Verifying PV Array Performance with the

Solmetric PVA-600 PV Analyzer March 18, 2011 Paul Hernday Applications Engineer [email protected] cell 707-217-3094

Solmetric Solutions

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

What Is Performance Verification? Acceptable limits

Performance Measurement

Performance Standard *

* A model, a specification, or a reference device (eg another PV string, in a troubleshooting situation)

Performance Verification Methods Inverter readout

String DC measurements

Basic

String I-V curve measurements

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

Benefits of I-V Curve Performance Testing Commissioning New PV Systems
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

Maintaining PV Systems (O&M, Asset Management)
Assure nervous owners that arrays are working properly
Troubleshoot more efficiently
Sort out module versus inverter issues
Provide convincing data for module warranty claims

Benefits of I-V Curve Performance Testing Enhancing Your Quality Assurance Capabilities
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

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 Curve Full I-V curve means more diagnostic value

I-V curve Isc Imp Current

Max power point

Voltage

Vmp

Voc

Array of Cells, Cell Strings or Modules I

Parallel

I-V building blocks

Total (net) I-V curve Series

V

This ‘building block’ graphic is useful in troubleshooting arrays

P-V Curve Calculated from the measured I-V curve

Pmax

Power

Current

Isc Imp

I-V curve

P-V curve

Voltage

Vmp

Voc

Fill Factor A measure of the square-ness of the I-V curve Max Power Point (Imp, Vmp)

Current

Isc Imp

Area A = Imp x Vmp (watts)

Voltage

Fill Factor =

Area A Area B

=

Area B = Isc x Voc (watts)

Vmp Voc

Imp x Vmp (watts) Isc x Voc (watts)

=

aSi:

0.50 – 0.70

xSi:

0.75 – 0.85

GaAs: 0.85 – 0.9

I-V Curve Signatures of PV Problems

Current (A)

Isc

Normal I-V curve

Any reduction of the knee of the curve means reduced output power.

Max power point

Shunt losses*

Series losses** Mismatch losses (incl. shading)

Voltage (V)

Voc

All of these impairments are easily observed using the PV Analyzer, but are not observed when measuring only Isc and Voc.

I-V Measurement Methods PV module, string, or sub-array

Current sense Voltage (simultaneous sense data acquisition)

Load can be: •Electronic •Resistive •Capacitive

Current

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. Voltage

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

Solmetric PV Analyzer: PVA-600 • 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.

Touch Screen User Interface PC is user-supplied (Samsung Q1 Ultra shown here)

Showing the Verify tab screen. Performance Factor (%) =

Measured Pmax Predicted Pmax

x100

Optional wireless sensor kit Irradiance & temperature sensors

PVA-600 Block Diagram (simplified) Battery charging connector (isolated)

CPU & wireless module

C (1 of 3)

V sense

I sense Control button w LED (isolated)

• 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 Analyzer To 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 Measuring Category CAT III, 600V

*Strings of high-efficiency modules should be measured singly, not in parallel

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 Setup Example: measuring strings at a combiner box

Test Process Example: 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 String measurement (repeat for each string): 1. Insert a string fuse 2. Press “Measure” 3. View and save results 4. Lift the fuse

15 seconds

Portland Habilitation Center PV System 860kW 7-inverter system by Dynalectric Oregon

Testing at combiner boxes 860kW System at Portland Habilitation Center, by Dynalectric Oregon

Combiner Boxes (two per inverter) 860kW System at Portland Habilitation Center, by Dynalectric Oregon

Combiner Box 860kW System at Portland Habilitation Center, by Dynalectric Oregon

Testing individual PV strings 860kW System at Portland Habilitation Center, by Dynalectric Oregon

Performance analysis Using the optional I-V Data Analysis Tool Distributions • Pmax • Fill factor • Imp/Isc • Vmp/Voc • Isc, Voc, etc Array Tree

Table of key performance parameters

Overlay plots of I-V curves

Table of Key Performance Parameters From I-V measurement of a population of PV strings

Example Histogram From I-V measurement of a population of PV strings 7 6

Frequency

5 4 3 2

2100

2050

1950

0

2000

1

Pmax (Watts)

Width is ~ +- 3.5%. Irradiance sensing was not used in this example.

Example I-V Curve Overlay From I-V measurement of a population of PV strings 7

Current (Amps)

6 5 4 3

Effect of shade from nearby fixture.

2 1 0 0

100

200

300

Voltage (Volts)

400

500

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 curves Red 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

Troubleshooting example Comparison of typical (blue) and atypical (red) I-V curves 8 7

Current - A

6 5 4 3 String 4B14

2

String 4B15

1 0 0

50

100

150

200 Voltage - V

250

300

350

400

Troubleshooting example Problem isolated to failed module

Irradiance changed between measurements

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 Diodes Typical 72-cell PV Module (bypass diodes shown)

+

Partial shading Inverter must find and track the right peak

Max Power (until the shade pattern changes!)

Current

Power

Isc

Voltage

Voc

Partially shaded residential array Single string, along lower edge of roof

I-V Curve of the partially shaded string Single 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-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

Shadow of leafless tree branch Single module with 72 cells and 3 bypass diodes

Verifying PV Array Performance with the

Solmetric PVA-600 PV Analyzer March 18, 2011 Paul Hernday Applications Engineer [email protected] cell 707-217-3094