Introduction
Modern grid-tied photovoltaic (PV) plants should be specifically designed for the project site conditions and the inverters to which the DC subsystem is connected. Important factors for string sizing fall under three categories: Environmental, Module and Inverter.
Environmental Considerations
The most important environmental factor is ambient temperature. PV module open circuit voltage is inversely proportional to temperature. PV plant designers must consider the temperature extremes for a particular project site and match the corresponding resultant string voltages to the inverter DC input characteristics. The PV module mounting method determines the module temperature rise. This value is low for free air and high for close to a rooftop. The global warming factor is another point of consideration when evaluating PV array performance 20 years in the future. The location of a solar PV site will also determine the optimal tilt angle of a fixed-tilt array. Optimizing the tilt angle is good practice as it maximizes energy yield. Wind is another consideration in the array design. High wind areas will have civil and structural impacts on the array design; most importantly if a tracker is included in a project. PV module soiling from dust/dirt and snow is also considered in the design of the PV array.
Module
PV modules are designs are varied with several types available today. Bifacial, or double-sided modules have a boost effect on the string current. PV Module electrical attributes include: maximum power voltage, maximum power current, open circuit voltage, short circuit current, temperature coefficient of voltage, and open circuit voltage degradation factor. These factors are used with site variables to develop the open circuit string voltage at the coldest site temperature. This value is also important at the warmest site temperature because inverters require a minimum DC voltage to start up and begin feeding into the grid.
Inverter
A modern PV inverter is design to maximize the energy harvest from the PV array. As a solar production day begins the inverter is in stand-by waiting for the DC voltage to increase above the minimum threshold. After the voltage is satisfied, the inverter begins the startup procedure. The procedure time varies by manufacturer; but, generally falls between 1-5 minutes. During the startup time the inverter performs a health check, a DC voltage check and AC grid check. Modern inverters are designed to stop operation if the DC voltage rises above the UL listing of the inverter. For a 1500 Vdc inverter, that value is 1500 Vdc. Plant designers must take care to ensure the PV string sizing does not result in voltages above this critical value which will occur on the coldest days. The opposite end of the DC voltage range is also critical. PV designers must ensure there is sufficient array voltage on the hottest days to allow an inverter restart. The worst case scenario would be a grid trip event during the hottest day in the middle of the afternoon.
Modeling
PV designers use modeling software to simulate all aspects of PV system design. The industry de facto standard for this is PVSyst. Many PV EPC companies, like Solar Support, use PVSyst when designing new PV generating plants.
For further reading
PVSyst Information
https://www.pvsyst.com/features/
Global Weather Data Resources
https://www.visualcrossing.com/weather-data?ga_h1&gclid=EAIaIQobChMI6r6l9N_n-wIVEvjICh1tKQuiEAAYASAAEgLK6vD_BwE
https://ggweather.com/climate/extremes_us.htm
Manual String Calculation
https://unboundsolar.com/blog/string-sizing-guide
Gary Custer, PE