Central Inverters for Commercial PV Applications (2012)

While the basic concept of how an inverter does its job remains the same regardless of its size, the design variables and equipment options that system designers and installers have to consider increase along with inverter capacity.

When SolarPro magazine published its “2010 Central Inverter Specifications Guide” (October/November 2010), the companion article focused on central inverter trends in PV power plant applications. While utility-scale PV systems represented the fastest-growing PV market segment in the US last year—the Solar Energy Industries Association (SEIA) and GTM Research report that 242 MW of PV were installed by utilities in 2010, up from 70 MW in 2009 (see Resources)—the largest market segment remained nonresidential PV installations, accounting for 372 MW of installed capacity.

The nonresidential PV market segment includes commercial, public sector and nonprofit projects, and has largely driven the growth of the PV industry in the US for the last decade. Typical nonresidential customers include Fortune 500 companies (Applied Materials, Google, Johnson & Johnson and so on), big-box retailers (Ikea, Target, Walmart and so on), airports, universities, military bases, VA hospitals and so forth. According to Paula Mints, principal PV Services Program analyst at Navigant Consulting, this market segment can be further divided into four capacity-driven subsegments: small systems of up to 100 kW, medium systems of 100 kW to 500 kW, large systems of 500 kW to 1 MW, and multi-megawatt systems larger than 1 MW in capacity (see Resources).

In this article, I focus on central inverter selection, specification and installation issues typical of medium to large commercial applications. These projects typically feature monolithic PV arrays in the sense that module type and orientation tend to be homogenous. As a result, hundreds of kilowatts of PV array capacity are often connected to a single MPPT channel. The inverters themselves interconnect to 3-phase services, often at 480 Vac.

System designers and installers have many opportunities to exercise their problem-solving skills when working with central inverters in the 100 kW to 1 MW range. If you are used to working with single-phase string inverters in residential and small commercial applications, making the transition to working with higher-capacity central inverters presents some new challenges.

Most notably, central inverter systems in medium and large nonresidential applications require additional levels of dc combiners, overcurrent protection devices (OCPDs) and servicing disconnects. On the ac side of the system, the output voltage for a central inverter is seldom field configurable or auto-ranging, meaning that the correct inverter must be ordered to interconnect to the specific 3-phase electrical service. In terms of array-to-inverter matching, however, the design process is generally more similar than different when compared to systems employing single-phase inverters. Since PV arrays are built up one source circuit at a time, once you have determined the optimal string configuration, it is not that hard to design a system with 20 or 200 strings.

To keep the scope of this article manageable, I focus on grounded, monopolar PV array applications and transformer-based, UL-listed central inverters, which covers the majority of the products and applications available. First, I go over common central inverter design parameters and discuss how these play into inverter selection on commercial projects. Then I present a detailed case study that outlines how to specify and integrate larger central inverters.

Design Parameters

Selecting a central inverter for a specific project is based on several electrical parameters, including maximum dc voltage, MPPT voltage range, rated power, nominal ac output voltage, and maximum input and output current. Even if you work with single-phase string inverters exclusively, you are familiar with these parameters. Most of the electrical parameters that impact system design are the same, regardless of inverter capacity. These are also the electrical parameters that are typically found in inverter data sheets.

Other design parameters, such as termination and input combiner specifications, are more commonly found in inverter installation manuals. These parameters include dc and ac terminal type and configuration, conductor size ranges, neutral requirement, maximum OCPD rating, PV startup and shutdown voltage, and integrated dc input combiner options.

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