One of the latest advancements in solar inverters is three-phase string inverters for the small-to-medium commercial market.
This innovation is in response to residential applications having single-phase electricity, while commercial buildings often have three-phase systems. As Verena Sheldon, senior manager of field applications at Advanced Energy explains, three-phase electric power means having three single phases synchronized and offset by 120°. Each of three conductors in the system carries an alternating current of the same frequency and voltage.
“Three-phase electric power is commonly used for power generation, transmission and distribution,” she says. “It is more cost effective than a single-phase or two-phase system with the same voltage, mostly because of savings in conductor material. Most commercial and industrial systems in North America connect to a three-phase service.”
Until now, installers have used three single-phase inverters or a small central inverter for most commercial applications, but manufacturers are starting to offer a single three-phase string inverters. This offers a variety of benefits.
“Using three-phase inverters ensures that the power fed into the grid is distributed evenly among the phases, creating a balanced system,” Sheldon says. “Single-phase inverters can only create a balanced system when used in sets of three with equal total power rating per phase.”
“The local utility interconnection standards typically set a maximum value for phase imbalances, since large imbalances between the phases can cause instabilities,” she adds. “Labor and material cost for inverters, balance-of system equipment and wiring is significantly higher with three, single-phase inverters compared to one three-phase model.”
Ben Marcus of Sungrow North America also stresses the benefits of using a single three-phase inverter. “If you had three separate inverters, you could have different production for each, especially if some of the modules are shaded and feed only one inverter,” he says. “Lower production on one inverter would effectively lower the output of the entire system.”
“If the difference is substantial enough, it could cause the entire system to shut down,” Marcus adds. “It could also take longer for systems to get enough voltage to start up in the morning, and they might shut down earlier in the evening. Additional communication and protection devices would be required if you used three, single-phase inverters.”
Marcus notes that using three-phase string inverters has been common practice in Europe — even in utility projects — whereas the United States is just starting to pick up the technology.
“Part of the reason is that string-level monitoring is more common in Europe,” Marcus says. “This feature is more standard with three-phase strings, while typically it’s optional on central inverters.”
But Marcus says the United States is now beginning to see three-phase strings on multi-megawatt ground-mounted projects as well.
“As transformerless three-phase string inverter technology has matured, prices have come down and efficiency has increased,” he says. “These inverters have lower installation costs versus centrals, are light-weight — so they don’t require cranes of forklifts — and can further increase production with multiple power point trackers.”
“On the operations and maintenance side, entire units can be swapped out quickly without highly trained technicians,” Marcus says. “You can even have some spares on site.”