New Tools for Verifying
PV Array Performance
f
Paul Hernday
Applications
pp
Engineer
g
PV Array Measurements
Solmetric
Mission: Improve the effectiveness of solar systems and
those who
ho install them by
b providing
pro iding high q
quality,
alit eas
easy-toto
use, accurate tools with embedded technical know-how.
Topics
• PV array performance verification
• Current-Voltage (I-V) curve measurement
• Array
A a performance
pe fo mance models
• I-V test setup and process
• Example I-V measurement results
• Electrical access to strings for testing purposes
• Thermography applied to PV arrays
What is performance verification?
Determination that measured performance meets
a performance standard, to within a specified margin.
Depending on the objective, the standard may be a
reference PV string, a contracted performance
specification, or a detailed array performance model.
Objectives of performance verification
•
•
•
•
•
•
•
Expose problems earlier
Troubleshoot more efficiently
Get better commissioning data in less time
Close out p
projects
j
earlier ($$$ flows earlier))
Check the health of older PV arrays
Localize problems to PV modules or inverter
Provide data for PV module warranty claims
Build better solar
Improve financial & energy payback
Cultivate public trust in PV
Performance Verification Applications
In the PV System Life Cycle
Prelim.
assessment
Site
survey
Final design
Startup
Prelim.
design
Installation
planning
Commissioning
C
i i i
Acceptance
Proposal
Installation
Maintenance
Contract
Checkout
Service
Performance Verification Tools
Inverter readout
compared with
system model
DC measurements
compared with
PV model
Basic
I V curve measurements
I-V
t
compared with
PV model
Comprehensive
I
V
Solmetric PV Analyzer
PVA-600
I V curve measurements
I-V
and on-board PV models
Wireless to your
notebook computer
Topics
• PV array performance verification
• Current-Voltage (I-V) curve measurement
• Array
A a performance
pe fo mance models
• I-V test setup and process
• Example I-V measurement results
• Electrical access to strings for testing purposes
• Thermography applied to PV arrays
Benefits of I-V Curve Measurement
Compared
p
with DMM & Clampmeter
p
Testing
g
Max Power Point
Current
Power
Isc
(Clamp meter)
Voltage
g
Voc
(Multi-meter)
• Full I-V curve  More diagnostic value
• Automated  Fast & less chance of record
record-keeping
keeping mistakes
Fill Factor
Reduced ‘square-ness’
q
of I-V curve can indicate aging,
g g, defects
Max Power Point
Currentt
Isc
Imp
Area A = Imp x Vmp
Vmp Voc
Voltage
Fill Factor =
Area B = Isc x Voc
Area A
Area B
Series and Shunt Losses
Contributors to reduced fill factor
Max Power
M
P
Point
P i t
Imp, Vmp
Isc
C
Current
Imp
Shunt
loss
Series
loss
Voltage
g
Useful indicators: • Imp / Isc ( shunt loss)
• Vmp / Voc ( series loss)
Vmp Voc
I-V measurement methods
I sense
V sense
Load can be:
•Electronic
•Resistive
•Capacitive
Cu
urrent
Load
L
d requirements:
i
t
• Voltage & current range
• Power dissipation
Voltage
• ‘Sweep’ dynamics, esp sweep
rate, which must be slower to
accurately test high-efficiency
modules
Topics
• PV array performance verification
• Current-Voltage (I-V) curve measurement
• Array
A a performance
pe fo mance models
• I-V test setup and process
• Example I-V measurement results
• Electrical access to strings for testing purposes
• Thermography applied to PV arrays
PV models
For p
predicting
g PV arrayy performance
p
• Sandia National Labs PV Array Model
– Most
M t comprehensive
h
i (30+ parameters)
t )
– ~400 modules modeled, more will flow starting ~ mid-2011
• 5-Parameter Model
– Developed at U. Wisconsin, used by CEC for NSHP program
,
PV modules modeled at this time
– ~1,700
• Simple Datasheet Model
– Translates Pmax ((STC)) to actual irradiance & temperature
p
– User enters data sheet parameters
These 3 methods are available in the Solar Advisor Model (SAM) from NREL and are
embedded in the Solmetric PV Analyzer.
Topics
• PV array performance verification
• Current-Voltage (I-V) curve measurement
• Array
A a performance
pe fo mance models
• I-V test setup and process
• Example I-V measurement results
• Electrical access to strings for testing purposes
• Thermography applied to PV arrays
I-V measurement setup
Example: measuring strings at a combiner box
Steps to test a PV array
at a combiner box
PC setup (can be done in advance):
• Select PV model p
parameters (type
( yp & # of modules,, etc))
• Customize a directory (‘array tree’) for saving string data
Hardware setup (onsite):
1 Locate
1.
L
t th
the iirradiance
di
& temperature
t
t
sensors
2. Open the combiner box’s DC disconnect
3. Lift the fuses to de-energize the buss bars
4. Connect test leads to the buss bars
String measurement (per string):
1 Insert
1.
I
t a ffuse
2. Press “Measure”
3. Observe and save results
Array tree for saving I-V data
Same hierarchy
as the PV system
Measurement data
from individual PV
string tests
Portland Habilitation Center PV System
860kW 7-inverter system
y
byy Dynalectric
y
Oregon
g
Testing at combiner boxes
860kW System
y
at Portland Habilitation Center,, byy Dynalectric
y
Oregon
g
Combiner Boxes (two per inverter)
860kW System
y
at Portland Habilitation Center,, byy Dynalectric
y
Oregon
g
Combiner Box
860kW System
y
at Portland Habilitation Center,, byy Dynalectric
y
Oregon
g
Testing individual PV strings
860kW System
y
at Portland Habilitation Center,, byy Dynalectric
y
Oregon
g
Max Power as % of Predicted
Example
Measured I-V & P-V curves
Red dots are the model predictions
p
•Shaded area is the inverter’s MPPT voltage range
•Model set to ‘array as sensor’ mode
Performance analysis
Evaluation of the ‘arrayy tree’ data
Overlay plots of II-V
V curves
curves,
raw and normalized
Histograms
Pmax, Fill
factor, Imp/Isc, Vmp/V
oc
Isc, Voc, etc
User-defined metrics
Reports
Topics
• PV array performance verification
• Current-Voltage (I-V) curve measurement
• Array
A a performance
pe fo mance models
• I-V test setup and process
• Example I-V measurement results
• Electrical access to strings for testing purposes
• Thermography applied to PV arrays
PV module with 100 ohm shunt resistance
Demonstration: Single PV module with external resistor
PV module with 2.5 ohm added series resistance
Troubleshooting example
Comparison
p
of typical
yp
(blue)
(
) and atypical
yp
(red)
( ) I-V curves
8
7
Current - A
6
5
4
3
String 4B14
2
String 4B15
1
0
0
50
100
150
200
250
300
Voltage - V
Troubleshooting example
Problem isolated to failed module
Irradiance changed between measurements
350
400
Field-aged thin film string
Array-as-sensor mode
Effect of shade on PV arrays
PV Module with Bypass Diodes
Example:
p 72-cell module
+
Partial shading
Inverter must identifyy and track the right
g peak
p
Max Power
(until the shade pattern changes!)
Current
Po
ower
Isc
Voltage
Voc
Partially shaded residential array
Single
g string,
g, along
g lower edge
g of roof
Partially shaded residential array
Single
g string,
g, along
g lower edge
g of roof
Shading Pattern Effects
Sub-cell shading
Business card on 1 cell in a string
g of 15,, 48-cell modules
Shade 2 cells in the same cell-string
Single module with 72 cells and 3 bypass diodes
Shade 2 cells in adjacent cell-strings
Single module with 72 cells and 3 bypass diodes
Shade ‘taper’ along a cell-string
Single
g module with 72 cells and 3 bypass
yp
diodes
Shade ‘taper’ across a cell-string
Single
g module with 72 cells and 3 bypass
yp
diodes
Shadow of leafless tree branch
Single
g module with 72 cells and 3 bypass
yp
diodes
Topics
• PV array performance verification
• Current-Voltage (I-V) curve measurement
• Array
A a performance
pe fo mance models
• I-V test setup and process
• Example I-V measurement results
• Electrical access to strings for testing purposes
• Thermography applied to PV arrays
String access points
Best choice depends
p
on system
y
layout
y
2
1
3
DC
Disco
DC
DC
Combiner Disco
5
In e te
Inverter
Service
Panel
DC
Disco
4
The challenges of string access
#1: Electrical isolation
• Easy when strings are individually fused
• Difficult in small, non-fused (2-string) residential systems
#2: Electrical connection
• Commercial combiners typically provide enough space, but safety
shields may make block alligator clip access to the buss bars
• Difficult in un-fused residential systems if the source conductors run
directly to inverter-integrated DC disconnect
Recommendation:
Every PV system should have means for isolating and connecting to
string conductors for testing purposes, typically at the combiner
box or the inverter
inverter.
Lifting conductors to isolate strings for measurement
Topics
• PV array performance verification
• Current-Voltage (I-V) curve measurement
• Array
A a performance
pe fo mance models
• I-V test setup and process
• Example I-V measurement results
• Electrical access to strings for testing purposes
• Thermography applied to PV arrays
Thermography* applied to PV
Example
p applications:
pp
• Evaluate roof integrity (aerial imaging)
• Evaluate temperature differences across an array
• Identify hotter or cooler strings or modules (aerial imaging)
• Identify hotter/cooler regions within a PV module
Cells
Ribbon bonds
J boxes
J-boxes
* IR iimaging
i and
d ttemperature
t
measurementt
Temperature variation across an array
From D.L. King, et al, Photovoltaic Array Performance Model, Sandia, 2004
Aerial Thermography
860kW System
y
at Portland Habilitation Center,, byy Dynalectric
y
Image courtesy of Oregon Infrared
http://www.oregoninfrared.com/
Conclusion
• Verification of PV array DC performance is a best practice for PV system
commissioning and ongoing maintenance.
maintenance
• I-V measurements compared with a detailed PV model gives the best
insight into performance and aging characteristics of a PV array.
• I-V testing is a natural troubleshooting tool for responding to
performance alerts,, thermographic
p
g p
surveys,
y , and other indicators.
• The automated nature of I-V measurements means more valuable
data, in less time, with fewer errors.
• The industry should adopt means, in inverters and combiner boxes, for
isolating and connecting to string conductors, for testing purposes.
New Tools for Verifying
PV Array Performance
f