BASIC PRINCIPLES FOR DESIGN AND CONSTRUCTION OF PHOTOVOLTAIC PLANTS

1. TRAINING COURSE BASIC PRINCIPLES FOR DESIGN ANDCONSTRUCTION OF PHOTOVOLTAIC PLANTS Ing. Salvatore Castello ENEA - Renewable Energy Technical Unit - Photovoltaic Lab

2. Summary • Criteria for selecting PV modules • Strings and PV generator • Supporting structures • Fire prevention • Power conditioning unit • The connection to the grid • Design documentation

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. SWITCHBOARD WITH MONITORING CAPABILITIES Current sensor

11. 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]

12. 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

13. 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

14. 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)

15. THE SUPPORTING STRCTURES OF MODULES

16. 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)

17. 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

18. 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

19. 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

20. 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

21. 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

22. POLE STRUCTUREStwo axis tracking

23. 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

24. FIRE PREVENTION IN PV PLANTS

25. 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)

26. 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)

27. 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

28. 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)

29. 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

30. 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,

31. 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)

32. 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

33. 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

34. THANK YOU FOR YOUR KIND ATTENTION for information: salvatore.castello@enea.it