SolmetricSunEye

Verifying PV Array Performance

And Shade Analysis

Presented by:

Veenu shekhar, SVEL

[email protected]

Slides provided by: Solmetric

Workshop customization by: SVEL

August 26th, 2011

SEMI Workshop

Jaipur India

mailto:[email protected]

About Solmetric Corporation

Leading tool maker of:

Company is based in Sebastopol, CA, USA

About SVEL

Bring technology driven products for the Indian solar industry including Solmetric Corporation

Company is based in Santa Clara, CA, USA

Develop hands-on curriculum and toolkit to provide educational training in photovoltaic and electricity for schools & colleges. More than 100 teachers &

several hundred students have been trained in various cities in US

Hands-on educational solar tracker circuit design workshops

Austin, Texas Greenfield, MA Soledad, CA

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

Performance Verification Methods

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 (rather

than up-front incentives).

When To Test Array Performance?

• Install QA/Start-up

• Commissioning

• Periodic checkups

• Service alarms

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

Benefits of I-V Curve Performance Testing

Commissioning New PV Systems

Assure nervous owners that arrays are working properly

Troubleshoot more efficiently

Sort out module versus inverter issues

Provide convincing data for module warranty claims

Maintaining PV Systems (O&M, Asset Management)

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

Benefits of I-V Curve Performance Testing

Enhancing Your Quality Assurance Capabilities

Testing Building-Integrated PV Installations (BIPV)

„Array as sensor‟ mode derives average module temperature and

irradiance from measured IV curve, for situations where sensor

deployment is not feasible.

Topics


• I-V Curves, and How They’re Measured






1. Charge generation by photons

2. Charge separation by built-in electric field

3. Charge collection at the contacts

Photon

Essential Functions of a Solar Cell

Cu

rre

nt

Voltage

Isc

Voc

Max power point

Vmp

Imp

I-V curve

I-V CurveFull I-V curve means more diagnostic value

P-V CurveCalculated from the measured I-V curve

Cu

rre

nt

Voltage

Isc

Voc

I-V curve

Vmp

Imp

Po

we

r

P-V curve

Pmax

Fill FactorA measure of the square-ness of the I-V curve

Cu

rre

nt

Voltage

Imp

Vmp

Max Power Point (Imp, Vmp)Isc

Voc

aSi: 0.50 – 0.70

xSi: 0.75 – 0.85

GaAs: 0.85 – 0.9

Fill Factor =Imp x Vmp (watts)

Isc x Voc (watts)=

Array of Cells, Cell Strings or Modules

This ‘building block’ graphic 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 curveCu

rre

nt

Voltage

Max power point

I-V Curve Signatures of PV Problems

Any reduction of the knee of the curve

means reduced output power.

All of these impairments are easily observed using the PV Analyzer,

but are not observed when measuring only Isc and Voc.

Cu

rre

nt

(A)

Voltage (V)

Isc

Voc

Shunt

losses*

Series

losses**

Mismatch losses

(incl. shading)

Normal I-V curve

Reduced

current

Reduced

voltage

Bypass Diodes

PV Module with Bypass DiodesTypical 72-cell PV Module (bypass diodes shown)

+

Bypass Diode Operation

Vmp

+

-

4 strings of 10 modules

72 cells/module

3 bypass diodes/module

Inverter

PV System Operating at Max PowerWith no shade

5A, 0.5V

+

-5A, 0.5V

BP

Vmp

+

-

Inverter

PV System Operating at Max Powerwith shade and no bypass diode

+

-

•Shade happens

•String current drops

•Cell voltages increase

•Shaded cell sees reverse voltage & avalanches

Shaded cell

Unshaded cell

Vmp

+

-

Inverter

PV System Operating at Max Powerwith shade and bypass diode

+

-

•Shade happens

•String current drops

•Cell voltages increase

•BP diode sees forward bias and turns on

Shaded cell

BP

I-V Measurement Methods

Load can be:•Electronic•Resistive

•Capacitive

Voltagesense

Cu

rre

nt

Voltage

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.

PV module, string,

or sub-array

Current sense

(simultaneous

data acquisition)

Topics








• 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.

Solmetric PVA-600 PV Analyzer

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

Showing the Verify tab screen.

Performance Factor (%) = Measured Pmax

Predicted Pmaxx100

Optional wireless sensor kitIrradiance & temperature sensors

PVA-600 Block Diagram (simplified)

CPU &

wireless

module

I sense

V senseC

Control button w LED(isolated)

Battery charging connector(isolated)

(1 of 3)

• 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 AnalyzerTo 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

*Strings of high-efficiency modules should be

measured singly, not in parallel

Measuring Category CAT III, 600V

Environmental ConditionsFor Array Performance Testing

•Cloudless sky

•2 hours either side of solar noon

•Little or no wind

This variation can be caused by

clouds you don’t even notice.

Topics








I-V Test SetupExample: measuring strings at a combiner box

Test ProcessExample: 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

1. Insert a string fuse

2. Press “Measure”

3. View and save results

4. Lift the fuse

String measurement (repeat for each string):

15 seconds

Portland Habilitation Center PV System860kW 7-inverter system by Dynalectric Oregon

Testing at combiner boxes860kW System at Portland Habilitation Center, by Dynalectric Oregon

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

Combiner Box860kW System at Portland Habilitation Center, by Dynalectric Oregon

Testing individual PV strings860kW System at Portland Habilitation Center, by Dynalectric Oregon

Overlay plots of I-V curves

Performance analysisUsing the optional I-V Data Analysis Tool

Distributions

• Pmax

• Fill factor

• Imp/Isc

• Vmp/Voc

• Isc, Voc, etc

Table of key performance

parametersArray

Tree

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)

1950

2000

2050

2100

7

6

5

4

3

2

1

0

Fre

qu

en

cy

Pmax (Watts)

I-V Data Analysis Tool

Topics








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 curvesRed 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 module with 100 ohm shunt resistance

Demonstration: Single PV module with external resistor

PV module with 2.5 ohm added series resistance

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

String of Field-aged, Early TF Modules

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

Topics








PV Module with Bypass DiodesTypical 72-cell PV Module (bypass diodes shown)

+

Partial shadingInverter must find and track the right peak

Cu

rre

nt

Voltage

Isc

Voc

Po

we

r

Max Power(until the shade pattern changes!)

Partially shaded residential arraySingle string, along lower edge of roof

I-V Curve of the partially shaded stringSingle 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-stringSingle module with 72 cells and 3 bypass diodes

Shade 2 cells in adjacent cell-stringsSingle module with 72 cells and 3 bypass diodes

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

Shade „taper‟ across a cell-stringSingle module with 72 cells and 3 bypass diodes

Edge and Corner SoilingCommon in low-tilt arrays

Cleaning the top 90% of

module area recovered

50% of the performance.

Cleaning the bottom 10%

recovered the rest.

Dirty

Clean

Edge and Corner SoilingCommon in low-tilt arrays

SunEye: Annual Sunpath View

View Solar Access View TOF & TSRF

Editing a Skyline

• Simulate…

– Tree trimming

– Tree removal

– Tree growth

– New building

construction

Conclusion

• Shade matters: A single business card shade on the panel can reduce

the performance by 35%

•Curve Tracing is Awesome!

•Fast and automated, & manual data recording is eliminated

•Provides a complete description of performance

•Measures max power of strings, independent of rest of array!

•Great diagnostic power when combined with onboard PV models

•Enables a deeper understanding of array operation

•Helps build skills across offices and crews

I

V

SolmetricSunEye

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