2012 International Conference on Lightning Protection (ICLP), Vienna, Austria Lightning Protection Practice for Large-Extended Photovoltaic Installations Dr. Nikolaos Kokkinos Research and Development Department Elemko SA Athens, Greece [email protected]Dr. Nicholas Christofides Department of Electrical Engineering. Frederick University Nicosia, Cyprus [email protected]Dr. Charalambos Charalambous Department of Electrical and Computer Engineering. University of Cyprus Nicosia, Cyprus [email protected]Abstract— This paper aims to analyse the lightning protection system (LPS) of an isolated large and extended Photovoltaic (PV) installation park. The area where the PV plant operates is characterised by the high ground flash density (25 thunderstorm days per year) and the extremely high soil resistivity value (i.e. pure rock with a resistivity of more than 2000m). The paper includes the LPS system design after experimental testing results, which were performed in the laboratory. It also includes solutions to some difficult overcoming problems that were faced during the application of the lightning protection design. Keywords-Photovoltaic park, Lightning protection system I. INTRODUCTION In a country like Greece where the sun is shining for most of the year round, the number of photovoltaic (PV) installations has been significantly increasing during the last years. Nowadays, the interest and investment in large scale PV parks in the MWp range is becoming very common. The knowledge however of a proper lightning protection system (LPS) design and installation, including surge protection, for such large and extended structure areas (with long cabling loops) is still under research. This is the reason for the development of the new CENELEC document; TS 50539-12: 2009 [1] describing application principles of surge protection in PV installations. The investments in such large scale PV parks are considerable and it is merely common sense that investors should choose to adopt a LPS for their systems. When compared to the income losses incurred due to a failure or damage resulting from a lightning strike, not to mention the technical and practical difficulties associated with the repairs or component replacements, the cost of a LPS system is negligible. It is therefore advisable, not to say self-evident, that a LPS is necessary. The main objective of this on-going work is to address the issues necessary to form a global framework for the lightning protection system (LPS) design of isolated large and extended photovoltaic installations - PV parks. In particular, this paper describes the preliminary work on LPS system designs with particular emphasis on experimental testing that is performed at ELEMKO’S H.V laboratory in Greece. This work aims in framing proposals and solutions to overcome challenges and problems that may rise during the installation of lightning protection designs. II. SITE SURVEY The particular PV park under study is installed on a mountain peak, flat area, occupying a total surface of around 115,000m 2 . In total it contains 180 DC/AC inverters of 11kW nominal power, operating at 800VDC and 7,300 solar panels of 270Wp nominal power with dimensions of 2m x 1m each. The PV park is connected to the 21kV medium voltage (MV) distribution system via 8 substations (MV/LV). The soil was rocky (>2000Ωm) and the support structure of the PV panels was a combination of concrete reinforced bases embedded in soil and aluminium supports above soil. III. INITIAL LIGHTNING PROTECTION SYSTEM DESIGN CONSIDERATIONS Due to the high resistivity of the soil, which was not promising for an effective earthing system and in conjunction with the high ground flash density of the area, the design of the lightning and surge protection was considered of high priority. Due to the extensive area coverage of the PV park, the design of the external LPS considered both possible cases, the one for an isolated application as well as the one for a non-isolated application design, as per IEC 62305 – 3 [2].
5
Embed
2012 International Conference on Lightning … › nlsi_lhm › LPLargePVArray.pdf2012 International Conference on Lightning Protection (I CLP), V ienna, Austria Lightning Protection
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
2012 International Conference on Lightning Protection (ICLP), Vienna, Austria
Lightning Protection Practice for Large-ExtendedPhotovoltaic Installations
Dr. Nikolaos KokkinosResearch and Development Department
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
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
SPDUoc=1000VdcUc=500VdcΙn=20kΑ (8/20μs)
Ιmax=40kΑ (8/20μs)
Up = <4kV
T2 CEPV - 1000T2
SPDUoc=1000VdcUc=500VdcΙn=20kΑ (8/20μs)
Ιmax=40kΑ (8/20μs)
Up = <4kV
T2 CEPV - 1000T2
_+ PE
SPDUoc=1000VdcUc=500VdcΙn=20kΑ (8/20μs)
Ιmax=40kΑ (8/20μs)
Up = <4kV
T2 CEPV - 1000T2
SPDUoc=1000VdcUc=500VdcΙn=20kΑ (8/20μs)
Ιmax=40kΑ (8/20μs)
Up = <4kV
T2 CEPV - 1000T2
SPDUoc=1000VdcUc=500VdcΙn=20kΑ (8/20μs)
Ιmax=40kΑ (8/20μs)
Up = <4kV
T2 CEPV - 1000T2
SPDUoc=1000VdcUc=500VdcΙn=20kΑ (8/20μs)
Ιmax=40kΑ (8/20μs)
Up = <4kV
T2 CEPV - 1000T2
SPDUoc=1000VdcUc=500VdcΙn=20kΑ (8/20μs)
Ιmax=40kΑ (8/20μs)
Up = <4kV
T2 CEPV - 1000T2
SPDUoc=1000VdcUc=500VdcΙn=20kΑ (8/20μs)
Ιmax=40kΑ (8/20μs)
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
SPDUC=275VacΙn=15κΑ (8/20μs)
Ιmax=40κΑ (8/20μs)
Up = <1.9kVTa = <25ns
T2 CE40T2
L
SPDUC=275VacΙn=15κΑ (8/20μs)
Ιmax=40κΑ (8/20μs)
Up = <1.9kVTa = <25ns
T2 CE40T2
L
SPDUC=275VacΙn=15κΑ (8/20μs)
Ιmax=40κΑ (8/20μs)
Up = <1.9kVTa = <25ns
T2 CE40T2
N
SPDUC=255VacΙn=20κΑ (8/20μs)
Ιmax=40κΑ (8/20μs)
Up = <1.2kVTa = <100ns
T2 CE40GT2
PE
L
SPDUC=275VacΙn=15κΑ (8/20μs)
Ιmax=40κΑ (8/20μs)
Up = <1.9kVTa = <25ns
T2 CE40T2
L
SPDUC=275VacΙn=15κΑ (8/20μs)
Ιmax=40κΑ (8/20μs)
Up = <1.9kVTa = <25ns
T2 CE40T2
L
SPDUC=275VacΙn=15κΑ (8/20μs)
Ιmax=40κΑ (8/20μs)
Up = <1.9kVTa = <25ns
T2 CE40T2
AC e
lect
ric p
anel
boa
rd String 1
String 2
Common earthing system
≈1m
N
SPDUC=255VacΙn=20κΑ (8/20μs)
Ιmax=40κΑ (8/20μs)
Up = <1.2kVTa = <100ns
T2 CE40GT2
PE
L
SPDUC=275VacΙn=15κΑ (8/20μs)
Ιmax=40κΑ (8/20μs)
Up = <1.9kVTa = <25ns
T2 CE40T2
L
SPDUC=275VacΙn=15κΑ (8/20μs)
Ιmax=40κΑ (8/20μs)
Up = <1.9kVTa = <25ns
T2 CE40T2
L
SPDUC=275VacΙn=15κΑ (8/20μs)
Ιmax=40κΑ (8/20μs)
Up = <1.9kVTa = <25ns
T2 CE40T2
N
SPDUC=255VacΙn=20κΑ (8/20μs)
Ιmax=40κΑ (8/20μs)
Up = <1.2kVTa = <100ns
T2 CE40GT2
PE
L
SPDUC=275VacΙn=15κΑ (8/20μs)
Ιmax=40κΑ (8/20μs)
Up = <1.9kVTa = <25ns
T2 CE40T2
L
SPDUC=275VacΙn=15κΑ (8/20μs)
Ιmax=40κΑ (8/20μs)
Up = <1.9kVTa = <25ns
T2 CE40T2
L
SPDUC=275VacΙn=15κΑ (8/20μs)
Ιmax=40κΑ (8/20μs)
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.