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Quality assurance of PV plants
connected to the grid
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Today widely extended technical quality assurance practises
STEP PROCEDURE OBJECTION
Design:
Energy yield forecast
Commercial software Based on non-guaranteed
information
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..for definitive simulations, the user is advised to carefully verify the library data
with the last manufacturers specificationsWe drop out any responsibility about
the integrity and the exactness of the data and performance including in the
library..
Disclaimer at PVSYS Users Guide
The PVsyst is based on the full I-V curve one-diode model. Required information
(series and shunt resistance, photo current, saturation current and diode quality
factor) is mainly obtained from I-V curves database from TISO (Centrale di prova
peer componente PV, Ticino, Switzarland) and Photon (German PV journal)
..These data are key parameters of the model, and should be part of the modules
specifications in the future..
A. Mermoud el al, 25thEuropean PVSEC (2010)
The database was compiled to the best of our knowledge and with the greatest
possible accuracy. At the same time, PHOTON cannot be held responsible from
any damage that results from the use of this database.
Disclaimer at Photon database
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Risk of relying on default settings that prescribe irradiance behaviour in PV*SOL
and Pvsyst module models
K. Sauer et al, from Yingly
PV Performance Modelling Workshop, Sandia Labs (2013)
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Today widely extended technical quality assurance practises
STEP PROCEDURE OBJECTION
Design:
Energy yield forecast
Commercial software Based on non-guaranteed
informationProcurement:
PV module sample
peak power testing
Prior to the
installation, at
qualified laboratories
Neither Light Induced
Degradation nor
Irradiance and temperature
behaviour are addressed
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Today widely extended technical quality assurance practises
STEP PROCEDURE OBJECTION
Design:
Energy yield forecast
Commercial software Based on non-guaranteed
informationProcurement:
PV module sample
testing
Prior to the
installation, al
qualified laboratories
Light Induced Degradation
not addressed
Commissioning:
PV plant production
testing
PRduring a week
(Often pass criteriaPR 80%)
-Time-dependence disturbs
technical qualityqualification
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Weekly PR evolution along the year
0,850
0,900
0,950
1,000
1,050
1,100
1,150
0 10 20 30 40 50 60
PRDC
PR
The weekly PR varies up to 10% along the year, and up to 5 % along a
same month.
c-Si PV array, Navarra (Spain), 2011
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Today widely extended technical quality assurance practises
STEP PROCEDURE OBJECTION
Design:
Energy yield forecast
Commercial software Based on non-guaranteed
informationProcurement:
PV module sample
testing
Prior to the
installation, al
qualified laboratories
Light Induced Degradation
not addressed
Commissioning:
PV plant production
testing
PRduring a week -Time-dependence disturb
technical qualityqualification
- Detailed characterization of
real PV plant behaviour not
addressed
Infrared inspection Hot-spot detection - Acceptance criteria scarcely
addressed
Operation
PV plant production
PRduring full years - Ageing measurement not
addressed
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IEC 62446:
Grid connected PV systemsMinimum requirements for system
documentation, commissioning tests and inspection
7.2.2.2 IR test resultsModule hot spots
Module temperature should be relatively uniform, with non areas of significant
temperature difference. However, it is to be expected than the module will be hotter
around the junction box compared to the rest as the heat is not conducted as well tothe surrounding environment. It is also normal for the PV modules to see a
temperature gradient at the edges and supports.
A hot spot elsewhere in a module usually indicates an electric problem, possibly series
resistance, shunt resistance or cell mismatch. In any case investigate the performance
of all modules that show significant hot spot(s). Visual inspection may show signs ofoverheating, for example a brown or discoloured area.
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Today widely extended technical quality assurance practises
STEP PROCEDURE OBJECTION
Design:
Energy yield forecast
Commercial software Based on non-guaranteed
informationProcurement:
PV module sample
testing
Prior to the
installation, al
qualified laboratories
Light Induced Degradation
not addressed
Commissioning:
PV plant production
testing
PRduring a week -Time-dependence disturb
technical qualityqualification
- Detailed characterization of
real PV plant behaviour not
addressed
Infrared inspection Hot-spot detection - Acceptance criteria scarcely
addressed
Operation
PV plant production
PRduring full years - Ageing measurement not
addressed
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SUMMARY:
Energy production of current large PV plants seems to satisfy investors
expectation
Nevertheless, Technical Quality Assurance procedures can be improved in
order to:
Clarify the rules for endorsement of responsibilities in the event of
problems (real production below expectation, hot-spots appearance, etc.)
Improve the technical soundness and the usefulness of in-field
commissioning testing
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QA Objective: Tightening real and predicted productions
Expectation in terms of , both, yearly energy production and degradation rate
Assumptions:
Phenomena Procedure Responsible
Weather evolution (G(0) and TA) Solar database Nobody
Operation conditions (G(in-plane)
and TC) evolution
Transposition models Nobody
PV plant response P*, INV, loses
PR, PRSTC
EPC; PV modules and
inverter manufacturers
Commissioning:
Testing the PV plant response during a few weeks period
Operation:
Monthly and yearly verification of PV plant response and
maintenance procedures
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Modelling the PV plant response
*
GPACP
G
TC
=
, , . .
- Dealing with the PV plant response requires modelling of= , ,
- Because associated responsibilities, involved specifications (P*, thermal
response, etc.) must be agreed with PV manufactures.
- Current specification practises:
- Guaranteed: P*, degradation rate in %/year
- Standard information: NOCT, Thermal coefficients
- Additional information: I-V graphics
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= , , modelling alternatives
1- Considering just the MPP
=
1
+ .
+ .
standard data sheet information (IEC 61215 and IEC 61646)
(a, b,c) a=1, b=0, c=0
200
/1000
(IEC 61215 and IEC 61646)
granting generality to published values (1)
Commissioning tests
2- Considering the full I-V curve
Adjusting 5 parameters (ISC, VOC, RS, RP and m) to a given I-V curve is not straightforward
Module to module parameters variation is typically larger than MPP variation
3Specific energy rating attempts
Kings model from Sandia Lab
Power matrix as defined at IEC 618532
(1) R. Kenny et al, Prog. Photovolt: Rs.Appl. (DOI: 10,1002/pip.2365 (2013)
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Available literature warns about scarce benefits from complexity
Surprisingly, there doesnt seem to be a need for overly complicated modelling to
achieve this accuracy for most technologies
PERFORMANCE final brochure (EPIA, 2009)
A simplified version of the Kings model, using a single expression for the maximum,
power point and requiring just 6 empirical coefficients performs as well as the original
(Huld, T. 2011)
The authors feeling is that the complexity of the standard [ IEC 618532] is actually not
beneficial for an accurate energy prediction, as it requires data which is actually
normally not know and the generation of this.. seems to affect the overall agreement
more than it would be without this complicated step
(Jyotirmoy Roy, 2008)
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Experimental comparison of different practical possibilities
- 6 PV arrays of different technologies (1.9 < P*(kW)
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Weekly energy errors [(ModelledExperimental)/Experimental ], for the c-Si
PV array, weighted by the daily irradiation and expressed in %.
DATA SHEET ADDITIONAL EXPERIMENTS
Full I-V MPP
MPP
, 200
Full I-V MPP
, ref (1)
MPP
, meas.
W1 0.4 1.85 2.67 -0.08 2.15 1.41W2 -1.69 0.84 1.81 -0.79 1.12 0.28
W3 -1.78 0.97 2.03 -0.8 1.26 0.32
W4 3.95 1.89 3.79 -0.64 2.93 1.17
Year 2.11 1.50 2.64 -0,83 1.86 0.85
1.42 1.27 1.29 1.07 0.9 0.97
- PREAL* 1% larger than PNOMINAL*
- Selected weeks are centred on the equinox (W1 and W3), on the summer solstice
(W2) and on the winter solstice (W4)
An MPP model considering just the power temperature coefficient given at
the data sheet explains up to 97% of the observed variability.
Considering also the efficiency variation with irradiance reduces uncertainty
by about 1%
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Yearly energy errors at 12 different commercial Spanish PV plants
- G and TC given by reference PV modules, MPP model with and 200/1000
- P* and INVERTERvalues obtained at Commissioning tests
Nominal power(MW)
Real yearly Yield(kWh/kW)
Modelled yearlyyield (kWh/kW)
Error (%)
2 2038 2067 1.4
2.16 2056 2095 1.9
2.95 2050 2096 2.2
2 2194 2163 -1.4
1.5 2074 2032 -2.1
1.4 1561 1597 2.3
2.03 2142 2140 -0.1
11.2 2016 2038 1.1
2.1 2204 2198 -0.3
1.9 2320 2279 -1.8
9.7 2108 2111 0.1
25.3 1594 1616 1.4
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Daily energy errors at 45.6 kW Amaraleja PV plant
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-5
0
5
10
15
20
KTd
heoretical-
easured(
-6 -4.5 -3 -1.5 0 1.5 3 4.5 6 7.5 90
5
10
15
20
25
Theoretical-Measured (%)
requency
Mean: 1.3 %
STD: 1.9 %
- G and TC given by 9 reference PV modules, MPP model with
- P* and INVERTERvalues first obtained at Commissioning tests andperiodically reviewed
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Commissioning testing: Current state of art - 1
- The simplest possibility rely on the Performance Ratio concept
- Acceptation , .
.PRGUARANTEED. (or PR PRGUARANTEED)
Given by the energy meter Specified at the contract (80%)
Nominal power Given by the in-plane solar radiation meter
- PRcalculation requires only Grecords
- Because it includes time-dependent unavoidable losses, the mere PRis generally
not adequate for sub-year periods (days, weeks, months ).
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Weekly PR evolution along the year
0,850
0,900
0,950
1,000
1,050
1,100
1,150
0 10 20 30 40 50 60
PRDC
PR
The weekly PR varies up to 10% along the year, and up to 5 % along a
same month.
c-Si PV array, Navarra (Spain), 2011
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Commissioning testing: Current state of art - 2
- Time-dependent unavoidable energy loses are removed at the Performance Ratio at
Standard Test Conditions concept:
=
(1)
- E means energy loses and I extends to all unavoidable phenomena:
- Thermal (due to TC TC* )- Efficiency variation with irradiance
- Anomalies: Shading, inverter saturation, PV plant disconnections, etc.
- PRSTCcalculation requires (additional to G) TC records .and modelling.
- PV reference modules are recommended to minimize uncertainty
- Acceptation PRSTC along T PRSTC, GUARANTEED
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Weekly PR and PRSTCevolution along the year - 1
0,850
0,900
0,950
1,000
1,050
1,100
1,150
0 10 20 30 40 50 60
PR
PRSTC
PR
PRSTC
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Commissioning testing: Advances procedures
- Not only energy production but also PV plant characterization is addressed
G
TCPDC PAC
PREAL* =PNOMINAL* . (1-FG); INV, REAL= INV, NOMINAL.(1-FINV
- Highly desirable for further careful operation surveillance
- PV plant in-field characterization requires G, TC, PDCand PACrecordsand modelling
- PV reference modules and accurate wattmeters are recommended to minimizeuncertainty
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PV reference modules and accurate wattmeters
I fi ld t ti AC
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In-field testing: AC power response
21
PAC,EXPPDC,EXPG, TC
I fi ld t ti STC f th PV t
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0
20
40
60
80
100
120
140
0 200 400 600 800 1.000 1.200
Gef(W/m2
)
PDC
Gef,
25C
W
PDC= 0,1087 Gef
R2= 1
In-field testing: STC power of the PV generator
*
CC
DCefDC
1
C25,
TT
PGP
PAC,EXPPDC,EXPG, TC
,EXP
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Mean
ISC*[A] 1240.0 6.4
VOC*[V] 862.3 5.1
IM*
[A] 1114.6 4.2VM
*[V] 703.4 10.6
PM, IEC-60891*[W] 784044 11067
FF* 0.733 0.003
PM,*[W] 742728 4894
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In-field testing: efficiency of inverters
0
10
20
30
40
50
60
70
80
90
100
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0
pac
(%)
Inversor A
Inversor B
Inversor C
Modelo
Fabricante
h
P
PAC,EXPPDC,EXPG, TC
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Dealing with hot-spots - 1
Hot-spots threaten PV module lifetime and reduce operation voltage
THS 5oC
VOP= 29.5 V
THS 17.4oC
VOP= 26.3 V THS 14.3oC; THS 11.8
oC
VOP= 22.6 V
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Dealing with hot-spots - 2
Proposal for contracts:
- THS 20oC PV module rejection
- 10oC THS< 20oC and V > 20%
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CONCLUSIONS
- c-SI PV array response is properly described by a simple model considering
just the maximum power point and the power temperature coefficient
- The power temperature coefficient must be included at the guarantees
provided by the PV module manufacturers
- In-field commissioning testing must preferably deal with PRSTC(over the
mere PR). That requires measuring Gand TC and modelling unavoidable
loses
- Commissioning testing can also include the characterization of the
effective behaviour of the PV plant. That requires measuring G,TC , PDCand
PAC.
- Reference PV modules are preferred for measuring, both, Gand TC
- Proposed rejection criteria for PV module with hot-spots:
- Temperature differences larger than 20oC
- Temperature differences between 10oC and 20oC, and voltage
operation loses larger than 20%
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