Tigo optimizers enable system designers to mix different PV module types within strings. For example, if a module needs replacing and its technology is now outdated or its manufacturer is not producing that module, you can replace it with a module with different characteristics. This can help eliminate keeping inventory that add substantial sums to the cost of ownership.
The 25% rule: mismatches up to 25% in string voltage or current are allowed within single strings or between parallel strings. Whether due to string length or panel characteristics, it is important that no string or module fall outside the 25% mismatch.
Note: Power mismatches between the different modules have no restrictions. If the current, voltage, and overall mismatches obey the 25% rule, you can mix and match modules with any power rating or technology: monocrystalline, polycrystalline, and thin film.
Calculate the PV Module Mismatches
Note: Each of the three mismatches, current, voltage, and overall, must adhere to the 25% rule.
When mixing modules within a string, current rating is the most important thing to consider. You can mix and match modules in the same string if the current ratio (Cr, lowest current divided by highest current) is larger than 0.75. Use this result to calculate the current mismatch (Cm):
Cm = (1-Cr); it must be less than or equal to 25%.
Calculating voltage mismatch is a bit more complex, because it affects the string-to-string mismatch. Consider these two scenarios:
Scenario 1: If you have one string per MPPT, Tigo doesn't limit you.
Scenario 2: If you have more than 1 string connected to an MPPT, you can mix and match voltage ratings if the voltage ratio (Vr, lowest string voltage divided by the highest string voltage) is greater than 0.75. Use this result to calculate the voltage mismatch (Vm):
Vm = (1- Vr); must be less than or equal to 25%.
The overall mismatch, Om, which must be less than 25%, is calculated in much the same way, using the Cr and Vr ratios:
Om = (1 - (Vr) * (Cr)) ; must be less than or equal to 25%.
This example considers a system with 2 strings of 15 PV modules per string for a total array of 30 PV modules. It is equipped with 2 different module types: 27 modules with Voc=33.5V @ 6.4 A and three modules with Voc = 43V @ 8.0A. This example examines this configuration: string A with 15 modules of 33.5V (string voltage is 15*33.5V = 502.5V); string B with the remaining modules (string voltage is 3*43V + 12*33.5V = 531V).
Current Mismatch Calculation
String A has no issue; for string B the Cr is 6.4/8.0 = 0.8.
The Cm is: (1-Cr) = 0.20 current mismatch. This value is acceptable.
Voltage Mismatch Calculation
The Vr is 502.5V/531V = 0.95.
The Vm is (1-Vr) = 0.05 voltage mismatch. This value is acceptable.
Overall Mismatch Calculation
The Om is: (1 – (Vr)*(Cr)) = 0.24 overall system mismatch. This value is acceptable.