After the inverter’s power output rating has been considered, the choices can be further narrowed down based on brand preference. You can then refine the list even more by determining the maximum and minimum number of modules allowed in a string. Inverter manufacturers always list the maximum allowable input voltage and the required maximum power point tracking (MPPT) range for their inverters’ operation. Make sure the array never exceeds the inverter’s maximum voltage and remains above its minimum voltage during operation.
Manufacturers report multiple voltage values, but focus on two: the maximum input voltage and the minimum MPPT voltage. These two extremes will define the DC voltage window. Remember that module voltage is inversely affected by the PV cell operating temperature—the higher the temperature, the lower the voltage and vice versa. The cell temperature is related to the ambient air temperature.
All inverter manufacturers maintain string sizing calculators on their websites. In addition, Blue Oak Energy and SolarPro magazine offer an online string-sizing tool (see Access). A good design practice is to perform the calculations manually and then use Internet resources to double-check your work.
The maximum input voltage an inverter can accept is the limit of the temperature-adjusted open-circuit voltage (Voc) of the PV array. This voltage is critical—if the array exceeds it, the inverter can be damaged. The other voltage parameters are the upper and lower ends of the MPPT window.
The array’s temperature-adjusted MPP voltage (Vmp) must always remain above the inverter’s minimum MPPT value while the array is operating, or the inverter may shut down and not go back on until the next morning—when the cooled-down array starts making energy again. For a good review of Voc, Vmp, maximum power point, and other PV-specific electrical parameters, see “Back Page Basics” in HP131.
Keeping the array’s voltage outputs within the voltage window for all temperature conditions will help ensure the inverter won’t be damaged or turned off during extreme temperature conditions. There are some generally accepted rules that most designers can agree on (see “Accounting for Photovoltaic Cell Temperature” sidebar at right).
Unless the inverter has multiple MPPT, all the module string lengths should be identical and should have the same orientation. While it may be possible to have strings at different tilt or azimuth angles, it isn’t ideal.
Once the inverter has been selected and the module strings have been determined, the next consideration is how to get the PV circuits off the roof and to the inverter. Typically, there will be a junction box near the PV array. Since most PV modules come with cables with quick connects, this box can transition from the outdoor-rated conductors used at the array to properly rated conductors in conduit. The box can also be used to combine the PV source circuits and run to the inverter with a single circuit.
Since it’s possible to see PV arrays with four strings serving a single inverter in residential applications, whether to combine or not deserves attention. If the circuits were to remain separate, that requires running more conductors. More conductors in conduit requires derating the conductors because of additional heat inside the conduit. The benefit of using separate circuits is to utilize the disconnect/combiner that is part of most residential-sized grid-tied inverters—saving time and money on additional boxes. And even with the additional conductors, in a typical four string residential application, you can still install 10 or 12 gauge conductors in 3/4–inch conduit.
The option of combining strings at the array should not always be dismissed, though. It comes down to analyzing the cost differences between the two methods. For those systems under the 2011 NEC, be aware that there is a change that requires a disconnecting means at the combiner boxes for fuse servicing. This may preclude the use of combiners in many residential applications since there isn’t always a good location on roofs.
The AC operating voltage range for all UL 1741 grid-tied inverters is relatively narrow. Inverters can operate from 12% below to 10% above nominal grid voltage (211 to 264 V for a 240 VAC system).
In the majority of installations, voltage drop between the inverter and the grid isn’t an issue. But if the conductors between the inverter and the main distribution panel (MDP) are too small, the inverter will interpret the grid voltage as being out of spec (greater than the nominal voltage +10%). The inverter’s automatic safety features will isolate it from the grid while it waits for the voltage to get back into spec.
Ideally, the voltage drop in the conductors between the inverter and the MDP will be less than 1.5%, to give the inverter the best opportunity to stay connected to the grid in all situations. Calculating voltage drop before specifying conductors is wise—upsizing conductors after they have been installed is never fun or easy.