Centralized & Decentralized PV Power Plants: Vendor Perspectives: Page 4 of 11
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“With the introduction of 1,000 Vdc 3-phase string inverters, utility-scale projects now have a distinct new option available. Companies can deliver, install and maintain string inverters differently from how they manage larger central units, with comparable capital expenditure (CAPEX) costs. For projects in difficult terrain, such as brownfields and landfills, and complex rooftops, the ability to distribute inverters throughout the array, and in many cases take advantage of the same mounting equipment the PV is already using, can make some projects possible that otherwise would not be due to accessibility or other site-specific restrictions.
“Utilizing string inverters in large-scale plants has had a game-changing effect on transportation logistics, installation practices and, perhaps most notably, on long-term O&M. One or two people can replace string inverters, and companies can keep spare units on-site. This reduces or eliminates altogether the need for coordinating specialized service personnel or lifting equipment. String inverters also distribute independent MPPT inputs throughout a site, which can lead to improved energy production, and by their nature these units provide exceedingly granular operating data, often down to just a few strings, all at no additional cost. The decentralized architecture can help stakeholders install more easily, identify problems faster and produce more power over the life of the plant.”
—Ryan LeBlanc, SMA America
“SolarEdge’s 1,000 Vdc inverters operate at a fixed voltage of 850 Vdc, providing an optimum conversion point from dc to 480 Vac without added buck or boost steps. Our module-level optimization system allows integrators to size longer strings, about 2.3 times longer than with traditional 1,000 Vdc systems and more than 4 times longer than with traditional 600 Vdc systems.”
—Dru Sutton, SolarEdge
“There is a perception in the industry that string inverters allow the designer to minimize land preparation because the granularity allows for greater tolerance in site topology. For utility-scale projects 5 MW and larger, these benefits are marginal. Customers are achieving impressive energy yields using central inverters on land with widely varying topology. Our customers continue to evaluate appropriate solutions on a case-by-case basis.”
—Eric Every, Solectria
“Metaphorical stars have aligned with the introduction of high-powered 3-phase string inverters, 1,000 Vdc systems and declining string inverter prices. Historically, the high cost per watt of string inverters limited their use in utility-scale projects. Substantial price drops in 3-phase string inverters have closed the gap. From a system perspective, deploying multiple string inverters in a utility-scale system is akin to the rapid growth in market share of microinverters in the residential segment. Modularity, granularity and redundancy all have played a role in the adoption of 3-phase string inverters on larger systems. With string inverters, you have more MPPTs, less downtime and greater energy harvest, all leading to a change in the design of utility-scale systems.”
—Paul Mync, Sungrow USA
What attributes of decentralized utility-scale string inverter systems make them a compelling value proposition for some projects? What are the limitations when compared to centralized systems?
“In terms of ABB products, 3-phase string inverters offer MPP optimization at a finer granularity, with one channel of the TRIO inverter optimizing the MPPT for each 8 kW–16 kW of the system. In contrast, the ULTRA inverter optimizes the MPPT for 390 kW of the system. The TRIO’s more-granular MPPT optimization supports installations with shading, noncontiguous land segments and uneven terrain. Additionally, TRIO string inverters, which weigh less than 170 pounds, obviate the costs for cranes or forklifts. For utility-scale sites, the 480 Vac output voltage requires a step-up transformer for the utility medium-voltage transmission connection at 2.3 kV–25 kV. String inverter outputs combine at ac panels before connecting to the transformer. When using a central inverter, you do not need to combine the ac. One clear limitation for string inverters is the current trend of utility-scale projects designed at 1,500 Vdc, a system voltage that string inverters do not yet address.”
—Sarah J. Ozga, ABB