Advancements in utility-scale solar inverter design give developers plentiful options
Utility-scale solar projects are getting a bigger and bigger, with the largest approved project in the United States (as of August 2021) at a whopping 690 MWAC. Choosing the best inverters for these sites is becoming increasingly important to generate the massive amount of energy these projects are pursuing.
Project developers select inverters even before submitting an interconnection application. Utilities need to know those details to conduct studies that determine the impact of the solar projects on the grid.
“The inverter is one of the very important components of the studies and in fact allows us to see the project at a very early stage of development,” said Vishrut Bhatt, industry segment manager of renewable energy systems at TMEIC.
Manufacturers of modules, racks and inverters typically work together at the beginning of a project’s lifecycle to give developers the best plan for their projects, said Maren Schmidt, director of the utilities sector 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 terms with the module manufacturers and the tracker manufacturers because the goal should be to optimize yields.”
During this planning period, developers look at utilities, financial targets 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 that the inverter they choose will still be around two or three years into the future when the project is actually groundbreaking.
“It looks at the history of these manufacturers and where we think they are, how healthy their business is at the time and their forecasts for those coming quarters,” Taddonio said.
Choose between string and central
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 centralized and string inverters – manufacturers are now creating “centralized” string inverter solutions where multiple string inverters are grouped together in one enclosure. String converters provide multiple MPPT (maximum power point trackers) to reduce shadowing and are easier to swap out in the event of failure, minimizing power loss. If a central inverter fails at a site, much more energy is lost until O&M crews can fix the problem.
Fimer recently released two new solutions for utility-scale projects: a traditional stand-alone string inverter and a skid solution with multiple string inverters centralized in one place. Schmidt said the company has moved forward with the new centralized design, as the cost of string inverters is now nearly equal to that of power plants.
TMEIC also offers centralized string inverter solutions through its Solar Ware Ninja Line, with powers between 730 and 920 kW. These converters are integrated on a single 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 of string solutions for utility-scale projects. Now the modular Ninja line is TMEIC’s main offering in the US market.
Still, some utility contractors are sticking to true central inverters. Blue Ridge chooses plants for most of its projects because of their ease of installation, lower costs and additional storage options.
“While there is always the argument that you can repair a string inverter much faster than you can replace an entire plant and your system is not [long]I think some of the other considerations outweigh that. And we are still able to properly inventory the spare parts needed for these larger central inverters,” said Taddonio.
Installing large inverters
It is important to choose the right place to mount the inverters if an installer is stacking them together. The inverters must be easily accessible to O&M technicians and kept out of harm’s way, Taddonio said. Floodplains should be avoided, or at the very least the land should be built up so that the water drains away. Blue Ridge also ensures that transducers are carefully placed away from occupied structures, as transducers do produce some noise.
Once the right spot has been determined, it’s time to get the inverters in. Although the transport of large central inverters requires some heavy machinery, installation is a relatively simple process, according to Taddonio.
“Assuming you have that space on the site and you can handle that equipment on the site well, it becomes very efficient to install those units, especially if they come on the packaged skid assemblies,” Taddonio said.
Blue Ridge Power hoists the inverters onto the site and 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 cooperates with Blue Ridge and produces skid mounting equipment for inverters. After working for some time to remediate sunken assemblies for solar inverters, Cantsink came up with the pre-construction solution to install spiral piles under skid assemblies to prevent future soil disturbances.
“The soils are all very, very different. That is not always taken into account, and so you have that risk and that high probability of solving problems over time. You don’t want that with your project,” says Dara Macias, sales director at Cantsink.
Once the skids are secured in place, the commissioning process is quite simple. Inverter systems like TMEICs are prewired, requiring installers to simply bring the DC inputs to individual inverters, connect the AC cables to the transformer, and set up any fiber or control networks they need, Bhatt said.
“They go to that unit, they get their test equipment out, they sit there. Whether it’s a large unit or small units in a cluster, they can do all their work there in that one location, and it really takes care for some efficiency in their movements around the site,” said Taddonio.
Designing and installing inverters on large scale projects sounds daunting, but good planning, engineering and collaboration between manufacturers help make the process as simple as possible.