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TRAINING COURSE
BASIC PRINCIPLES FOR DESIGN AND
CONSTRUCTION OF PHOTOVOLTAIC PLANTS
Ing. Salvatore Castello
ENEA - Renewable Energy Technical Unit - Photovoltaic Lab
Summary
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Criteria for selecting PV modules
Strings and PV generator
Supporting structures
Fire prevention
Power conditioning unit
The connection to the grid
Design documentation
STRINGS AND PV GENERATOR
CELLA
MODULE
(Pnom)
STRING
Set of modules series connected
to obtain the same voltage of the
PV generator
PHOTOVOLTAIC GENERATOR
Set of strings parallel connected (in string
switchboards) to obtain the power required
SIZING OF PV GENERATOR
• the PV generator is characterized by 2 main electrical parameters
• Pnom
• Working voltage.
• The choice of the Pnom is made ​on the basis of:
• economic availability
• Small sdize
2 €/W
• large size
1.2 €/W)
• desired reduction in energy expenditure (self producer)
• energy gain estimated (production plants)
• availability of spaces on which to install the PV generator
• single row
7m2 for 1 KW
• multi row
14m2 for 1 KW
• 2axis tracking 28m2 for 1 KW
THE CHOICE OF DC VOLTAGE
It should be made ​on the basis of:
• safety limits: standard fix the limit of the Low Voltage at 1500 Vdc
• current values ​(Vm = Pm / Im) and consequent losses
• Inability to put the system out of tension in the presence of light
• photovoltaic module: characterized by a maximum permissible voltage.
• Switching devices: commercial components often rated up to 600V. Higher
voltages mean more expensive devices
• Inverter: input voltage window, taking into account that the array voltage
depends on Irradiance and Temperature
MINIMUM AND MAXIMUM VOLTAGE
V(Irr, temp)= Vstc [ 1 - 0,0037 (Tm - 25) ]*[ 0,05 ln(Irr) + 0,655]
Vmin_2
Vmin_1
Vmax
inverter start-up
threshold
THE STRINGS
• The strings are constituted by the series of individual photovoltaic modules
• In order to minimize mismatch in current, the string should be formed by
modules
• of the same type and class of current
• with the same exposure
• The electrical characteristics of parallel strings must be as uniform as
possible (mismatch in voltage)
• same type and number of modules in series
• same exposure
• Different strings can be used with different inverter
• In systems with a high number of strings (> 3), each string must be
provided with blocking diode
SECTIONING OF STRINGS
• Each string must be individually disconnectable
• The switching device
• will be able to
• Switck on and off the string in open circuit conditions
• withstand the nominal and maximum current (Isc)
• It must be all-pole
• It can consist of:
• Power switch
• Circuit-breaker (expensive and suject to accidental interventions)
• Replaceable fuses
• Connector for PV modules
SWITCHBOARDS
• In DC (string box and subarray switchboard)
contain:
• the switching devices for strings
• overcurrent protective devices (blocking
diodes / fuses)
• String monitoring devices
• Overvoltage protection devices against
induced surges
• bars for parallel of strings (also in different
levels)
• in AC contain
• Switches for parallel connection of inverter
• Grid interface protection devices
• Inverter
SWITCHBOARD WITH MONITORING CAPABILITIES
Current sensor
THERMAL DISSIPATION IN SWITCHBOARDS
•
Switchboard must be sized in order to maintain the interior temperature
< maximum permissible temperature of the components inside
•
taking into account
•
dissipated power by the components (cables, switches, diodes,
inverter)
•
thermal resistance of the framework (provided by the manufacturer)
inversely proportional to the external surface of the framework
•
in presence of highly dissipative components (inverter in containers) is
necessary to use extractor fan
•
The Fan flow rate Q [m3 / h] is chosen according to the Dissipated power
Pd [W]
ELECTRIC CABLES
• Requirements
• Resistance to UV rays, weathering and moisture
• Not propagator of fire
• Low emission of toxic gases (indoor installation)
coeff . sicurezza
• rated voltage compatible with Vmax of circuit
• section (resistance) sized in order to have
• Max. voltage drop of <2%
• cable capacity (maximum value of current that can flow without cable
insulation damaging) > Imax of circuit
• It is advisable to use
• unipolar cables laid in separate cable ducts (+ / – poles) or cables with
double insulation
• “solar cables” for module connections (more resistant to atmospheric
agents)
PROTECTION DEVICES
• Indirect lightning can generate (inductive coupling) an overvoltage in the
circuits of string that typically have the form of closed-loop
• in order to reduce or eliminate the overvoltage each string should be
equipped with protection devices:
• varistor type
• with visual indication of the state
• with built-in fuse or external
• following the formation of the arc, can persist the current Isc
• series connected with spark gap
• facilitates the extinction of the arc
• Provides high insulation in operating conditions
• It is recommended, when possible, a proper wiring of strings
STRING LAY-OUT
Large
inductive coupling facilitated in
large surface of closed-loop
Narrow
box at the bottom
sometimes not allowed
Crossed
opposite overvoltage
in the two coils
(balanced output voltage)
THE SUPPORTING STRCTURES OF MODULES
SUPPORTING STRUCTURES
• Represent a significant share in plant cost distribution (6÷12%)
• Systems formed by the assembly of elements, typically metallic,
capable of
• support the PV modules
• anchor them to soil or a building structures
• optimize exposure
• Are distinguished in
• Free standing structure (ground, flat roofs)
• Pole structures for tracking or fixed systems (ground)
• structure for the integration or retrofit (on buildings, urban or
rural infrastructures)
SIZING OF SUPPORTING STRUCTURES
• Must be performed according to technical norms for constructions
(Eurocode) to withstand the various stresses of load
Should be verified combining the stress load in the most unfavorable
conditions. Typical verifications:
• structure overturning
• support surface
• Resistance of the individual elements of the structure (if not
certified)
• This evaluation shall be made ​by qualified technician
In the case of installation on existing building it is appropriate have the
approval of:
• the designer of such a facility or
• of a qualified technician
STRESS LOAD
• Permanent loads
• Weight: modules, support structures, ballast
• Wind pressure
• Geographical area (reference wind speed)
• altitude
• height of the structure from the ground
• roughness and topography of the land
• site exposure
• shape and dimensions of the structure
• Snow load
• area (reference snow load on ground)
• altitude
• shape of the structure
• Generally are not taken into acconut:
• seismic actions
• Thermal effects
FREE STANDING STRUCTURE
Back view
1.00 m
Lateral view
0.50 m
PV modules
Junction
box
Galvanized steel
elements
Telescopic supports
concrete ballasts
14 m
2m
Front view
STRUCTURAL VERIFICATIONS
FREE STANDING STRUCTURE
OVERTURNING VERIFICATION
Overturning wind pressure
overturning moment (wind)
<
Structure
Weight
Ballast weight
or
tie rod resistance
resultant of stabilizing moments due to
- structure weight
- ballasts weight (ballast sizing)
STRUCTURAL VERIFICATIONS
FREE STANDING STRUCTURE
SUPPORT SURFACE VERIFICATION
stabilizing wind
pressure
resistant action of the support surface
(backplates sizing)
>
combined actions of
- ballast weight
- structure weight
- stabilizing wind pressure
- snow load
Weight
Ballast weight
POLE STRUCTURES
two axis tracking
STRUCTURAL VERIFICATIONS
structures for tracking or a pole
OVERTURNING VERIFICATION
SUPPORT SURFACE VERIFICATION
stabilizing
wind
pressure
Overturning wind pressure
Weight
Stucture weight
Weight of foundations
Weight of foundations
FIRE PREVENTION IN PV PLANTS
FIRE RISK
• photovoltaic plants are not, in themselves, among the activities
subject to fire prevention inspections
• However, if the PV plant is installed on a building, could result
(depending on the electrical and construction characteristics and /
or its mode of installation) in an increase of the pre-existing level
of safety in case of fire
• The PV plants could in fact:
• interfere with the ventilation system
• obstacle fighting operations in the event of a fire
• constitute an electrocution risk during the day
• facilitate (through their components) the propagation of flames
among fire compartments (part of building bounded by
constructive elements of adequate resistence to fire)
FIRE EVENTS
Events recorded by the Fire
Brigade: 300/400.000 (Italy)
• Causes
– Poor design
– Bad installation
– Hotspot
– Defects modules:
connections strips or
terminal box
• Consequences
– Damage to glass / Tedlar
– Faults in the junction box
– Loss of insulation
– Arcing
– Probable local fire /
extended (materials close
to modules)
RISK OF FIRE SPREAD
the design and the installation of PV plant must be
carried out in order to avoid the spread of a fire from
the PV generator to the building and / or between its
compartments. This condition can be fulfilled:
Installing PV modules
on roofing elements
and / or façade
incombustible
PV
Structure
incombustible
Covering
incombustible
interposing
between the PV
modules and the
support surface, a
layer of material of
adequate fire
resistance and
incombustible
PV
Layer continuous
and incombustibile
specific assessment
of the risk of spread
of fire taking into
account:
- reaction class of
roofs to external fire
and
- behavior to fire of
PV modules (certified
in accordance to
specific norms)
TECHNICAL REQUIREMENTS
• PV plant should be installed at adequate distances from:
• possible ways of fire vehiculation (skylights, chimneys, etc. )
• smoke and heat evacuator systems (in order not to interfere with their
operation)
• projection of any vertical elements of fire partitioning (avoiding the pread of
fire among compartments)
skylights
d>1m
or
risk assesment
PV strings
d >1m
d >1m
(see note 10)
Projection of fire partition
element
conduits
EFC
TECHNICAL REQUIREMENTS
The emergency button
PV system must be equipped with a device, installed in an easy accessible position, that
disconnect the User grid from the PV plant and the Utility grid
PV generator
grid
Emergency
device
(Signalled and
accessible)
Fire compartment
Switching device
PV generator
Inverter
grid
Emergency
device
Technical
compartment:
Fire compartment
two possible modes of implementation of emergency device, with reference to the
location of the disconnecting device
TECHNICAL REQUIREMENTS
• PV systems components should not be installed
• in places defined as "safe" (where people can be considered safe from
the effects of fire)
• in area with the presence of flammable gases, vapors, mists or
combustible dusts, in order to avoid the hazards originating from
electric ignition
• in areas with the presence of explosive materials shall be installed at
safe distances established by the norms
The PV photovoltaic generator constitute a potential sources of
ignition,
SAFETY SIGNS
• The area, in which is installed the PV plant, must be marked warning signs
• the sign must report the statement :
Attention
photovoltaic plant
energized during daylight
hours
(…. Volt)
• The safety signs, resistant to ultraviolet rays, must be installed along the
pipeline
• In case of PV plants installed on buildings, the sign shall be installed at the
front door
• Also the emergency button must be indicated by proper safety signs
TECHNICAL REQUIREMENTS
system components must not be a hindrance to the escape ways
(Path without obstacles, that allows people who occupy a building to reach a
safe place)
If PV installed along escape ways:
PV generator bounded by
fences with signs
Exit with
signs
exit
Excape way
accessible area
accessible area
Fire escape ladder
fire compartment
PV plant
with signs
fire compartment
PLANT INSPECTIONS
• Periodically and at any
extension or modification of
the installation, the PV plant
must be test for the
purposes of fire risk
• Tests and inspections
– on joint and torque
– IR test
– visual inspection
– IV curve measuremet
THANK YOU FOR YOUR
KIND ATTENTION
for information:
[email protected]