Commissioning and Troubleshooting PV Arrays. Solmetric PV Analyzer

Transcription

1 Commissioning and Troubleshooting PV Arrays with the Solmetric PV Analyzer November 14, 2013 Paul Hernday Senior Applications Engineer cell

2 Topics Review of I-V Curves Introduction to the Solmetric PV Analyzer Setup & Measurement Live demo of PVA software Irradiance and module temperature measurement Data analysis and reporting Measurement examples Bypass diodes Effects of shading, soiling, snow Troubleshooting using the selective shading method

3 Review of I-V Curves

4 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

5 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

6 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

7 Solmetric PV Analyzer

8 Solmetric 1000V PV Analyzer Kit PVA-1000S I-V Curve Tracer & SolSensor Wireless Reference Sensor 1000V, 20A Measured vs. predicted (3 dots) PC-based (control, display, storage) Wireless interface for convenience and workplace safety Automated data analysis & reporting Database of >10,000 modules, with automatic updates

9 SolSensor Wireless PV Reference Sensor Clamps to module frame in plane of array Silicon photodiode irradiance sensor with temperature compensation and angle of incidence corrections Two thermocouples Tilt sensor Sensors triggered at same time as I-V sweep to optimize accuracy in unstable sky conditions >300ft wireless range Auxiliary input for external irradiance sensors (software under development) Uses same wireless USB adapter as the PVA Rechargeable battery Optional tripod mounting kit

10 Optional SolSensor Tripod Kit Under development Allows locating SolSensor for best line-of-sight transmission across large PV arrays. Also useful in situations where array is not accessible. Thermocouple lead can be extended to reach array, or temperature can be determined from the measured I-V curve. Leveling unit, tilt and pan unit, mounting clip

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

12 How It Works Irradiance Temperature Tilt PV Module Irradiance Module temperature Tilt Azimuth Latitude Longitude Date & time Built-in PV models 3 dots predict curve shape I-V Data

13 How It 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.

14 PV Analyzer Benefits Greater productivity One measurement per string (& just 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 users think like a PV array and develop troubleshooting skills

15 Setup & Measurement Large Systems

16 Setup and Measurement Example: Measuring strings at a combiner box Hardware setup (do once at each combiner box): 1. Deploy or move the sensors (to stay in wireless range) 2. Open the DC disconnect of the combiner box 3. Lift the string fuses 4. Clip the 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 seconds, typically

17 Example Measurement Setup Courtesy of Chevron Energy Solutions 2011

18 Measurement Process 1. Open the DC disconnect for the sub-array that you want to test. Courtesy of Portland Habilitation Center and Dynalectric Oregon

19 Measurement Process 2. Locate the combiner box Courtesy of Portland Habilitation Center and Dynalectric Oregon

20 Measurement Process 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 Combiner box

21 Measurement Process 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. Combiner box Courtesy of Portland Habilitation Center and Dynalectric Oregon

22 Setup & Measurement Residential Systems

23 SolSensor Setup Clamp SolSensor to a module frame Tape thermocouple tip to backside Press the ON button (glows red)

24 Accessing PV Source Circuits for performance measurements Warning: Shut down inverter and open DC disconnect before accessing PV source circuits 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 = Isolation + Connection Choose the safest, most convenient point to isolate and connect to strings Sometimes you ll need two access points for one string

25 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 Test Lead kit. Alligator clip the positive lead to the fuse clip. Connect the negative lead to the negative terminal block via a pigtail or using a probe in place of the alligator clip (for example the Fluke Fused Test Probe, 10A max).

26 Unbroken Conductors In systems with non-integrated DC disconnects, the grounded conductors often pass unbroken through a DC disconnect switch. You can isolate the PV source circuits by opening the switch and clipping your test lead to a supply side terminal. Connect to the grounded conductors by attaching a pigtail at the inverter terminal block.

27 Live Demo of PV Analyzer Software

28 Irradiance & Temperature Measurement

29 Low Irradiance Problem: When you measure array performance at very low irradiance, the data is a poor basis for estimating performance at high irradiance, where performance matters most. At low irradiance the I-V curve changes shape, and this causes greater error in translating the data to STC. Solution: Negotiate for new date. If that s not possible, test for function, and re-test later for performance.

30 Unstable Irradiance Problem: Unstable irradiance introduces scatter in the Performance Factor values, especially if there is a time delay between I-V and irradiance measurements.. Solution: Use SolSensor, which is triggered simultaneously with the I-V measurement. Trigger tests at moments when irradiance is high

31 Irradiance (W/m 2 ) Effect of Time Delay Between I-V and irradiance measurements s 10s 10s 10s 10s 10-second intervals If the irradiance changes significantly between the I-V curve and irradiance measurements, it introduces irradiance error in the performance prediction against which the I-V curve is evaluated. The longer the time delay and the steeper the irradiance ramp, the greater the irradiance error. The PVA software sends simultaneous trigger signals to the I-V curve tracer and SolSensor, assuring that irradiance samples are accurate. Manual measurement and entry of irradiance usually causes long delays.

32 Mounting the backside thermocouple ** MOCAP MCD-PE 1.75 poly dot ~$80/roll of 1000 dots Like irradiance, module temperature is used by the PV model to predict expected performance. Choose a location that is typical of the average temperature of the array. Avoid the edges of the array, which are cooler than average. Avoid areas where heat is trapped or concentrated (for example, the upper edge of a flush-mounted rooftop array). For single module tests (shown here), mount the thermocouple 2/3 of the way between the corner and center of a module. Use high-temperature tape (eg 1-3/4 inch Kapton dots**). Roll the tape firmly across the thermocouple to force out wrinkles and assure firm contact with backside.

33 I-V Data Analysis Tool

34 Frequency Current (Amps) Displays Generated by the I-V Data Analysis Tool Table Limits Statistics I-V curve overlay graph (one per combiner) Voltage (Volts) String ID s Parameter Values Histogram (one for each parameter) 0 Pmax (Watts)

35 Example Report

36 Measurement Examples

37 Current - A High Series Resistance Faulty module Neighboring strings String 4B14 String 4B Voltage - V

38 Example of a High Resistance Failure Module cord separates from buss ribbon in J-box Probably failure mode: Heat cycling bond degradation resistive heating

39 Example of a Hot Spot Failure Backside view Backside view, closeup Frontside view

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

41 Dropped Cell String Conducting or shorted bypass diode

42 Bypass Diodes

43 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.

44 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.

45 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.

46 Shading

47 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

48 Partially shaded residential array Measure the single string mounted along lower edge of roof

49 I-V Curve of the partially shaded string Single string mounted along lower edge of roof Approximately 40% reduction in string s output power

50 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

51 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

52 Tapered Shading

53 Current Tapered shading From adjacent row, parapet wall, railing, etc Isc Effect of tapered shade rows not parallel 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 Voltage Voc

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

55 Non-Uniform Soiling

56 Random Non-uniform Soiling (seagull example) Effect is similar to random partial shading Shows up as steps or notches in the I-V curve

57 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

58 Snow

59 Snow on Array (light cover)

60 Snow on Array (heavier cover)

61 Troubleshooting Using Selective Shading

62 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

63 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)

64 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

65 Under development Input invited (examples, failure modes, I-V curves and photos) Will be made available as a wall poster

66 Commissioning and Troubleshooting PV Arrays with the Solmetric PV Analyzer November 14, 2013 Paul Hernday Senior Applications Engineer cell