Allan Gregg’s phone has been ringing a lot recently.
As the chief technology officer for the inverter manufacturer Sungrow USA, Gregg has been the go-to person for utility-scale solar power plant developers looking to deploy the company’s virtual central inverter concept, unveiled last year.
Unsurprisingly, there are a lot of questions about exactly how it would work in the field, particularly using Sungrow’s 1,500-volt, 125-kilowatt inverter, which will begin shipping in July. But it’s not just the virtual concept that needs explaining.
Gregg says that many of the utility-scale solar developers need help moving past an outdated understanding of string inverters. He also has to make the case for marrying string inverters for large-scale solar power plants with the centralized controls that are one of the key advantages of central inverters.
“It’s a little bit of a paradigm shift, because classic string inverters, over the last four or five years, have been like 25...or 30 kilowatts, up to 60 kilowatts,” said Gregg. “We have to get people out of that paradigm and into the new world -- the 1,500-volt world, which has a new process for designing systems that have more solar panels per string, fewer strings, fewer combiner boxes and longer runs.”
From 2014 to 2016, three-phase string inverters gained global market share in utility-scale projects over 5 megawatts, growing from 10 percent to more than 30 percent last year, according to The Global PV Inverter and MLPE Landscape report from GTM Research. But the trend has been more prevalent in China and Europe, with less than 5 percent of utility-scale projects in North America using string inverters.
Sungrow’s central inverter concept brings together the company’s 1,500-volt, 125-kilowatt inverter to replace each 2.5-megawatt central inverter with twenty 125-kilowatt inverters. It then combines a collection of power modules with a centralized point of control and communication.
Ditching the integrated combiner box
The different features of Sungrow’s 125-kilowatt string inverter have important implications for designing the overall system. For example, each Sungrow string inverter has a single DC input and just one maximum power point tracker. It also does not have an integrated string combiner box, which most string inverters do. It’s another design element that often needs some explaining.
“If someone is having a tough time with the concept, I tell them to think about it like a mini central inverter and think about what kind of architecture makes sense,” said Gregg. “Architecturally, it makes more sense to not have the string combiner box integrated.”
One reason is size and weight. If a combiner box is integrated into the inverter, it increases the total weight enough that two people can no longer install it. As a result, this increases the time, complexity, and -- most importantly -- the cost of installation and maintenance. At just 150 pounds, Sungrow’s 125-kilowatt string inverter has the highest power output to weight ratio for any string inverter.
There are other reasons why Sungrow’s 125-kilowatt string inverter does not have an integrated combiner box. Traditionally, 1,000-volt string inverters with integrated combiner boxes have been located near the solar array in order to keep DC wiring runs short while utilizing the AC output for longer wire runs to the point of connection. Because of IEEE regulations about voltage drop, this meant AC wiring had to be more extensive -- and more expensive.
By contrast, the Sungrow string inverter is best located near the point of connection, rather than within the PV array.
Lowering costs, increasing flexibility
Sungrow’s 125-kilowatt inverter has an AC output voltage of 600, compared to 480 volts for most other string inverters. The AC output voltage also has important implications for choosing the correct wire size; for example, higher AC voltage allows for smaller and less expensive wire.
“Higher voltage is lower current, and that’s why we have a higher AC voltage,” said Gregg. “This is not a string inverter on the customer side of the meter. They have to get away from that idea and think of it as a mini central inverter. I want the highest AC voltage I can get so I can run my smaller wires from the inverter to the transformer.”
Not all of the advantages of Sungrow’s new inverter design come from jettisoning combiner boxes and lower wiring costs. Gregg argues that another benefit of the virtual central inverter is that it features a centralized command control interface.
“I can’t think of a situation where decentralized command and control would be desired,” said Gregg. “One of the main features of the virtual central inverter concept is the simplification of command and control.”
As string inverters have become larger and more sophisticated, their costs have also come down. Central inverters are also continuing to see price declines. “When you get the price point down to where the hardware costs are comparable to the classic central inverter, then all of a sudden the other benefits are gravy,” Gregg said of string inverters.
The virtual central inverter concept allows solar power plant designers to potentially mix traditional central inverters with the more customizable virtual central inverters.
For instance, a project may just want it to be possible to use virtual central inverters for the outer edges of an array that may be 1.2 or 1.6 megawatts in size. “They can customize the virtual central inverter to match the array size instead of being stuck with one-size-fits-all central inverters," said Gregg.