Distributed Inverter Design

Utilizing String Inverters in Large Commercial Systems

For the past decade, designers and installers working in the North American PV industry have welcomed the challenges presented by larger, more complex systems. Inverter manufacturers also embraced the challenge and began to offer high-capacity string and central inverters to meet the design and power-conditioning demands of these larger solar projects. My company, Sunlight Electric, completed its first PV installation in 2002. The project utilized a single SMA America Sunny Boy 2,500 W inverter. As system sizes grew, we installed multiple string inverters coupled with an ac aggregation panel to meet a given system’s capacity requirements. In 2004 we installed our first project that utilized a central inverter. It was an important milestone for the company, and we had a sense of accomplishment in knowing that we had graduated from “beginner” to “expert” inverters. However, in recent years we have taken a step back and have been developing large commercial systems based on distributed designs that utilize multiple string inverters.

Today, for projects between 100 kW and 1 MW, designers can specify one or more central inverters, or they can create a distributed design that utilizes numerous wall-mountable string inverters. In the abstract, designing a 1 MW PV system consisting of 400 2.5-kW string inverters does not seem all that far-fetched. Intuitively we appreciate that there are major benefits to distributed designs in terms of fault tolerance and performance. After all, the Tesla Roadster battery pack consists of approximately 7,000 individual cells, and Google employs nearly 2 million servers across 12 data centers around the world.

Despite this intuitive understanding, if you asked PV system designers to consider using 400 2.5-kW inverters on a 1 MW PV system, it would probably be difficult to engage in a serious discussion of the relative merits of such a design. The foregone conclusion is that this approach would not be cost effective due to greater inverter, labor and BOS costs. But consider two 500 kW inverters versus a single 1 MW inverter. Savvy designers and engineers recognize the benefit of the increased fault tolerance. After all, the chances of both inverters failing at the same time would be considerably less than the chance of a single inverter failure. But what about specifying four 250-kW inverters, 10 100-kW inverters or even 40 25-kW inverters? This is where the discussion starts to get interesting.

Despite tendencies in the US toward utilizing central inverters for commercial- and small utility-scale projects, Henry Dziuba, the president and general manager of SMA America, points out that “distributed inverter designs have long been a best practice in Europe.” New string inverter products allow integrators to deliver robust and cost-effective distributed systems for their customers. In this article, I present the potential benefits of the distributed inverter design approach and discuss engineering and installation best practices for integrating these systems based on the evolution of our design approach at Sunlight Electric.

Benefits of a Distributed Inverter Design

For commercial-scale projects, we began using central inverters at the earliest possible opportunity. In 2004 we specified our first central inverter: a Xantrex PV225 (225 kW) model for a 179 kWdc project at Frog’s Leap Winery in Napa, California. We did not even consider utilizing multiple string inverters, and we ruled out using two 75 kW central inverters because the cost of the single 225 kW inverter was actually lower.

A few years later, when we were designing a 125 kWdc ground-mount project for ZD Wines in Napa, we added the distributed approach to our list of options. We considered specifying a Xantrex PV225 central inverter, but the cost penalty of purchasing an additional 100 kW of capacity would have made the economics prohibitive. By then, SMA America had introduced the Sunny Boy 6000-US, and we realized that the distributed approach allowed a better matching of the PV array and inverter capacities. In this case, the string inverter option was also the most economical solution. With a higher weighted efficiency and a lower cost per watt, the 6 kW string inverter also won out over the 30 kW and 40 kW central inverter models that were available at the time. After we ruled out using a single large central inverter and multiple smaller central inverters, the best option appeared to be specifying 18 string inverters. As part of our typical stakeholder sign-off process, I contacted SMA’s technical support staff to discuss integrating 18 SB 6000-US inverters and learned there were even more compelling benefits associated with a distributed inverter design.

Optimized inverter-to-array ratio. Central inverters have larger power-capacity increments than string inverters do, making it more difficult to optimize the ratio between the inverter and array capacities. The distributed approach gives designers more flexibility to optimize the ratio between the inverter’s dc input capacity and the size of the PV array, and can reduce a project’s total inverter cost and improve the owner’s ROI.

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