Newbie Query: Understanding Mismatch Losses

[EYAlex]

Hi all – I’m fairly new to the PV EYA world and have a few questions which will help me understand mismatch losses I was hoping someone could answer. Apologies in advance for any silly queries! The first query I have is as follows:

I understand that for modules connected in series, the string current is equal to the current of the ‘weakest’ modules’ current and that the string’s voltage is calculated through adding each module’s voltage together.

When is comes to connecting the strings in parallel (which is how they are connected into the inverter inputs?) I am under the impression that the currents are added together. What is the theory behind calculating the voltage at the inverter? Is the voltage equal to the voltage of the ‘weakest’ string’s voltage?

Again, my understanding is that strings connected in parallel must have the same voltage so perhaps there is some sort of conversion going on at the inverter input that equates all the string’s voltages?

[André Mermoud]

The mismatch effect is calculated by adding the I/V curves:

- In the string (modules in series) we add the Voltages

- In the array (several strings in parallel): we add the currents of each string.

Now the result of the mismatch loss is the difference of the Pmpp of the complete array curve with respect to the Pmpp of identical modules.

The position of the Pmpp (voltage or current) on the curve is "unpredictable". Sometimes you can have 2 different maxima on the P/V curve.

This is deeply explained in the tool "Project's dialog > Detailed losses", page "Module quality - LID - Mismatch", button "String voltage study > Detailed study". Especially the first option "Mismatch: General principles".

Here you can press F1 for an explanation of the mismatch issues.