Utility-scale solar projects are getting bigger and bigger, with the largest project approved in the United States (as of August 2021) coming in at a whopping 690 MWAC. Choosing the best inverters for these sites is increasingly important to generate the massive amount of energy these projects strive for.
Project developers select inverters before they even submit an interconnection application. Utilities must know those details in order to perform studies that determine the impacts of the solar projects on the grid.
“The inverter is one of the very important components to the studies and basically allows us to see the project at the very early stage of development,” said Vishrut Bhatt, industry segment manager of renewable energy systems at TMEIC.
Manufacturers of modules, racking and inverters typically collaborate at the beginning of a project’s lifecycle to give developers the best plan for their projects, according to Maren Schmidt, managing director of the utility line of business at Fimer.
“We all have to work together,” Schmidt said. “We have the project developers, the EPCs, the technical advisors — we have to think in system together with the module producers and the tracker producers because the aim has to be to optimize the yield.”
During this planning period, developers are looking at utility requirements, financial objectives and product forecasts, said Brian Taddonio, VP of engineering for Blue Ridge Power, the newly formed EPC arm of Pine Gate Renewables. Developers need to be sure the inverter they’re choosing will still be around two or three years in the future when the project actually breaks ground.
“It’s looking at the past history of these manufacturers and where we believe they are, how healthy their company is at that point in time and their projections for those upcoming quarters,” Taddonio said.
Choosing between string and central
Utility solar project developers have more options than ever for the type of inverter that best suits each site. It’s no longer just a choice between central and string inverters — manufacturers now make “centralized” string inverter solutions where numerous string inverters are grouped together in one enclosure. String inverters offer multiple MPPT (maximum power point trackers) to mitigate shading effects and are easier to swap out in case of failure, minimizing power loss. If one central inverter goes down on a site, much more energy is lost until O&M crews are able to fix the problem.
Fimer recently released two new solutions for utility-scale projects: a traditional standalone string inverter and a skid solution featuring multiple string inverters centralized in one spot. Schmidt said the company moved forward with the new centralized design since the cost of string inverters is now nearly on par with centrals.
TMEIC also offers centralized string inverter solutions via its Solar Ware Ninja line, with power ratings between 730 and 920 kW. These inverters come integrated on one skid, with up to three Ninjas on each side of a centered medium-voltage transformer. The inverter manufacturer previously only made central inverters but saw the value in string solutions for utility-scale projects. Now the modular Ninja line is TMEIC’s primary offering to the U.S. market.
Still, some utility-scale contractors are sticking with true central inverters. Blue Ridge chooses centrals for the majority of its projects for their ease of installation, lower cost and add-on storage capability.
“While there is always the argument that you can fix a string inverter much quicker than you can replace a whole central and your system’s not down as [long], I think some of the other considerations outweigh that. And we’re still able to properly inventory the spare parts that would be needed for these larger central inverters,” Taddonio said.
Installing large-scale inverters
Choosing the right spot to mount the inverters if an installer is banking them together is important. The inverters should be easily accessible for O&M techs and kept out of harm’s way, Taddonio said. Flood zones should be avoided, or the land should at least be built up so water runs away. Blue Ridge also makes sure to be conscientious about siting inverters away from occupied structures, since inverters do emit some noise.
Once the right spot is determined, it’s time to bring in the inverters. Although transporting large central inverters does require some heavy machinery, Taddonio said installation is a relatively simple process.
“Assuming you have that space on the site and you can properly handle that equipment on the site, it gets very efficient to install those units, especially if they come on the packaged skid assemblies,” Taddonio said.
Blue Ridge Power cranes the inverters onto the site, then drops them onto pre-drilled screw systems to hold them in place.
“You put the screws in, then you drop it right on, you do some welding, you attach it and you’re good to go,” Taddonio said.
Cantsink works with Blue Ridge and manufactures skid mounting equipment for inverters. After working to remediate sinking solar inverter mounts for some time, Cantsink came up with the pre-construction solution to install helical piles under skid mounts to avoid future soil disturbances.
“The soils are all very, very different. That’s not always taken into consideration, and so you do have that risk and that high chance of having settling issues over time. That’s not what you want on your project,” said Dara Macias, director of sales for Cantsink.
After the skids are secured in place, the commission process is pretty straightforward. Inverter systems like TMEIC’s come pre-wired, requiring installers to simply bring in the DC inputs to individual inverters, connect the AC cables to the transformer and set up any fiberoptics or control networks they need, Bhatt said.
“They get to that unit, they get their test equipment out, they sit right there. Whether it’s a large unit or small units in a cluster, they can get all their work done right there in that one location, and it really creates some efficiency in their movement around the site,” Taddonio said.
Inverter design and installation on large-scale projects sounds daunting, but proper planning, engineering and collaboration among manufacturers help to make the process as simple as possible.