PV Array Commissioning and Troubleshooting with the Solmetric PV Analyzer April 11, 2013 Paul Hernday Senior Applications Engineer paul@solmetric.com cell 707-217-3094
Review of I-V Curves
I-V and P-V Curves Expect this shape for un-shaded, healthy modules & strings Current Power I-V curve Isc Pmax Imp P-V curve Voltage Vmp Voc *P-V curve is calculated from the measured I-V curve
I-V Curve Deviations Each represents a reduction of generating power Current (A) Isc Reduced current Normal I-V curve Increased slope Any reduction of the knee of the curve means reduced output power. Reduced slope Max power point Mismatch losses (incl. shading) Voltage (V) Reduced voltage Voc Conventional measurements do not reveal many of these effects
Current How I-V Curve Tracing Works (in concept) Current Voltage (I) 2 (V) Read I & V 1 Adjust the load resistor 3 Plot the point (V) (I, V) (I) A curve tracer instrument does all of this automatically The load may be resistive, capacitive, or electronic Voltage
Solmetric PV Analyzer
Solmetric PVA-600 PV Analyzer 600V, 20A (1000V version coming) Measured vs predicted (5 dots) PC-based (control, display, storage) Wireless interface for convenience and workplace safety
How it works Irradiance sensor (in plane of array) Temperature sensor (module backside) All wireless Module make & model Tilt Azimuth Irradiance Module temperature Latitude Longitude Date & time Built-in PV models 5 dots predict curve shape PV Module or string
How the Solmetric PV Analyzer Works Bleed resistor Switch Controller & Wireless Capacitor (1 of 3) V sense PV Test Leads I sense NEMA 4X FG Enclosure The PV Analyzer uses a capacitor as the load. Current and voltage change smoothly, at controlled rate, across the voltage range, ideal for accurately testing high efficiency modules.
Solmetric PV Analyzer Users I-V curve tracing is embedded in the PV community EPC organizations System Integrators Electrical contractors Module Manufacturers I Consulting Engineers Inverter Manufacturers V O&M Companies Technical colleges IBEW Training Centers Training Organizations
PV Analyzer Benefits Greater productivity One measurement per string (& one hookup at combiner box) Allows testing array performance before inverter is online Instant performance check via built-in PV models Automated data analysis and reporting Faster troubleshooting Greater insight I-V curve is the most complete performance measurement possible, capturing Isc, Voc, Imp, Vmp, Pmax, plus the entire I-V curve Independent Pmax measurement for each string Helps us think like a PV array
Setup & Measurement - Commercial Systems
Setup and Measurement Example: Measuring strings at a combiner box Hardware setup (do once at each combiner box): 1. Move the sensors (if necessary to stay in wireless range) 2. Open the DC disconnect of the combiner box 3. Lift the string fuses 4. Clip PV Analyzer 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 10-15 seconds, typically
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
SolSensor Wireless PV Reference Sensor Integrated construction Silicon irradiance sensor with temperature compensation and software Compatible with external irradiance sensors via auxiliary port Two thermocouples Tilt sensor Clamps to module frame in plane of array 100m wireless range Tripod mounting option Rechargeable
Measurement Process 860kW 7-inverter system 1. Open the DC disconnect for the sub-array that you want to test. Courtesy of Portland Habilitation Center and Dynalectric Oregon
Measurement Process 2. Locate the combiner box Courtesy of Portland Habilitation Center and Dynalectric Oregon
Measurement Process Combiner box wiring 3. With a clamp-meter, verify that the load has been disconnected. 4. Then lift all of the fuses. Courtesy of Portland Habilitation Center and Dynalectric Oregon
Measurement Process Insert a single fuse to test the corresponding string 5. Clip the PV Analyzer to the buss bars. 6. Push down a fuse and make an I-V curve measurement. Lift fuse again. 7. View and save results. 8. Repeat for the other strings. Courtesy of Portland Habilitation Center and Dynalectric Oregon
Setup & Measurement Residential Systems
SolSensor Setup Clamp SolSensor to a module frame Tape thermocouple tip to backside Press the ON button (glows red)
Accessing PV Source Circuits for performance measurements Warning: Shut down inverter and open DC disconnect before accessing PV source circuits. 1 2 5 3 J-Box Small Residential System DC Disco 4 Inverter DC Disco AC Larger Residential System 1 2 DC Combiner Box 3 DC Disco 5 4 Inverter DC Disco AC Access = Isolating the source circuit + Connecting to it Choose the safest, most convenient point Sometimes you ll need two access points for one string
Accessing PV Circuits at an integrated DC disconnect 1. Open the AC and DC disconnects 2. Lift the string fuses 3. Connect the PV Analyzer test leads to the grounded conductor terminal strip and to the ungrounded conductor fuse clip (supply side), observing correct polarity. Use the Solmetric armored, high-current test leads. Attach the supplied alligator clips or high-current probes like the Fluke model FTP. Usually can t alligator clip to a terminal block because of the insulating plastic cover. Install a short, insulated, temporary wire pigtail in the terminal block, to clip to.
Unbroken Conductor Runs The most common example is a negative-grounded system with the negative conductors passing unbroken through a DC disconnect switch. Grounded conductors passing through the enclosure unbroken Can isolate the PV source circuits by opening the switch, and connect to them at the top (supply side) of the switch. Connect to the grounded conductors by attaching a pigtail to their terminal block at the inverter.
Live Demo of PV Analyzer Software
Irradiance Issues
Low Irradiance Problem: Max power performance at low irradiance is not a good predictor of performance at high irradiance. Solution: Negotiate for new date. If that s not possible, test for function, and re-test later for performance.
Unstable Irradiance Problem: Unstable irradiance introduces error in irradiance and temperature measurements, making data analysis less accurate. Solution: Trigger tests at moments when irradiance is high Use real-time, wireless sensors rather than manual sensors & data entry.
Irradiance (W/m 2 ) Effect of Time Delay Between I-V and irradiance measurements 980 PV performance measurements can be less accurate under conditions of unstable irradiance, even if the irradiance is nominally high. Here is why. 960 940 920 30 There is typically some small time delay between the I-V curve and irradiance measurements. As shown in this graph, if the irradiance changes during this time delay, the measured irradiance value will not be representative of the condition under which the I-V curve was measured. In other words, under unstable irradiance conditions, a time delay between the two measurements translates into irradiance measurement error. The longer the time delay, the more vulnerable the performance measurement is to this source of irradiance error. 900 10s 10s 10s 10s 10s 10-second intervals This error is minimized when using real-time, wireless irradiance sensing with very short time intervals between irradiance measurements. At the other extreme, the error is maximized when you measure and enter irradiance values manually, because of the time these steps require.
Data Analysis
1950 2000 2050 2100 Frequency Current (Amps) Displays Generated by the I-V Data Analysis Tool String table (one row per string) 7 6 5 4 3 2 1 0 String I-V combo graph (one per combiner) 0 100 200 300 400 500 Voltage (Volts) 7 6 Distribution graph (for each parameter) 5 4 3 2 1 0 Pmax (Watts)
Tour of the Data Analysis Tool
Measurement Examples
Current - A High Series Resistance 8 7 6 5 4 Faulty module Neighboring strings 3 2 1 String 4B14 String 4B15 0 0 50 100 150 200 250 300 350 400 Voltage - V
J-box failure Example of a series resistance problem Probably failure mode: Heat cycling bond degradation resistive heating
Hot Spots Backside view Backside view, closeup Frontside view
String of Field-aged, Early TF Modules Degraded fill factor, lower output power Array-as-sensor mode for viewing relative changes in curve shape
Dropped Cell String from shorted bypass diode Shorted bypass diode, or Mismatch causing diode to turn on when current starts flowing
Bypass Diodes
Bypass Diodes PV modules designed for grid-tie systems have extra components semiconductor bypass diodes designed to protect shaded, badly soiled, or cracked cells from electrical and thermal damage. Bypass diodes also allow nonshaded modules to keep producing, by shunting current around groups of shaded cells. In most module designs, the bypass diodes are mounted in the junction box on the module backside. In many cases, the junction box can be opened to test and replace the bypass diodes. Each bypass diode protects a different group of cells within the module. For example, in a 72-cell crystalline silicon module there may be three bypass diodes, each protecting a group of 24 cells, usually laid out as two adjacent columns as viewed in portrait mode.
Bypass Diodes This sketch shows current flow in a typical 72- cell PV module with 3 bypass diodes (shown at top). If none of the cells is seriously shaded or otherwise impaired in its ability to generate current, the current flows as shown by the green path. The bypass diodes do not conduct current. In the next slide, we shade a cell and the bypass diode protecting that cell group turns on, routing current around the partially shaded group.
Bypass Diodes In this sketch, a cell at lower right has been shaded. Assuming this module is loaded, the bypass diode protecting that group of cells turns on, routing current around that group, protecting the shaded cell. Another benefit of bypass diodes is that by eliminating the shade-induced restriction to current flow, they preserve the production of the non-shaded modules and cell groups.
Shading
Power Current 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 Isc Depth of step is proportional to shading factor on most shaded cell Bypass diode turning on Voltage Voc
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 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
Tapered Shading
Current Tapered shading From adjacent row, parapet wall, railing, etc Isc Effect of tapered shade Voltage rows not parallel Voc This effect produces an I-V curve deviation similar to that of shunt loss The tapered sliver of shade causes a slight current mismatch across cell groups and modules In tilt-up system, the impact of this shade is felt only early and late in the day, at low sun angles In general, inter-row shading losses are greater if rows are crowded to increase peak capacity
Shade taper across a cell-string Single module with 72 cells and 3 bypass diodes
Non-Uniform Soiling
Random Non-uniform Soiling (seagull example) Effect is similar to random partial shading Shows up as steps or notches in the I-V curve
Lower Edge Soiling ( dirt dam ) Common in arrays with tilt less than 10 degrees Dirty Clean 50% of the power loss 50% of the power loss
Snow
Snow on Array (light cover)
Snow on Array (heavier cover)
Troubleshooting Using Selective Shading
I-V Curve Deviations Each represents a reduction of generating power Current (A) Isc Reduced current Normal I-V curve Increased slope Any reduction of the knee of the curve means reduced output power. Reduced slope Max power point Mismatch losses (incl. shading) Voltage (V) Reduced voltage Voc Conventional measurements do not reveal many of these effects
Selective Shading Technique String with one bad module Key to the technique: Blocking the light turns on the bypass diode, shorting out the cell group. Photo courtesy of Harmony Farm Supply and Dave Bell (shown)
Troubleshooting using selective shading to identify a bad module Current Isc Shading the bad module produces this blue curve, with no step. Shading any good module produces this red curve, still showing the step This is the string with no shade, showing a step. Voltage Voc The method can also be used to identify a bad cell string in a single module
PV Array Commissioning and Troubleshooting with the Solmetric PV Analyzer April 11, 2013 Paul Hernday Senior Applications Engineer paul@solmetric.com cell 707-217-3094