BASIC PRINCIPLES FOR DESIGN ANDCONSTRUCTION OF PHOTOVOLTAIC PLANTS

Ing. Salvatore CastelloENEA - Renewable Energy Technical Unit - Photovoltaic Lab

TRAINING COURSE TRAINING COURSE

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

CELLAMODULE(Pnom)

STRING

PHOTOVOLTAIC GENERATOR

Set of modules series connected to obtain the same voltage of the PV 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

inverter start-up threshold

Vmax

Vmin_2

V(Irr, temp)= Vstc [ 1 - 0,0037 (Tm - 25) ]*[ 0,05 ln(Irr) + 0,655]

Vmin_1

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)

• 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

• 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

Largeinductive coupling facilitated in

large surface of closed-loop

Crossedopposite overvoltage

in the two coils(balanced output voltage)

Narrowbox at the bottom

sometimes not allowed

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

Junction box

PV modules

Lateral view

Back view

Front view

0.50 m

1.00 m

concrete ballasts

14 m

2 m

Galvanized steel elements

Telescopic supports

STRUCTURAL VERIFICATIONS FREE STANDING STRUCTURE

Overturning wind pressure

Ballast weightor

tie rod resistance

Structure Weight

OVERTURNING VERIFICATION

overturning moment (wind)

<resultant of stabilizing moments due to- structure weight- ballasts weight (ballast sizing)

STRUCTURAL VERIFICATIONS FREE STANDING STRUCTURE

stabilizing wind pressure

snow load

SUPPORT SURFACE VERIFICATION

Weight

Ballast weight

resistant action of the support surface (backplates sizing)

>combined actions of - ballast weight - structure weight - stabilizing wind pressure- snow load

POLE STRUCTUREStwo axis tracking

STRUCTURAL VERIFICATIONS structures for tracking or a pole

Weight of foundations

Overturning wind pressure

WeightStucture weight

stabilizing wind pressure

Snow load

OVERTURNING VERIFICATION SUPPORT SURFACE VERIFICATION

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:

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

Installing PV modules on roofing elements and / or façade 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)

Structure incombustible

Coveringincombustible

PV

PV

Layer continuous and incombustibile

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)

PV strings

cond

uits

skylights

EFC

d >1m (see note 10)

d >1m

Proj

ectio

nof

fire

parti

tion

elem

ent

d>1m or

risk assesment

TECHNICAL REQUIREMENTS

The emergency buttonPV 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

Fire compartment

PV generator Switching device

Emergencydevice

(Signalled and accessible)

Inverter

Technicalcompartment :

Emergencydevice

Fire compartment

PV generator

gridgrid

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 :

• 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 REQUIREMENTS

fire compartment

If PV installed along escape ways:PV generator bounded byfences with signs

Excape wayaccessible area

exit

fire compartment

PV plantwith signs

accessible area

Exit with signs

Fire

esc

ape

ladd

er

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)

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:

salvatore.castello@enea.it