Centralized & Decentralized PV Power Plants: Vendor Perspectives

As the capacity of utility-scale PV plants has increased in the US, so has the capacity of the centralized power-conditioning units used in these projects. Today, many plants utilize factory-integrated skids that combine inverters, medium-voltage transformers and switchgear into packages that range in capacity from 1 MW to 2.5 MW. These integrated systems offer project developers many advantages, including optimized component compatibility, as well as reduced installation time and expense. While the size and sophistication of centralized solutions continues to grow, an increasingly compelling trend is occurring in what are best described as small utility-scale systems (<50 MW) that challenges the centralized “bigger is better” power conditioning system approach.

In the last two years, the industry has seen the development and introduction of a new class of high-capacity string inverters that are well suited for both commercial and industrial use and increasingly are showing up in utility-scale power plants. Many of these units are rated for 1,000 Vdc and allow string lengths (and corresponding material and installation cost reductions) matching those of designs using central inverters. In addition, most of these string inverter models offer native 3-phase 480 Vac output that is well suited for integration with the medium-voltage transmission systems used in utility-scale PV power plants.

To get a well-rounded perspective on the project variables and deliverables that drive the centralized or decentralized design decision and how installers are deploying large-scale string inverter systems in the field, I reached out to nine power-conditioning system manufacturers active in the US market. Of these nine vendors, six manufacture or distribute both high-capacity string inverters and central inverters suitable for utility-scale projects. These companies include ABB, AE Solar Energy, KACO new energy, SMA America, Solectria and Sungrow. The remaining three vendors—Chint Power Systems, Fronius and SolarEdge—offer string inverters but not large central inverters in the US.

As with many PV design decisions, an individual project’s characteristics and site challenges, as well as the capabilities and limitations of the available equipment, ultimately drive a system’s general architecture and product specification. While both centralized and decentralized designs have bright futures in the North American market, contemporary string inverter–based power-conditioning solutions offer project developers an additional and potentially compelling option to consider.

What are the central and high-capacity string inverter models in your product portfolio?

“ABB’s ULTRA line of central inverters has three models: 750 kW, 1 MW and 1.5 MW. These central inverters are liquid cooled, have NEMA 4X enclosures and are available with up to four MPPT channels. The ULTRA models offer wide dc-input voltage ranges and operate with peak efficiencies of 98.4%. Skids and stations are available that pair all the necessary components to create up to 2 MW blocks.

“ABB’s high-capacity string inverter line is the TRIO family with 20 kW and 27.6 kW models. TRIO inverters are currently installed at ground-mount sites as large as 30 MW and are commonly deployed on commercial rooftops and carports. The TRIO inverter has dual MPPT, 1,000 Vdc input and 98.2% peak efficiency. Equipped with advanced grid-management features, including programmable power factor, and voltage and frequency ride through, the TRIO has features comparable to those of the ULTRA inverter. Engineered with an integrated switch box with options for ac and dc surge protection, and ac and dc disconnecting means, TRIO inverters eliminate installation of separate ac and dc BOS components, saving the installer time and capital investment.”

Sarah J. Ozga, product manager of commercial inverters, North America, ABB

“For the North American market, Advanced Energy (AE) offers 3-phase string inverters ranging from 12 kWac to 23 kWac, as well as central inverters up to 1 MWac per single inverter. In late 2015, AE will complement the portfolio with a higher-capacity string inverter targeting the large commercial and utility-scale segment, based on the topology of the 40/46 kWac inverter already available in the International Electrotechnical Commission (IEC) markets. All products are suitable for and proven in utility-scale applications and are aimed to achieve lowest cost of energy throughout the project life cycle.”

Verena Sheldon, senior manager of field applications, AE Solar Energy

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