Opportunities, Strategies & Best Practices for Electrical Balance of Systems Optimization: Page 3 of 5
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SP: What are your preferred wire and cable management solutions? Have you identified any products that improve system safety and long-term performance while driving down up-front costs?
BOE: While financial considerations are typically the biggest variable driving our choice of cable management solutions, we also consider whether the developer will own the asset long term or whether the developer plans to sell the asset. If the developer plans to keep the asset, that weights choices affecting long-term maintenance cost more heavily against first cost to purchase. In other cases, lowest first costs primarily drive choices, which may lead to higher maintenance costs in the long term.
We have a few favorite products that provide an optimal balance between first costs and long-term maintenance costs. For source-circuit conductor management, we like wire clips—such as the SunRunner clips from Heyco—that attach directly to the module frame. For dc homerun conductor management, we like Heyco’s SunBundler stainless steel cable ties and messenger wire systems such as CAB’s solar hangers. [See “Aerial Cable Support Systems.”] In rooftop systems, we prefer cable tray over other cable management systems, as this protects conductors while reducing temperature derate effects. Overall, we believe that cable tray provides lower long-term costs for replacement and maintenance.
Distributed Generation Perspective
Standard Solar Inc. (SSI), based in Rockville, Maryland, is a full-service project developer that specializes in nonresidential distributed generation (DG). The company offers design, construction and installation services to commercial, utility and public sector customers. I interviewed the company’s director of engineering, C. J. Colavito, to learn about eBOS trends and best practices in DG applications.
SP: What types of eBOS optimization strategies has SSI employed in recent years?
SSI: From a DG perspective, some of the things that work well for utility-scale designs do not provide the same value for one-off 2 MW projects. That said, one of the largest changes we have adopted is the use of 3-phase string inverters for just about all our projects, including rooftops, parking canopies and ground mounts. Moving to 3-phase string inverters has allowed for more system design standardization and simplified compliance with new Code requirements for rapid shutdown and arc-fault detection and interruption. The shift to 3-phase string inverters also eliminates the need for source-circuit combiner boxes and recombiners, as well as for any additional dc disconnecting means beyond the inverter-integrated dc disconnect. Access to larger-capacity 3-phase string inverters in the 50–60 kW range further simplifies things for ground-mounted systems over 2 MW and makes it easier to integrate this technology on larger sites.
SP: Has SSI developed any techniques or strategies for optimizing ac circuit aggregation on these 3-phase inverter systems?
SSI: Off-the-shelf ac panelboards are easier to source, have fewer application-specific requirements and are less specialized than dc combiners. However, a challenge we have faced moving to string inverters is that the nominal voltage for our long wire runs from the various portions of the PV array to the main equipment pad or transformer have gone from 700-plus Vdc to 480 Vac, which introduces some voltage drop complications. The fact that the ac output terminals of many 3-phase string inverters have limited compatibility with larger-diameter aluminum conductors exacerbates these considerations. One approach SSI has adopted to combat this challenge in ground-mount applications is to install small subgroups of four to six string inverters adjacent to a small panelboard that is compatible with aluminum compression lugs, allowing for a bolted connection to the busbar. This lets us upsize the aluminum conductors out of the panelboard as needed.
SP: Are there situations where you like to use wire harnesses to precombine circuits in the array field to reduce or eliminate combiners?
SSI: We have experimented with prefabricated wire harnesses for ground-mount applications. The use of wire harnesses can save time and money from a labor and material standpoint. However, it requires very careful, accurate planning and coordination from the wire harness supplier. It also requires collaboration and sharing of savings with installation labor, which a third-party subcontractor rather than in-house workers typically provides. We have had a mixed experience on 2 MWac ground-mount DG applications. Even when competitively bidding installation subcontractors, we encounter inconsistent and unreliable results with respect to extracting the savings from the subcontractor’s fixed price to complete the work. Our experience is similar for other factory-assembled solutions, such as prefabricated inverter pads or partially preassembled racking systems. To truly extract value from prefabricated and preassembled solutions, I think you need to either use in-house labor or have a very strong and open relationship with the installation subcontractor.