As 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.
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.
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.
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 and the temperature voltage coefficient is 0.0032V/℃.
- 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, . We can apply equations from Lesson 2.
The maximum module’s voltage will be:
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
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.
ANSWER: The NEC table method is more conservative and easy to use without tedious mathematical calculations. Designers usually use NEC values to ensure code compliance. However, educated designers are encouraged to argue their calculations with utility design reviewers when they use precise calculations based on actual weather values and a PV manufacturer's datasheet for voltage coefficients.