1. 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

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
without insulation
with insulation
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 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

7. 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

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 dimensions drawbacks: - 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)
Vpv,max
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

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
% LF transformer
% 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 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

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 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

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 %)
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 could shutdown 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
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

35. THANK YOU FOR YOUR KIND ATTENTION
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