Power on MMPT1 vs MMPT2

Questions
#1

Hi Guys, Wanted to check if the power generation on mmpt1 and mmpt2 have to be similar and that they can’t deviate from each other too much?

Reason I’m asking is because with mmpt1 and 2 more or less matching, I’ve realized that my total generation is much lower than one with one mmpt much higher than the other. I have 3 strings of 11 panels (33 in total). 22 of them (2 strings) are facing north west, 11 of them are facing south west. When one string from NW is parallel to the SE panel, and the other string is one NW string, this results in an even mmpt1 and 2 and much lower generation, like scenario 1.

When the two NW strings are parallel together and the SE one is by itself, it results in scenario 2.

e.g.

at 10am, total generation varies from 5kw to 7kw.

Scenario 1: mmpt1 2.5kw, mmpt2 2.5kw, total 5kw

Scenario 2: mmpt1 5kw, mmpt2 2kw, total 7kw.

Any issues with having the 2 NW panels in parallel and SE one alone vs one NW and SE in parallel vs one NW one on its own?

Thanks in advance guys!

#2

What panels? Reason I ask is I have a string of ET panels and the VOCs are all over the place.

I’m currently going through warranty.

#3

If you put strings in parallel, you’ll want same-direction strings paralleled together.
The voltage across each of the paralleled strings has to be the same (because it is wired together) which means that if you have two strings that have the same amount of sun, you’ll be able to hit the maximum power point in the PV curve of both strings. If they have different sunlight then you can either hit the maximum point on the curve of one string or the other, not both (and the MPPT will get confused trying to target one or the other).

What inverter are you using? Solaredge can work around this since it can MPPT each panel individually. Generac could also work around it since it does MPPT at each string.

#4

That’s not necessary. And it’s not always the most desirable outcome either.

All that is required for parallel strings is that the panels be of the same type/rating and there be the same number of panels in each string.

It’s perfectly fine to parallel strings of different orientations, provided the difference in orientation relative to the sun is not too large.

One example is paralleling an East and a West facing array. Provided the tilt of the panels is not steep (e.g. up to 20-ish degrees) then this sort of arrangement will still perform very well.

More common is paralleling strings with an azimuth 90 degrees apart.

I have in one MPPT with two strings in parallel, a NW string and a NE string. It works very well, extends the production curve over a longer period of the day compared with all panels facing same orientation.

The peak output is a little lower but that’s not always a bad thing as in many cases a charge controller can’t use the peak power, or there may be export limits which cap peak production anyway.

#5

That’s a good point. I get hung up on the voltage equal thing, but forget that even a panel producing 1 watt can be doing it at rated voltage.