New Tools for PV Array Commissioning and Troubleshooting

New Tools for PV Array Commissioning and Troubleshooting November 10, 2011 Paul Hernday Applications Engineer paul@solmetric.com cell 707-217-3094 Bryan Bass R&D Engineer bryan@solmetric.com

Solmetric Solutions www.solmetric.com

Annual Insolation Tool (web based)

Roof Azimuth Tool (web based)

Compound Angle Tool (downloadable)

Today s Topics PV array performance verification Introduction to I-V curves The Solmetric PVA-600 PV Analyzer Live demo of the PV Analyzer user interface Commissioning PV Arrays Troubleshooting PV arrays De-mystifying shading

Array Performance Test Methods For Startup/Commissioning/Checkups/Service Alarms 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).

What is I-V Curve Tracing? Adjustable Load Measure voltage Measure current Built-in PV models mean user can instantly check performance against expectations for the existing irradiance and temperature. Current Voltage Load can be Resistive Capacitive Electronic

I-V Curve Tracing I-V curve measurements provide direct performance characterization and verification, as well as a diagnostic tool for periodic PV system performance assessments. I-V curve tracing is the most informative measurement that can be performed on a PV module or array. David King DK Solar Works Developer of the Sandia PV Array Model at Sandia National Labs SolarPro, Aug/Sep 2011 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. Bill Brooks Brooks Engineering SolarPro, Aug/Sep 2011

PV Analyzer Users EPC Organizations System Integrators O&M Companies PV Module Manufacturers Consulting Engineers PV Inverter Manufacturers College PV Instructors Training Organizations IBEW Training Centers

Benefits of I-V Curve Performance Testing Commissioning New PV Systems A single electrical connection & a single measurement The most comprehensive PV measurement possible 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 Comply with IEC-62446 (commissioning grid-tie PV systems) Maintaining PV Systems (O&M, Asset Management) Troubleshoot more efficiently Sort out module versus inverter issues Provide convincing data for module warranty claims Keep arrays producing maximum energy

Benefits of I-V Curve Performance Testing Developing an Analytical Capability Take advantage of string-level performance statistics to set appropriate system margins & set appropriate pass-fail criteria Better understand and account for PV module degradation Provide stronger foundation for performance estimates and guarantees.

Solar Cell Photon 3 1. Charge generation 2. Charge separation 3. Charge collection 3 2 1 Cells are in series a single cell can be a bottleneck. A 12-module string may have 12 x 72 = 864 cells in series.

I-V and P-V* Curves Expect this shape for healthy cells, modules, strings, arrays Isc Imp I-V curve Pmax Current P-V curve Power Voltage Vmp Voc *P-V curve is calculated from the measured I-V curve

Stacking PV Modules I Current Parallel I-V building Series Total (net) I-V curve blocks V Voltage This building block graphic is useful in troubleshooting arrays. Blocks can also represent cell strings within modules.

I-V Curve Signatures of PV Problems Isc Current (A) Reduced current Normal I-V curve Shunt losses* Mismatch losses (incl. shading) Any reduction of the knee of the curve means reduced output power. Series losses** Max power point Voltage (V) Reduced voltage Voc Conventional measurements do not reveal many of these effects.

Useful diagnostics Fill Factor, Current Ratio, Voltage Ratio Isc Imp Current ratio Imp/Isc Max Power Point Current Voltage ratio Vmp/Voc Voltage Vmp Voc 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

Solmetric PVA-600 PV Analyzer Single connection & measurement Detailed performance picture Comparison to built-in model (5 dots) Convenient wireless interface Rugged and easy to use String measurement showing I-V and P-V curves, and comparison of I-V curve with model (5 dots).

Example Measurement Setup Courtesy of Chevron Energy Solutions 2011

Wireless Sensor Kit Irradiance & temperature sensors Irradiance transmitter. K-type thermocouple Omega Part # 5SRTC-GG-K- 30-72 Receiver (USB) Temperature transmitter

Mounting the Thermocouple Use metalized HVAC tape or Kapton tape for high temperature stability Press firmly to assure intimate contact between TC and backside Fold a tab so you can remove the tape Suggested replacement TC: Beaded K-type Omega Part No. 5SRTC-GG-K-30-72

PVA-600 Block Diagram (simplified) Battery charging connector CPU & wireless module C (1 of 3) V sense PV Source I sense Control button with LED indicator NEMA 4X FG Enclosure Capacitive load method. Electrically isolated. Ground lead is not required. Protected for over-voltage, -current, -temperature, & reverse polarity.

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

Comparing PVA measurement with resistive load method

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 Analyzer Predict 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 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.

Example Measurement Setup

Example Measurement Setup Courtesy of: Integrated Energy Systems Pittsburg Unified School District Sage Renewables Stellar Energy Solutions

I-V Measurement Setup Example: Measuring strings at a combiner box * *Electrically isolate buss bars, then connect PVA.

Test Process Example: Measuring strings at a combiner box Hardware setup (do once at each combiner box): 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 Electrical measurement (repeat for each string): 1. Insert a string fuse 2. Press Measure 3. View and save results 4. Lift the fuse 15 seconds typically

Example Measurement Setup 860kW 7-inverter system Courtesy of Portland Habilitation Center and Dynalectric Oregon

Example Measurement Setup Combiner boxes for one inverter Courtesy of Portland Habilitation Center and Dynalectric Oregon

Example Measurement Setup Combiner box wiring Courtesy of Portland Habilitation Center and Dynalectric Oregon

Example Measurement Setup Insert a single fuse to test the corresponding string Courtesy of Portland Habilitation Center and Dynalectric Oregon

How Data Is Stored This format allows the I-V Data Analysis Tool to: Array Tree Automatically analyze your data Identify non-conforming strings Provide convincing charts for your commissioning report

Displays Generated by the I-V Data Analysis Tool* Current (Amps) 7 6 5 4 3 2 1 0 0 100 200 300 400 500 Voltage (Volts) 7 6 5 Frequency 4 3 2 *Optional, MS Excel-based tool, $95 1 0 1950 2000 2050 2100 Pmax (Watts)

Recommended Sky Conditions For Array Performance Testing Clear sky (for high, stable irradiance) Irradiance affects mainly the height of the I-V curve (Isc, Imp) Consistent irradiance makes it easier to show uniformity, or to spot performance issues I-V curves change shape as irradiance decreases Translation to STC from low light conditions is much less accurate 4 hour window centered on solar noon* Low light issue as mentioned above Angle of incidence and spectral effects come into play Low/No wind (for more consistent module temperature) Temperature affects mainly the width of the I-V curve (Voc, Vmp) Temperature is not uniform across an array under any conditions Wind keeps changing the temperature pattern, making it difficult to compare one string result with another. *Solar Noon Calculator: http://www.esrl.noaa.gov/gmd/grad/solcalc/

Problem Sky Conditions Small clouds Easily overlooked Causes 20% variation in irradiance Changes in seconds.

Problem Sky Conditions Cirrus clouds Easily overlooked Causes 10% variation in irradiance Changes in seconds.

Problem Sky Conditions Cloud Cloud edge effect effect Boosts irradiance up to 30% Changes in seconds.

String of Field-aged, Early TF Modules Degraded fill factor, lower output power Array-as-sensor mode for viewing relative changes in curve shape

Troubleshooting example Anomalous slope in string I-V caused by single high-resistance module 8 7 6 Current - A 5 4 3 2 1 0 String 4B14 String 4B15 0 50 100 150 200 250 300 350 400 Voltage - V

Example of a series resistance failure inside a module J-box

Dropped Cell String Shorted bypass diode, or Mismatch causing diode to turn on when current starts flowing

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

Shading Myth #1 Myth: If you completely shade one cell, that entire PV module or PV string stops producing energy. Fact: In grid-tie systems, bypass diodes turn on, shunting current around the shaded cell strings (sub-strings) and keeping the rest of the string in production.

Review of Bypass Diodes The purpose of bypass diodes: Preserve performance and prevent damage under mismatch conditions, including non-uniform shading

PV System in Full Sun Thought experiment - No bypass diodes 360v Inverter dc input voltage Each column represents a string of PV modules System operating at Pmax Each cell represents ½ volt + + + + We re going to shade this cell 5A, 0.5V Inverter 0v

Effect of Shading One Cell Thought experiment - No bypass diodes 360v Inverter dc input voltage + 5A, 0.5V + When this voltage becomes + + These cells shift to a lower current, higher voltage operating point: large enough, the cell breaks down and conducts in the reverse direction. 3A, 0.52v Inverter 0v

Effect of Shading One Cell When bypass diode is not present or has failed open 85 Heating of a Single Shaded Cell Backside temperature with no bypass diodes 80 Un-shaded (control) cell Shaded cell Temperature (C) 75 70 65 60 55 50 Shaded cell Unshaded cell Single module, full sun Output short circuited 3 cell strings All 3 BP diodes removed Single cell shaded 45 40 0 5 10 15 20 25 30 Shade Duration (seconds)

Introducing Bypass Diodes Example: Current flow in an un-shaded 72-cell PV Module Cell String Cell String Bypass Diodes Cell String + Bypass diode turns on when the shaded cell(s) can no longer pass as much current as the non-shaded cells.

Shade One Cell Example: 72-cell PV Module Cell String Cell String Bypass Diodes Cell String + Bypass diode turns on when the shaded cell(s) can no longer pass as much current as the non-shaded cells.

Which shading pattern is worse? 1 2 +

I-V Curve of a Partially Shaded String Isc Multiple knees in the I-V curve means multiple peaks in the P-V curve Inverter should track the highest peak Tricky when shading is changing and thus the peaks are changing! Power Current Voltage Voc

Shading Myth #2 Myth: Reduction in array output power is proportional to the amount of array area that is shaded. Fact: Shading has a very disproportional impact. Output depends strongly on where the shade falls on a module.

Shading one cell string drops 1/3 of PV module voltage and power Shade 2 cells in the same cell-string 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. Shade 2 cells in adjacent cell-strings Single module with 72 cells and 3 bypass diodes

Shading Myth #3 Myth: Shading from power lines has no effect. Fact: A PV cell s current drops linearly in proportion to the shaded area. Any amount of shading has an effect.

Line Shade Narrow Shade on of two series connected modules Voc Isc Vmpp Impp Pmax 79.29 5.128 62.06 4.603 285.6 79.28 5.141 64.02 4.380 280.4 Shade 98% 13 12 12

Line Shade Medium Shade on of two series connected modules Voc Isc Vmpp Impp Pmax 79.19 5.219 61.92 4.660 288.6 79.09 5.196 66.21 Shade 3.987 264.0 91% 6 5 5

Line Shade Thick Shade on of two series connected modules Voc Isc Vmpp Impp Pmax 79.62 5.267 62.41 4.712 294.1 79.05 5.244 68.50 3.343 229.0 Shade 77% 1 3 3

Edge and Corner Soiling Common in low-tilt arrays Dirty Cleaning the top 90% of module area recovered 50% of the performance. Cleaning the bottom 10% recovered the rest. Clean

What is the PV Analyzer all about? Commission and troubleshoot PV arrays faster and better Curve tracing is fast - a single measurement for each string Curve tracing is the most complete performance measurement possible Independently measures each string s max power Built-in PV models give instant performance check Does not require bringing the inverter online Software tool automates data analysis Helps us learn to think like a PV array I V

5-Minute PV Analyzer Training Videos http://www.solmetric.com/videos1.html

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

SolarPro Magazine, Aug/Sep 2011

New Tools for PV Array Commissioning and Troubleshooting November 10, 2011 Paul Hernday Applications Engineer paul@solmetric.com cell 707-217-3094 Bryan Bass R&D Engineer bryan@solmetric.com