Array Voltage Considerations: Page 6 of 6

Comparison of results. It is now possible to recalculate the acceptable source-circuit configurations using standard assumptions for a string-sizing program. There are two main differences in the calculations.

First, use the record low temperature for the location in place of the ASHRAE Extreme Annual Mean Minimum Design Dry Bulb Temperature for the VMAX and NMAX calculations:

VMAX = 37.0 V + ((-21°C - 25°C) x -0.137 V/°C)
= 37.0 V + (-46°C x -0.137 V/°C)
= 37.0 V + 6.3 V = 43.3 V

NMAX = 600 Vdc / 43.3 V = 13.9
= 13 modules in series

Second, do not apply a 0.85 multiplier as part of the VMIN calculations. This means that the minimum number of modules per source circuit is calculated using a Vmp of 25.6 Vdc:

NMIN = 305 V / 25.6 V = 11.9
= 12 module in series

The best array design for this case study calls for 14 modules per source circuit. However, the simplest string-sizing program specifies 12 to 13 modules per source circuit. More sophisticated string-sizing programs apply a margin of safety to the minimum expected dc voltage to account for high ac grid voltage, array degradation and module-to-module voltage mismatch. Using PVSelect.com, for example, to calculate the acceptable string sizes for this case study disqualifies source circuits of 12 modules. However, if the designer is not using ASHRAE low design temperature data, even PVSelect.com cannot identify the best design option. Assuring the best design requires both accurate calculations and proper data.

SIZING THINGS UP

I am not suggesting that inverter manufacturers do not provide a valuable service with their string-sizing tools. Without these resources, the number of array design mistakes would undoubtedly be many times what it is today. Nevertheless, system designers routinely make mistakes, in spite of the fact that they have ready access to many easy-to-use string-sizing tools. The results of the low voltage mistakes described here are not dangerous; they do not pose a hazard to persons or property; they do not violate Code. They simply miss the mark of reducing up-front system cost and optimizing long-term performance. Designers need to keep in mind that all “approved” string sizes are not created equal.

From an installed cost point of view, it is always better to put the maximum number of modules in series. This delivers the greatest amount of power per pair of source-circuit conductors. Longer strings also increase the array voltage, which has voltage drop benefits when cables are sized. Getting the array voltage up also provides insurance when it is needed most against insidious low dc voltage problems that result in poor system performance precisely when the solar resource is greatest. On 5 kW or 50 kW net-metered projects, the difference in performance between having 14 modules in series or 12 or 13 modules in series might not register with the customer. However, on 500 kW or 5 MW projects that are PPA financed, this will make a world of difference in both the installed costs and the revenue generated over the life of the systems.

CONTACT

Bill Brooks / Brooks Engineering / Vacaville, CA /  brooksolar.com

RESOURCES

American Society of Heating, Refrigeration, and Air-Conditioning Engineers / ashrae.org

Blue Oak PV Selection Tool / pvselect.com

Solar America Board for Codes and Standards (Solar ABCs) / solarabcs.org/permitting

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