NREL is a national laboratory operated by the Alliance for Sustainable Energy, LLC for the US Department of Energy. The document discusses a new method for monitoring series resistance in photovoltaic systems in real-time without calibration. By observing open-circuit voltage values at different irradiances and temperatures, the method can calculate a time series for series resistance that provides insight into potential problems. This automated monitoring could help issue alerts for failures or conditions that increase fire risks like degraded connections.

1. NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.
Applying the principles of suns-Voc
to PV system monitoring
Michael G. Deceglie
Timothy J Silverman
Sarah R. Kurtz

2. 2NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Goal: Rs monitoring
• Beyond performance indices - automated diagnostic alerts
• Provides actionable intelligence about incipient problems such
as failing contacts or connections
• Increases in Rs associated with fire risks
opped by opening the series circuit in which they are
curring.
rallel arcs occur when two different dc polarities
me in close proximity. In a PV module, a parallel arc
n occur due to a ground fault. Parallel arcs are more
fficult to detect and much more difficult to stop, as
e current flow is either directly from plus to minus or
rough a ground loop.
terial selection or module design is going to prevent a
rom catching fire once an arc is sustained because of
mely high temperatures within an arc.
Resistive heating of solder bonds connecting two ribbons
ame cell.
Fig. 6. An arc across the broken interconnect on a cell.
The most likely place for an open circuit to occur
module is at the output leads because:
• These connections are usually not protected by by
diodes;
No material selection or module design is going to prevent a
module from catching fire once an arc is sustained because of
the extremely high temperatures within an arc.
Fig. 4. Resistive heating of solder bonds connecting two ribbons
from the same cell.
Fig. 5. An arc caused by failure of the solder bond that attached an
output lead to the module circuitry.
IV. IMPROVING MODULE DESIGN AND CONSTRUCTION
Wohlgemuth and Kurtz, PVSC 2012

3. 3NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Automated Rs monitoring
• Avoid hands-on, equipment-intensive
measurements
• Automatically adapt to other changes in the module
o e.g. change in shunt resistance
NREL Image 6326716

4. 4NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Background: Suns-Voc
• Measure irradiance
dependent Voc values
• Construct an Rs free IV
curve
• Rs from Ohm’s law:
• More accurate than diode
model curve fitting1
• Most often used for
measuring cells
• Recently demonstrated
outdoors on modules and
systems2
1Bowden and Rohatgi, PVSEC 2001 v.2 p.1802
2Forsyth et al., PVSC 2014, p.1928
Rs-free curve
Actual curve
ΔV

5. 5NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Operating principles
Irradiance E0
Irradiance Eʹ
Isc,0
Translate Voc from low irradiance to:

6. 6NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Operating principles
Translate Voc from low irradiance to:

7. 7NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Real-time series resistance (RTSR)
• Observe:
o Operating point current I0
and voltage V0 at
irradiance E0
o Low-irradiance Voc values
• Calculate target irradiance, Eʹ
Irradiance E0
Isc,0
(V0, I0)

8. 8NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Real-time series resistance (RTSR)
• Observe:
o Operating point current I0
and voltage V0 at
irradiance E0
o Low-irradiance Voc values
• Calculate target irradiance, Eʹ
• Find recent Vʹoc at Eʹ
• Calculate series resistance:
Irradiance E0
Isc,0
(V0, I0)
Irradiance Eʹ

9. 9NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Real-time series resistance (RTSR)
• Module will generally be cooler
at Eʹ
• Temperature coefficient
depends on irradiance
• Goal: adaptive, calibration-free
Irradiance E0
Isc,0
(V0, I0)
Irradiance Eʹ

10. 10NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Handling temperature
10 20 30 40
17
18
19
20
21
Module temperature (°C)
V'oc(V)

11. 11NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Handling temperature
• Look at recent Voc
measurements near
Eʹ
o Here, recent = 1 week
• Regress Voc vs. T
• Extrapolate

12. 12NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Handling temperature
• Capture Voc vs. T at relevant irradiance
• Automatically adapt to changes in the module
• Look at recent Voc
measurements near
Eʹ
o Here, recent = 1 week
• Regress Voc vs. T
• Extrapolate
Deceglie et al. IEEE J. PV 5:6 p1706 (2015)

13. 13NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Irradiance dependence of Voc vs T
• Voc temperature coefficient depends on irradiance
• Important to extract the correct value for the target irradiance
• Using the 1-sun value causes ~20% error in series resistance
NREL field data for Si module showing Voc
temperature coefficient for varying irradiance

14. 14NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Typical target irradiance
• For 1-sun operation, typically comparing Voc points from 50–80 W/m2
• Most of the time, irradiance is all diffuse at these times, limiting
effects of shade
NREL field data for Si module target irradiance
for Voc values for given operating irradiance

15. 15NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Example application
• Public dataset
• IV curves collected on
modules deployed in Cocoa
FL, Eugene OR, and Golden
CO
• IV curves measured at 5-
minute intervals
• Meteorological monitoring
• RTSR: Use subset of data,
not full IV curve
Marion et al. NREL Report: TP-5200-61610
Contact: bill.marion@nrel.gov

16. 16NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Generating an Rs time series
For each operating
point:
• Calculate target
irradiance
• Regress Voc vs. T
(at target
irradiance) for prior
week
• Calculate Rs
→ Time series of Rs

17. 17NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Generating an Rs time series
For each operating
point:
• Calculate target
irradiance
• Regress Voc vs. T
(at target
irradiance) for prior
week
• Calculate Rs
→ Time series of Rs
Rs losses, aggregated weekly
CIGS1
CIGS2
Si

18. 18NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Rs time series
CIGS1
CIGS2
Si
Ohmic loss
CIGS2
Si
First and final 4 weeks of Rs losses
Beginning End
Can identify increases in Rs
Rs losses, aggregated weekly
CIGS1
CIGS2
Si
Deceglie et al. IEEE J. PV 5:6 p1706 (2015)

19. 19NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Validation with IV curves
Weekly fractional ohmic loss
IV curves
CIGS1
Can identify increases in Rs
Beginning
End
Si
Deceglie et al. IEEE J. PV 5:6 p1706 (2015)

20. 20NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Adaptive capability
Weekly fractional ohmic loss
Adapts to other changes in module
IV curves
CIGS1
CIGS1
CIGS2
Si
CIGS2
Beginning
End
Deceglie et al. IEEE J. PV 5:6 p1706 (2015)

21. 21NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Implementation
• Readily applicable at
module level
o Integrated electronics
o Power optimizers
• Possible at string-
level
o Challenges:
– Variation in
mounting angle
– Partial shading
NREL Image 6326716

22. 22NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Conclusion – real time series resistance
• Automated,
module/array
integrated
• Adaptive,
calibration-free
• Benefits
o Beyond performance
indices
o Diagnostic
o Early alert for fire
risks / connection
problems
CIGS1
CIGS2
Si
stopped by opening the series circuit in which they are
occurring.
2. Parallel arcs occur when two different dc polarities
come in close proximity. In a PV module, a parallel arc
can occur due to a ground fault. Parallel arcs are more
difficult to detect and much more difficult to stop, as
the current flow is either directly from plus to minus or
through a ground loop.
No material selection or module design is going to prevent a
module from catching fire once an arc is sustained because of
the extremely high temperatures within an arc.
Fig. 4. Resistive heating of solder bonds connecting two ribbons
from the same cell.

23. 23NREL M. Deceglie PV Performance Modeling and Monitoring Workshop 2017
Acknowledgements
• Thank you for insightful discussion:
o Ron Sinton (Sinton Instruments)
o Chris Deline (NREL)
o Bill Marion (NREL)
• Contact: michael.deceglie@nrel.gov
• Further reading:
o Deceglie et al. IEEE J. PV 5:6 p1706 (2015)
• This work was supported by the U.S. Department of Energy under
Contract No. DE-AC36-08GO28308 with the National Renewable
Energy Laboratory. Funding provided U.S. Department of Energy
Office of Energy Efficiency and Renewable Energy Solar Energy
Technologies Office.