thermal temperature tools. These periodic audits can provide useful trend analysis reports to identify and prevent future problems. Equipment servicing per manufacturer’s recommendations is a must and vegetation management for larger arrays could become part of a scheduled maintenance routine.
NABCEP Certified PV Installer Brian Mehalic, project engineer for O2 Energies in North Carolina, has over a decade of PV experience and has commissioned hundreds of residential and large commercial plants. With more than 23 MW of PV quality-assurance inspections, Mehalic highly recommends a sensible O&M strategy for larger PV systems. When teaching the Solar Energy International (SEI) operations and maintenance course3, he emphasized the need to, “Schedule early follow up...one month, 90 days, whatever is necessary, and keep an eye on the production early on. Failures/issues tend to follow a bathtub curve.”
All three stages are important for the long-term health of the PV system, but most questions arise during the design stage - and it is easy to understand why. It is the stage most rooted in tradition, tribal knowledge or perhaps even ignorance of alternatives. Most solar industry professionals do not want to debate the best inverter topology, but it is definitely worth spending some time discussing.
INVERTER SELECTION
When considering inverter selection, one must ask, “What problem am I trying to solve?” Generally speaking, for residential applications there are two main categories of inverters: string inverters and module-level power electronics (MLPE). Commercial applications favor string inverters and/or central inverters, with the latter typically chosen for large utility-scale applications. Each has its strengths and weaknesses and there is no one-size-fits-all approach if you value a positive ROI and consider the O&M involved with the system.
While there are commercial installations using MLPE, typically they do not yield a reasonable ROI since the higher capital expenses are often not offset by the O&M system costs. In the end, more profit can be made over the lifetime of the PV system using a string inverter for large commercial applications rather than an MLPE.
STRING INVERTERS
String inverters are well known in the industry and have been the de facto standard for PV installations for decades. There are three topologies associated with string inverters: low frequency transformer, high frequency transformer and transformerless, with the latter being the lightest and most efficient of the three. Leading string inverters are easy to maintain, require little O&M and have become lighter, more reliable and more efficient as the years have passed.
String inverters are named for the process of connecting a string of modules in a series to increase the system voltage. Depending on the size of the inverter, multiple strings are connected in parallel to increase the total current of the PV array, thus increasing the DC power of the array.
MODULE-LEVEL-POWER ELECTRONICS
This broad category includes devices that are mounted onto or behind the solar modules and consist of two different types of converters: micro inverters and DC optimizers. Both types of devices are connected to an individual module, but they differ in their conversion process.
Micro inverters convert the DC directly into AC at the module level, usually either 240 V or 208 VAC. This power is then sent to the main breaker in the installation via each
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Optimzing PV Systems eFeature | July 2014
OPTIMIZING PV SYSTEMS - PART 1: INVERTERS