Array Voltage Considerations

Source-circuit configuration is arguably the most important aspect of PV system design. The electrical and mechanical characteristics of a PV array follow from this fundamental design decision, which has a bearing on both labor and material costs. In addition, source-circuit configuration impacts system performance, in some cases negatively. Low dc array voltage, for example, is a common cause of substandard performance that occurs when open-circuit or operating voltages for an array persistently fail to meet minimum inverter dc input voltage thresholds over time. In this situation, the system design does not take into account the cumulative effects of a variety of real-world circumstances, including high ac grid voltage, array degradation, module-to-module voltage tolerance and high ambient temperatures. Fortunately, low dc array voltage is avoidable.

In this article, I detail array design best practices for determining the maximum number of modules in a source circuit. My approach is slightly less conservative than the industry standard and is supported by changes to the National Electrical Code that are introduced in the 2011 cycle. I also present recommendations for determining the minimum number of modules per source circuit. While these may be more conservative than current design standards, my opinions are based on years of experience. They are not influenced by the desire to sell more or less of any specific product but rather by the general desire to propagate well-designed PV systems that perform optimally for decades.

CONSIDER THE SOURCE

Interestingly enough, over the past decade inverter manufacturers have been the primary source of education regarding array design and source-circuit sizing. With all due respect, these companies usually have expertise in power electronics and not necessarily in PV array design. However, since the advent of the first string-sizing program—which was developed by John Berdner while he was the president of SMA America—it has become the industry standard for inverter manufacturers to provide PV array configuration advice.

The main drawback to having inverter manufacturers dictate array design is that they have a conflict of interest. Manufacturers want their products to be used as often as possible, and this is facilitated in part by allowing the maximum number of module configurations. In addition, although most manufacturers have stern warnings about exceeding the maximum inverter input voltage, they generally have little to say about circumstances where there is too little voltage for the inverter to fully operate the PV array.

This skewed perspective informs both the string-sizing tools and the training materials that inverter manufacturers develop. The upshot is that inverters in the field seldom have a problem with high array voltage but routinely have problems with low array voltage. While low array voltage will not damage the inverter, it will compromise system performance.

If the inverter cannot operate the array at its MPP, for example, then power production and energy harvest suffer. Problems can also result from open-circuit voltage being too low. On hot days, an array’s Voc can pass below the restart voltage of the inverter. The consequence is that if the inverter shuts down in the middle of the day due to a utility disturbance, it will not restart until the late afternoon when the Voc increases. This can reduce the system’s operating availability by several percentage points annually if utility disturbances are common in the summer, such as when utilities switch in distribution capacitors around noon on hot days to accommodate high air conditioning loads. To design a PV array that is well-matched to an inverter’s operating window, system designers need to pay attention to the low end of the inverter operating voltage range, as well as to the maximum voltage allowed.

HIGH DC VOLTAGE

The maximum dc voltage for an inverter is clearly stated on the product specification sheet, installation manual or in tables, such as the one from the SolarPro article, "Central Inverter Trends in Power Plant Applications". While relevant UL standards and NEC requirements certainly apply, the maximum voltage is generally set by the input capacitors and the ratings of the transistors in the inverter, so it is a constant rather than a variable limit.

Because it is possible to create overvoltage in an inverter by putting too many modules in series, some manufacturers keep the maximum dc input voltage in nonvolatile memory for warranty purposes. This allows the manufacturer’s service technicians to verify the maximum dc voltage input to any inverter that is returned from the field under warranty. If the inverter was exposed to overvoltage conditions, then the manufacturer may choose not to provide a free replacement inverter. Historically, the most common cause of over-voltage is putting two source circuits in series rather than in parallel. This is a relatively easy mistake to make, especially in a small system with only two source circuits. Failure to properly account for low ambient temperatures is another potential cause of inverter overvoltage.

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