BASIC PRINCIPLES FOR DESIGN AND CONSTRUCTION OF PHOTOVOLTAIC PLANTS 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 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 test if DP >0 then V’=V+DV else V’=V-DV P V Pm DV DP Starting point MPPT point Multi Operating modes Indirect (try and test) Direct measurement of Irr and T V scannering Relative maximum

DC/AC BRIDGE Configurations PUSH-PULL BRIDGE 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 PWM MODULATION

INVERTER SINGLE-STAGE WITH LF TRANSFORMER Varistors DC filter DC/AC Bridge AC filter Trafo interface device EMC filter Var Controller advantages: - ability to manage the photovoltaic generator with one pole-to-ground limitations: - lower efficiency than systems transformerless; - high weight, dimensions and noise

INVERTER SINGLE-STAGE WITHOUT TRANSFORMER Varistors DC filter DC/AC Bridge AC filter interface device dc protect. EMC filter advantages: - high efficiency - circuit simplicity - low weight and dimensions drawbacks: - photovoltaic generator necessarily "floating" (NO 1-pole to ground capability); - limited range of input voltage

INVERTER DUAL-STAGE WITH HF TRAFO Varistors DC/DC isolated dc filter DC/AC Bridge AC filter interface device EMC filter HF trafo controller 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);

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 The voltage of the PV array must be compatible with the input voltage range of the inverter (Vmi, Vmax) Vpv,max Voc @Tmin PV Generator Vpv,min V Vmin Vmax Inverter Input voltage Low voltage field (not sufficient to startup) Safety operation field possible damage field startup threshold (depending on grid voltage) overvoltage protection mode

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) h eur = 0,03h5% + 0,06 h10% + 0,13h20% + 0,1h30% + 0,48h 50% + 0,2h100%

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 PROS Conten cost per unit 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 Aurora PVI-CENTRAL

CENTRALIZED SYSTEM ARCHITECTURE Source: Elettronica Santerno

CENTRALIZED PLANT LAYOUT

COMMERCIAL CENTRALIZED INVERTER 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%. SMA Elettronica Santerno Power One

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 ADVANTAGES Good flexibility Shadowing management Source: Power One ADVANTAGES 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

INVERTER FOR DISTRIBUTED LAYOUT 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

LAYOUT OF DISTRIBUTED SYSTEM

CENTRALIZED VS. DISTRIBUTED 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

THE OPTIMIZER 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)

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 + pnegative pole to ground CdTe (First Solar) All (subject to verification of FS) Thin Film (TF) –substrate (Unisolar o affini, senza parti metalliche limitrofe) Unisolar o similar on metal substrate + TL in “quiet rail” technology

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