Centralized & Decentralized PV Power Plants: Vendor Perspectives: Page 8 of 11

“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

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