Selecting the correct string size is one of the most important aspects of PV system design. A string is multiple solar modules connected together and the number of modules in that string is its size. Systems with strings that are too small will sacrifice efficiency. Over sizing strings can damage inverters, void equipment warranties, and violate the electrical code.
STRINGS IN SERIES OR PARALLEL
Solar modules can be connected in series or parallel to obtain different output characteristics. Series connections will increase the voltage of the string while keeping the current constant. Parallel connections will increase the current of the string while the voltage remains constant.
Connecting solar modules in series is most common in grid tied systems while parallel strings are often used in small battery charging systems.
TEMPRATURE AND STRING VOLTAGE
The relationship between ambient temperature and string voltage must be taken into consideration when calculating string size. An inverse relationship exists between the ambient temperature and PV array energy output. Cold temperatures will result in increased output and higher string voltage. Hot temperatures will decrease output and string voltage.
If the minimum and maximum ambient temperature is ignored while calculating string size a cold winter morning could result in over voltage to your inverter or charge controller causing it to shut down when the array is in its most efficient temperature range. On the other hand, a hot summer afternoon could cause the string voltage to drop below the maximum power range on your grid tied inverter.
GRID TIED HIGH VOLTAGE PV ARRAYS
For Grid Tied systems, it is critical to match the string size to the selected inverters operating parameters. The output of a PV array with correctly sized strings will stay within the optimum operating range of the inverter through varying conditions at the site.
Typically, grid tie systems in the U.S. are rated at a maximum DC voltage of 600V. Solar modules in series need to typically produce a DC voltage between 240-480V. Software tools like SMA’s Sunny Design can help by letting you know if the module you are purchasing will work with a certain inverter.
For example, the Sunny Boy 5000US has an MPP voltage range of 250-480VDC.
That means as long as the module string falls within that voltage, the inverter will continue to operate as intended. Although it will track down the 250VDC, the initial input voltage is 300VDC. The inverter requires this minimum voltage to start up. Once the inverter starts, the voltage can dip to 250VDC before it shuts down. However, if the inverter never gets to 300VDC, the inverter will never wake up, and the system will not operate. A standard grid tie voltage for a solar module is 30Vmp; meaning 10 in series gives us 300VDC. Ideally, we would use 12 panels per string so there is no issue with the inverter hitting the voltage it needs to turn on.
OFF GRID BATTERY CHARGING PV ARRAYS
Unlike grid tie systems, off grid systems have a much lower voltage input from the solar modules. Typical MPPT charge controllers accept inputs up to
150VDC. this is usually the maximum rating, so it’s best to stay well under this threshold.
Assuming we have solar modules rated nominally at 24V (meaning 44VDC open circuit/ 36VDC under load), we would use strings of 3, giving us 132VDC as our open circuit voltage. Using 4 in series provides us with 176VDC, which could cause damage to the charge controller.
Charge controllers typically do not have a BOOST feature, meaning your modules cannot be below the accepted voltage of the battery bank. For instance, a panel with 44VDC would not be able to charge a 48V battery, as the controller is unable to step up the voltage. String sizing can be confusing, but we are here to help. Contact any of our sales engineers and we will be glad to make sure all of your system components are compatible with one another.