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?

“Solectria offers three 1,000 Vdc, 3-phase 480 Vac transformerless string inverters with capacities of 23 kWac, 28 kWac and 36 kWac. The inverters feature dual MPPT with four fused inputs per tracker. Solectria’s 1,000 Vdc, 3-phase 380 Vac central inverter models include the 500 kW SGI 500XTM and the 750 kW SGI 750XTM. Both models are designed for direct connection to an external transformer and can be configured as 1 MW or 1.5 MW Solar Stations. Available utility-scale options include a plant master controller and advanced grid-management features such as voltage and frequency ride through, reactive power control, real power curtailment and power factor control.”

—Eric Every, senior applications engineer, Solectria—A Yaskawa Company

How has the introduction of high-power 1,000 vdc, 3-phase 480 vac string inverters impacted the design choices available for utility-scale PV plants?

“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

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?

“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

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

“Owners who are just becoming familiar with decentralized systems are finding that commissioning costs and operating costs are higher than they originally expected. Consider a system with string inverters that have an uptime three times better than that of a central inverter. For the same array size, a decentralized array would require about 30 times more inverters. This means the decentralized array would require 10 times the number of inverter service calls. Since LCOE accounts for operating costs, additional service visits increase cost per kWh. The reality is that developers and investors should evaluate both solutions, and they will find they have two good options to choose from.”

—Eric Every, Solectria

How do centralized and decentralized designs affect the BOS requirements and construction costs for the dc and ac sides of the system?

“For short distances, it is generally more cost effective to route power through ac conductors as opposed to dc conductors. DC circuit ampacities have larger safety factors, and dc circuit conductors carry 73% more current than the 3-phase equivalent, which reduces wire size requirements. If circuit length exceeds about 200 feet, it is more cost effective to install dc circuits over ac circuits. Since the ac conductors operate at 480 V, they require larger conductors to achieve 1% voltage drop. Keeping voltage drop low is important because it equates to system energy loss. For utility-scale PV plants that require long runs, lower conductor costs make centralized solutions with combiner boxes much more attractive.”

—Eric Every, Solectria

How do commissioning and O&M activities compare when considering centralized and decentralized designs? How does the design approach impact plant availability?

“A 10 MW centralized interconnect could use 10–20 central inverters. For the same project, a decentralized system design would require 350–450 string inverters. That is significantly more inverters to maintain and service, which means higher annual O&M costs. Commissioning can take longer for string inverters because each and every inverter needs attention. Every inverter needs the dc input and ac output voltages verified, terminal torques confirmed and Modbus communications ID set. Additional time is necessary if the interconnection agreement requires specific voltage and frequency settings or power factor settings.”

—Eric Every, Solectria

What implications do centralized and decentralized designs have on inverter-based grid management and control?

“While both topologies offer great grid-management functions and value to the utilities, decentralized systems have significantly more inverters to monitor and control. These projects will typically use a plant master controller to provide a single communication interface with the utility. That controller then distributes commands to all of the inverters. System control becomes more complicated and costly as the number of inverters increases. Also, anti-islanding coordination is more difficult with a higher inverter count.”

—Eric Every, Solectria

How do you see the relationship between utility-scale project requirements and centralized and decentralized designs evolving in the next few years?

“Expect to see more-advanced inverter controls implemented more often. California Rule 21 and amendments to IEEE 1547 will remove restrictions on grid support features that inverters are capable of performing. Solectria will be able to meet these needs with both system designs, which are reasonable and have strong futures.”

—Eric Every, Solectria

CONTACT

Joe Schwartz / SolarPro magazine / Ashland, OR / solarprofessional.com

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