PrintAs we learned earlier in previous lessons, the maximum possible voltage is the array’s open-circuit voltage measured at the lowest expected temperature for the specific location (the lowest record temperature). There are two main methods for PV professionals to determine the maximum voltage of a PV array.
Method 1
As we learned in Lesson 2, the voltage can be calculated using the PV module manufacturer’s temperature coefficient for voltage. Temperature information is usually gathered from weather stations for each location.
Method 2
The voltage can be easily identified using an NEC correction table, depending on the ambient temperature ranges of the location. The NEC provides a temperature correction factor found in table 690.7. The factors of the NEC table make calculations for estimating the maximum voltage as easy as multiplying the string voltage by a single number.
Example
Assuming you have a PV system with the following specification:
- There are 10 modules in series that form a PV string.
- The individual PV module open circuit voltage is 38V, with 60 cells, and the temperature voltage coefficient is 0.0032V/℃/Cell.
- The lowest ambient temperature is -10℉.
Then the maximum voltage according to method 1 can be calculated as follows:
We can find the Tcell to be -23°C at very low irradiance or in other words, Tcell=Tambient. We can apply equations from Lesson 2.
V=Vstc+(Vt−coeff
Vmod=38+[(60
Then the maximum voltage of the string will be:
The maximum voltage according to method 2 can be calculated as follows:
Looking up the voltage correction factor corresponding to -10℉, the factor is 1.20 (found on NEC table 690.7).
The maximum voltage in this case is
Reflection
We can see that both methods gave results with increased voltage by a factor. However, these factors are not equal. What value should a Pv designer use when designing a PV string for maximum voltage.