Tigo optimizers enable system designers to mix different PV module types within strings such as polycrystalline with monocrystalline types. For example, if a module needs replacing and its technology is now outdated, or its manufacturer is not producing that module any longer, you can replace it with a module that has different electrical characteristics.
The 25% mismatch rule: electrical characteristics of power, voltage, and current may be different as much as 25% in the total string voltage and/or total string current and the resulting total string power are allowed within any single string, and between paralleled strings. Whether due to the number of modules in a string (length) or individual module characteristics, it is required that a strings total voltage, total current and the resultant total power, as well as that of any individual modules voltage and current in the string, not exceed 25% mismatch.
In other words: all modules in a string must be within 25% of each other from minimum to maximum voltage and from minimum to maximum current. Also, the different strings must be within 25% for minimum and maximum voltage, current, and power produced by a string of modules.
Module Power Note: Module power mismatches between the different modules are not restricted as long as the voltage and current differences are less than 25%. Assuming that the the current and voltage mismatches obey the 25% mismatch rule, you can freely mix and match modules of any power rating and of any technology: for example monocrystalline, polycrystalline, and thin film types.
How to Calculate the PV Module Mismatches
Note: Each of the three electrical mismatch limits of 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.