Commercial PV System Data Monitoring, Part One: Page 10 of 11
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Freeman Corbin, director of sales at DECK Monitoring, says: “O&M service structure is the most important consideration for determining the appropriate monitoring level.” After all, the costs associated with future O&M services are typically going to be paid by the integrator.
If a company has a large crew that performs regular on-site system checks and the cost of this service is built into the system operating costs, then inverter-level monitoring might be sufficient. Similarly, if the O&M model pays the integrator to send someone into the field to check every string manually, then there is little incentive to install a granular monitoring system.
However, as integrators install more systems and their service territory grows, centralizing O&M by using a more sophisticated monitoring system might prove to be a better option. This can allow one person at the company to monitor an entire fleet of PV systems from a single web portal. More granular monitoring data can then be used to send troubleshooting crews out with a focused agenda or maintenance schedule, which may save time and money over the life of the system.
Beware of putting systems on autopilot, though. According to Bill Brooks, principal engineer at Brooks Engineering: “Too often, system operators think that with string-level monitoring they can sit back and wait for things to go wrong. This is a flawed approach. Data monitoring systems should never take the place of regularly scheduled preventative maintenance.”
GRANULARITY PROS AND CONS
The higher the level of granularity in your monitoring system, the more complex the monitoring system becomes. If you are collecting inverter-level data and information from a single energy meter, the number of communication circuits and data collectors is relatively small and thus simple to design and implement. However, when you are collecting large amounts of string-level data, the network becomes necessarily more complex, more devices are involved and the conduit and cable schedules get larger as well.
Inverter level. Total system or inverter-level monitoring has the advantage of being the least expensive option and the least complex to install. Typically the inverter or inverters are daisy-chained with cable appropriate for an RS-485 based Modbus network to the datalogger, revenue-grade meter and weather-station hardware.
This level of monitoring provides data on total system performance only. If there is a single inverter, it is difficult to benchmark production, because there is nothing to compare inverter output against. If issues arise in a system that is monitored at the inverter level, the entire array or subarray must be examined. Another disadvantage to this monitoring scheme is that performance problems can easily go unnoticed and persist until the next scheduled maintenance interval, if not longer.
When more than one inverter is used, at least some level of performance variance analysis is possible. With multiple inverters in the system, the inverters’ output power can be compared for consistency. This can reduce troubleshooting time by enabling a process of comparison and elimination. When many smaller-capacity inverters—single-phase string inverters, for example—are individually monitored and compared, the net effect is equivalent to combiner box outputlevel monitoring and may even provide string-level insight into system performance.
Combiner box output level. The next level of complexity and granularity is combiner box output-circuit monitoring, which is also referred to as zone or subarray monitoring. This level of monitoring can be accomplished in several different ways, depending upon the system configuration and the locations where measurements are taken.
The simplest way to achieve zone monitoring for commercial-scale PV systems is generally to specify subarray monitoring at a central inverter’s fused input combiner. In some cases, the manufacturer offers this option.
“We currently offer system-, inverter- or subarray-level monitoring,” notes Michael Zuercher-Martinson, CTO at inverter manufacturer Solectria Renewables. “Subarray monitoring comes factory-installed within the inverter and gets us 80% of the string-level monitoring benefits for 20% of the equipment cost,” he continues. “No additional wiring or IT setup or configuration is required.”
Alternatively, system designers can specify smart subarray combiners within the array field. These large combiner boxes are outfitted with CTs and bolt-in fuses that aggregate PV output conductors from source-circuit combiner boxes. Subarray combiners are useful on large commercial systems. For example, if a 500 kW central inverter with six 450 A fused inputs is monitored at the input combiner only, then there is very little granularity to zone monitoring. The loss of a single string is very difficult, if not impossible, to detect. However, if six smart subarray combiners are specified upstream from the inverter, then the granularity of the zones being monitored can be improved until string-level resolution is achieved.
While combiner box output-level monitoring is more expensive than inverter-level monitoring, it provides much more information about the performance of the system. It improves the resolution of the data used for comparative analyses, allowing for quick baseline system performance verification. If differences are detected in combiner output circuits, troubleshooting efforts can be targeted at a much smaller section of the array.