Array Voltage Considerations: Page 2 of 6
Inside this Article
Some inverter manufacturers have claimed in their trainings that a 600 Vdc inverter will spontaneously combust if the array reaches 601 Vdc. While the inverter warranty may be voided if the array goes above the published maximum voltage, it is inconceivable that the capacitor or transistor tolerances are tight enough for the devices to operate well at 600 Vdc and explode at 601 Vdc. If that were true, inverters would also explode at 580 Vdc and they (usually) do not—at least not because of component tolerance.
Low temperature calculation. Most inverter manufacturers recommend using the site’s record low temperature to determine the maximum number of modules per source circuit. While the record low temperature is easily attainable (see “Low Design Temperature,” below), it is also overly conservative for maximum voltage calculations. The record low temperature is usually too conservative for design calculations because temperature is only one of two major factors that impact array open-circuit voltage. The other major factor is irradiance. As an example, look at the set of I-V curves in Figure 1, which assumes constant cell temperature and variable irradiance, and notice where the I-V curves intersect the horizontal axis. As irradiance decreases, so does open-circuit voltage.
The NEC, however, uses temperature only to determine maximum system voltage. The criterion for determining the maximum PV system voltage, according to Article 690.7(A), is to correct the source circuit open-circuit voltage for the “lowest expected ambient temperature.” Prior to the 2011 cycle, the NEC did not define the term lowest expected ambient temperature. However, the 2011 NEC will define it in an Informational Note (formerly known as a Fine Print Note) as follows: “One source for statistically valid, lowest expected ambient temperature design data for various locations is the Extreme Annual Mean Minimum Design Dry Bulb Temperature found in the American Society of Heating, Refrigeration, and Air Conditioning Engineers’ ASHRAE Handbook—Fundamentals. These temperature data can be used to calculate maximum voltage using the manufacturer’s temperature coefficients relative to the rating temperature of 25°C.”
An Informational Note is not a Code requirement and cannot be interpreted as such. System designers can use any authoritative source of data for the lowest expected ambient temperature. However, this Note is intended to help the designer and the AHJ focus on the most appropriate data for balanced array design. Since many system designers may not have ready access to the ASHRAE Handbook, the Extreme Annual Mean Minimum Design Dry Bulb Temperature data—hereafter referred to as the ASHRAE low design temperature data—is included in Appendix E of the Expedited Permit Process for PV Systems document that I wrote for the Solar America Board for Codes and Standards (Solar ABCs). This document is readily available on the SolarABCs website (see Resources) and includes data for more than 650 cities in the US.
Some may ask why ASHRAE data is better to use than the record low temperature. One reason is that using the record low temperature sometimes excludes acceptable source-circuit configurations that may in fact be preferred over shorter source circuits. (This is illustrated in “Case Study: Example dc Voltage Calculations,” below.) In addition, the extra margin of safety that the record low temperature design provides is often statistically insignificant when compared to the ASHRAE design.
System designers must consider three important issues when determining an appropriate design temperature. First, statistically, the record low temperature may never occur again. Second, lower irradiance conditions in winter make it even less likely that peak irradiance (1,000 W/m2) will accompany the record low temperature, which is a necessary coincidence to achieve the calculated maximum voltage based on temperature. Third, to achieve in the field the maximum voltage that is possible on paper, the PV array must be in a condition that is as good as new. The modules cannot be soiled, mismatched or degraded; the maximum voltage for each of the installed modules must equal its published rating. The statistical likelihood of these conditions occurring at the same time is low.
The ASHRAE data provide statistically derived expected low temperatures. Although ASHRAE processes National Weather Service data for use by engineers sizing heating and cooling equipment, the data are also relevant to many other fields, including the electrical industry. The ASHRAE low design temperature data is derived by averaging the annual low temperature for every year on record. The result is a low temperature that has a 50% chance of occurring once a year at a specific location. Statistically, 50% of the years that a PV system is in service, the low for the year will be colder than this value—and for the other 50%, the low will never reach this value.