Verifying PV Array Performance Webinar, 3Dec2010

Verifying PV Array Performance

Peter Hoberg VP Sales and Marketing

Bryan Bass Sales Engineer

Paul Hernday Applications Engineer [email protected]

Solmetric Solutions

Topics • PV Array Performance Verification • I-V Curve Measurement • The Solmetric PV Analyzer: PVA-600 • Test Process

• Live Demo • Example Measurement Results • Effect of Shade on array performance • Access to PV Strings for Testing

What is Performance Verification? Showing that measured performance meets a performance standard, to within a specified margin. Depending on the objective, the standard may be: • a reference PV string • a performance spec

• a performance model

Performance Verification Methods Inverter readout compared with system model

Basic

DC measurements compared with PV model

I-V curve measurements compared with PV model

Comprehensive

I V

When Do You Do Performance Testing? • Construction (in-process QA) • Commissioning – Verify correct installation & design – Assure client that contract has been met – Create a performance baseline

• Regular audits/checkups • Service Calls – Troubleshoot poor performance

Additional Objectives • Expose problems earlier

• Troubleshoot more efficiently • Get better commissioning data in less time • Close out projects earlier ($$$ flows earlier) • More rapidly localize problems to PV modules or inverter • Provide data for PV module warranty claims

Topics • PV Array Performance Verification • I-V Curve Measurement • The Solmetric PV Analyzer: PVA-600 • Test Process

• Live Demo • Example Measurement Results • Effect of Shade on array performance • Access to PV Strings for Testing

Benefits of I-V Curve Measurement Compared with DMM & Clampmeter Testing

String-level max power value and max power point

Current

Power

Isc (Clamp meter)

Voltage

Voc (Multi-meter)

• Full I-V curve  Much more diagnostic value • Automated  Fast, & less chance of record-keeping mistakes

Fill Factor

Reduced „square-ness‟ can indicate aging, defects Max Power Point

Current

Isc Imp

Area A = Imp x Vmp

Voltage

Area B = Isc x Voc

Vmp Voc

Area A Fill Factor = Area B

Series and Shunt Losses Contributors to reduced fill factor

Max Power Point Imp, Vmp

Isc

Current

Imp Shunt loss Series loss

Voltage

Vmp Voc

•Shunt losses reduce the ‘current ratio’ Imp / Isc •Series losses reduce the ‘voltage ratio’ Vmp / Voc

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

I sense

Current

V sense

Voltage

Load can be: •Electronic •Resistive •Capacitive

Load considerations: • Voltage & current range • Power dissipation • ‘Sweep’ dynamics, esp sweep rate, which must be slower to accurately test high-efficiency modules

Topics • PV Array Performance Verification • I-V Curve Measurement • The Solmetric PV Analyzer: PVA-600 • Test Process

• Live Demo • Example Measurement Results • Effect of Shade on array performance • Access to PV Strings for Testing

Solmetric PV Analyzer: PVA-600 • • • • •

Measurement of I-V curve Comparison to model results Quick Verify readout Wireless interface Low-cost

CONFIDENTIAL

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 I-V method, 3 auto-selected capacitor values • Capacitors over-sized to accurately test high-efficiency PV modules • Electrically isolated circuitry means a system ground is not required • Protection for over-voltage, -current, -temperature, & reverse polarity

PV Models in the PVA-600 For predicting PV array performance

• Sandia National Labs PV Array Model – Most comprehensive (30+ parameters) – ~400 modules modeled, more available mid-2011

• 5-Parameter Model – Developed at U. Wisconsin, used by CEC for NSHP program

– ~1,700 PV modules modeled at this time

• Simple Datasheet Model – Translates Pmax (STC) to actual irradiance & temperature

– User enters data sheet parameters 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 Voc:

20V

• Min 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)

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

Topics • PV Array Performance Verification • I-V Curve Measurement • The Solmetric PV Analyzer: PVA-600 • The Test Process

• Live Demo • Example Measurement Results • Effect of Shade on array performance • Access to PV Strings for Testing

Steps to test a PV array at a combiner box

PC setup (can be done in advance): • Set up the PV model (type & number of PV modules, etc) • Choose an „array tree‟ for saving test results Hardware setup (onsite): 1. Place the irradiance & temperature sensors 2. Isolate the combiner box (open DC disco) 3. De-energize the buss bars (lift the fuses) 4. Clip test leads to the buss bars String measurement (per string): 1. Insert a fuse 2. Press “Measure” 3. View and save results

I-V measurement setup

Example: measuring strings at a combiner box

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

Array tree for saving I-V data

Same hierarchy as the PV system

Measurement data from individual PV string tests

Performance analysis

Evaluation of the „array tree‟ data

Overlay plots of I-V curves, raw and normalized Histograms Pmax, Fill factor, Imp/Isc, Vmp/Voc Isc, Voc, etc User-defined metrics

Reports

Topics • PV Array Performance Verification • I-V Curve Measurement • The Solmetric PV Analyzer: PVA-600 • The Test Process

• Live Demo • Example Measurement Results • Effect of Shade on array performance • Access to PV Strings for Testing

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 Curve Measurement • The Solmetric PV Analyzer: PVA-600 • The Test Process

• Live Demo • Example Measurement Results • Effect of Shade on array performance • Access to PV Strings for Testing

Setup for Static Load I-V Measurement Comparison

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

Field-aged thin film string

Array-as-sensor mode

Topics • PV Array Performance Verification • I-V Curve Measurement • The Solmetric PV Analyzer: PVA-600 • The Test Process

• Live Demo • Example Measurement Results • Effect of Shade on array performance • Access to PV Strings for Testing

PV Module with Bypass Diodes Example: 72-cell module

+

Partial shading

Inverter must identify 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

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

Shading Pattern Effects

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

Shade „taper‟ along a cell-string Single module with 72 cells and 3 bypass diodes

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

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

Topics • PV Array Performance Verification • I-V Curve Measurement • The Solmetric PV Analyzer: PVA-600 • The Test Process

• Live Demo • Example Measurement Results • Effect of Shade on array performance • Access to PV Strings for Testing

String Access Points

Best choice depends on system layout

2 1 DC DC Combiner Disco

3 DC Disco

5

Inverter

DC Disco 4

Service Panel

The challenges of string access #1: Electrical isolation • Easy when strings are individually fused

• Difficult in small, non-fused (2-string) residential systems #2: Electrical connection

• Commercial combiners typically provide enough space, but safety shields may make block alligator clip access to the buss bars • Difficult in un-fused residential systems if the source conductors run directly to inverter-integrated DC disconnect Recommendation: Every PV system should have means for isolating and connecting to string conductors for testing purposes, typically at the combiner box or the inverter.

Lifting Conductors for String Access

Recommendation

Every PV system should have means to access PV strings at combiner box (or residential inverter) for testing purposes. Access means safely and quickly isolating and connecting to individual strings.

Verifying PV Array Performance