Winter Commissioning

Commissioning PV systems in winter months presents many challenges to gathering useful and valid data. It is difficult, if not impossible, to follow industry-standard commissioning practices—such as those outlined in IEC 62446—when irradiance levels are low and unstable, or when snow covers the modules. According to Cameron Steinman, PE, the president of Endura Energy, an Ontario-based solar consulting firm: “Winter weather is unpredictable, and this negatively impacts resourcing, as commissioning is typically rescheduled multiple times.”

Nonetheless, a PV system may need to be commissioned whether conditions are ideal or not. (See Figure 1 for the time-of-year impacts on irradiance levels.) Contractors have construction deadlines and year-end goals to meet. Performance payments may be at stake. Owners and developers need to beat expiration dates for tax incentives or feed-in tariff contracts.

This article details a two-step process for winter commissioning. Tests in the first phase verify system safety and can be performed at any time. When appropriate irradiance levels are available—perhaps in early spring—technicians can undertake the second phase, which includes performance tests and measurements.

Phase 1: Safety Tests

Table 1 outlines the activities performed in the first phase of winter commissioning, in the recommended order of completion. Here I provide some brief application notes. These activities are discussed in greater detail in previous SolarPro magazine articles: “PV System Commissioning” (October/November 2009) and “Data Acquisition System Installation and Commissioning” (April/May 2013).

Inspect and photograph. The inspection process should follow IEC Standards 62446 and 60364-6, which detail verification requirements for grid-connected PV systems in particular and low-voltage electrical systems in general. The goal is to verify that system components are installed according to the engineered drawings and per the manufacturer’s instructions. This inspection includes verifying that all bolted connections—mechanical and electrical—have been torqued to manufacturer-recommended values. In addition to checking for visible signs of damage, commissioning agents verify that the installation complies with applicable safety codes and standards. They also photograph the as-built condition of the installation, including all major BOS equipment, using a high-resolution camera for the project's records.

Insulation resistance test. Commissioning agents test each installed cable with a megohmmeter and record the results. These tests require technicians to isolate conductors from the common bus and from the common ground. (See “Standardizing PV System Documentation and Verification,” SolarPro magazine, February/March 2012, for minimum acceptable insulation resistance values for PV circuits.) [Editor’s note: Ideally, technicians will complete insulation resistance tests prior to terminating the cables to avoid having to disassemble all of the connections later.]

Earth continuity check. Earth continuity is essential for the proper operation of ground-fault and overcurrent-protection devices. Commissioning agents use a digital multimeter (DMM) or similar tool to verify the continuity of the equipment-bonding conductor to the system ground. On smaller systems, it may be possible to clip one test lead to a module frame and the other directly to the system ground. Otherwise, a series of tests must be performed, accounting for each component and grounding conductor, to verify the continuity of the equipment bonding back to the system ground.

Polarity check. Commissioning agents verify the polarity of all dc cables, using a DMM or similar tool. This is one of the simplest and most important of the safety commissioning tests. Several rooftop fires involving PV systems have been traced back to reverse polarity.

VOC and ISC measurements.  After verifying the polarity, commissioning agents complete open-circuit voltage (VOC) and short-circuit current (ISC) tests. They look for consistency in the measured values, with a maximum variance of ±5% across all strings in the same source-circuit combiner box and across all PV output circuits in an array combiner.

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