Transcript PRESENTATION NAME

TRAINING COURSE
BASIC PRINCIPLES FOR DESIGN AND
CONSTRUCTION OF PHOTOVOLTAIC PLANTS
Ing. Salvatore Castello
ENEA - Renewable Energy Technical Unit - Photovoltaic Lab
Summary
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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
without insulation
with insulation
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 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
• BOOST
• FLAYBACK
Vo < Vi
Vo > Vi
Vo < > Vi
MAXIMUM POWER POINT TRACKING
Indirect method: try and test
if DP >0 then V’=V+DV
else V’=V-DV
P
Pm
DV
Starting
point
DP
MPPT
point
V
Relative
maximum
Multi Operating modes
• Indirect (try and test)
• Direct
• measurement of Irr and T
• V scannering
DC/AC BRIDGE
DC/AC INVERTER BASIC CIRCUITS
Configurations
PUSH-PULL
BRIDGE
LOAD
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
LOAD
PUSH - PULL
PWM MODULATIONBRIDGE
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)
Voc @Tmin
V
0
Inverter
Input
voltage
Vpv,max
Vpv,min
PV
Generator
Vmin
Low voltage field
(not sufficient
to startup)
Vmax
possible
damage
field
Safety operation 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
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
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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%.
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:
Power One
FRONIUS MIX™
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
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 shortcircuit 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:
[email protected]