Verifying PV Array Performance

with the

Solmetric PVA-600 PV Analyzer

March 18, 2011
Paul Hernday Applications Engineer paul@solmetric.com cell 707-217-3094

Solmetric Solutions

Topics
PV Array Performance Verification I-V Curves, and How Theyre 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

What Is Performance Verification?

Acceptable limits

Performance Measurement

Performance Standard *

* A model, a specification, or a reference device (eg another PV string, in a troubleshooting situation)

Performance Verification Methods

Inverter readout String DC measurements String I-V curve 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

Benefits of I-V Curve Performance Testing

Commissioning New PV Systems
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

Maintaining PV Systems (O&M, Asset Management)

Assure nervous owners that arrays are working properly Troubleshoot more efficiently Sort out module versus inverter issues Provide convincing data for module warranty claims

Benefits of I-V Curve Performance Testing

Enhancing Your Quality Assurance Capabilities
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

Topics
PV Array Performance Verification I-V Curves, and How Theyre 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

I-V Curve
Full I-V curve means more diagnostic value

I-V curve Isc Imp Current Max power point

Voltage

Vmp

Voc

Array of Cells, Cell Strings or Modules

I

Parallel

I-V building blocks

Total (net) I-V curve Series

This building block graphic is useful in troubleshooting arrays

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

Isc Imp Current

I-V curve

Pmax

P-V curve

Voltage

Vmp

Voc

Power

Fill Factor
A measure of the square-ness of the I-V curve
Max Power Point (Imp, Vmp) Isc Imp Current

Area A = Imp x Vmp (watts)

Area B = Isc x Voc (watts)

Voltage

Vmp Voc
aSi: 0.50 0.70 0.75 0.85

Fill Factor =

Area A Area B

Imp x Vmp (watts) Isc x Voc (watts)

xSi:

GaAs: 0.85 0.9

I-V Curve Signatures of PV Problems

Normal I-V curve
Shunt losses*

Any reduction of the knee of the curve means reduced output power.

Isc Current (A)

Max power point

Series losses** Mismatch losses (incl. shading)

Voltage (V)

Voc

All of these impairments are easily observed using the PV Analyzer, but are not observed when measuring only Isc and Voc.

I-V Measurement Methods

PV module, string, or sub-array

Current sense Voltage (simultaneous sense data acquisition)

Load can be: Electronic Resistive Capacitive

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.
Voltage

Current

Topics
PV Array Performance Verification I-V Curves, and How Theyre 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

Solmetric PV Analyzer: PVA-600

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.

Touch Screen User Interface

PC is user-supplied (Samsung Q1 Ultra shown here)

Showing the Verify tab screen. Performance Factor (%) = Measured Pmax Predicted Pmax x100

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 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 Analyzer

To 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 cos) 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: Max DC input current: Maximum DC power: Min recommended Voc: Min recommended Isc: I-V measurement time: Points per I-V trace: Storage capacity: Safety: 600V 20A* 12 KW (instantaneous) 20V 1A 80-240mS typical Up to 100, depends on test device 1,000+ (PC running PVA SW) IEC-61010 Measuring Category CAT III, 600V
*Strings of high-efficiency modules should be measured singly, not in parallel

Topics
PV Array Performance Verification I-V Curves, and How Theyre 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

I-V Test Setup

Example: measuring strings at a combiner box

Test Process
Example: 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 String measurement (repeat for each string): 1. Insert a string fuse 2. Press Measure 3. View and save results 4. Lift the fuse
15 seconds

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

Performance analysis
Using the optional I-V Data Analysis Tool
Distributions Pmax Fill factor Imp/Isc Vmp/Voc Isc, Voc, etc
Array Tree

Table of key performance parameters

Overlay plots of I-V curves

Table of Key Performance Parameters

From I-V measurement of a population of PV strings

Example Histogram
From I-V measurement of a population of PV strings
7 6 5 Frequency 4 3 2 1 2000 1950 2050 2100 0

Pmax (Watts)

Width is ~ +- 3.5%. Irradiance sensing was not used in this example.

Example I-V Curve Overlay

From I-V measurement of a population of PV strings
7 6 Current (Amps) 5 4 3 2 1 0 0 100 200 300 400 500 Voltage (Volts)
Effect of shade from nearby fixture.

Topics
PV Array Performance Verification I-V Curves, and How Theyre 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

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 curves

Red dots are the model predictions

Shaded area is the inverters 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 Curves, and How Theyre 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

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 6 Current - A 5 4 3 2 1 0 0 50 100 150 200 Voltage - V 250 300 350 400
String 4B14 String 4B15

Troubleshooting example
Problem isolated to failed module

Irradiance changed between measurements

String of Field-aged, Early TF Modules

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

Topics
PV Array Performance Verification I-V Curves, and How Theyre 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

PV Module with Bypass Diodes

Typical 72-cell PV Module (bypass diodes shown)

Partial shading
Inverter must find and track the right peak

Isc

Max Power (until the shade pattern changes!)

Current

Power

Voltage

Voc

Partially shaded residential array

Single string, along lower edge of roof

I-V Curve of the partially shaded string

Single 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-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

Shadow of leafless tree branch

Single module with 72 cells and 3 bypass diodes

Verifying PV Array Performance

with the

Solmetric PVA-600 PV Analyzer

March 18, 2011
Paul Hernday Applications Engineer paul@solmetric.com cell 707-217-3094