PV System Commissioning: Page 3 of 11

Especially on small projects, the tendency is to try to commission the system at the end of the last day of installation. This strategy is efficient but not effective. Often, the sun is too low in the sky to provide sufficient irradiance for proper performance verification. Shading is also more likely at the end of the day, and any shade on the array makes performance verification difficult. Finally, commissioning demands focus, clear thinking and sufficient time. If the end of the day is near, the crew may be cold, hot, hungry, thirsty or just ready to go home. None of these conditions are likely to produce accurate commissioning results. It is better to clean up, go home and come back another day when the sun is out and minds are fresh.

Who does it? For commissioning to be most effective, the commissioning party should not be inclined towards a cursory process with a guaranteed positive outcome. The contractor who installed the system will usually have this bias. Ideally, the commissioning party should represent the system owner, not the installer, and should be able to act completely in the owner’s interest without conflict. For large systems, the owner should contract with an outside commissioning specialist to oversee the commissioning process.

If the scope of the project is not sufficient to bring in an outside specialist and a direct representative of the owner is not available, commissioning should be performed by an objective party under the original system integration contract. For small commercial projects, for example, an engineer or system designer might be the best choice for the job. Even though the engineer works for the integrator, he or she is at least one level removed from the physical project.

At a bare minimum, a person with sufficient knowledge about PV in general and the system being commissioned in particular must undertake commissioning. This is true even for small residential projects, where the best person to commission might be the crew leader who was in charge of the installation. As someone with a supervisory role, the crew leader can delegate any corrective tasks and focus on effective commissioning, leaving behind a system to be proud of. Regardless of the level of objectivity, whoever carries out the commissioning must have the proper tools and sufficient training.

Documentation. The commissioning agent must start with all of the available system documentation. Before undertaking commissioning, relevant documentation such as the following must be on hand: drawings, ideally as-built; cut sheets for modules and inverters; specifications, especially as they pertain to commissioning; special requirements or forms for rebate programs or other incentives; and equipment manuals.

The commissioning agent must also understand what deliverables are required at the end of the process. For a municipal or federal PV project, for example, the deliverables are often extensive and detailed in the bid package. For a residential project, the commissioning documentation may be part of a post-installation punch list that remains in the customer file for internal use.

Recommissioning and Retro-commissioning of Existing Systems

Recommissioning. Repeating the commissioning of a system that was previously commissioned is called recommissioning. Usually, recommissioning should be the last step in any substantial maintenance project, such as after replacing major components, especially inverters; after adding additional modules; after a non–self-clearing alarm is diagnosed and repaired, such as a ground fault; and as part of a system checkup or regular annual maintenance visit.

In addition, if the original commissioning was performed during less than optimal seasonal conditions, like shading or extended poor weather, a recommissioning event may be called for during better conditions or in the summer. Recommissioning results should be closely compared to those from the original commissioning. If the results are inconsistent (after accounting for shading or other changes), the system integrator should track down the source of the inconsistencies. Recommissioning performance results should also be compared to updated expected performance numbers and discrepancies addressed.

Retro-commissioning. For PV systems that were not properly commissioned in the first place, all hope is not lost. Retro-commissioning can be performed at any point in the system’s lifetime. Although commissioning is a good idea for any system and is better done late than never, the additional expense of retro-commissioning may be best justified whenever there is a significant concern that the system is underperforming. Several indications of possible underperformance include:

  • Monitoring system reports faults, alarms or low performance.
  • Utility bills are higher than expected, after taking into account any new loads.
  • One inverter shows significantly less accumulated kWh than others, even though they all have the same size arrays and no difference in shading.
  • Total accumulated kWh, read from the inverter, is significantly less than predicted for the relevant time period.

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