New Tools for PV Array Commissioning and...

New Tools for PV Array

Commissioning and Troubleshooting

Megger®

MIT430

Solmetric

PVA-600

Paul HerndayApplications [email protected] 707-217-3094

April 5, 2012

Audio is available by

telephone or your computer

Solmetric Solutions

www.solmetric.com

Sales CallPreliminary Site survey

Preliminary design

Proposal Contract

StartupPerformance Verification

Performance Monitoring Service

Solar PV Installation Life Cycle

Detailed Site Survey

Final Design

Installation Checkout

Topics

• I-V curve tracing and PV performance verification

• Introduction to I-V curves

• The Solmetric PVA-600 PV Analyzer

• Demo of the PV Analyzer user interface

• Features of the new PV Analyzer software

• 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 – a single electrical connection & a single measurement

� Most complete test method

� 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)

Solmetric PV Analyzer Users

EPC organizations

System Integrators

Consulting Engineers

Training OrganizationsTechnical colleges

IBEW

Training Centers

O&M Companies

Electrical contractors

I

V

Module Manufacturers

Inverter Manufacturers

Topics








Solar Cell

1

2

3

3

Ideally, the same current flows through series cells.

In practice, any 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

Incoming 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

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








Easy model setup

-New Project Wizard-Site information from the web

Easy data management

-Database replaces existing folder tree-Array Navigator™ graphical interface (touch to save or recall data)-Outputs data in the traditional csv format (compatible with Data Analysis Tool)

Translation to STC (built in)

History tab

-Shows tabular values for the last 20 measurements

Archiving of Meg Test data

-Saves manually entered meg test data to the new database(NEW - Solmetric now offers the Megger MIT-430 Insulation Tester)

Features of the new PVA software

Traces tab

System name & path screen

Performance model screen

Site info screen

Roof Measurement Tool

Latitude – manual entry

Longitude – manual entry

Array Navigator™ screen

Add inverter screen

Array Navigator screen

Saving a meg test result

Meg Test tab

Topics






• Commissioning & troubleshooting PV arrays


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.

Test 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

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 exported

This format makes it easy to locate

particular data.

It also allows automating the

analysis of large amounts of data. Created

when you

install the

PVA PC

software

• Identify non-conforming strings

• Generate 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

– 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 issues

• 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 for any test method or meter, making strings look

less consistent and making it harder to spot performance issues.

800W/m2

Problem Sky Conditions

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

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

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

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.

IR signature of bypassed cell string

Image 383

22 C 45 C

Cardboard shading a cell

Measured usingthe FLIR i7

infrared camera

What is the PV Analyzer all about?

•Single measurement for each string

•Most complete performance measurement possible

• Independent max power measurement for each string

•Built-in PV models give instant performance check

•Don’t need to bring inverter online

•Automated data analysis

•Teaches us to “think like a PV array”

I

V

Testing PV array performance faster and better

Next topic: Insulation resistance testing

Topics






• Commissioning & troubleshooting PV arraysrays


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 it is less common 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

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

April 5, 2012

New Tools for PV Array Commissioning and...

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