(2013) Central Inverters for Utility-Scale Applications

A Designers Perspective on Product Selection and Specification

The US solar industry has reached a crossroads: The utility sector is starting to eclipse the traditionally dominant nonresidential sector. Over the past decade, the nonresidential sector—government buildings, retail stores and military installations—was the largest PV market sector in the US. This general trend is evident in Figure 1, which originally appeared in IREC’s annual report, “US Solar Market Trends 2011” (see Resources). The report’s author, Larry Sherwood, president of Sherwood Associates, also notes: “Utility-sector PV installations more than doubled in 2011 compared to 2010.” While the annual installed PV capacity in the utility sector did not exceed that of the nonresidential sector in 2011, it almost certainly did in 2012. According to the most recent “US Solar Market Insight Report” from SEIA and GTM Research (see Resources), “Q2 2012 was the largest quarter ever for utility PV installations, as more than 20 projects were completed, totaling 447 MW.”

Given the robust nonresidential and utility sectors, it should come as no surprise that market analysis by IMS Research indicates that the largest grid-connected PV inverter market in the world today is the market for central inverters. As shown in Figure 2, IMS Research expects this trend to continue. According to Cormac Gilligan, a PV market analyst at IMS Research, “Standard central inverters will continue to be the most widely used inverter type in 2016 due to the growing demand for large commercial and utilityscale projects, particularly in China, India, North America and other emerging markets.” Gilligan also notes that demand for turnkey substations is forecast to grow quickly in emerging markets, since these products “help speed up installation times or simplify designs for large projects.”

In this article, I discuss some of the innovative architectures and controls employed in central inverters for utility-scale PV applications. Since high dc utilization voltages are characteristic of these applications, I look only at inverters intended for use with 1,000 Vdc PV arrays. I also limit my discussion to inverters with a rated capacity of 500 kW or larger that employ both centralized maximum power point tracking and power conversion. This specific class of PV inverter serves an important market segment throughout the world, one in which very large, unshaded PV arrays are directly interconnected with utility distribution or transmission grids.

As a system designer, there are four questions I ask myself when I specify central inverters for utility-scale applications:

  1. Does the inverter meet all of the project-specific objectives, now and for the duration of its warranty?
  2. Are the inverter and its manufacturer bankable, and does the product have a documented track record of success?
  3. Has the manufacturer supplied me with everything I need to fully assess the cost, functionality, safety and performance of the inverter and the associated balance of system components?
  4. Does using this inverter result in the lowest levelized cost of energy (LCOE) for the entire PV system, compared to my other inverter choices?

While I do not discuss bankability in this article, I touch on each of the other considerations. Keep in mind that the first three questions are prerequisites to answering the last, and that it is essential to look beyond the information provided in product specification sheets. You cannot appreciate all of the innovation happening in this important inverter space—let alone put it to use—if you are simply comparing the peak efficiency of inverter A to inverter B. You need to understand what accounts for differences in specifications and ratings, and what this might mean in terms of a PV system’s overall functionality, reliability and profitability.

Inverter Topology 101

The power train of a central inverter is where all of the electrical transformations take place. The term topology describes the arrangement and control of the devices that make up the inverter power train. Central inverter topology is very complex, and in this article I am just scratching the surface of the topic. As a system designer, I am primarily interested in understanding how it relates to product cost, performance and reliability.

Table 1 (below) describes 14 attributes of central inverters used in large-scale PV applications, broken into seven categories: switching device type, dc source type, ac waveform control, switching bridge topology, conversion stages, modularity and galvanic isolation. In addition to describing these attributes, the table provides a brief summary of the pros and cons associated with each. Lastly, I have indicated whether the attribute is common in utility-scale PV inverters.

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