2010 Grid-Direct String Inverter Guide: Page 3 of 9

Common Design and Installation Considerations

In order to provide the most useful information regarding string inverter design and installation considerations, all of the manufacturers included in the companion table were surveyed, as well as several large integrators. Wherever possible, engineering and technical support personnel were queried. The interviews were intended to identify common design or installation errors regarding string inverters and best practices. In general, the responses were very consistent with my own experience, which dates back to 2003. My background includes integrating string inverters and BOS equipment into power panels (before those BOS components were integrated with the inverters themselves), installing systems, providing technical support to installing contractors, and years designing and managing both residential and commercial projects from permitting through inspection. Though I learned a lot in that time, the subject matter experts who helped inform this article have seen it all.

In many ways, the most important design and installation considerations today are no different than they were when I attended my first string inverter training. What has changed enormously since then, however, is the depth and breadth of the combined knowledge about the use of string inverters in North America. As I tapped into that reservoir of expertise for this article, I was looking for themes that were repeated, across product platforms, across the country, year after year. Not all of the most common mistakes are obvious, but many are—and all are easily avoided with a little foresight and diligence. The following advice mixes some very fundamental concepts with nuggets of wisdom from industry veterans.

INVERTER SPECIFICATION

Chose the right inverter for the job. This is so fundamental that it is easy to overlook or underestimate. However, this is precisely the PV system designer’s role: to specify appropriate products for specific applications. Representatives from both the integration side and the manufacturing side of the industry identified inverter specification as the single most important consideration for the correct use of string inverters.

This might mean any number of things. For example, is the project even appropriate for string inverters? According to Ryan LeBlanc, lead project designer at SPG Solar in Novato, California, this is the first box that needs to be checked off before moving forward with a proposed design. “Knowing when string inverters provide the most efficient and cost effective total system is critical to delivering the best solar solution,” says LeBlanc. This is easy to establish when the use of string inverters offers the lowest installed price to your customer. When it does not, the question that needs to be answered is whether the use of string inverters results in significantly improved system performance, such that the string inverter approach provides financial savings or some other value to your client over the life of the system.

From the string inverter manufacturer’s point of view, correct inverter specification generally means sizing the PV array appropriately relative to the inverter capacity. Just about all of the surveyed manufacturers identified oversizing PV arrays as a common mistake. My down and dirty rule of thumb is to ensure that array capacity does not exceed inverter capacity by more than 15%. This is closely aligned with a common allowance that Blake Gleason, engineering services manager for Berkeley, California-based Sun Light & Power, feels is generally defendable. That process is to size the PTC rating of the array so that it does not exceed the nominal power rating of the inverter. Additional mitigating circumstances to consider when sizing an array include placement of the inverter, array orientation and the ambient temperature range of the location.

Last but not least, a really good item to check off early in a design is whether the interconnection voltage and service is compatible with the proposed device. This is far less an issue today than it was five years ago when separate inverter models were generally used for different interconnection voltages. Though many of today’s string inverters are autoranging and compatible with multiple service voltages, it is still possible to specify the wrong product if you do not pay attention to the product specifications. The Fronius IG Plus 11.4-3, for example, is appropriate for 3-phase 208 or 240 delta interconnections, but not for interconnection at 277 V in a 3-phase 277/480 wye configuration. The IG Plus 12.0-3 will interconnect only with a 3-phase 277/480 service. All of the single-phase IG Plus products, meanwhile, can interconnect at 208, 240 or 277 Vac. This is a lot to keep track of, especially manufacturer to manufacturer, but the accompanying specifications table will be a valuable aid.

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