Solar Builder

MAY-JUN 2018

Solar Builder focuses on the installation/construction of solar PV systems. We cover the latest PV technology (modules, mounting, inverters, storage, BOS) and equip installers/contractors with tips and tools to make informed purchasing decisions.

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54 M AY / J U N E 2 0 1 8 The following is an excerpt from the 2018 Inverter Buyer's Guide, now available to download at solarbuildermag.com/solar- inverter-buyers-guide-2018-edition. I n large-scale PV design, the discus- sion is changing to distributed string vs. virtual centralized string. Flipping through the 2018 Inverter Buyer's Spec Guide, you'll see a bunch in the 100- to 125-kW range, and those that aren't rated at 1,500 volts will be soon. The cost is now closer to 5 to 6 cents per watt, with central inverters still sitting at a cost per watt similar to two years ago. That 3 to 5 MW cap on string inverter-based projects is about to be a thing of the past. CPS has its 1,500-volt product coming out in June and says it is rocketing past that 3 to 5 MW sweet spot. "We are having regular discussions about projects 20 to 30 MW in size now when before, that was extremely rare," says Sarah J. Ozga, product manager North America for CPS America. This could possibly go as high as 100 MW in the not- too-distant future. Centralized string explained Some inverter manufacturers, like Sungrow and CPS, are starting to roll out a "virtual central" concept in which, instead of every row getting its own inverter, the inverter company partners with a skid inte- grator to aggregate a group of 20 or more inverters to deploy systems in 2 to 3 MW blocks. This accomplishes several things: • The obvious O&M benefit of having a ton of inverters centrally located while still reaping the string inverter energy genera- tion benefits and increased uptime. • A huge reduction in communication wiring costs. This is a big one. The RS485 communication wiring that connects each string inverter could run a tab well north of $10,000. Instead of going row to row with RS485, you're only going a few feet. • Installation time is also reduced. "At first that sounds silly, but when you have a 30 MW array out there, that's a lot of land if you have inverters spread across the whole thing," Ozga says. Differences in the details Choosing among distributed string or centralized string options will be site and customer specific. CPS America provides both options, and Ozga says some custom- ers see the wiring costs are greater in dis- tributed while others see them higher in an aggregated virtual central system. The differences can often be fractions of percentage points, but the larger the site, the more these fractions matter, and the considerations usually come down to: expe- riences with DC and AC wiring, labor rates, trenching, DC and AC cabling costs, voltage drop calculations, data monitoring costs, etc. Virtual central string will also need combiner boxes out in the array, which will add costs and a little bit more to the O&M picture to consider. It is worth the exercise to calculate both scenarios in full to see how they pencil out for every project. "We've decided to offer both for any customers that feel they are not seeing the savings and want to stay distributed, but I think it is a decision that comes down to system size," Ozga says. Beyond the decentralized vs. virtual cen- tralized string decision, there are other small features you can find among today's larger string inverters that also make a big impact. For example, a big benefit of string inverters vs. central inverters is the uptime. If a string inverter or two has to come off line for maintenance, only a fraction of the genera- tion of the system is affected compared to the large chunks connected to a central inverter. Doubling down on this advantage, the CPS inverter comes with a separable but integrated wiring box where all of the con- ductors go in and out. Sold together, every inverter comes with its mated wire box. Four bolts hold the wire box to the inverter body — they slide together with guide pins and one large electrical connector — leaving no wires to play with between the inverter and the wire box. "If there is a problem with the inverter, 99 percent of the time, it will be in the inverter not the wiring box," Ozga says. "And what takes so much time when swapping out inverters is disconnecting the wires to remove it. So with the inte- grated but separable wire box, you never have to touch a wire — just leave the wires and conduits installed in the wire box, disconnect the inverter and slide in the new one in." Centralized string design PV POINTERS String inverters are more cost effective for large-scale solar than ever — if you know your options INVERTER Buyer 's Guide 2018 SPONSORED BY Monitoring platform updates Inverter selection advice Designing for solar + storage 136 INVERTERS SPECS

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