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. 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

4. 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 withoutinsulation withinsulation without insulation with LF insulation with HF isolation

5. 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

6. MAXIMUM POWER POINT TRACKING Indirect method: try and test if DP >0 then V’=V+DV else V’=V-DV P Pm • Multi Operating modes • Indirect (try and test) • Direct • measurement of Irr and T • V scannering DP DV MPPT point Starting point V Relative maximum

7. DC/AC BRIDGE PUSH-PULL BRIDGE Configurations 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

8. 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

9. 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 dimensionsdrawbacks:- photovoltaic generator necessarily "floating" (NO 1-pole to ground capability);- limited range of input voltage

10. 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);

11. 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

12. INVERTER – ARRAY COUPLING • The voltage of the PV array must be compatible with the input voltage range of the inverter (Vmi, Vmax) Voc @Tmin Vpv,max Vpv,min PV Generator V 0 Vmin Vmax Inverter Input voltage Low voltagefield (notsufficient tostartup) Safety operation field possibledamage field startup threshold (depending on grid voltage) overvoltage protection mode

13. 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)

14. 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

15. 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%

16. 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

17. 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)

18. INVERTER TIPOLOGIES CENTRALIZED INV. STRING INVERTER MULTISTRING INV. MODULE INV.

19. CENTRALIZED LAYOUT 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

20. CENTRALIZED SYSTEM ARCHITECTURE Source: Elettronica Santerno

21. CENTRALIZED PLANT LAYOUT

22. 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

23. 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

24. DISTRIBUTED LAYOUT 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

25. 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

26. LAYOUT OF DISTRIBUTED SYSTEM

27. 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

28. 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)

29. 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

30. 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

31. 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)

32. 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

33. 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 couldshutdown inverter • grounding a pole of the array has a stabilizing action of the fluctuations • . Inverter

34. INVERTER SELECTION AND MODULE TECHNOLOGY Some manufacturers provide tables in order to facilitate the selection of the inverter suitable for the various module technologies

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