Parallel Strings with Optimizers

[teal95]

Have 20 kW of panels with 6 strings of 7 panels each, going into a SolArk 15k, so only 3 MPPT. I have shading issues and knew it would be a problem, hence the optimizers, especially since I already had to get RSD as panels are roof mounted.

I only got the system operational right after Christmas so only the last few days have I actually have sun. Peak of the building is N-S and only 4-12 pitch. MPPT 1 is 2 rows of panels on the east side of the roof. MPPT 2 is 2 strings on the west side of the roof. MPPT 3 is the bastard, the bottom row of panels on the west side is 1 string and the other string is the 2 panels at the end of each of the first 4 strings. So I know #3 is going to behave oddly. I have a plan to improve things, but I want to get what I have inspected first as it's what's on the drawing.

As I finally have some sun I can see how bad it's doing. At ~1:30 every day as the shadows start creeping up the bottom of the west side the power production drops off a cliff (from ~6 kW down to 2 kW), while the voltage spikes (from 250 to 350).

Does this make sense to anyone? I would expect the optimizers to drop the voltage, not increase it.

 

[DIYrich]

Putting 2 strings into one mppt is the equivalent of paralleling them. The strings should be identical. The mppt maximizes watts, not volts or amps. The mixed strings can cause wacky results.

I would get a separate mppt for the last string, and have it charge the battery directly. Or, put micros on the 8 panels and have it come in the smart load port. Get it permitted the way you have it, disconnect the last string, and see if production improves. When SA releases their micros at the end of the year, put those on the panels. If your wiring can't handle the micros, and you can't run additional wire, then an mppt solar charge controller is your ticket.

 

[teal95]

Thanks for confirming my understanding of what's happening.

This is stage one. Once it's inspected I'll start stage 2. Part of the reason for building this is power generation and backup, but part is also to make 3 phase power. I have 3x 6500ex going in after it's inspected to run some machinery. With each of them having 2 MPPT I will have plenty of inputs to divide things up, and add more panels. And then more batteries. And then more panels...