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

“Chint Power Systems (CPS) is currently shipping four 3-phase string inverter models. The 14 kW and 20 kW units are for 600 Vdc applications, and the 23 kW and 28 kW models are designed for 1,000 Vdc systems. While we do have large multi-MW installations with our 600 Vdc line, the majority of large-scale systems are designed with the 1,000 Vdc product line. The 23 kW and 28 kW inverters are dual MPPT, with best-in-class efficiency at 98% CEC, and ship with a standard integrated wiring box that is also separable. They offer integrated AFCI functionality and high-power output at 480 Vac. Users can install the inverters in vertical or horizontal (15°) orientations. The entire product line ships standard with advanced utility-interactive control features including remote power-factor control, reset and dynamic ramp-rate control.”

Sukriti Jain, applications engineer, Chint Power Systems, North America

“Fronius now offers the Fronius Symo line of string inverters designed for commercial and utility 3-phase systems. The Symo portfolio comprises nine lightweight, transformerless 1,000 Vdc inverters with dual MPPTs. Symo 240/208 Vac and 480 Vac models are available from 10 kW to 24 kW.”

Moe Mahone, key account manager of commercial sales, US, Fronius

“KACO new energy’s central inverter family includes the XP500U-TL, XP550U-TL, blueplanet 750 TL3, blueplanet 875 TL3 and blueplanet 1000 TL3. All models have a large range of reactive power support (0.2 lead/lag) and full apparent power (kVA) at 50°C, and are compatible with most global grid codes for utility interaction. Distributed 3-phase string inverter models include blueplanet 32.0, 40.0 and 50.0 TL3, and Powador 60.0 and 72.0 TL3. The blueplanet inverters are listed to UL 1741 standards. The Powador models are CE certified for international use. All string inverter models have a large range of reactive power support (0.6 lead/lag) and full apparent power (kVA) at 50°C or higher, and are compatible with most global grid codes for utility interaction.”

Bill Reaugh, senior director of product management, KACO new energy

“SMA’s utility-scale product portfolio includes string and central inverters for systems 2 MW or greater and interconnecting at medium voltage. SMA’s string inverter solution for projects of this size—where array-mounted dc combiners are not required, since the inverter itself takes their place—is the Sunny Tripower. The Tripower series allows for a decentralized design for utility-scale plants and includes 3-phase 480 Vac, 4-wire models from 12 kW to 24 kW with built-in AFCI, dc step-up and dual MPPT inputs for maximum string flexibility. The 60 kW MLX 60 string inverter completes the product family.

“For central inverters, SMA offers 19 Sunny Central models ranging from 500 kW to 2,500 kW. For a complete system solution, the Compact Medium Voltage Power Platform and Utility Power System solutions package central inverters, medium-voltage transformers and all related BOS equipment as a complete assembly built in a controlled environment. SMA’s central inverter portfolio gives designers opportunities for 600 Vdc, 1,000 Vdc and 1,500 Vdc systems; a variety of MPPT and operating ranges; multiple inputs and fuse sizes; proven grid support functionality; reactive power capabilities; class-leading efficiencies; and the highest power densities available.”

Ryan LeBlanc, senior application engineer, SMA America

“SolarEdge’s SE20k model is a 20 kWac inverter that provides a distributed generation solution for today’s utility PV plants. Combined with our P700 2-to-1 power optimizers that allow connection of two 72-cell modules in series, the SE20k offers high-energy yield, increased design flexibility, module-level monitoring and SafeDC architecture.”

Dru Sutton, technical marketing manager, SolarEdge

“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

“Sungrow central units include the 600 Vdc SG500LV and three 1,000 Vdc models, the SG750MX, SG800MX and SG1000MX. We offer three high-capacity string inverters, including the 480 Vac SG30KU and SG36KU, and the 380/400 Vac SG60KU. All inverters feature 10% overload capability at a 0.9 power factor up to 50˚C ambient. The central inverters have customizable dc input options, including fuses or a direct busbar connection, and an optional dc breaker cabinet. The 30 kW and 36 kW string inverters feature 10 monitored dc inputs. The 60 kW unit has eight MC4 inputs. The use of Branch ‘Y’ MC4 connectors in the PV array allows for cable savings by requiring only one homerun to the SG60KU per two strings. The SG60KU is one of the first units to use silicon carbide power components, allowing us to nearly double the power density. This, combined with advanced cooling techniques, allows for a lower cost per watt, as well as lower labor and BOS costs.”

Paul Mync, technical sales manager, Sungrow USA

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?

“1,000 Vdc string inverters open up potential sites for utility-scale plants that central inverters previously could not serve due to shading, noncontiguous land segments or uneven land requiring arrays of different tilts or azimuths. The decentralized architecture of string-inverter systems also distributes the weight load, allowing installations on landfill sites with weight restrictions. The 3-phase string inverters allow more flexibility in the PV plant size, so you can size plants in increments of 20 kW and not in 100 kW blocks. Additionally, ABB 3-phase string inverters have a 10-year warranty in contrast to the standard 5-year warranty for central inverters.”

Sarah J. Ozga, ABB

“The availability of high-efficiency transformerless 3-phase string inverters and code changes that facilitate 1,000 Vdc systems make string inverters a competitive option for utility-scale applications. Giving system designers and integrators the opportunity to choose from these different topologies increases design flexibility. The specific system details, financial metrics or site-specific design challenges or limitations drive the decision. With string inverters becoming a viable option for utility-scale applications, system designers and installers can transition more easily from commercial applications to utility scale, since they would be using the same equipment.”

Verena Sheldon, AE Solar Energy

“The impact is positive. We see an increasing number of developers evaluating 3-phase string inverters for utility-scale power plant applications. The adoption of a decentralized approach for projects ranging from 2 MW to 5 MW began in 2013, and we see this trend continuing into the future. CPS is releasing a 36 kW string inverter this year and will follow with higher-power units designed specifically for utility-scale applications.”

Sukriti Jain, Chint Power Systems, North America

“Fronius has been a front-runner in the US solar industry’s shift of consciousness toward a distributed inverter topology. The European market frequently leads design trends, such as allowing 1,000 Vdc residential and commercial systems. The flexibility that string inverters provide makes them an ideal choice for both simple and complex designs.”

Moe Mahone, Fronius

“The introduction of 1,000-volt high-power (30 kWac–60 kWac) string inverters provides more design choices to developers and system integrators. The major benefits of string inverters include improvements in energy harvest due to a larger number of MPPTs, an increase in availability and a reduction in downtime when issues occur. However, the increase in the number of inverters can make interconnection design more complex. The communications and control infrastructure can also be more complicated due to the larger number of devices.”

Bill Reaugh, KACO new energy

“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

“The decentralized approach offers greater design flexibility and can solve a number of challenges that can be harder, or in some cases impossible, to overcome with a centralized approach—space constraints or weight limitations, for example. Because string inverters are light and have small footprints, installers can mount them directly at the back of the array. With this placement option, shading and required row space are far less significant issues than they would be with a central inverter. Weight per unit can become a decision criterion on sites with unstable ground, such as landfills. The main challenge in utility-scale string inverter systems is the increased complexity of the monitoring and control system, which the use of equipment suitable for the specific application can easily overcome.”

Verena Sheldon, AE Solar Energy

“The primary benefit of a decentralized approach is to increase system uptime and performance. If an inverter failure occurs with a central inverter, a large loss of production occurs. A smaller inverter building block minimizes this risk. In addition, the typical MTTR for a central inverter often exceeds a week or more. Technicians can replace string inverters within days. The secondary benefits of a decentralized design include elimination of combiner box costs, inherent zone and string-level monitoring, installer preference to work with ac wiring (even given 1,000 Vdc systems), and the design and procurement efficiencies of smaller building blocks. String inverters have no real limitations to use. However, the uptime benefits do decrease as system size dramatically increases. For example, a single 500 kW inverter failure for a 5 MW system has more impact (10% loss) than a failure in a 50 MW project (1% loss).”

Sukriti Jain, Chint Power Systems, North America

“Design flexibility, reliability, system uptime, and low LCOE and BOS costs, as well as ease of installation and logistics, all make decentralized systems a compelling value. The size of decentralized utility-scale string inverter systems grows each year. Our customers have found few limitations and multiple benefits.”

Moe Mahone, Fronius

“The largest cost-related benefit to distributed string inverter systems in utility-scale projects is a vast simplification of the medium-voltage interconnection. While the low-voltage ac network is more complex due to the larger number of inverters, the output LCL [inductor-capacitor-inductor] filter design of these inverters can reduce the medium-voltage (MV) transformer  requirements. Specifically, most central inverters require a custom MV transformer with three or even four windings to provide the necessary impedance between the inverters to prevent recirculating currents. String inverters have this additional impedance built in, and a standard MV transformer with two windings is possible on these types of systems.”

Bill Reaugh, KACO new energy

“Compelling value propositions of decentralized utility-scale systems include simplified shipping and logistics (for example, humans or animals can carry string inverters to remote sites in the developing world); increased ease of installation (reduced ground work and concrete); increased performance data granularity; increased MPPT for high ground–cover ratios due to interrow shading, rolling clouds and tree lines; and reduced O&M (you can simply swap inverters out rather than bringing in repair specialists). Limitations of string inverters for utility-scale systems compared to central inverters are very few, because both have nearly identical grid management capabilities, efficiencies, loading capabilities and so on. However, cost is one differentiator since many smaller units tend to cost more than one big one, due to redundancy of components. Central inverters also do not require as much ac collection, so costs associated with this can be a factor as well.”

Ryan LeBlanc, SMA America

“A benefit of decentralized utility-scale string inverters is the ability to swap dc combiners for ac combiners. Furthermore, the use of string inverters simplifies the maintenance process, which becomes similar to that for maintaining a residential inverter.”

Dru Sutton, SolarEdge

“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

“The cost of string inverters in recent years has declined significantly—more rapidly than that of central inverters—due to increased power classes and improved production methods. Therefore, LCOE has seen a convergence in recent years to a point where distinguishing factors are project specific, with the costs of decentralized systems equaling—and then perhaps improving upon—centralized system costs. Some sites favor one approach, while other sites are more favorable to the other. What the solar industry is experiencing today is almost a push at the CAPEX level, but modestly improved performance and lower O&M costs improve LCOE for string inverter systems.”

Ryan LeBlanc, SMA America

“The biggest change we notice is a greater appreciation of factors such as risk, O&M and system lifetime. Previously, most of the attention was on the initial capital investment; now this has shifted to considerations such as O&M stipulations, warranty period and system uptime calculations.”

Dru Sutton, SolarEdge

“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

“String inverters still cost more than central inverters, but that difference continues to shrink rapidly. Fortunately, the market is advanced enough to where the dollar-per-watt cost is not the only consideration. When you factor in the other benefits of decentralized designs—such as built-in combiner boxes with string-level monitoring; multiple MPPTs; savings because you do not need cranes, skids and in many cases concrete pads; and lower downtime and O&M costs, especially in remote locations—we have seen string inverters deployed in larger and larger projects.”

Paul Mync, Sungrow USA

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

“Centralized systems have higher dc BOS costs due to the need for multiple dc combiner boxes to feed into the central inverters. Most string inverters used in large plants include an integrated dc combiner that reduces the dc BOS costs. In addition to eliminating external dc combiner panels, string inverter systems eliminate large PV output conductors and conduit. Designers can achieve additional savings on the dc side by placing the string inverters closer to the arrays, which minimizes the length and size of PV source-circuit conductors and conduit. Reducing the size of these conductors reduces the associated labor costs as well. Decentralized systems require the use of ac combiner panels to parallel the output of the string inverters to one point of connection with the utility. While this is an additional cost that centralized systems usually do not require, the cost of ac combiner panels is lower than that of dc combiners and offers a net BOS savings.”

Sarah J. Ozga, ABB

“For decentralized systems, the majority of the BOS cost is on the ac side for collecting and combining ac runs from the individual inverters. However, these systems have dramatically reduced dc BOS compared to centralized systems because they do not require external dc combining if the inverter comes with this capability integrated, as does the AE 3TL product. The cost for dc wiring including conduits generally exceeds the wiring cost on the ac side. Also, the costs for fused dc combiners with disconnect, which centralized designs require, run higher than for the panelboard typically used to combine and protect the ac side in decentralized designs. Overall, there is a potential for BOS savings using the decentralized approach.”

Verena Sheldon, AE Solar Energy

“String inverters from CPS come with an integrated combiner box that has prefused string inputs, ac and dc disconnects, AFCI functionality, and a communication connection for monitoring all built in. This minimizes BOS on the dc side for decentralized plants by combining all these functions in the inverter itself. You can mount string inverters within the array to minimize long dc homeruns and reduce wire costs on the dc side. The ac collection scheme requires panelboards with circuit breakers, which you can also mount either within the array or next to the transformers, depending on the project layout.”

Sukriti Jain, Chint Power Systems, North America

“With the availability of dual MPPT 1,000 V string inverters, dc strings can now be much longer and more efficient, and require less, if any, string fusing before the inverter. Due to the mass production of standard ac components, the cost is typically much lower than for specially produced dc components. Mounting inverters at the array alleviates the need for the costly engineering, parts and labor involved in building a concrete pad or the crane lifting sometimes needed for mounting a central inverter.”

Moe Mahone, Fronius

“With regard to distributed architectures, the smaller number of conductors and the lower currents reduce the cost per combiner box for the dc collection; however, the potential for an increased quantity of combiner boxes may offset that advantage. On the ac side, we see a significant potential for cost savings. Manufacturers sell most central inverters to projects in pairs integrated onto skids. To reduce inverter cost, most central inverters require the use of three- or four-winding MV transformers. These custom-designed transformers lead to higher first costs and higher maintenance costs down the road. With distributed inverters, the additional impedance required to prevent recirculating currents already exists, so you can use utility-standard two-winding MV transformers. This seemingly small change can significantly reduce up-front costs and ongoing maintenance costs.”

Bill Reaugh, KACO new energy

“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

“Decentralized design eliminates dc combiner boxes, recombiner boxes and the need for dc monitoring. The elimination of dc recombiners, required for central inverters, offers some immediate cost reductions. Sungrow’s SG60KU offers built-in MC4 connectors, which can save labor time on the inverter installation and wiring. Monitoring becomes more inclusive, as you can monitor the inverter and the strings together with 3-phase string inverters through one Modbus connection, as opposed to central inverters that require external string monitoring or internal zone monitoring. Sungrow has done BOS analysis, and we see the costs close to equal or perhaps slightly lower for 3-phase string inverters. One thing is for sure: using ac circuits to parallel large numbers of 3-phase string inverters eliminates the use of PV-specific high-voltage dc components and allows the system designer to specify common, readily available 3-phase ac collection equipment that electricians are familiar with. The large number of 3-phase string inverters can definitely impact labor costs, so construction efficiency becomes more important in systems with hundreds of inverters.”

Paul Mync, Sungrow USA

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

“While performing O&M on decentralized plants, you need to shut down only a small portion of the plant at any one time, which provides greater power production compared to a centralized plant.”

Sarah J. Ozga, ABB

“Generally speaking, the commissioning of a central inverter requires more highly skilled laborers who are specifically trained for the equipment you are commissioning. Some manufacturers make it mandatory for the electrical personnel to attend factory training and get certification prior to commissioning. In contrast, the level of knowledge and training required to install and commission a 3-phase string inverter in commercial or utility-scale applications is comparable to that required for a residential installation—installers need electrical knowledge that they can apply to the specific guidelines given in the manufacturer’s installation manual.

“Looking at O&M plans and activities, the decentralized approach presents a potential for additional savings. Although there are differences between individual brands, it is safe to assume that string inverters typically require less preventive maintenance than do central inverters. During a maintenance event, the technician must de-energize the equipment. The decentralized design approach allows the technician performing the maintenance to turn the system off in smaller sections, enabling the remaining sections to continue to operate. This decreased system downtime during a maintenance event translates into overall higher system availability and higher energy yield.”

Verena Sheldon, AE Solar Energy

“Centralized designs follow a repair model, whereas decentralized designs mostly follow a replacement model. Thus, O&M activity for a decentralized power plant is minimal and limited to swapping the inverter in the field. A centralized approach requires regular maintenance of parts and time-consuming repair in the field in case of failure. Plant availability, or uptime, increases significantly when you reduce the mean time to repair field issues. In addition, a centralized design approach can reduce the budget requirements for general system O&M.”

Sukriti Jain, Chint Power Systems, North America

“Commissioning decentralized systems is a simple prospect that experienced solar installers are familiar and comfortable with. Commissioning string inverters one at a time decreases stress and liability. New software and monitoring options allow application of settings to a fleet of inverters from a central on-site or remote location. A major benefit for O&M comes from the ability to field-service inverters. If a central inverter goes off-line, the production lost waiting for a technician to repair the system can have a substantial impact on plant availability. Central inverters come with a shorter standard warranty, and manufacturers usually sell them with additional service packages to cover regular inspections and servicing. Data from independent O&M companies show that central inverters have a 13% higher O&M cost. Decentralized systems provide reliability by eliminating the single point of failure.”

Moe Mahone, Fronius

“There is a significant and measurable difference in central versus distributed architectures when considering O&M and availability. Generally, distributed inverter systems have significantly lower maintenance requirements. This makes the skill level of the required maintenance personnel less important, and it significantly reduces the cost of spare parts inventories. In some cases, the repair program is ‘rip and replace,’ similar to that for small commercial and residential systems. When an inverter fails in a distributed system, the percentage of the total system that goes offline is substantially less than in a typical central project. This, combined with the typically faster return to service on distributed inverters, means that distributed architectures significantly improve availability.”

Bill Reaugh, KACO new energy

“You can generally achieve faster commissioning with string inverters, while central inverters require more specialized, highly skilled technicians to adequately and rapidly commission the larger units.”

Ryan LeBlanc, SMA America

“Decentralized system designs simplify the maintenance process, which becomes similar to that for maintaining a residential inverter. These designs also enable the installation of replacement inverters on-site. Furthermore, they allow for greater visibility into plant operations with string-level or module-level diagnostics. We can do some of the commissioning on the fly by using the SafeDC feature to check voltages before connecting to an inverter. Finally, decentralized designs allow phased commissioning, such as 20 kW at a time, which is a huge advantage when you are dealing with large installations.”

Dru Sutton, SolarEdge

“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

“O&M and commissioning are key factors when looking at decentralized design. Three-phase string inverters offer quick installation and relatively simple commissioning procedures. With built-in combiner boxes, you can do string testing essentially after wiring up the inverter. You can set parameters via the LCD screen or remotely via Modbus. The commissioning of 3-phase string inverters does not require specially trained technicians. Central inverter commissioning can often require manufacturer technicians and an on-site supply of spare parts, and it can take more time than commissioning string inverters.”

Paul Mync, Sungrow USA

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

“Comparing the grid management capabilities and limitations of the inverters themselves, there are no significant differences between central and string inverters. The main difference lies in the system control structure, which at first sight can be more challenging using a distributed approach. For system coordination, it is critical to provide a control structure that allows communication with and control of several inverters at once. Readily available equipment ensures this coordination. One of the benefits of distributed designs is the added granularity, which can be beneficial for certain requirements, such as a controlled staged ramp-up.”

Verena Sheldon, AE Solar Energy

“Decentralized designs offer better granularity of control than centralized designs. While it is true that these systems have more inverters to control, modern communication methods and the use of a plant control scheme can support and control a large number of inverters.”

Sukriti Jain, Chint Power Systems, North America

“Decentralized systems allow a more granular and flexible system for grid management and control. Combining output from multiple units allows a plant-level controller to offer multiple options for controlling power. For example, reducing power from all units, or alternatively, taking some units off-line completely, could meet a power reduction command. Intraplant communications could be more specialized and possibly more complex in a distributed system.”

Moe Mahone, Fronius

“From the KACO perspective, choosing a distributed or central inverter has no significant impact with respect to grid management, support and control. Distributed inverters have a smaller range of reactive power support than central inverters, but for projects within the Americas, the available range even from the distributed inverters is more than sufficient for most projects.”

Bill Reaugh, KACO new energy

“Only very minor differences in grid management and control are present with respect to SMA’s solutions. SMA offers both small and large inverters with advanced grid support functionality. In Europe, the industry often requires system sizes down to 3 kW to provide fault ride through capabilities, among other factors, to help support the grid in the event of a fault rather than drop off and make it worse. With string inverters, there are more nodes on the network, but both designs can meet the features just about any project requires.”

Ryan LeBlanc, SMA America

“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

“Most available 3-phase string inverters have the advanced grid-support features that have become standard with central inverters. These include LVRT, reactive power injection, power curtailment, remote shutdown and frequency response. The interconnection standards in Germany have driven this functionality, so if an inverter manufacturer is active in the German market, its inverters most likely have these capabilities. Communication capabilities are also important, and both central and string inverters offer Modbus RTU and TCP/IP as standard offerings, required for communication with utility plant controllers.”

Paul Mync, Sungrow USA

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

“Due to the economy of scale, very large utility-scale projects will continue with central inverters and migrate from 1,000 Vdc to 1,500 Vdc systems for BOS cost savings. As developers build out large 100 MW utility-scale projects on available, primarily flat desert parcels, decentralized string inverter systems will be more common for smaller utility-scale plants on less homogeneous sites that are better served by more granular MPPT optimization.”

Sarah J. Ozga, ABB

“String inverters are packing more power into smaller enclosures and becoming smarter with their utility interactive control features and commands. With increased plant availability and ease of installation, as well as replacement, decentralized power plants will become the norm rather than an exception over the next few years.”

Sukriti Jain, Chint Power Systems, North America

“At Fronius, we see a continuing trend toward decentralized systems in utility-scale projects. With the availability of larger and more efficient string inverters and the introduction of favorable new building codes, the goal of driving down system costs and decreasing the LCOE is now more attainable than ever.”

Moe Mahone, Fronius

“The biggest evolution in utility-scale projects going forward will be the change to 1,500 Vdc and the concurrent change to 690 Vac output. These changes will impact the BOS costs on the dc and ac sides of the project, but will not necessarily affect any of the points discussed above when comparing the benefits of central and distributed inverters.”

Bill Reaugh, KACO new energy

“With each new record year for solar, in North America in particular, the industry develops the low-hanging, easiest sites first, and then tackles the more difficult sites, which in turn increases the demand on manufacturers to innovate even more flexible and robust equipment. In the next few years, SMA anticipates that utility-grid management requirements will drive increased interactivity and a broader range of applications. Both string and central inverters can take this on without either having a particular advantage, but we expect the flexibility of string inverters to become a more important trait as project sites become more remote and oddly shaped, and owners become more cost conscious. SMA’s 2 MW Sunny Central 2200-US is feature rich, cost effective, compact and efficient. This will be a difficult machine for string inverters to compete with on cost alone. However, where site access for larger machinery and higher-skilled labor is limited, or where O&M needs to be lean, we can expect to see some very large projects installed with some modestly sized inverters.”

Ryan LeBlanc, SMA America

“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

“The evolution of utility-scale project requirements will likely involve storage systems integrated into the PV plant. Large-scale storage will help with the intermittent nature of PV and provide ancillary services that customers can monetize on the utility level. These developments may lead to the combination of 3-phase string inverters with a central, four-quadrant inverter that allows for battery-bank charging and discharging. Communication and control will also drive utility-scale project requirements, because utilities will need to monitor and control these generation assets through advanced communication schemes. For both centralized and decentralized designs, the plant controller will evolve, as well as the on-board web servers inside the inverters.

“Asset availability will also play a deciding factor in utility-scale plant design as we gather more data from existing systems. Three-phase string inverters provide more redundancy, which developers will find compelling as they execute PPAs based on performance. In addition, 3-phase string inverters offer increased energy production due to the number of MPPTs and the ability to generate power early in the day and late in the afternoon—the ‘shoulders’ of the daily power curve get wider. As utility-scale systems become more advanced, uptime and control will become paramount, and 3-phase string inverters provide less risk in terms of downtime. Additionally, 3-phase string inverters will also increase in size, eventually breaking the 100 kW mark, while remaining lightweight enough for a two-person installation. Finally, 1,500 Vdc 3-phase string inverters will hit the market in the coming years, which will increase efficiency while further lowering the LCOE.”

Paul Mync, Sungrow USA

CONTACT

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

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