Commissioning and O&M Applications of the Solmetric PV ...resources.solmetric.com/get/Solmetric-PVA-Webinar-Jan-10-2013.pdf · Your PC WUSB2 WUSB1 I-V Measurement Unit ... Summary

Commissioning and O&M Applications of the Solmetric PV Analyzer

Paul Hernday

Senior Applications Engineer

[email protected]

cell 707-217-3094

January 10, 2013

mailto:[email protected]

Topics

• Introduction to I-V curves and the PV Analyzer

• Introducing free Version 2.0 PVA Software Upgrade

• Commissioning PV arrays

• Troubleshooting PV arrays

• Meg testing with the Megger® MIT430

What Is An I-V Curve? Those combinations of I,V at which the module can be loaded

• Load can be resistive, capacitive, or electronic

• Curve can be measured from either direction

Current

(I)

Cu

rre

nt

Voltage

(I, V)

Voltage

(V)

+

-

I-V Curve for Resistive Loads The (I, V) combinations at which the PV module(s) can be loaded

Cu

rre

nt

Voltage

Isc

Voc

I-V curve of PV module

I-V curve of resistor

Higher resistance

Lower resistance

How Are I-V Curves Measured? Concept

Current

(I) Voltage

(V)

Cu

rre

nt

Voltage

(I)

(V)

(I, V)

Adjust the

load

1

3 Plot the

point

Read I & V

2

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

Cu

rre

nt

Voltage

Isc

Voc

I-V curve

Vmp

Imp

Po

we

r

P-V curve

Pmax

Max power point

I-V Curve Abnormalities

Any reduction of the knee of the curve

means reduced output power.

Cu

rre

nt

(A)

Voltage (V)

Isc

Voc

Increased

slope

Reduced

slope Mismatch losses

(incl. shading)

Normal I-V curve

Reduced

current

Reduced

voltage

Conventional Isc and Voc measurements do not reveal many of these effects

Solmetric PV Analyzer Users

EPC organizations

System Integrators

Consulting Engineers

Training Organizations Technical colleges

IBEW

Training Centers

O&M Companies

Electrical contractors

Module Manufacturers

Inverter Manufacturers

Array

Irradiance sensor

Module backside

temperature sensor

Your PC WUSB2

WUSB1

I-V Measurement

Unit

Built-in PV models

Irradiance & temperature

Module, tilt, orientation…

5 dots predict the shape of the curve

Wireless

How Performance Prediction Works

Data analysis and reporting are automated

Screen

shot

Wireless Measurement Network Built-in Analysis

• The five dots indicate the expected curve shape

• The dots appear whenever an advanced performance model (Sandia or 5-Parameter) is selected

• The middle red dot is the predicted max power point

• The yellow dot on the P-V curve marks the peak of that curve, and is not a prediction of the PV model.

Expected I-V Curve Shape determined by the advanced PV models

Dot 1 (Isc, 0)

Dot 2 (I, Voc/2)

Dot 3 (Imp, Vmp)

Dot 4 (I, [Vmp + Voc]/2)

Dot 5 (0, Voc)

1

5

3 2

4

I-V Measurement Setup Example: 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.

Example Measurement Setup

Courtesy of Chevron Energy Solutions © 2011

Wireless Sensor Kit Irradiance & temperature sensors

Irradiance

transmitter

Receiver (USB)

Temperature

transmitter

K-type

thermocouple

Omega Part #

5SRTC-GG-K-

30-72

.

Deploying irradiance & temperature sensors

Guarantee that the irradiance sensor

is in the plane of the array by placing it

on a module (that’s not in the string

you are measuring) or mounting it to a

corner of a module.

Mount the thermocouple 2/3 of the way

between the corner and center of a

module. Use high-temperature tape (eg

1-3/4 inch Kapton dots**). Press TC into

contact with backside. ** MOCAP MCD-PE 1.75 poly dot

~$80/roll of 1000 dots

[email protected]


Summary of PV Analyzer benefits

•Single connection at combiner box

•Single measurement for each string

•Don’t need to bring inverter online

•Built-in PV models for instant performance check

•More effective troubleshooting

•Automated data analysis and reporting

•Most complete performance measurement possible

• Independent Pmax measurement for each string

•Teaches us to “think like a PV array”

Testing PV array performance faster and better

Next topic: Insulation resistance testing

Lower cost

Satisfied clients

Topics

• Brief introduction to I-V curves and the PV Analyzer





Faster setup

-Project Wizard

-Web tool for azimuth, latitude, longitude

Large displays of Irradiance & temperature on every measurement screen

Array Navigator™ for touch-based data save and recall

Translates I-V curves to STC

5000 more module models, and automatic update of equipment databases

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

(Solmetric now offers the Megger MIT-430 Insulation Tester)

Report generation capability via the updated I-V Data Analysis Tool (optional

accessory for analyzing large quantities of test data)

Features of v2 PVA PC software

Array Navigator™

•Represents the PV system architecture

•Created by the Project Wizard

•Touch a string location to save or recall data

•Also contains information needed by the PV

model (eg wire gauge and lengths)

•Currently limited to sequential labeling

schemes (numbers or letters)

Array Navigator™

The Project File

• Contains array details, performance model, and measurement data

• Easy to share between offices

• Foundation for future advanced analysis and reporting capabilities

Version 2.X Software

(legacy)

(legacy)

Project file

Saving a measurement result with V2.0

v1.2.4440 v2.0.5644

5param: 4,991 8,722

Sandia: 517 554

Simple: 0 11,454

Unique modules: 11,821

Additions to the Module Database

Roughly 2,178 module models

exist with only the simple model

(11,454 - 8,722 - 554).

Live Software Demo • Demo uses saved data

• Screen images are also shown in the following slides

Traces tab

Project Wizard, Step 1 Notes & Info

Project Wizard, Step 2 Performance Model

Project Wizard, Step 3 Site Info

Roof Measurement Tool

Autofill icon

Project Wizard, Step 4 Array Navigator™

Autofill Tool

System Tree

Saving a test result

Traces screen

Environmental Inputs controls

Table screen

Verify screen

History screen

Meg Test tab

Topics






• High irradiance

– I-V curve shape changes significantly at low (<400W/m2) irradiance

– Translation to STC introduces more error if translation is large

• Clear sky

– Stable irradiance means better correspondence between I-V, irradiance, and temperature data, and thus better analysis statistics

• 4 hour window centered on solar noon*

– For high irradiance and low error from spectral and reflective effects

• Low wind

– For stable temperature and thus better analysis statistics

Recommended environmental conditions for performance testing with any type of instrument

http://www.esrl.noaa.gov/gmd/grad/solcalc/

*Solar Noon Calculator:

In practice, sometimes you will need to measure

under less than optimum conditions!!

http://www.esrl.noaa.gov/gmd/grad/solcalc/

Unstable sky conditions

Why do we want stable conditions?

• Meaningful analysis of ANY array performance requires that irradiance and temperature

(the sensor data) be measured at the same time as the array output.

• When irradiance and temperature are rapidly changing, a delay between the sensor and

array output measurements translates into sensor error, which introduces random variation

in the PV model’s performance prediction. This affects the consistency of Isc, Pmax, and

Performance Factor.

• The errors are greater when the time delay is greater, and when the conditions are

changing more rapidly. Manual measurement and entry of irradiance or temperature values

suffer most from this issue, because of the time required for measurement and data entry.

• Accuracy & precision cannot be regained by translating measured I-V curves to STC.

• The Solmetric Wireless Sensor Kit keeps this delay minimal, for most accurate performance

analysis.

Example Measurement Setup

I-V Measurement Setup Example: 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.

Measurement Process 860kW 7-inverter system

Courtesy of Portland Habilitation Center

and Dynalectric Oregon

1. Open the DC disconnect for the

sub-array you want to test.

Measurement Process



2. Locate the

combiner box

Measurement Process



3. Lift all of the fuses

Measurement Process



5. Push down one fuse

at a time, take the I-V

trace, view and save

results. Takes ~ 10-15

seconds per string.

4. Clip the PV Analyzer

to the buss bars

Displays Generated by the I-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 accessory $95

• A histogram worksheet is created for each performance parameter, revealing patterns in the data

• Each histogram represents the frequency of different values of a measured parameter, across a group of string measurements

• In this example, the spread of Pmax values (upper graph) is largely explained by variation of irradiance (lower graph)

Histograms

Pmax

Irradiance

• Fill Factor is a very useful diagnostic tool (details in the next slides)

• Compared with Pmax or Isc, Fill Factor has a weak dependency on irradiance, making it a useful metric even under conditions of rapidly changing irradiance

• In this example, notice the clean, bell-shape of the fill factor (upper graph), compared with the irradiance distribution (upper graph)

Fill Factor Histogram

FF

Irradiance

Topics






Max power point

I-V Curve Signatures of PV Problems

Any reduction of the knee of the curve

means reduced output power.

Cu

rre

nt

(A)

Voltage (V)

Isc

Voc

Shunt

losses*

Series

losses**

Mismatch losses

(incl. shading)

Normal I-V curve

Reduced

current

Reduced

voltage

Conventional measurements do not reveal many of these effects.

Isc

Voc

Useful diagnostics Fill Factor, Current Ratio, Voltage Ratio

Cu

rre

nt

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 ratio Imp/Isc

Voltage ratio Vmp/Voc

Imp

Vmp

Max Power Point

String of Field-aged, Early TF Modules Degraded 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 example Anomalous slope in string I-V caused by single high-resistance module

Example of a series resistance failure inside 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

Cu

rre

nt

Voltage

Isc

Voc

Po

we

r

Typical 72-cell grid-tie module

+

Cell String

Cell String

Bypass

Diodes

Cell String

Shade One Cell

+

Cell String

Cell String

Cell String

Bypass

Diodes

Bypass diode turns on when the shaded cell(s) can

no longer pass as much current as the non-shaded

cells.

Shade 2 cells in the same cell-string Single 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-strings Single module with 72 cells and 3 bypass diodes

The same

amount of

shade,

oriented

differently,

drops 2/3 of

PV module

voltage and

power.

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

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

Approximately 40% reduction in string’s output power

Selective shading troubleshooting technique

• Traditional troubleshooting techniques can isolate a problem to a given

string, but it is often difficult to identify the failing module

• Selective shading and I-V curve tracing can often identify the failing

module without disconnecting modules from one another

• This greatly reduces the cost of troubleshooting

• Selective shading involves measuring the string repeatedly, with a

different module shaded each time

• Comparing the results leads us to the failed module

Troubleshooting using selective shading to identify a bad module

Photo courtesy of Harmony Farm Supply and Dave Bell

• Selective shading causes the

shaded module to be

bypassed, removing its

influence on the I-V curve

shape

• If there are N modules in a

string, measure the string N

times, with a different module

shaded each time. Go from

one end of the string to the

other to make the analysis

easier.

• All of the traces will look alike,

except for the case where the

failing module was shaded

Cu

rre

nt

Voltage

Isc

Voc

Entire string with no

applied shade (our

starting point)

Any good module shaded

Bad module shaded

Troubleshooting using selective shading to identify a bad module

The method can also be used to identify

a bad cell string in a single module

Summary of PV Analyzer benefits

•Single connection at combiner box

•Single measurement for each string

•Don’t need to bring inverter online

•Built-in PV models for instant performance check

•More effective troubleshooting

•Automated data analysis and reporting

•Most complete performance measurement possible

• Independent Pmax measurement for each string

•Teaches us to “think like a PV array”

Testing PV array performance faster and better

Next topic: Insulation resistance testing

Lower cost

Satisfied clients

Topics






Insulation Resistance Testing of PV Arrays Where are we?

1. The industry is trending toward the best practice of meg testing all

systems during commissioning and at maintenance intervals.

2. Meg testing is not yet being done on all commercial systems, and is

rarely done on residential systems.

3. Source circuits are often tested in aggregate, with string-level testing

if the aggregate resistance is low.

4. Methods for sub-array test are not standardized or widely

understood. Best reference is Bill Brooks’ ‘re-wrenches’ post.

5. In general, string or sub-array resistance values are compared for

consistency rather than held to a specific spec.

6. PV module leakage is usually the dominant effect.

7. 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

Upcoming Webinars

Tools for Solar Site Assessment and Design January 8, 2013 10am PST

To register:

http://www.solmetric.com/webinar.html

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

SolarPro Magazine, Aug/Sep 2011

Paul Hernday

Senior Applications Engineer

[email protected]

cell 707-217-3094

January 10, 2013

Commissioning and O&M Applications of the Solmetric PV Analyzer


I-V Curve The (I, V) combinations at which the PV module(s) can be loaded

Cu

rre

nt

Voltage

Isc

Voc

I-V curve of PV module

I-V curve of resistor

Higher resistance

Lower resistance

Documents

Commissioning and O&M Applications of the Solmetric PV ...resources.solmetric.com/get/Solmetric-PVA-Webinar-Jan-10-2013.pdf · Your PC WUSB2 WUSB1 I-V Measurement Unit ... Summary