Voltage drops, current increases on one string - Page 1
Forum section: electronics / general

author: mcbota
author_rank: Newbie
timestamp: July 12, 2022, 11:27:07 am
content: Hello everyone. I have two string panels and on one string composed of 13 panels of 400 W, with Vmp of around 31V and Voc around 37V, in the morning, almost in each day, starting around 7:40, for about two-three hours the voltage drops around 220 V and the current increase comparing with normal value, something like trying to keep the optimal power output. I really dont observe any shaded area or to have some obstacle to shade the panels. On the other string everything is going to be OK. I have a Huawei three phase inverter, 15 KW KTL m0. Thanks for every possible suggestion.

author: Zero999
author_rank: Super Contributor
timestamp: July 12, 2022, 12:44:25 pm
content: You might get more help if you posted a simple drawing with the voltages on it.The controller will adjust the voltage to get peak power and solar cells have a negative temperature voltage coefficient.

author: mcbota
author_rank: Newbie
timestamp: July 12, 2022, 01:39:45 pm
content: Normal voltage of the string

author: mcbota
author_rank: Newbie
timestamp: July 12, 2022, 01:40:30 pm
content: Dropped voltage

author: Siwastaja
author_rank: Super Contributor
timestamp: July 13, 2022, 06:15:31 am
content: It's producing barely anything?Seems like it's too early in the morning and if it's shining at all, the angle is so far off the panels don't get direct light at all.A photo taken of the array during the same time would be helpful.In low light conditions it's perfectly normal to get roughly similar power output at significantly different voltages. Global MPPT algorithm can then jump between those equally good choices.

author: mcbota
author_rank: Newbie
timestamp: July 13, 2022, 07:42:39 am
content: The voltage starts ok in the morning on both strings, is just that after a while of functionality the voltage drops, the current increases and stays in this situation for two-three hours, then for the rest of the day, everything is normal. I really dont understand the technical side, what really makes the string/inverter to act in this manner. On that string I have 8 panels on S-SE oriented and 5 on S oriented(being a flat roof), but I observe no shaded area/zone.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 13, 2022, 10:27:32 am
content: Oh, each string must face in the same direction. Having 8 in one direction and 5 in a different one is far from optimum.String inverters are OK with a few panels getting occasional shading, but cannot really deal with half of the panels facing different way.That's your problem. If it was professionally designed and installed, you should complain, because this is basics 101.As a result, the power curve will have two optimum peaks and the MPPT algorithm is oscillating between those. The bigger problem is, neither of these optimum peaks are not as good as an installation facing in uniform direction would be.Really, you would need an inverter which has one string input and MPPT tracker more, and while at it, make sure it has low enough minimum voltage. Your Huawei seems to be 200V minimum so 5 panels in a string won't work well.Microinverters could also be an option.

author: mcbota
author_rank: Newbie
timestamp: July 13, 2022, 10:42:07 am
content: Thanks for the answer. The theory is known, but because of the area of the roof that wasnt enough, I had to install also on S-SE orientation. But because the inverter has only two MPPT, I tried to reconfigured the strings putting the maximum number of panels on the same orientation on one string and on the other string to put the panels from two orientations. I suppose that putting 7 panels from S-SE on one string and the other 29 panels from S tot he other string is not working, what other suggestion do you have to make the most efficient configuration? You have the orientation of the panels in the picture (the connections are not the real, but those 23 from the first three lines are on one string connected now)

author: Siwastaja
author_rank: Super Contributor
timestamp: July 13, 2022, 10:48:45 am
content: Did I get it right, you have 37 panels, out of which 29 panels in direction 1 (S) and 8 panels in direction 2 (S-SE)?29 panels could theoretically go to a single string but clearly it will exceed the voltage limit. I would consider dividing the 29 panels in two strings (e.g. 14 and 15) in your existing Huawei inverter, and get some cheap second-hand inverter for the remaining 8-panel string.

author: mcbota
author_rank: Newbie
timestamp: July 13, 2022, 10:53:58 am
content: Are 36 panels (one line connection on the third line was put by mistake). I understand your solution. Do you think that will be a high gain in efficiency and output power to be worthy the investment in another inverter, please? Thanks

author: Siwastaja
author_rank: Super Contributor
timestamp: July 13, 2022, 11:04:16 am
content: It could be, especially if you can find a used inverter for cheap. Some old bargain 2.5kW inverter with single MPPT.Another option would be to sell the 8 panels and just live without them. PV panels are getting more expensive so you could get most of your money back on those.As it is now, it's very hard to say if those 8 panels are net negative or positive. Likely the thing still produces more than if those panels did not exist at all, but not necessarily much, and it's hard to say without better modeling or monitoring. So frankly I don't know.

author: mcbota
author_rank: Newbie
timestamp: July 13, 2022, 11:09:23 am
content: I appreciate all your answers and suggestions. I will think at the best solution from those that you proposed me.Thanks.