Centralized & Decentralized PV Power Plants: Vendor Perspectives: Page 5 of 11

“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

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