Commercial PV System Data Monitoring, Part One: Page 11 of 11

Planning becomes extremely important when monitoring at this level, especially if communication circuits extend into the array field. Additional components are needed, such as a smart combiner box with CT equipment. A communication line needs to be run to each combiner box location, unless a wireless solution is available. Each combiner box requires its own power supply. For best results, work directly with a monitoring solutions provider on the specific design details as the project is conceived and developed.

String level. If more granular data is desired, the resolution of the data monitoring system can be extended to the level of the source circuits or module strings. Further dividing the system in this manner allows owners and operators to pinpoint performance issues and reduce on-site troubleshooting time.

The equipment necessary for this level of monitoring is very similar to that needed for smart subarray combiner monitoring. However, additional CTs are needed within the monitoring system. Instead of CTs located at the subarray level on PV output circuits, string-level monitoring requires smart sourcecircuit combiner boxes, which typically have a CT for every one to two source circuits.

A string-level CT unit—like the Multi Circuit DC Monitor manufactured by Ovius—typically has eight noncontact Hall Effect sensors and provides a Modbus RS-485 output for monitoring eight module strings. The units are modular in the sense that multiple CT units can be incorporated into a single enclosure for 16-, 24- or 32-circuit combiner boxes, which are available from companies like AMtec Solar and SolarBOS. The Ovius CT units require a 24 Vdc power source, which typically means that a 120 Vac circuit needs to be run to each combiner box in order to power a 120 Vac/24 Vdc power supply.

While string-level monitoring can be effective for detecting faulty equipment and low-performing modules, it also adds a great deal of complexity to the PV data acquisition system. For this monitoring to be effective in the long term, a thorough review of the plant is necessary during system commissioning to map each string to its corresponding sensor. Despite the best intentions of power system and data acquisition system designers, changes in the field do happen. A well-documented set of as-built drawings is crucial for troubleshooting later with this level of data analysis.

Module level. Module-level monitoring is not currently offered by independent third-party monitoring solutions providers. Rather, it is a side benefit of installing module-level power electronics: ac modules, microinverters or dc-to-dc optimizers. At present, these three technologies are rarely seen in commercial PV applications due to the economy of scale that larger string or central inverters provide.

While this level of monitoring can obviously be very effective in detecting low-performing modules, the sheer amount of data generated in a large array necessitates a computer-driven model for detecting anomalies. As with string monitoring, for the data to be useful later for troubleshooting in the field, mapping the devices to the sensors is a crucial commissioning step—one that becomes more important with each higher level of monitoring system granularity.

“The best practice that we can recommend is to make sure that the asset owner or maintenance team has a good plan for making use of the data we are providing,” says Krisa of Tigo Energy. He explains: “Instrumenting a system well is just the first step. The system owners must also ensure that the organization and business processes are in place to use the data once they have it. We can help by providing the tools to manage fleets of systems and summary metrics that can mitigate the issue of having too much data. But our best customers also have the team and operational plan in place to make use of the tools that we provide.”

System Implementation

In Part Two of this article, we will consider commercial PV data monitoring system selection and specification criteria in more detail. Completing a site survey is an important early step. You need to decide what data must to be collected and what web portal views are necessary. You need to provide Internet access. The location of data acquisition system components drive conduit and circuit routing. Making these design decisions early is critical, so that installation crews can efficiently execute the scope of work associated with the data monitoring system.

Special thanks to Bill Reaugh at Draker Laboratories for providing expert technical review services and input during the preparation of this article.

CONTACT

Kyra Moore / Southern Energy Management / Durham, NC / southern-energy.com

Rebekah Hren / 02 Energies / Durham, NC / 02energies.com

RESOURCES

Third-Party Commercial PV Monitoring Providers:

AlsoEnergy / 866.303.5668 / alsoenergy.com

ArgusON / 866.459.4103 / arguson.com

DECK Monitoring / 503.224.5546 / deckmonitoring.com

Draker Laboratories / 866.486.2717 / drakerlabs.com

Fat Spaniel (Power-One) / 408.785.5200 / fatspaniel.com

Locus Energy / 877.562.8736 / locusenergy.com

Solar-Log / 203.702.7189 / solar-log.com

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