In a recent webinar, Allan Gregg, director of applications engineering at Sungrow, discussed some best practices for utility solar PV plants in terms of choosing the right inverter. Here’s a Q&A from the presentation. You can watch the full webinar here.

SPW: Can you discuss the advantages and disadvantages of using string or central inverters when it comes to projects using storage? 

Gregg: PV string inverters and PV central inverters must be tied to a grid to operate. As soon as the grid goes away, they must shut down to meet UL 1741. The only way you’re going to put energy storage with PV inverters is to parallel them on the AC side. In this case, it doesn’t really matter which inverters you use because string inverters and central inverters will respond the same. When you talk about hybrid inverters that can handle both PV and batteries, you’re talking about a totally different animal because those inverters have to be “grid forming,” or they have to be able to operate either tied to a grid or in island mode. PV inverters by themselves can operate in island mode, at least longer than about three seconds for low level ride through.

SPW: Many times inverters have warranties of five or 10 years, when solar projects are meant to last 25 years. Why is that?

Gregg: No manufacturer is going to warranty their out-of-the-box product for 20 to 25 years for a variety of reasons. The key thing to keep in mind is that you can pay extra and get extended warranties for 20 to 25 years with most companies, including Sungrow. But when the money experts look at those numbers, they believe it’s better to budget to replace the inverters after about 10 years. It could actually cost less to have an opportunity to upgrade a technology if you budget this way.

With central inverters, this type of budgeting isn’t really practical because it will cost more to remove and replace the central inverter that is mounted on a concrete pad. But with string inverters, since you just bring in the string and the AC connection to the site or to the string inverter itself, you could just swap that with a different string inverter 10 years down the road. So really, the biggest differentiator is not so much the factory warranty, but what it is going to cost to maintain the inverters over a set number of years, and to have the option to upgrade if possible.

SPW: If the failure rate for string inverters is the same or higher than central inverters and since there are more string inverters than central inverters in the system, how do you look at repair and maintenance costs when their repair and maintenance costs may be higher?

Gregg: The biggest thing to keep in mind is that with string inverters, you have more inverters that can fail. Typically, the failure rate between the two inverter types is about the same, but if you have 20 times as many inverters, you’re going to have 20 times as many failures. You have to weigh that in but the difference is that with a 60-kW inverter versus a 1-MW inverter, if you lose a 60-kW inverter, you don’t have to bring in a maintenance crew. If you lose a 1-MW inverter, you have to bring in a crew because it’s considered an emergency. With string inverters, you can wait until you lose four or five inverters before you bring in a crew. That’s what drives the cost. It’s not so much the cost of repairing the inverter, it’s bringing the service and maintenance people out to the site, and the more remote the site, the higher the cost.

SPW: Do you think central inverters will eventually be replaced by string inverters in utility scale systems, or do you think they will definitely have a place?

Gregg: I think both types of inverters have a place. I think the home for central inverters is going to be in the mega size system, 100-MW, 200-MW sites, which actually aren’t uncommon. I think the line separating central and string is probably going to increase to at least the 250-MW range and maybe beyond in the near future.