New Tools for PV Array Commissioning and Troubleshootingresources.solmetric.com/get/Solmetric-PVA-Webinar-Feb-9-2012.pdf · New Tools for PV Array Commissioning and Troubleshooting

New Tools for PV Array

Commissioning and Troubleshooting

Megger®

MIT430

Solmetric

PVA-600

Paul HerndayApplications [email protected] 707-217-3094

February 9, 2012

Solmetric Solutions

www.solmetric.com

Solar PV Installation Life Cycle

Topics

• PV array performance verification

• Introduction to I-V curves

• The Solmetric PVA-600 PV Analyzer

• Demo of the PV Analyzer user interface

• Commissioning & troubleshooting PV array performance

• Meg testing with the Megger® MIT430

Array Performance Test MethodsFor Startup/Commissioning/Checkups/Service Alarms

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

“I often tell students in my classes to learn to think like a PV array.

Thinking like a PV array requires understanding the I-V curve and how it

changes based on ambient conditions and array problems. An I-V curve

tracer is the best way to gain an understanding of these changes, since it provides a graphical representation of the array operating characteristics.”

“I-V curve tracing is the most informative measurement that can be performed

on a PV module or array.”

I-V Curve Tracing is a PV Best Practice

David KingDK Solar Works

Developer of the Sandia PV Array Model at Sandia National Labs

SolarPro, Aug/Sep 2011

Bill BrooksBrooks Engineering

SolarPro, Aug/Sep 2011

� Fast – it’s a single electrical connection & a single measurement

� The most comprehensive PV measurement possible

� 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

� Troubleshoot more efficiently

� Sort out module versus inverter issues

� Provide convincing data for module warranty claims

� Keep arrays producing maximum energy

Maintaining PV Systems (O&M, Asset Management)

PV Analyzer Users

EPC organizations

System Integrators

Consulting Engineers

Training OrganizationsTechnical colleges

Module Manufacturers

Inverter Manufacturers

IBEW

Training Centers

O&M Companies

Electrical contractors

Topics







Solar Cell

1

2

3

3

Cells are in series �� a single cell can be a bottleneck.

A 12-module string may have

12 x 72 = 864 cells in series.

Charge generation

Charge separation

Charge collection

1

2

3

Photon

Top layer

Bottom layer

Fingers

Backside

What is an I-V curve?And how is it measured?

AdjustableLoad

Load can be•Resistive•Capacitive•Electronic

MeasurecurrentMeasure

voltage

Current

Voltage

Built-in PV models mean

user can instantly check

performance against

expectations for the

existing irradiance and

temperature.

I-V and P-V* CurvesExpect this shape for healthy cells, modules, strings, arrays

Current

Voltage

Isc

Voc

I-V curve

Vmp

Imp

Power

P-V curve

Pmax

*P-V curve is calculated from the measured I-V curve

‘Stacking’ PV Modules into Arrays

This ‘building block’ analogy 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 curveCurrent

Voltage

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.

Topics







• Complete performance measurement up to 600V, 20A

• Comparison to built-in models

• Wireless interface to your PC

String measurement showing I-V and P-V curves,

and comparison of I-V curve with model (5 dots).

Solmetric PVA-600 PV Analyzer

Courtesy of:Integrated Energy SystemsPittsburg Unified School DistrictSage RenewablesStellar Energy Solutions

Example Measurement Setup


Courtesy of Chevron Energy Solutions © 2011

PVA-600 Block Diagram(simplified)

Controller&

Wireless

I sense

V senseC

Control button withLED indicator

Battery charging connector

(1 of 3)

• Capacitive load method (3 capacitor values, auto-selected)

• Electrically isolated. Ground lead is not required

• Protection for over-voltage, -current, -temperature, & reverse polarity

PV TestLeads

NEMA 4X FG Enclosure

Wireless Sensor KitIrradiance & temperature sensors

Irradiance

transmitter

Receiver (USB)

Temperature

transmitter

K-type

thermocouple

Omega Part #

5SRTC-GG-K-

30-72

.

Comparing PVA measurement with

resistive load method

Resistive

load

Resistive

load

Switch

Data points from the

resistive load method

Method:•Clear sky, solar noon.

•Quickly alternate the two methods.

•New resistance at each load point.

PV Models in the PV AnalyzerPredict PV array performance for immediate comparison

• Sandia National Labs PV Array Model

– Most comprehensive (30+ parameters)

– ~500 PV modules

• 5-Parameter Model

– Developed at U. Wisconsin, used by CEC for NSHP program

– ~5000 PV modules

– Less reliable results for amorphous silicon technologies

• Simple Datasheet Model (predicts Pmax)

– 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.

Topics







Topics





• Commissioning & troubleshooting PV arrays



I-V Measurement SetupExample: Measuring strings at a combiner box

Mounting the Thermocouple

•Use metalized HVAC tape or Kapton tape for high temperature stability

•Press firmly to assure intimate contact between TC and backside

•Fold a tab so you can remove the tape

Suggested replacement TC:Beaded K-typeOmega Part No.5SRTC-GG-K-30-72

Test ProcessExample: Measuring strings at a combiner box

Hardware setup (do once at each combiner box):

1. Move the sensors (if necessary)

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

Example Measurement Setup860kW 7-inverter system

Courtesy of Portland Habilitation Center and Dynalectric Oregon

• Open the DC disconnect

for the sub-array you

want to test.

Example Measurement SetupCombiner boxes for one inverter


• Locate the

combiner box

Example Measurement SetupCombiner box wiring


• With a clamp-meter,

verify that the load has

been disconnected.

• Then lift all of the

fuses.

Example Measurement SetupInsert a single fuse to test the corresponding string


• Clip the PV Analyzer to

the buss bars.

• Push down one fuse at

a time and make I-V

curve measurements.

• View and save results.

How Data Is Stored

Saving data in the Array Tree

allows the I-V Data Analysis Tool

to:

Created

when you

install the

PVA PC

software

• Automatically analyze your data

• Identify non-conforming strings

• Provide convincing charts for your

commissioning report

Demo of I-V Data Analysis Tool

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

• Clear sky (for high, stable irradiance)

– Height of I-V curve varies directly with irradiance

– I-V curve changes shape at low irradiance

– At low light, comparison to models or STC is much less accurate

• 4 hour window centered on solar noon*

– Avoids the low-light issues mentioned above

– Avoids any major angle of incidence and spectral effects

• 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:

Inconsistent conditions (clouds, wind) cause more variation in PV measurement results,

making strings look less consistent and making it harder to spot performance issues.

800W/m2

Problem Sky ConditionsCause variability in PV measurement results

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.

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

String of Field-aged, Early TF ModulesDegraded fill factor, lower output power

Array-as-sensor mode for viewing relative changes in curve shape

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

Example of a series resistance failureinside a module J-box

Probably failure mode:

Heat cycling � bond degradation � resistive heating

Dropped Cell String

• Shorted bypass diode, or

• Mismatch causing diode to turn on

when current starts flowing

Partially shaded residential arrayMeasure the single string mounted along lower edge of roof

I-V Curve of the partially shaded stringSingle string mounted along lower edge of roof

Approximately 40% reduction in string’s output power

What bypass diodes doExample: Normal current flow in an un-shaded 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.

Shade one cell

+

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.

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

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

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.

What is the PV Analyzer all about?

•Single measurement for each string

•Most complete performance measurement possible

•Measures each string’s max power independently

•Built-in PV models give instant performance check

•Does not require bringing the inverter online

•Software tool automates data analysis

•Helps us learn to “think like a PV array”

I

V

Commission and troubleshoot PV arrays faster and better

Topics







Insulation Resistance Testing of PV ArraysWhere are we?

• Best practice is to test all systems during commissioning and maintenance

• Today, meg testing is not done on all commercial systems, and is rare in residential systems.

• Source circuits are often tested in aggregate

– Individual strings are tested only if aggregate resistance is low

• Methods for subarray test are not standardized or widely understood

– Best reference is the “re-wrenches” blog (Home Power Magazine)

• Trend is toward specifying meg testing for commercial systems

• Insulation problems will be much more common as PV systems age

Preparation: Open the DC Disconnect � Lift string fuses � Lift negative feeder cable(s)

Lift

Meg Testing at the Subarray Level

Lift

Example assumes a

negative-grounded array

Lift

Combiner box

Preparation: Open the DC Disconnect � Lift string fuses � Lift negative source cable

Lift Lift

Meg Testing at the String Level

Example assumes a

negative-grounded array

Lift

Saving Meg Test Data in the PV Analyzer

1

2

3

4

5

6

500

500

500

500

500

500

1.604

1.003

1.427

1.769

1.191

0.826

C1 S1

C1 S2

C1 S3

C1 S4

C1 S5

C1 S6

Me

g

Te

st

5-Minute PV Analyzer Training Videoshttp://www.solmetric.com/videos1.html

Free I-V Curve Posterhttp://www.solmetric.com/specialoffers.html

SolarPro Magazine, Aug/Sep 2011

New Tools for PV Array

Commissioning and Troubleshooting

Paul Hernday

Applications Engineer

[email protected]

cell 707-217-3094

February 9, 2012

Documents

New Tools for PV Array Commissioning and Troubleshootingresources.solmetric.com/get/Solmetric-PVA-Webinar-Feb-9-2012.pdf · New Tools for PV Array Commissioning and Troubleshooting