Opportunities, Strategies & Best Practices for Electrical Balance of Systems Optimization: Page 2 of 5

SP: What is BOE’s experience with prefabricated wire harnesses or homerun assemblies? Do you ever precombine circuits in the array field to reduce or eliminate combiners?

BOE: We have worked with various forms of wire harnesses for the last 7 years. We have found that wire harnesses provide installation and maintenance cost reductions with thin-film modules when a single series string does not take full advantage of the available conductor ampacity. String inverters, which we have recently begun to deploy on larger systems, in particular on carports and rooftops, reduce the need for harnesses.

Large, repeatable, fixed or tracking ground-mount systems represent an area where wire harnesses can have a great effect in reducing costs. However, you have to consider three major areas of concern in the use of wire harnesses: manufacturing quality, logistics and SKU management, and module series fuse ratings.

The quality of the weld, crimp or solder connections and the overwrap that protects them is of critical importance, as these are part of a system designed to operate for 20 years or more. BOE has observed connection failures in all types of harnesses from many different manufacturers. It is critical to thoroughly vet manufacturer quality control and factory acceptance test processes when procuring wire harnesses.

While wire harnesses are considered laborsaving devices, SKU management and design coordination become critical to avoid ballooning project logistics costs. You must have a strong project management and inventory tracking system in place to achieve cost savings.

Module electrical characteristics and series fuse ratings are key drivers in determining whether harnesses are feasible for a given project. Wire harnesses that do not require inline fusing are preferable to those that do. Wire harnesses with inline fuses introduce a potential failure point that is not obvious when troubleshooting.

—Ryan Zahner, PE, VP of operations, and Tim Brown, construction manager

SP: In what applications does BOE use 3-phase string inverters in place of larger central inverters? Have you developed any strategies for optimizing ac circuit aggregation on these systems?

BOE: For BOE, any project size up to about 10 MWac is a potential candidate for string inverters. We routinely use them on projects smaller than 2 MWac. The advantages of string inverters on these smaller projects—including shorter installation time, less complexity, increased energy harvest, reduced O&M expenses and less downtime—outweigh concerns we might have in using these types of inverters. We achieve savings with 3-phase string inverters in part by using off-the-shelf main lug only (MLO) panelboards with standard ampacity ratings as part of the ac aggregation design. These panelboards are routinely available at local electrical supply houses, which saves time and money.

Nearly all the commercially available 3-phase string inverters output at 277/480 Vac. With only a few exceptions, central inverters output at a custom ac voltage, so generally we use them only in large-scale projects with medium-voltage ac collection systems. It is impractical to use central inverters for systems that tie directly to a customer’s main power system at typical utilization voltages. When we compare installation costs and factor in the expense of specialized labor, ac aggregation systems are substantially more cost-effective in most cases than dc aggregation systems up to a project capacity of 5 MW. Over 5 MW, variables such as the size of the inverter, the project layout and other design considerations tend to drive the choice of equipment.

—Bill Reaugh

SP: To what extent does BOE’s approach to eBOS optimization vary based on application specifics?

BOE: In rooftop systems, the key drivers to eBOS design are the ability to modify the building envelope for ideal circuit routing, main service equipment design and thermal management. New construction provides many opportunities for reducing eBOS cost by coordinating the needs of the building with the needs of the PV system design. In ground-mount systems, placing equipment near access roads and power block pad locations, direct-burying conductors when conditions permit, and aligning tracker rows can all contribute to minimizing BOS costs.

—Joe Kopp, engineering project manager

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