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Commissioning tests performed on the Cloudview PV array and balance-of-system (BOS) equipment indicate a safe and wellperforming system. |
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In the spring of 2012, EcoDepot Inc. installed a 4.8 kW PV array on single-axis trackers on the Cloudview Ecofarm’s cold storage building. The system commissioning was performed by my company on the last day of the installation. Although the weather was less than ideal for the IV curve tracing tests, it was adequate for the IEC 62446 tests and performance evaluation.
As with any commissioning testing, each installation has different requirements, but a detailed test plan will help you complete the tasks efficiently. Here are the results of the Cloudview commissioning tests.
Polarity: All modules passed. The Seaward 150 automatically measures polarity upon attaching the module or string MC4 connectors to the meter’s MC4 output cables. If the polarity is reversed, a “cross arrows” symbol will flash and no further tests can be completed. The system uses microinverters, so there were no high-voltage strings to test.
Voc: 35.63 VDC (average). All measurements taken were within the IEC standard of +/- 5% of each other. The PV150 calculates this value automatically. Although the measurements were a little lower than the manufacturer’s cell-temperature-adjusted STC values, they were still well within the manufacturer’s tolerance of +/- 3%.
Isc: 9.2 A (average). All measurements taken fell within the IEC 62446 guidelines of +/5%. Although they were slightly higher than the manufacturer’s STC value of 8.7 amps, they still were well within the tolerance of +/-3% when irradiance was accounted for.
Grounding continuity: Because we used an unconventional ground connection between the pivot pole and tracker frame, this measurement was important to confirm that our technique would pass inspection. We measured ground continuity from the module frames to the tracker’s main equipment grounding conductor (EGC). The average was 0.014 Ohms (continuity for a 10-foot piece of copper wire without connectors would be 0.004 Ohms). This result is reasonable, considering the continuity pathway—from the frame, through the WEEBs and the grounding connector on the pivot pole, and then through an additional Ilsco clamp connected to the main EGC conductor. There is no standard in IEC 62446 for this, but certainly high resistance would indicate a poor ground connection, requiring troubleshooting.
Insulation resistance testing found no faults in the modules or in the wires, including the extensions from the modules to the microinverters, which were required for this particular installation (see “Increasing Production with Single-Axis Trackers” in this issue). The Seaward PV150 meter applies 250 VDC (the maximum voltage recommended for a less than 120-volt module circuit, see “Insulation Resistance” table) to the shorted output wires of each individual module at the point of connection at the microinverters, and automatically gives a pass/fail result based on the test voltage.
The resistance measurement was greater than 199 megohms. For a voltage of less than 120 VDC, IEC 62446 requires a minimum resistance of 0.5 megohms. All the modules passed.
IV curves were taken for each module to verify baseline performance. The shape of an IV curve can tell a lot more about a module’s performance than its Voc and Isc measurements—it can indicate bad diodes, shading, or a damaged module. In this case, the testing results were submitted to the module manufacturer as data for degradation studies.
All modules had a normal-shaped IV curve and were within the manufacturer tolerances of +/- 3% once converted to STC. Curve tracers take a measurement at the specific field conditions and can make adjustments for irradiance and cell temperature automatically to compare to the manufacturer’s STC values. The Solar IV will give an immediate “pass” or “fail” when its measurement is compared against the manufacturer’s specifications. In addition, the IV curve tracer can calculate the maximum power point (Vmp, Imp, and Pmp) values of the array. Because IV curve tracing takes several seconds to perform, results can be impacted by changing weather conditions during the testing. During the Cloudview commissioning, intermittent sunshine caused edge-of-cloud irradiance changes and a 10 mph wind influenced cell temperatures.
A final performance measurement was taken for each microinverter branch circuit (two branch circuits of 10 in parallel) during normal operation to ensure that they were within +/-10% of the expected power output relative to the irradiance and cell temperature at the time of the measurement. (Note: There is not an established standard for this.) This was done using the Seaward 200R for irradiance and cell temperature, and a handheld digital multimeter and clamp-on meter. Calculations determined the expected output, while considering the irradiance, cell temperature, inverter efficiency, and wire losses (see the “Cloudview Commissioning Power Tests” table). In this situation, the branch circuits were tested, since we did not want to measure all 20 inverters and modules individually. These final measurements revealed that both strings were exceeding the calculated output, although this may have been due to edge-of-cloud effects.
