2012 International Conference on Lightning Protection (ICLP), Vienna, Austria
Lightning Protection Practice for Large-Extended
Photovoltaic Installations
Dr. Nikolaos Kokkinos
Dr. Nicholas Christofides
Research and Development Department
Elemko SA
Athens, Greece
nkokkinos@elemko.gr
Department of Electrical Engineering.
Frederick University
Nicosia, Cyprus
n.christofides@frederick.ac.cy
Dr. Charalambos Charalambous
Department of Electrical and Computer Engineering.
University of Cyprus
Nicosia, Cyprus
cchara@ucy.ac.cy
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
2000m). 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.
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.
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.
SITE SURVEY
The particular PV park under study is installed on a
mountain peak, flat area, occupying a total surface of around
115,000m2. 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].
Fig. 1 and Fig. 2 illustrate the 2 cases considered when
designing the LPS. In Fig. 1, for the non-isolated case, the air
terminals for every 2 consecutive rows are connected to one to
one electrode installed between the 2 rows. In this scenario, it
is advised that the distance between the PV frame of each row
and the earth electrode does not exceed 3m. Fig. 2, illustrates
the isolated case, where the air-terminal of each PV row is
connected to a separate electrode directly and not through the
PV framework. Each PV frame is also bonded on the same
earth electrode but with independent bonding conductors.
In addition, the PV park under study was designed utilizing
small 11kW inverters and not central inverters. Therefore, the
DC cable loop was small and there was no need to use T1+T2
SPDs on the DC side of the inverter even for the non-isolated
LPS case. Fig. 4, illustrates the installation of T2 only type
SPDs on the DC side resulting from the fact that the lightning
loop formation is limited or non-existent.
1
3
2
4
Figure 3. Acceptable materials for earthing system depending on the type of
foundation used for the PV framework
≤ 3m
≤ 3m
Figure 1. Design of Type B earth electrode using non-isolated LPS. The airterminals of 2 consecutive PV rows are connected to 1 electrode which is
installed between them
TABLE 1. DETAILED DESCRIPTION OF MATERIALS DEPICTED IN
FIG. 1
Type of Foundation of the PV
Allowed material for earthing
framework
system driven into the soil
Galvanized steel profile directly
1
Galvanized steel, Stainless steel
buried into the soil
Steel profile embedded in
Copper coated steel, Copper,
2
concrete
Stainless steel
Reinforced concrete block placed Galvanized steel, Copper coated
3
at ground level not into the soil
steel, Copper, Stainless steel
Reinforced concrete foundation
Copper coated steel, Copper,
4
into the soil
Stainless steel
Note 1: Copper conductor may also be tinned
Note 2: Aluminum is not allowed to be used into the soil
≤ 3m
L
L
L
N
L
L
L
N
SPD
SPD
Uoc=1000Vdc
SPD
UC=275Vac
SPD
UC=275Vac
Ιn=15κΑ (8/20μs)
SPD
UC=255Vac
Ιn=15κΑ (8/20μs)
Ιn=20κΑ (8/20μs)
Ιmax=40κΑ (8/20μs) Ιmax=40κΑ (8/20μs) Ιmax=40κΑ (8/20μs) Ιmax=40κΑ (8/20μs)
Up = <1.9kV
Ta = <25ns
T2
For PV system installations in open fields, in order to
minimize the cost and at the same time increase the efficiency
and life time of the earthing system (by avoiding
electrochemical corrosion) it is very important that during the
design the type of foundation used for the PV façade –
framework is taken into consideration.
In this particular project, the foundation material was
reinforced concrete buried into the soil, therefore copper coated
and solid copper earth electrodes were used. Fig. 3 and Table 1,
depict information about the selection criteria of the materials
used for the earthing system, depending as mentioned above,
on the type of the foundation used for the PV framework or
structures.
SPD
UC=275Vac
Up = <1.9kV
Ta = <25ns
CE
40GT2
SPD
UC=275Vac
Ιn=15κΑ (8/20μs)
T2
Up = <1.9kV
Ta = <25ns
CE T2
40T2
Ιn=15κΑ (8/20μs)
Up = <1.9kV
Ta = <25ns
CE T2
40T2
Ιn=20kΑ (8/20μs)
Uoc=1000Vdc
Uc=500Vdc
Ιn=20kΑ (8/20μs)
UC=255Vac
Ιmax=40kΑ (8/20μs) Ιmax=40kΑ (8/20μs) Ιmax=40kΑ (8/20μs)
Ιn=20κΑ (8/20μs)
Up = <4kV
Up = <4kV
Up = <4kV
T2
CE T2
CE T2
CE
PV - 1000T2
PV - 1000T2
PV - 1000T2
Up = <1.2kV
Ta = <100ns
CE T2
40T2
PE
Uc=500Vdc
Ιn=20kΑ (8/20μs)
SPD
UC=275Vac
Ιn=15κΑ (8/20μs)
Ιmax=40κΑ (8/20μs) Ιmax=40κΑ (8/20μs) Ιmax=40κΑ (8/20μs) Ιmax=40κΑ (8/20μs)
Up = <1.2kV
Ta = <100ns
CE T2
40T2
CE
40GT2
+
PE
AC electric panel board
For the isolated LPS it was necessary to provide an earth
electrode behind each PV row so as to earth the isolated air
terminals directly. For the non-isolated case however, an earth
electrode for every 2 consecutive PV rows was necessary,
combining the earthing of the air-terminals.
Up = <1.9kV
Ta = <25ns
CE T2
40T2
AC electric panel board
Figure 2. Design of Type B earth electrode using isolated LPS. Each isolated
air terminal is connected to a separate electrode directly and not through the
metal PV framework
Up = <1.9kV
Ta = <25ns
CE T2
40T2
≈1m
PE
DC Junction Box
SPD
UC=275Vac
SPD
Uoc=1000Vdc
Uc=500Vdc
SPD
Ιn=15κΑ (8/20μs)
_
String 1
String 2
>10m
Common earthing system
Figure 4. Installation of SPDs for PV application inside a junction box
situated at a distance >10m from the inverter at the DC & AC side. Only T2
SPDs 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
AC Input
DC & AC Termination
Set
String 1
SURGE PROTECTION
8899kWhr
DC Input
PV Generators
Inverter
kW Meter
String 2
DC Output
AC Output
Figure 5. DC & AC termination set for optimum protection of the inverter
against surges arriving either from the AC or DC cabling
Figure 6. Effect on the extended wiring of an open field large PV system
installation due to a direct or indirect lightning strike
For the effective protection of the inverters, a combined
protection was developed allowing a combined termination of
all DC & AC cables and at the same time providing protection
from coupled surges. By using such a combined termination
set, 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 voltages
requires specific isolation distances. Fig. 5, illustrates this
arrangement, where 1 termination box is used for both DC and
AC cable terminations
IV.
EXAMINED CASE STUDIES
Due to the extensive and long cable loops which are formed
by the DC cables, any direct or nearby lightning will cause
high induced surges, Fig. 6. Since screening of the DC cables is
difficult to provide for in large PV parks, the question that
arises is which external LPS provides a more suitable and more
effective protection against over-voltages induced on the
cables. A scaled down experiment was performed in the
laboratory in order to obtain measurements and information
that would assist in the external LPS design of the 2MWp PV
park under study. The purpose of the laboratory tests was to
evaluate the performance and apply it later on.
The scaled down version of the experimental setup in the
laboratory was a 2kWp PV system depicted in Fig. 7. The
system consists of 9 PV modules connected in series giving a
total of 200V output voltage and 10A current. During the
impulse experiments the laboratory lights were switched off in
order to have zero volts at the DC cable loop, which was
approximately 18m long.
Figure 7. PV panels tested in the laboratory and the formation of the DC cable
loop (red +ve and black –ve)
The initial purpose of the test was to investigate which of
the following two possible cases would give a lower induced
voltage across the DC cable loop of the string. In case A the
impulse current was injected directly on the framework of the
PV panels through an air termination rod, which was supported
on the edge of the PV support framework. In case B the
impulse current was injected directly on the on an isolated air
termination rod, which was supported on the laboratory floor at
a distance of approximately 700mm from the PV support
framework. The results are summarized in Fig. 8, Fig. 9 and
Fig. 10.
α
Case Α
≈ 25%
Isolated LPS at a distance of 0,7m behind
the steel frame of the PV structure. The
lightning current is driven to earth via a
dedicated rod and down conductor. The
metallic structure of the PV is only
connected to the LPS via the earth.
Case Β
α
Non isolated LPS, the lightning current is
distributed along the metallic frame of the
PV structure. The metallic structure is
used a natural down conductor.
T1+T2 SPDs for the DC was not mandatory. Furthermore, due
to the fact that the earthing system arrangement was a cost
effective solution for the non-isolated case compared with the
isolated LPS case, the investor decided to use a non-isolated
LPS for the particular Photovoltaic park.
V.
CONCLUSIONS
100%
Figure 8. Examining the behavior of isolated and non isolated LPS in case of a
direct lightning strike and the effect of inducted over-voltages in the DC
cabling of a PV installation
The design of a lightning protection system for large scale PV
systems may depend on various factors and parameters. It is
very important to take into consideration the installation
arrangement and design adopted for large scale PV parks as it
involves crucial parameters that must be taken into
consideration for the effective external and internal LPS
design. There are numerous large scale PV system designs all
of which require a LPS that satisfies the unique, in some
times, conditions present.
α
Case Α
≈ 25%
1
1
The open circuit voltage across the (+)
and (-) negative pole of the string was
82V and the injected current was 100kA
(10/350μs).
Figure 9. Experimental results of scaled down experiment examining the
behavior of a non-isolated LPS with respect to the induced over-voltages on
the DC cabling of a PV string
α
Case Β
100%
1
1
The open circuit voltage across the (+)
and (-) pole of the string was 390V and
the injected lightning current 100kA
(10/350μs). It was almost 480% than the
non isolated case. This is due to the
reason that the current was not
distributed but driven 100% through one
path creating a high magnetic field.
Figure 10. Experimental results of scaled down experiment examining the
behavior 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 provide
lower induced over-voltages on the cabling in case of a direct
lightning strike. Additionally, since small inverters were used
(therefore no parallel path for the DC to earth) the need of
The need of an efficient lightning protection system is
mandatory for photovoltaic installations by virtue of its
preventing nature. Primarily, the need is imperative to prevent
any physical damage to structures and life hazards. It is worth
noting that the damage of the electrical and/or electronic
equipment of a PV installation, due to surges originates from
Lightning Electromagnetic Impulse (LEMP) as well as from
Switching Electromagnetic Impulse (SEMP) [8].
However, literature survey reveals that there is still very
little information published regarding the design of lightning
and surge protection for large and extended PV installations.
In particular, reference [9] comprehensively covers the related
scientific background by emphasizing on the aspects of
standardisation that should be addressed in the near future. As
quoted, the current practice, for protecting PVIs from
lightning surges, rests with adopting (partly) protective
measures described in standards for conventional low-voltage
power distribution systems.
It is vital that the LPS provide an effective protection
against direct or indirect lightning strikes in order avoid the
destructive effects of lightning. As previously mentioned, the
investment cost is very considerable and 1 thunderstorm can
be catastrophic with inestimable financial consequences.
The results presented in this paper propose the most cost
effective and technically correct solution for the LPS design of
the large scale photovoltaic system under study. The need
however, for deeper and more detailed analysis is required so
that amendments are made to the current standard in order to
include guidance and regulations for common large scale PV
installation practices and examples.
ACKNOWLEDGMENT
The authors wish to thank the engineering team of BIOSAR
SA (GREECE) for their contribution to the photovoltaic park
project.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
TS 50539-12:2009, Protection of PV installation against overvoltages
IEC 62305 – 3:2010, Protection against lightning Part 3: Physical
damage to structures and life hazard
EN 60439-1: Low voltage switchgear and controlgear assemblies – Part
1: Type tested and partially type tested assemblies.
EN 60439-3: Low voltage switchgear and controlgear assemblies – Part
3: Particular requirements for low voltage switchgear and controlgear
assemblies intended to be installed in places where unskilled persons
have access for their use – Distribution boards
HD 60304-7-712: Electrical installations of buildings – Part 7-712:
Requirements for special installation locations – Solar photovoltaic (PV)
power supply
EN 60664-1: Insulation co-ordination for equipment within low voltage
systems – Part 1: Principles, requirements and tests
EN 62446:2009: Grid connected photovoltaic systems – Minimum
requirements for system documentation, commissioning tests and
inspection.
IEC – 62305 “Protection against Lightning”
Jesus C. Hernandez, Pedro G. Vidal, Francisco Jurado, “Lightning and
Surge Protection in Photovoltaic Installations’’, IEEE Transcactions on
Power Delivery, Vol. 23, No 4. Oct. 2008 pp. 1961-1971.