PV System Commissioning

The modules are mounted, the inverter placed, and the wiring is done—but before you (or your installer) put away the tools, commissioning procedures are necessary to verify system safety and initial performance.

Although recommended in the North American Board of Certified Energy Practitioners (NABCEP) resource guide, a universal procedure for PV system commissioning has not been implemented in the United States. In Europe, many agencies are following the commissioning recommendations of the International Electrotechnical Commission (IEC). Extensive commissioning testing is commonplace for commercial-scale PV systems, and having a solid procedure for residential systems helps ensure the system is properly installed and performing optimally. This article focuses on the tests performed and tools used to commission the PV system at the Cloudview Ecofarm (see the “Commissioning Cloudview” sidebar in this article and ”Increasing Production with Single-Axis Tracking” in HP153).

The Tests

The following electrical tests are recommended by IEC 62446 (and by NABCEP) for commissioning:

• Polarity of each module string to verify that wires are marked and connected properly—check before connecting to the inverter, since reverse polarity could harm it.
• Open-circuit voltage (Voc) to confirm that series strings are properly wired. All parallel strings’ Voc should be within 5% of each other and within manufacturer’s tolerances for the given irradiance and cell temperatures of the test conditions.
• Short-circuit current (Isc) to confirm that all strings’ short-circuit currents are within 5% of each other.
• Grounding continuity to verify that adequate grounding has been established by measuring the array to earth ground resistance. A very high resistance means that there’s a bad connection (for example, an improperly torqued ground screw) in the grounding system.
• Insulation resistance testing (sometimes referred to as “megger testing”) confirms that there is no short-circuit connection between the positive and negative conductors and ground (see “Insulation Resistance” table for the pass/fail criteria and test voltages as recommended by IEC62446).
• Performance verifications ensure that the inverter is supplying the predicted output under operation based on the irradiance, ambient and cell temperatures, AC and DC voltage drop, and the inverter’s efficiency.