Centralized & Decentralized PV Power Plants: Vendor Perspectives: Page 6 of 11

“Incremental flexibility allows the project to approach a maximum interconnection limit without constraining it to multiples of 500 kW. When using many string inverters, most inverter brands generate sufficient harmonics to impact system uptime. It is important to use multiple transformers for multi-MW string inverter projects to minimize nuisance tripping and inverter downtime due to the increased harmonics.”

Eric Every, Solectria

“String inverters are slightly more expensive on a dollar-per-watt basis but significantly reduce BOS, O&M and downtime costs, a combination that creates more overall value and a lower LCOE compared to a centralized approach. Projects in remote locations can benefit from distributed designs due to maintenance concerns and a lack of on-site technical resources. System downtime and O&M take on a different perspective when coupled with decentralized string inverters. A failed inverter does not necessarily mean a developer has to roll a truck immediately. A previously scheduled routine preventative maintenance visit could include the service call. You have lower operational costs since you do not need to dispatch a high-level inverter technician to troubleshoot and replace a string inverter. Of course, there are limitations. For large projects, the sheer number of 3-phase string inverters required is often enough to make a developer nervous. While it may not be an accurate viewpoint, the concept of having more points of failure can deter developers. Also, central inverters are slightly less expensive than string inverters. Because larger projects operate on slim margins, the increased cost of 3-phase string inverters could be just enough to keep a project from penciling out.”

Paul Mync, Sungrow USA

Considering financial metrics such as LCOE, how has the financial comparison between centralized and decentralized systems changed in recent years?

“Due to the higher cost per watt of string inverters and the added cost for ac combining panels and lower capacity power transformers, the LCOE for central inverters is advantageous for systems over approximately 2 MW.”

Sarah J. Ozga, ABB

“When comparing centralized and decentralized systems based on financial metrics, the advantages and cost savings of the decentralized approach are most obvious for smaller systems. With increasing system size, the delta becomes smaller and eventually reaches an inflection point at which the lines cross and the centralized approach becomes more competitive. While the general shape of the curve has remained very similar over the years, the inflection point has moved to much larger system sizes. Changes in equipment cost, as well as the acceptance and availability of 1,000 Vdc–rated products, are factors in this development. The availability of higher-capacity string inverters will shift that crossover point to even larger system sizes.”

Verena Sheldon, AE Solar Energy

“In addition to providing the benefits described earlier, 3-phase string inverters have dropped significantly in cost over the last two years as volumes have increased, with 3-phase string prices approaching central inverter pricing.”

Sukriti Jain, Chint Power Systems, North America

“As always, the goal of any system designer, operator or owner is to maximize energy production. Decentralized systems have benefited from the price drop in string inverters, but the change to 1,000 V systems allowing for longer dc strings and lower BOS costs has probably had a larger impact.”

Moe Mahone, Fronius

“The LCOE comparisons have narrowed considerably when you factor in  O&M costs associated with central inverters. In most cases, ongoing O&M and warranty service costs, particularly on smaller (<20 MW) projects, have more than offset the reduced first cost of central inverter systems. For the smallest (<5 MW) projects, the LCOE for a distributed architecture is actually better than for central inverter designs. Of course, you need to consider the cost of labor. As overall system costs have decreased, the labor component has become an increasingly large portion of the first system cost. In states with high labor costs, this alone can be the deciding factor between central (skid-integrated) and distributed designs.”

Bill Reaugh, KACO new energy

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