Field Applications of I-V Curve Tracers in the Solar PV ... · Field Applications of I-V Curve...
Transcript of Field Applications of I-V Curve Tracers in the Solar PV ... · Field Applications of I-V Curve...
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Field Applications of I-V Curve Tracers
in the Solar PV Industry
Paul HerndaySenior Applications [email protected] 707-217-3094
November 14, 2012
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Solmetric Solutions
www.solmetric.com
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Topics
• Field portable I-V curve tracers
• Array performance testing
• Measurement conditions
• Data analysis
• Troubleshooting
• I-V curves of partially shaded arrays
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Introduction
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Trend in PV Incentives
• Performance based
– Net metering
– Solar Renewable Energy Credit (S-REC) programs
– Feed-in tariffs
• Expected performance based (cap/perf hybrid)
– State/utility buy-down programs
• Capacity based
– Rebate programs
– Tax credits
Goals of performance based incentives: Incentivize proper system design and installation, improve ROI, and help states meet renewable energy targets.
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Array Performance Test MethodsFor Startup/Commissioning/Checkups/Alarm response
Inverter readout
String I-V curve measurements
String DC measurements
Basic Comprehensive
I
V
Monitoring also plays a key role in
maintaining performance
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Curve Tracers – a Long and Proud HistoryMaking electrical relationships visible
• The Tektronix 575 (c1957) was the first widely used transistor curve tracer
• Curve tracers have been essential tools for PV research and manufacturing
• Today, I-V curve tracers are rugged, simple, and affordable enough for field in commissioning and maintaining PV systems.
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Max power point
Five Classes of I-V Curve Anomalies
Current (A
)
Voltage (V)
Isc
Voc
Increasedslope
Reducedslope
Mismatch losses (incl. shading)
Normal I-V curve
Reducedcurrent
Reducedvoltage
Conventional measurements do not reveal many of these effects.
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Benefits of I-V Curve Testing
• Most complete performance test possible for PV modules & strings
• Much faster than conventional methods – one connection, one test
• Allows fully testing the array before the inverter comes online
• Provides high-value data for the commissioning or O&M report
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Benefits of I-V Curve Testing
• Provides a detailed baseline for comparison as systems age
• Provides authoritative evidence for module warranty return
• Speeds troubleshooting – selective shading method
• Using the “best tools” helps win new business
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Curve Tracer Users
EPC organizations
System Integrators
Consulting Engineers
Training OrganizationsTechnical colleges
IBEW
Training Centers
O&M Companies
Electrical contractors
I
V
Module Manufacturers
Inverter Manufacturers
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Curve Tracers for PV Field Applications
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Loading the test device
Load
Load can be
•Resistive
•Capacitive
•Electronic
MeasurecurrentMeasure
voltage
Current
Voltage
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I-V curve tracing with a resistive load
V sense
R1
Current
Voltage
R2
I sense
R3 R4 R5 R6
• Resistors have their own I-V curves
• They are straight lines starting at I=0, V=0
• The lower the resistor, the steeper the line
• Switch in one load resistor at a time
• Each resistor identifies one operating point on the I-V curve
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I-V curve tracing with a capacitive load
I sense
V senseC
Capacitorvoltage
Vcap
Current
Voltage
Vcap
Time
Voc
Illustrate charging with an animation. When switch closes the circuit, the capacitor charges from 0V to Voc as shown in graph. I-V curve is swept out from left to righ, quickly at first, and gradually slowing above the knee of the I-V curve (the knees of the two curves coincide in voltage).
Bleeder resistor
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Rseries
Rshunt CI V sense
I sense
PV module equivalent circuit I-V curve tracer
I-V sweep rate effectsin testing high efficiency modules
• High efficiency PV modules store more charge, symbolized by the shunt capacitor
• If the load changes too fast, the charge does not have time to settle at each new load point
• This results in overshoot or undershoot of the measured curve, relative to the actual maximum power point
• The Solmetric PV Analyzer is designed for accurately testing high efficiency PV technologies
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Influence of solar cell capacitance on the
measurement of I-V curves of PV modules, Stefan Mau, Thomas Krametz
I-V sweep rate effectsin testing high efficiency modules
• These curves show the effect of different sweep times and directions
• Overshoot occurs if sweeping too fast from from open circuit to short circuit
• Undershoot occurs if sweeping too fast from short circuit to open circuit
Check for permission to use figure
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V sense
R1 R2
I sense
R3 R4 R5 R6
I sense
V senseC
Capacitorvoltage
Vcap
Measurement throughputThermal limitations
• Every I-V curve tracer absorbs a small amount of energy with each I-V curve measurement
• This energy must be managed to control instrument temperature
Resistive load
Capacitive load
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Built-in PV models
Irradiance & temperature
Module, tilt, orientation…
5 points predict the shape of the curve
Built-in PV models
• Built-in PV models provide an immediate check on the health of the module/string under test
• Models in use today include the Sandia, 5-Parameter, and simple datasheet models with STC translation.
• The curve tracer may predict the performance at existing conditions, or translate the I-V measurement to Standard Test Conditions (or other user-selected conditions)
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Solmetric PV Analyzer
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Solmetric PV Analyzer
• 20A, 600V
• 1000V in development
• Wireless interface to your tablet or notebook PC
• Extensive touch features
• Automated data analysis
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Irradiance sensor
Module backside
temperature sensor
Your PCWUSB2
WUSB1
I-V Measurement
Unit
Built-in PV models
Irradiance & temperature
Module, tilt, orientation…
5 points predict the shape of the curve
All wireless
How it works
Screen shot
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Typical Measurement Setup
Courtesy of Chevron Energy Solutions © 2011
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Wireless Sensor Kit
Irradiance
transmitter
Receiver (USB)
Temperature
transmitter
K-type
thermocouple
Omega Part #
5SRTC-GG-K-
30-72
.
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Deploying the sensors
Mount the irradiance sensorin the plane of the array.
Mount the thermocouple away from the cool edges of the module/array. Press the thermocouple into firm contact with
the module backside.** MOCAP MCD-PE 1.75 poly dot
~$80/roll of 1000 [email protected]
Irradiance Temperature
**
TCwire
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Software User Interface
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Traces tab
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Array Navigator™
Saving a Measurement ResultTouch location of test device, then save
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Environmental Inputs controls
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Verify screen
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Table screen
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History screen
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Measurement Conditions
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• Clear sky (for high, stable irradiance)
– Height of I-V curve varies directly with irradiance
– Shape of I-V curve changes at low irradiance
– Translation to STC is much less accurate from low light conditions
• 4 hour window centered on solar noon*
– Avoids low-light, spectral, and angle of incidence induced errors
• Low/No wind (for more consistent module temperature)
– Width of I-V curve varies inversely with temperature
– Temperature is not uniform across an array under any conditions
Recommended Sky ConditionsFor Array Performance Testing
http://www.esrl.noaa.gov/gmd/grad/solcalc/
*Solar Noon Calculator:
Good conditions mean less scatter in your data, which means:
• Easier to spot non-conforming strings, and
• More convincing demonstration of consistent string-level performance
800W/m2
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Problem Sky Conditions
Cirrus
Scattered
Edge effect
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Measurement Process - Commissioning
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I-V Measurement SetupExample: Measuring strings at a combiner box
Attach the thermocouple well away from module edges, using polyimide (Kapton) tape for best mechanical properties at high temperatures.
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Measurement ProcessExample: Measuring strings at a combiner box
Hardware setup (do once at each combiner box):
1. Move the sensors (if necessary to get wireless range)
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
Electrical measurement (repeat for each string):
10-15 seconds, typically
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Measurement Process Example
Courtesy of Portland Habilitation Center and Dynalectric Oregon
1. Open the DC disconnect
for the combiner where you will be making the
measurement
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Courtesy of Portland Habilitation Center and Dynalectric Oregon
2. Locate and
open the combiner
Measurement Process Example
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Courtesy of Portland Habilitation Center and Dynalectric Oregon
3. With a clamp-meter,
verify that the load
has been
disconnected.
Then lift all of the
fuses.
Measurement Process Example
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Courtesy of Portland Habilitation Center and Dynalectric Oregon
4. Clip the curve tracer leads to the
buss bars.
5. Push down one fuse at a time and
make I-V curve
measurements.
6. View and save
results.
Measurement Process Example
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Data Analysis
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Displays Generated by theI-V Data Analysis Tool*
1950
2000
2050
2100
7
6
5
4
3
2
1
0
Fre
qu
en
cy
Pmax (Watts)
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)
*Optional, MS Excel-based tool, $95
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Troubleshooting
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Max power point
I-V Curve Signatures of PV Problems
Any reduction of the knee of the curve
means reduced output power.
Current (A
)
Voltage (V)
Isc
Voc
Shunt losses*
Series losses**
Mismatch losses (incl. shading)
Normal I-V curve
Reducedcurrent
Reducedvoltage
Conventional measurements do not reveal many of these effects.
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Isc
Voc
Useful diagnosticsFill Factor, Current Ratio, Voltage Ratio
Current
Voltage
Fill Factor =Imp x Vmp (watts)
Isc x Voc (watts)
aSi: 0.50 – 0.70
xSi: 0.75 – 0.85
GaAs: 0.85 – 0.9=
Current ratioImp/Isc
Voltage ratioVmp/Voc
Imp
Vmp
Max Power Point
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String of Field-aged, Early TF ModulesDegraded fill factor, lower output power
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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
High-resistance StringAnomalous slope in string I-V caused by single high-resistance module
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Example of a Series Resistance FailureAt solder bond of PV output conductor to ribbon buss bar
Probably failure mode:
Heat cycling � bond degradation � resistive heating
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Example of catastrophic bond failure
• Stress induced failure
• Failures of this type tend to occur at buss bar bonds and at connections of PV output cables to ribbon conductors
• I-V curve tracing can detect this problem before it becomes catastrophic
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Dropped Cell String
• Shorted bypass diode, or
• Mismatch causing diode to turn on
when current starts flowing
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I-V Curves of Shaded Arrays
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Bypass Diode Function
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Bypass Diodes
Purpose:
• Prevent damage
• Preserve performance under mismatch conditions
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Typical PV Module
+
Cell String
Cell String
Cell String
BypassDiodes
Bypass diode turns on when the shaded cell(s) can no longer pass as much current as the non-shaded cells.
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Shade One CellExample: 72-cell PV Module
+
Cell String
Cell String
Cell String
BypassDiodes
Bypass diode turns on when the shaded cell(s) can no longer pass as much current as the non-shaded cells.
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I-V Curve of a Partially Shaded String
• Multiple ‘knees’ � multiple ‘local’ power peaks
• Peaks evolve as shading configuration changes
• Inverter tries to find and track the highest peak
Current
Voltage
Isc
Voc
Power
Bypass diode turns on
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Bypass diode thought experimentV
olta
ge
0v
360v + + +
Inverteroperating at MPP
All cells:
+In the next slidewe’ll shade this cell
String1
String2
String3
String4
Pmax
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0v
360v +
+
+ +
String1
String2
String3
String4
Inverteroperating at MPP
Bypass diode thought experimentV
olta
ge
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0v
360v +
+
+ +
String1
String2
String3
String4
Inverteroperating at MPP
Bypass diode thought experimentV
olta
ge
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Shading an Unprotected CellBypass diode failed ‘open’
Shadedcell
Control(not shaded)
Conditions:
•All 3 BP diodes removed
•Single cell shaded
•Output short circuited
(worst case)
Heating of a Single Shaded CellBackside temperature with no bypass diodes
40
45
50
55
60
65
70
75
80
85
0 5 10 15 20 25 30
Shade Duration (seconds)
Te
mp
era
ture
(C
)Shaded cell
Unshaded cell
Rise of 1°C per second
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I-V Curve of a Partially Shaded String
• Multiple ‘knees’ � multiple power peaks
• Peaks evolve as conditions change
• Inverter tries to find and track the highest peak
Current
Voltage
Isc
Voc
Power
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V
I
‘Modular’ view of shading
3 cell strings/module
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V
I
‘Modular’ view of shading
Signature of
shading one full
cell anywhere in
the actual array
3 cell strings/module
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Shade Examples
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Partially shaded residential arrayMeasure the single string mounted along lower edge of roof
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I-V Curve of the partially shaded stringSingle string mounted along lower edge of roof
Approximately 40% reduction in string’s output power
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Shade 2 cells in the same cell-stringSingle module with 72 cells and 3 bypass diodes
Shading one
cell string
drops 1/3 of
PV module
voltage and
power
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Shade 2 cells in adjacent cell-stringsSingle module with 72 cells and 3 bypass diodes
The same
amount of
shade,
oriented
differently,
drops 2/3 of
PV module
voltage and
power.
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Line Shade – NarrowShade one of two series connected modules
12
12
Pmax: 98%
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Line Shade – MediumShade one of two series connected modules
5
5
Pmax: 91%
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Line Shade – ThickShade one of two series connected modules
3
3
Pmax: 77%
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Tapered shadingFrom adjacent row, parapet wall, railing, etc
• This effect produces an I-V curve deviation similar to that of shunt loss
• The tapered sliver of shade causes a slight current mismatch across cell groups and modules
• In tilt-up system, the impact of this shade is felt only early and late in the day, at low sun angles
• In general, inter-row shading losses are greater if rows are ‘crowded’ to increase peak capacity
Current
Voltage
Isc
Voc
rows not parallel
Effect of tapered shade
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Shade ‘taper’ across a cell-stringSingle module with 72 cells and 3 bypass diodes
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Intimate ShadingSolution: Rent a goat
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Soiling
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Soiling
• Uniform
– Dust or grime, evenly deposited
– Decreased irradiance, lower PV current
• Non-uniform
– Dust ‘patterns’, debris, bird droppings
– Mismatch effects, steps in the I-V curve
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Examples of uniform & non-uniform soiling
Uniform
Non-uniform
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The dirt dam hurt performance as much as all of the uniform soiling.
Dirty
Clean
Edge soiling (dirt dam)Common in low-tilt arrays
Dirty
Clean
50%50%
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Troubleshooting Techniques
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Selective Shading
Photo courtesy of Harmony Farm Supply and Dave Bell (shown)
3 cell strings/module
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Current
Voltage
Isc
Voc
Initial string measurement:
Step at 15% of normal current, 10-12 volt wide
Entire string
Any good module shaded
Bad module shaded
Selective Shading
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Infrared imaging of PV arrays
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IR signature of bypassed cell string
Image 383
22 C 45 C
Cardboard shading a cell to force the bypass diode ‘on’
Measured usingthe FLIR i7
infrared camera
Center two columns of cells are slightly hotter
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Module Inspection
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Hot Spot on Module Buss Bar
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Aerial Thermography860kW System at Portland Habilitation Center, built by Dynalectric
Image courtesy of Oregon Infraredhttp://www.oregoninfrared.com/
Open strings
Module issues
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Field Applications of I-V Curve Tracers
in the Solar PV Industry
Paul HerndaySenior Applications [email protected] 707-217-3094
November 14, 2012