 22 33 44

Figure 3. Acceptable materials for earthing system depending on the type offoundation used for the PV framework

TABLE 1. DETAILED DESCRIPTION OF MATERIALS DEPICTED INFIG. 1

Type of Foundation of the PVframework

Allowed material for earthingsystem driven into the soil

1 Galvanized steel profile directlyburied into the soil

Galvanized steel, Stainless steel

2 Steel profile embedded inconcrete

Copper coated steel, Copper,Stainless steel

3 Reinforced concrete block placedat ground level not into the soil

Galvanized steel, Copper coatedsteel, Copper, Stainless steel

4 Reinforced concrete foundationinto the soil

Copper coated steel, Copper,Stainless steel

Note 1: Copper conductor may also be tinnedNote 2: Aluminum is not allowed to be used into the soil

_+ PE

SPDUoc=1000VdcUc=500VdcΙn=20kΑ (8/20μs)

Ιmax=40kΑ (8/20μs)

Up = <4kV

T2 CEPV - 1000T2



Up = <4kV

T2 CEPV - 1000T2



Up = <4kV

T2 CEPV - 1000T2

_+ PE



Up = <4kV

T2 CEPV - 1000T2



Up = <4kV

T2 CEPV - 1000T2



Up = <4kV

T2 CEPV - 1000T2



Up = <4kV

T2 CEPV - 1000T2



Up = <4kV

T2 CEPV - 1000T2



Up = <4kV

T2 CEPV - 1000T2

>10m>10m

N

SPDUC=255VacΙn=20κΑ (8/20μs)

Ιmax=40κΑ (8/20μs)

Up = <1.2kVTa = <100ns

T2 CE40GT2

PE

L



Up = <1.9kVTa = <25ns

T2 CE40T2

L



Up = <1.9kVTa = <25ns

T2 CE40T2

L



Up = <1.9kVTa = <25ns

T2 CE40T2

N



Up = <1.2kVTa = <100ns

T2 CE40GT2

PE

L



Up = <1.9kVTa = <25ns

T2 CE40T2

L



Up = <1.9kVTa = <25ns

T2 CE40T2

L



Up = <1.9kVTa = <25ns

T2 CE40T2

AC e

lect

ric p

anel

boa

rd String 1

String 2

Common earthing system

≈1m

N



Up = <1.2kVTa = <100ns

T2 CE40GT2

PE

L



Up = <1.9kVTa = <25ns

T2 CE40T2

L



Up = <1.9kVTa = <25ns

T2 CE40T2

L



Up = <1.9kVTa = <25ns

T2 CE40T2

N



Up = <1.2kVTa = <100ns

T2 CE40GT2

PE

L



Up = <1.9kVTa = <25ns

T2 CE40T2

L



Up = <1.9kVTa = <25ns

T2 CE40T2

L



Up = <1.9kVTa = <25ns

T2 CE40T2

AC e

lect

ric p

anel

boa

rd

DC

Jun

ctio

n Bo

x

Figure 4. Installation of SPDs for PV application inside a junction boxsituated at a distance >10m from the inverter at the DC & AC side. Only T2SPDs are needed on the DC side since there is no lightning loop formation.On the AC side however, T1 + T2 type SPDs are required due to the cable

loop, which may allow the lightning current to flow in a parallel path to earth

String 1

String 2

DC & AC TerminationSet

DC Input

DC Output

AC Output

AC Input

SURGE PROTECTION

8899kWhr8899kWhr

InverterkW Meter

PV G

ener

ator

s

String 1

String 2

DC & AC TerminationSet

DC Input

DC Output

AC Output

AC Input

SURGE PROTECTION

8899kWhr8899kWhr8899kWhr8899kWhr

InverterkW Meter

PV G

ener

ator

s

Figure 5. DC & AC termination set for optimum protection of the inverteragainst surges arriving either from the AC or DC cabling

For the effective protection of the inverters, a combinedprotection was developed allowing a combined termination ofall DC & AC cables and at the same time providing protectionfrom coupled surges. By using such a combined terminationset, the cable lengths are kept to an absolute minimum.However, such a combination should fulfill certain standards[3-7] since coexistence of DC and AC at relative high voltagesrequires specific isolation distances. Fig. 5, illustrates thisarrangement, where 1 termination box is used for both DC andAC cable terminations

IV. EXAMINED CASE STUDIES

Due to the extensive and long cable loops which are formedby the DC cables, any direct or nearby lightning will causehigh induced surges, Fig. 6. Since screening of the DC cables isdifficult to provide for in large PV parks, the question thatarises is which external LPS provides a more suitable and moreeffective protection against over-voltages induced on thecables. A scaled down experiment was performed in thelaboratory in order to obtain measurements and informationthat would assist in the external LPS design of the 2MWp PVpark under study. The purpose of the laboratory tests was toevaluate the performance and apply it later on.

The scaled down version of the experimental setup in thelaboratory was a 2kWp PV system depicted in Fig. 7. Thesystem consists of 9 PV modules connected in series giving atotal of 200V output voltage and 10A current. During theimpulse experiments the laboratory lights were switched off inorder to have zero volts at the DC cable loop, which wasapproximately 18m long.

Figure 6. Effect on the extended wiring of an open field large PV systeminstallation due to a direct or indirect lightning strike

Figure 7. PV panels tested in the laboratory and the formation of the DC cableloop (red +ve and black –ve)

The initial purpose of the test was to investigate which ofthe following two possible cases would give a lower inducedvoltage across the DC cable loop of the string. In case A theimpulse current was injected directly on the framework of thePV panels through an air termination rod, which was supportedon the edge of the PV support framework. In case B theimpulse current was injected directly on the on an isolated airtermination rod, which was supported on the laboratory floor ata distance of approximately 700mm from the PV supportframework. The results are summarized in Fig. 8, Fig. 9 andFig. 10.

Figure 8. Examining the behavior of isolated and non isolated LPS in case of adirect lightning strike and the effect of inducted over-voltages in the DC

cabling of a PV installation

α

Case Α

≈ 25%

The open circuit voltage across the (+)and (-) negative pole of the string was82V and the injected current was 100kA(10/350μs).

1

1

Figure 9. Experimental results of scaled down experiment examining thebehavior of a non-isolated LPS with respect to the induced over-voltages on

the DC cabling of a PV string

The open circuit voltage across the (+)and (-) pole of the string was 390V andthe injected lightning current 100kA(10/350μs). It was almost 480% than thenon isolated case. This is due to thereason that the current was notdistributed but driven 100% through onepath creating a high magnetic field.

1

α

Case Β

100%100%

1

Figure 10. Experimental results of scaled down experiment examining thebehavior of an isolated LPS with respect to the induced over-voltages on the

DC cabling of a PV string

The results show that the non-isolated LPS will providelower induced over-voltages on the cabling in case of a directlightning strike. Additionally, since small inverters were used(therefore no parallel path for the DC to earth) the need of

T1+T2 SPDs for the DC was not mandatory. Furthermore, dueto the fact that the earthing system arrangement was a costeffective solution for the non-isolated case compared with theisolated LPS case, the investor decided to use a non-isolatedLPS for the particular Photovoltaic park.

V. CONCLUSIONS

The design of a lightning protection system for large scale PVsystems may depend on various factors and parameters. It isvery important to take into consideration the installationarrangement and design adopted for large scale PV parks as itinvolves crucial parameters that must be taken intoconsideration for the effective external and internal LPSdesign. There are numerous large scale PV system designs allof which require a LPS that satisfies the unique, in sometimes, conditions present.

The need of an efficient lightning protection system ismandatory for photovoltaic installations by virtue of itspreventing nature. Primarily, the need is imperative to preventany physical damage to structures and life hazards. It is worthnoting that the damage of the electrical and/or electronicequipment of a PV installation, due to surges originates fromLightning Electromagnetic Impulse (LEMP) as well as fromSwitching Electromagnetic Impulse (SEMP) [8].

However, literature survey reveals that there is still verylittle information published regarding the design of lightningand surge protection for large and extended PV installations.In particular, reference [9] comprehensively covers the relatedscientific background by emphasizing on the aspects ofstandardisation that should be addressed in the near future. Asquoted, the current practice, for protecting PVIs fromlightning surges, rests with adopting (partly) protectivemeasures described in standards for conventional low-voltagepower distribution systems.

It is vital that the LPS provide an effective protectionagainst direct or indirect lightning strikes in order avoid thedestructive effects of lightning. As previously mentioned, theinvestment cost is very considerable and 1 thunderstorm canbe catastrophic with inestimable financial consequences.

The results presented in this paper propose the most costeffective and technically correct solution for the LPS design ofthe large scale photovoltaic system under study. The needhowever, for deeper and more detailed analysis is required sothat amendments are made to the current standard in order toinclude guidance and regulations for common large scale PVinstallation practices and examples.

Non isolated LPS, the lightning current isdistributed along the metallic frame of thePV structure. The metallic structure isused a natural down conductor.

Isolated LPS at a distance of 0,7m behindthe steel frame of the PV structure. Thelightning current is driven to earth via adedicated rod and down conductor. Themetallic structure of the PV is onlyconnected to the LPS via the earth.

α Case Β

100%

α Case Βα Case Β

100%100%

α

Case Α

≈ 25%

α

Case Α

≈ 25%

ACKNOWLEDGMENT

The authors wish to thank the engineering team of BIOSARSA (GREECE) for their contribution to the photovoltaic parkproject.

REFERENCES

[1] TS 50539-12:2009, Protection of PV installation against overvoltages

[2] IEC 62305 – 3:2010, Protection against lightning Part 3: Physicaldamage to structures and life hazard

[3] EN 60439-1: Low voltage switchgear and controlgear assemblies – Part1: Type tested and partially type tested assemblies.

[4] EN 60439-3: Low voltage switchgear and controlgear assemblies – Part3: Particular requirements for low voltage switchgear and controlgearassemblies intended to be installed in places where unskilled personshave access for their use – Distribution boards

[5] HD 60304-7-712: Electrical installations of buildings – Part 7-712:Requirements for special installation locations – Solar photovoltaic (PV)power supply

[6] EN 60664-1: Insulation co-ordination for equipment within low voltagesystems – Part 1: Principles, requirements and tests

[7] EN 62446:2009: Grid connected photovoltaic systems – Minimumrequirements for system documentation, commissioning tests andinspection.

[8] IEC – 62305 “Protection against Lightning”[9] Jesus C. Hernandez, Pedro G. Vidal, Francisco Jurado, “Lightning and

Surge Protection in Photovoltaic Installations’’, IEEE Transcactions onPower Delivery, Vol. 23, No 4. Oct. 2008 pp. 1961-1971.

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