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

THE INVERTER

• Converts the DC to AC

• It must be fit to transfer the power from the PV array to the distributor grid, in compliance with regulatory requirements, technical and safety standards.

• Features:• PWM technique• able to operate in automatic mode (on, off)• able of tracking the maximum power point (MPPT) of the PV generator

• typologies:• LINE-Commutated: for grid-connected systems• SELF-Commutaded: for stand alone or grid-connected systems

CLASSIFICATION

single-stage Inverter : integrate into one section the DC / AC converter and MPPT algorithm

dual-stage Inverter : the DC / AC conversion and the MPPT are made by two distinct stages. The PV voltage can also be raised

without insulation

with insulation

without insulation

with LF insulation

with HF isolation

DC/DC CONVERTER

• Converts the DC voltage at its input according to a variable conversion ratio : Vo = k Vi

• Formed by: chopper / HF trafo (optional) / rectifier

• Typical functions• Voltage regulator in systems with storage• Control and voltage regulation (grid connected)• Maximize the energy produced by the photovoltaic generator

(MPPT)

• Efficiency: 98-99% in a broad range of input power

• Basic circuits:• BUCK Vo < Vi• BOOST Vo > Vi• FLAYBACK Vo < > Vi

MAXIMUM POWER POINT TRACKING

Indirect method: try and testif P >0 then V’=V+V else V’=V-V

Multi Operating modes • Indirect (try and test)• Direct

• measurement of Irr and T• V scannering

P

V

Pm

VP

Starting point

MPPT point

Relative maximum

DC/AC INVERTER BASIC CIRCUITS

LOAD

LOAD

PUSH - PULL

BRIDGE

DC/AC BRIDGE

ConfigurationsPUSH-PULL BRIDGE

PWM MODULATION

The Output frequency and phase are generated electronically by controlling the width of voltage pulses

These techniques require switching power devices (transistors or IGBTs) in order to generate a proper voltage level

INVERTER SINGLE-STAGE WITH LF TRANSFORMER

advantages:- ability to manage the photovoltaic generator with one pole-to-ground limitations:- lower efficiency than systems transformerless;- high weight, dimensions and noise

Varistors DC filter DC/AC Bridge AC filter Trafo interface device EMC filter Var

Controller

INVERTER SINGLE-STAGE WITHOUT TRANSFORMER

advantages:- high efficiency - circuit simplicity- low weight and dimensionsdrawbacks:- photovoltaic generator necessarily "floating" (NO 1-pole to ground capability);- limited range of input voltage

Varistors DC filter DC/AC Bridge AC filter interface device dc protect. EMC filter

INVERTER DUAL-STAGE WITH HF TRAFO

advantages:- galvanic insulation- 1-pole to ground possibility- weight and overall dimensions smaller than with LF transformer- extended imput voltage range (dc / dc converter)limitations:- lower efficiency of transformerless (2 stages);

Varistors DC/DC isolated dc filter DC/AC Bridge AC filter interface device EMC filter

HF trafo

controller

THE INVERTER

• three-phase connection can be obtained using• three-phase inverter

• 3 single-phase inverters connected between one phase and neutral (maximum unbalance allowed is fixed by the Utility)

• The values of the output voltage and frequency must be consistent with those of the grid at which it is connected

INVERTER – ARRAY COUPLING

Vpv,min Voc @Tmin

Low voltage field (not sufficient

to startup)

Vpv,max

Vmin

Safety operation fieldpossibledamage

field

overvoltage protection

mode

Vmax

PV Generator

InverterInput

voltage

V0

startup threshold

(depending on grid voltage)

The voltage of the PV array must be compatible with the input voltage range of the inverter (Vmi, Vmax)

INVERTER

• The inverter is sized taking into account the rated power of the PV field (typically Pnom_inv = 0.85 * Pnom_pv)

• Typically equipped with • Grid interface protection device• device to check the insulation of the PV field• transformer to ensure the metal separation (LF or HF)

• In case of absence of the transformer (TL), the metal separation can be replaced by a DC overcurrent protection device

(which acts when the level of DC componed fed into the grid > allowed threshold)

INVERTER

• may be suited for indoor or outdoor installations, depending on the degree of protection

• is characterized by a range of ambient temperatures. Beyond which the inverter can limit the power output or shutdown

• electromagnetic interferences should remain within prescribed values. Are generated by switching devices and are• induced in the cables• air radiated

• To minimize the interferences is appropriate to comply manufacturer instructions• Grounding• Do not installed in proximity to sensitive equipment

INVERTER EFFICIENCY

Losses:• constant = power absorbed by the control circuitry + magnetic losses• proportional to Pi = switching losses• proportional to Pi2 = losses due to joule effect (inductors and transformer)

EU efficiency (weighted average over operation time at specific levels of Pi)

eur = 0,035% + 0,06 10% + 0,1320% + 0,130% + 0,4850% + 0,2100%

INVERTER EFFICIENCY

Typical average values of effiency: - 94-97% TL - 92-96% LF transformer - 92-94% HF transformer- 98,5% devices based on silicon carbide

INVERTER FEATURES• General

• Efficiency• ambient temperature range• Insulation level between the DC and AC• Protections for internal faults• Noise Level• EM emissions• Compliance standards for grid connection• Monitoring capability

• Input• Voltage range• Pnon, Pmax, Imax, Vmax, Pmin• # MPPT• Protections (over-voltage, inversion, array insulation)

• Output• Voltage, frequency and number of phases• Voltage and frequency ranges• Pnon and Pmax, Imax• Harmonic distortion total and single, power factor• Protections (islanding, over voltage and over-current)

INVERTER TIPOLOGIES

CENTRALIZED INV. STRING INVERTER MULTISTRING INV. MODULE INV.

CENTRALIZED LAYOUT

AuroraAuroraPVI-CENTRALPVI-CENTRAL

Example: Fonte Power One

PROS• Conten cost per unit • Speed and ease of installation (cabins

provided in turnkey solution)• connection directly in MV grid• Highest levels of efficiency - up to

98.6% (inverter)

DRAWBACKS• Low continuity of exercise (for inverter

fault);• Complex wiring of DC side

(switchboard and protections required)

• Reduced efficiency of the PV generator due to mismatch

• Constrained exposure and string configuration

CENTRALIZED SYSTEM ARCHITECTURE

Source: Elettronica Santerno

CENTRALIZED PLANT LAYOUT

Realized within a wide range of Pnom (50 ÷300 kW. Large size plant assembly several banks. For LV connection, have integrated transformer (eff. 95.7%) For MV connection, are TL configuration (dedicated external) allowing to

achieve efficiencies up to 97.5%.

COMMERCIAL CENTRALIZED INVERTER

Elettronica Santerno Power One SMA

MODULAR CENTRALIZED INVERTER

The inverter consists of several modules (ranging from 30 to 300 kW) the number of modules in operation depends on array output power

(irradiation) In the event of a module failure the remaining modules configurate

their contribution performing also the function of the fault module

commercial solutions:

FRONIUS MIX™Power One

DISTRIBUTED LAYOUT

Source: Power OneADVANTAGES• Good flexibility

• Shadowing management• Strings differently oriented• “Mixed“ module technologies

• Simplified installation • Standard design

• High plant availability• In case of failure quick

replacement executable by unqualified personne

DRAWBACKS• Higher unit cost • AC side wiring more complex

• String Inverters: Each string has its own dedicated inverter• Multi-input inverter: DC side act like a string inverters, while the AC side

works as a central inverter.• Module inverter (microinverter): devices of small power (a few hundred

W), suitable for direct AC connection of modules• High unit cost• Technical rules still lacking for safety aspects

INVERTER FOR DISTRIBUTED LAYOUT

LAYOUT OF DISTRIBUTED SYSTEM

In economic terms, there are not definite advantages in favor of one or the other solution

In real operating conditions should be taken into account inverter faults and the consequent reduction of energy production This factor lean toward distributed solutions

However, with the modular technology applied to large size inverter is possible to balance pro and cons of centralized and distributed generation, ensuring an effective reduction of energy losses associated with the single

failure in centralized configurations

CENTRALIZED VS. DISTRIBUTED

Module DC/DC Buck converter:Raises the current of the shaded module to align it to that of the string (reducing the voltage, compatible with the power that can deliver the module)

THE OPTIMIZER

TL INVERTERS WITH ONE POLE TO GROUND

Typologies born to exploit the benefits of -higher efficiency of TL inverters and-allow the management of a PV array with one pole to ground

It is said that the inverter can operate in dual ground configuration (both in AC and DC side), even in absence of transformer

The grounding of one pole of the array is necessary in some technologies-thin-film modules; to prevent premature degradation-back contact modules: reduce efficiency- metal substrate thin-film: limit leakage currents

TL INVERTERS WITH ONE POLE TO GROUND

Are based on the principle of disconnecting the DC from the AC side of the inverter during the period of freeweeling, (when the grid could be put in short-circuit through the array pole to ground)

high efficiencies (up to 97 - 98%) ground connection is made using special kits For safety reasons, is continuously monitored the level of insulation of the PV

array

POLARIZZATION OF C-SI BACK CONTACT MODULES

If the cell contact are located on the same side, the electric field is not uniform

a static charge occurs on the surface of cells that cause a reduction of the efficiency

The effect is reversible, as soon as the charges are removed The removal of the charges is performed grounding the positive pole of the

PV array (during freeweeling period)

TCO CORROSION IN THIN FILM MODULES

Regards a-Si and CdTe modules in superstrate configuration (deposition on the cover glass).

The TCO corrosion is caused by the reaction between moisture (from the edges) and the sodium, present in the glass

The corrosion is proportional to the potential of PV generator poles to ground

with the grounding of the positive pole of the PV generator is generated an electric field that reject the positive ions of sodium from the TCO layer.

In this way it is possible to prevent corrosion

HIGH LEAKAGE CURRENTS

In TL inverter the grid alternate voltage reflect a fluctuation on the DC side The problem arise in metal substrate thin-film modules that have high

parasitic capacitance (large surfaces and small distance between electrodes)

The undulations generate a high leakage current that could shutdown inverter

grounding a pole of the array has a stabilizing action of the fluctuations

.

Inverter

INVERTER SELECTION AND MODULE TECHNOLOGY

Some manufacturers provide tables in order to facilitate the selection of the inverter suitable for the various module technologies

Module technology Compatible inverter

Cristalline silicon (c-Si) All (TL; HF; LF)

All back-contact With transformer (HF; LF)+ positive pole to ground

Thin Film (TF) - superstrate With transformer (HF; LF)+ pnegative pole to ground

CdTe (First Solar) All (subject to verification of FS)

Thin Film (TF) –substrate (Unisolar o affini, senza parti metalliche limitrofe)

All (TL; HF; LF)

Unisolar o similar on metal substrate

With transformer (HF; LF)+ TL in “quiet rail” technology

THANK YOU FOR YOUR KIND ATTENTION

for information:

salvatore.castello@enea.it