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PV measurementsTRANSCRIPT
Verifying PV Array Performance
And Shade Analysis
Presented by:
Veenu shekhar, SVEL
Slides provided by: Solmetric
Workshop customization by: SVEL
August 26th, 2011
SEMI Workshop
Jaipur India
About Solmetric Corporation
Leading tool maker of:
Company is based in Sebastopol, CA, USA
About SVEL
Bring technology driven products for the Indian solar industry including Solmetric Corporation
Company is based in Santa Clara, CA, USA
Develop hands-on curriculum and toolkit to provide educational training in photovoltaic and electricity for schools & colleges. More than 100 teachers &
several hundred students have been trained in various cities in US
Hands-on educational solar tracker circuit design workshops
Austin, Texas Greenfield, MA Soledad, CA
Topics
• PV Array Performance Verification
• I-V Curves, and How They‟re Measured
• The Solmetric PVA-600 PV Analyzer
• Test Process (example: commissioning)
• Live Demo of the PV Analyzer User Interface
• I-V Signatures of Electrical Impairments
• I-V Signatures of Shading Effects
Performance Verification Methods
Inverter readout
String I-V curve measurements
String DC measurements
Basic Comprehensive
I
V
Verification methods are evolving in response to
increasing emphasis on energy production (rather
than up-front incentives).
When To Test Array Performance?
• Install QA/Start-up
• Commissioning
• Periodic checkups
• Service alarms
Get much more informative data, in less time
No need to bring the inverter on-line to fully test the array
Close out projects earlier ($$$ flow earlier)
Detailed baseline for comparison as arrays age & degrade
Benefits of I-V Curve Performance Testing
Commissioning New PV Systems
Assure nervous owners that arrays are working properly
Troubleshoot more efficiently
Sort out module versus inverter issues
Provide convincing data for module warranty claims
Maintaining PV Systems (O&M, Asset Management)
Make visible ALL of the array performance parameters
Develop a strong intuitive understanding of PV operation (think
like a PV system!)
Take advantage of string-level performance statistics to set
appropriate system margins & set appropriate pass-fail criteria
Benefits of I-V Curve Performance Testing
Enhancing Your Quality Assurance Capabilities
Testing Building-Integrated PV Installations (BIPV)
„Array as sensor‟ mode derives average module temperature and
irradiance from measured IV curve, for situations where sensor
deployment is not feasible.
Topics
• PV Array Performance Verification
• I-V Curves, and How They’re Measured
• The Solmetric PVA-600 PV Analyzer
• Test Process (example: commissioning)
• Live Demo of the PV Analyzer User Interface
• I-V Signatures of Electrical Impairments
• I-V Signatures of Shading Effects
1. Charge generation by photons
2. Charge separation by built-in electric field
3. Charge collection at the contacts
Photon
Essential Functions of a Solar Cell
Cu
rre
nt
Voltage
Isc
Voc
Max power point
Vmp
Imp
I-V curve
I-V CurveFull I-V curve means more diagnostic value
P-V CurveCalculated from the measured I-V curve
Cu
rre
nt
Voltage
Isc
Voc
I-V curve
Vmp
Imp
Po
we
r
P-V curve
Pmax
Fill FactorA measure of the square-ness of the I-V curve
Cu
rre
nt
Voltage
Imp
Vmp
Max Power Point (Imp, Vmp)Isc
Voc
aSi: 0.50 – 0.70
xSi: 0.75 – 0.85
GaAs: 0.85 – 0.9
Fill Factor =Imp x Vmp (watts)
Isc x Voc (watts)=
Array of Cells, Cell Strings or Modules
This ‘building block’ graphic is useful in troubleshooting arrays
I
V
I-V
building
blocks
Series
Parallel
Total (net) I-V curve
I
V
I-V
building
blocks
Series
Parallel
Total (net) I-V curveCu
rre
nt
Voltage
Max power point
I-V Curve Signatures of PV Problems
Any reduction of the knee of the curve
means reduced output power.
All of these impairments are easily observed using the PV Analyzer,
but are not observed when measuring only Isc and Voc.
Cu
rre
nt
(A)
Voltage (V)
Isc
Voc
Shunt
losses*
Series
losses**
Mismatch losses
(incl. shading)
Normal I-V curve
Reduced
current
Reduced
voltage
Bypass Diodes
PV Module with Bypass DiodesTypical 72-cell PV Module (bypass diodes shown)
+
Bypass Diode Operation
Vmp
+
-
4 strings of 10 modules
72 cells/module
3 bypass diodes/module
Inverter
PV System Operating at Max PowerWith no shade
5A, 0.5V
+
-5A, 0.5V
BP
Vmp
+
-
Inverter
PV System Operating at Max Powerwith shade and no bypass diode
+
-
•Shade happens
•String current drops
•Cell voltages increase
•Shaded cell sees reverse voltage & avalanches
Shaded cell
Unshaded cell
Vmp
+
-
Inverter
PV System Operating at Max Powerwith shade and bypass diode
+
-
•Shade happens
•String current drops
•Cell voltages increase
•BP diode sees forward bias and turns on
Shaded cell
BP
I-V Measurement Methods
Load can be:•Electronic•Resistive
•Capacitive
Voltagesense
Cu
rre
nt
Voltage
Sweeping too fast distorts the I-V
curve when testing high-efficiency
PV modules. There is an I-V curve
‘speed limit’ in volts/second/cell.
PV module, string,
or sub-array
Current sense
(simultaneous
data acquisition)
Topics
• PV Array Performance Verification
• I-V Curves, and How They‟re Measured
• The Solmetric PVA-600 PV Analyzer
• Test Process (example: commissioning)
• Live Demo of the PV Analyzer User Interface
• I-V Signatures of Electrical Impairments
• I-V Signatures of Shading Effects
• Measures the I-V curve
• Compares to a model curve (5 dots)
• Displays & saves data
• Wireless interface
• Rugged and easy to use
String measurement showing I-V and P-V curves.
Five red dots represent the PV model prediction.
Solmetric PVA-600 PV Analyzer
Touch Screen User InterfacePC is user-supplied (Samsung Q1 Ultra shown here)
Showing the Verify tab screen.
Performance Factor (%) = Measured Pmax
Predicted Pmaxx100
Optional wireless sensor kitIrradiance & temperature sensors
PVA-600 Block Diagram (simplified)
CPU &
wireless
module
I sense
V senseC
Control button w LED(isolated)
Battery charging connector(isolated)
(1 of 3)
• Capacitive load method with 3 auto-selected, large-value* capacitors
• Electrically isolated circuitry, no ground lead required
• Protection for over-voltage, -current, -temperature, & reverse polarity
*For best accuracy measuring high-efficiency PV modules
Static Load I-V
Measurement Setup
PV Models in the PV AnalyzerTo predict PV array performance
• Sandia National Labs PV Array Model
– Most comprehensive (30+ parameters)
– ~400 modules, soon 520, more available mid-2011
• 5-Parameter Model
– Developed at U. Wisconsin, used by CEC for NSHP program
– ~1,700 PV modules, soon 3,200
• Simple Datasheet Model
– User enters data sheet parameters (Isc, Voc, Pmax & temp co‟s)
– Translates datasheet Pmax (STC) to actual irradiance & temperature
These 3 methods are available in the Solar Advisor Model (SAM) from NREL and are
embedded in the Solmetric PV Analyzer.
PVA-600 Specifications
• Max DC input voltage: 600V
• Max DC input current: 20A*
• Maximum DC power: 12 KW (instantaneous)
• Min recommended Voc: 20V
• Min recommended Isc: 1A
• I-V measurement time: 80-240mS typical
• Points per I-V trace: Up to 100, depends on test device
• Storage capacity: 1,000+ (PC running PVA SW)
• Safety: IEC-61010
*Strings of high-efficiency modules should be
measured singly, not in parallel
Measuring Category CAT III, 600V
Environmental ConditionsFor Array Performance Testing
•Cloudless sky
•2 hours either side of solar noon
•Little or no wind
This variation can be caused by
clouds you don’t even notice.
Topics
• PV Array Performance Verification
• I-V Curves, and How They‟re Measured
• The Solmetric PVA-600 PV Analyzer
• Test Process (example: commissioning)
• Live Demo of the PV Analyzer User Interface
• I-V Signatures of Electrical Impairments
• I-V Signatures of Shading Effects
I-V Test SetupExample: measuring strings at a combiner box
Test ProcessExample: testing at a combiner box
Hardware setup:
1. Place the irradiance & temperature sensors
2. Isolate the combiner box (open the DC disconnect)
3. De-energize the buss bars (lift the string fuses)
4. Clip test leads to the buss bars
1. Insert a string fuse
2. Press “Measure”
3. View and save results
4. Lift the fuse
String measurement (repeat for each string):
15 seconds
Portland Habilitation Center PV System860kW 7-inverter system by Dynalectric Oregon
Testing at combiner boxes860kW System at Portland Habilitation Center, by Dynalectric Oregon
Combiner Boxes (two per inverter)860kW System at Portland Habilitation Center, by Dynalectric Oregon
Combiner Box860kW System at Portland Habilitation Center, by Dynalectric Oregon
Testing individual PV strings860kW System at Portland Habilitation Center, by Dynalectric Oregon
Overlay plots of I-V curves
Performance analysisUsing the optional I-V Data Analysis Tool
Distributions
• Pmax
• Fill factor
• Imp/Isc
• Vmp/Voc
• Isc, Voc, etc
Table of key performance
parametersArray
Tree
7
6
5
4
3
2
1
0
Cu
rren
t (A
mp
s)
0 100 200 300 400 500
Voltage (Volts)
7
6
5
4
3
2
1
0
Cu
rren
t (A
mp
s)
0 100 200 300 400 500
Voltage (Volts)
1950
2000
2050
2100
7
6
5
4
3
2
1
0
Fre
qu
en
cy
Pmax (Watts)
I-V Data Analysis Tool
Topics
• PV Array Performance Verification
• I-V Curves, and How They‟re Measured
• The Solmetric PVA-600 PV Analyzer
• Test Process (example: commissioning)
• Live Demo of the PV Analyzer User Interface
• I-V Signatures of Electrical Impairments
• I-V Signatures of Shading Effects
Live Demo
PVA-600 PV Model
• Built-in models predict shape of IV curve
• Sandia, 5-Parameter, Simple
CONFIDENTIAL
PVA-600 Model
• Built-in models predict shape of IV curve
• Sandia, 5-Parameter, Simple
CONFIDENTIAL
Measured I-V & P-V curvesRed dots are the model predictions
•Shaded area is the inverter’s MPPT voltage range
•Model set to ‘array as sensor’ mode
Table View
CONFIDENTIAL
Verify View
CONFIDENTIAL
Irradiance and Temperature
CONFIDENTIAL
Topics
• PV Array Performance Verification
• I-V Curves, and How They‟re Measured
• The Solmetric PVA-600 PV Analyzer
• Test Process (example: commissioning)
• Live Demo of the PV Analyzer User Interface
• I-V Signatures of Electrical Impairments
• I-V Signatures of Shading Effects
PV module with 100 ohm shunt resistance
Demonstration: Single PV module with external resistor
PV module with 2.5 ohm added series resistance
0
1
2
3
4
5
6
7
8
0 50 100 150 200 250 300 350 400
Voltage - V
Cu
rren
t -
A
String 4B14
String 4B15
Troubleshooting exampleAnomalous slope in string I-V caused by single high-resistance module
String of Field-aged, Early TF Modules
Array-as-sensor mode for viewing relative changes in curve shape
Topics
• PV Array Performance Verification
• I-V Curves, and How They‟re Measured
• The Solmetric PVA-600 PV Analyzer
• Test Process (example: commissioning)
• Live Demo of the PV Analyzer User Interface
• I-V Signatures of Electrical Impairments
• I-V Signatures of Shading Effects
PV Module with Bypass DiodesTypical 72-cell PV Module (bypass diodes shown)
+
Partial shadingInverter must find and track the right peak
Cu
rre
nt
Voltage
Isc
Voc
Po
we
r
Max Power(until the shade pattern changes!)
Partially shaded residential arraySingle string, along lower edge of roof
I-V Curve of the partially shaded stringSingle string, along lower edge of roof
Effects of Particular Shading
Patterns
Sub-cell shading Business card on 1 cell in a string of 15, 48-cell modules
Shade 2 cells in the same cell-stringSingle module with 72 cells and 3 bypass diodes
Shade 2 cells in adjacent cell-stringsSingle module with 72 cells and 3 bypass diodes
Shadow of leafless tree branchSingle module with 72 cells and 3 bypass diodes
Shade „taper‟ across a cell-stringSingle module with 72 cells and 3 bypass diodes
Edge and Corner SoilingCommon in low-tilt arrays
Cleaning the top 90% of
module area recovered
50% of the performance.
Cleaning the bottom 10%
recovered the rest.
Dirty
Clean
Edge and Corner SoilingCommon in low-tilt arrays
SunEye: Annual Sunpath View
View Solar Access View TOF & TSRF
Editing a Skyline
• Simulate…
– Tree trimming
– Tree removal
– Tree growth
– New building
construction
Conclusion
• Shade matters: A single business card shade on the panel can reduce
the performance by 35%
•Curve Tracing is Awesome!
•Fast and automated, & manual data recording is eliminated
•Provides a complete description of performance
•Measures max power of strings, independent of rest of array!
•Great diagnostic power when combined with onboard PV models
•Enables a deeper understanding of array operation
•Helps build skills across offices and crews
I
V