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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. Criteria for selecting PV modules Strings and PV generator Supporting structures Fire prevention

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slide1

TRAINING COURSE

BASIC PRINCIPLES FOR DESIGN ANDCONSTRUCTION OF PHOTOVOLTAIC PLANTS

Ing. Salvatore Castello

ENEA - Renewable Energy Technical Unit - Photovoltaic Lab

summary
Summary
  • 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
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
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
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
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
  • 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
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
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
thermal dissipation in switchboards
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
ELECTRIC CABLES
  • Requirements
    • Resistance to UV rays, weathering and moisture
    • Not propagator of fire
    • Low emission of toxic gases (indoor installation)
  • 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)

coeff . sicurezza

protection devices
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
STRING LAY-OUT

Narrow

box at the bottomsometimes not allowed

Large

inductive coupling facilitated in

large surface of closed-loop

Crossed

opposite overvoltagein the two coils

(balanced output voltage)

supporting structures
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
SIZING OF SUPPORTING STRUCTURES
  • Must be performed according to technical norms for constructions (Eurocode) to withstand the various stresses of loadShould 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 technicianIn 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
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
FREE STANDING STRUCTURE

1.00 m

Back view

Lateral view

0.50 m

PV modules

Junction

box

Galvanized steel elements

Telescopic supports

concrete ballasts

14 m

2 m

Front view

structural verifications free standing structure
STRUCTURAL VERIFICATIONS FREE STANDING STRUCTURE

OVERTURNING VERIFICATION

Overturning wind pressure

overturning moment (wind)

<

resultant of stabilizing moments due to

- structure weight

- ballasts weight (ballast sizing)

Structure Weight

Ballast weight

or

tie rod resistance

structural verifications free standing structure1
STRUCTURAL VERIFICATIONS FREE STANDING STRUCTURE

SUPPORT SURFACE VERIFICATION

stabilizing wind pressure

snow load

  • resistant action of the support surface (backplates sizing)
  • >
  • combined actions of
  • ballast weight
  • structure weight
  • stabilizing wind pressure
  • - snow load

Weight

Ballast weight

structural verifications structures for tracking or a pole
STRUCTURAL VERIFICATIONS structures for tracking or a pole

OVERTURNING VERIFICATION

SUPPORT SURFACE VERIFICATION

stabilizing wind pressure

Overturning wind pressure

Snow load

Weight

Stucture weight

Weight of foundations

Weight of foundations

fire risk
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
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
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

interposing between the PV modules and the support surface, a layer of material of adequate fire resistance and incombustible

  • 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)

PV

Structure incombustible

PV

Covering

incombustible

Layer continuous and incombustibile

technical requirements
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)
technical requirements1

PV

generator

PV

generator

Switching

device

Inverter

grid

Emergency

grid

device

Technical

Emergency

(

Signalled

and

compartment

:

device

accessible

)

Fire

compartment

Fire

compartment

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

two possible modes of implementation of emergency device, with reference to the location of the disconnecting device

technical requirements2
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
SAFETY SIGNS
  • The area, in which is installed the PV plant, must be marked warning signs
  • the sign must report the statement :
  • 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

Attention

photovoltaic plant energized during daylight hours

(…. Volt)

technical requirements3
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 generatorboundedby

fenceswithsigns

PV plant

with signs

Exit with signs

exit

Excape way

accessible area

accessible area

Fire escape ladder

fire compartment

fire compartment

plant inspections
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
    • visualinspection
    • IV curve measuremet
slide34

THANK YOU FOR YOUR KIND ATTENTION

for information:

salvatore.castello@enea.it