AE 868
Commercial Solar Electric Systems

Stringing PV inverter

PrintPrint

We discussed the effect of cell temperature on the I-V curve and the operating voltage and current in Lesson 2. Now it is time to apply this knowledge to calculate minimum and maximum operating voltage of module or string.

The NOCT and % temperature coefficients from the modules datasheet can be used to determine the min and max voltage levels and the range of MPP corresponding to it. PV designers are interested in the lowest recorded temperature for a location to determine the highest possible Voc for the string and Vmp for MPP range. Furthermore, the average highest temperature is used to find the lowest voltage from the module or string.

Note

Check out Weather.com. On this site you can search for the lowest and highest recorded temperatures by location.

We can also define the string voltage as the individual module’s voltage multiplied by the number of modules connected in series.

Assuming we are stringing the PV string shown in Figure 4.4, the I-V curves of the PV string vary depending on the temperature. When the temperature is higher than the standard 25ºC, both of the MPP voltage and the open circuit voltage Voc decrease. On the contrary, when the temperature is lower than 25ºC , both the MPP voltage and Voc increase, as illustrated in Figure 4.4. If we define operating ranges for both the MPP voltage and Voc, we can see that the inverter should be able to operate within the MPPT range at the lowest record and average highest MPP voltage of the PV string. Similarly, the inverter absolute maximum voltage should be at least equal to the maximum Voc of the string at the lowest record temperature and the minimum voltage of the inverter should not exceed the minimum Voc of the PV string at the highest temperature.

Inverter stringing voltage range. See paragraph above for description.
Figure 4.4: Inverter stringing voltage range
Credit: Mohamed Amer Chaaban

As PV designers, and when stringing the PV inverter with the PV string, we should make sure that the MPP voltage doesn't fall bellow the lowest voltage at the average high temperature and doesn't exceed the maximum voltage of the inverter.

Reflection

Example:

String the inverter with the following parameters:

  1. MPP voltage range between 250- 480 V
  2. Maximum allowable voltage of 600V
  3. Startup voltage of 300V

What are the stringing voltage values of the PV string that a PV designer should consider? (Hint: Compare values to Figure 4.4, above)

Click for answer...

ANSWER:

  1. Minimum open circuit voltage at average highest temperature doesn't fall bellow 300V to satisfy startup voltage requirements of the inverter.
  2. Minimum MPP voltage at average highest temperature doesn't fall bellow 250V to satisfy minimum MPPT voltage requirements of the inverter.
  3. Maximum open circuit voltage at lowest record temperature doesn't exceed 600V to satisfy maximum voltage requirements of the inverter.
  4. Maximum MPP voltage at lowest record temperature doesn't exceed 480V to satisfy maximum MPPT voltage requirements of the inverter.

Inverter stringing tool

As it seems, the math can get very tedious, so most Inverter manufacturers create their own sizing tools that are available online for free, where you can choose:

  1. PV module brand and model
  2. The desired inverter model
  3. The average highest ambient temperature and lowest (record) ambient temperature ranges (some tools allow user to choose the location and these temperature values will be already available).

The tool will then give all possible configurations (series and parallel) and the capacity factor. An example of inverter stringing will be available in the Lesson Activity, where you can apply the knowledge to a real world example.

DC to AC ratio and inverter design

As a rule of thumb, designers choose DC to AC ratio (AKA capacity factor) range not to be less than 80% and not to exceed 125% depending on the location and the irradiance and type of inverter used. For example, for a system in Seattle, WA, it is recommended to oversize the PV array since it is very unlikely to overload the inverter since radiation is less than the STC power rating of the modules. In contrast, in Miami, FL you cannot exceed 110% since radiation there reaches the STC level and may exceed it. For best stringing choices, use the manufacturer's datasheet to determine the maximum and minimum allowed PV array sizes.