Causes of blown solar panel diodes

[michaelniszl]

Good day,

I'm looking for some help. I've got a 6KW off-grid system. It has a 120V 40A MPPT charge controller, 6KW 120V inverter. The there are 24 X 250W panels connected in 4 sets of 6 series connected panels. I've had a problem with the junction boxes on the back of the panels blowing (Diodes completely melting and cracking). Can anyone advise what causes the diodes to blow? Could it occur if the MPPT were faulty or can that only occur from lighting or when a panel is shaded?

Thank you very much for the assistance

Mike

[sensij]

6 panels in series? On a 120 V charge controller? What is the Voc of your panels?

[michaelniszl]

The Voc is 38V. The manufacturer recommended connecting the panels in such a way. The controller does not charge the batteries at 200+V. It displays a charge voltage of between 126V to 140V at 30A at mid day depending on the charge level of the batteries. The DC range of the controller is 100V to 240V.

[bcroe]

Good day,

I'm looking for some help. I've got a 6KW off-grid system. It has a 120V 40A MPPT charge controller, 6KW 120V inverter. The there are 24 X 250W panels connected in 4 sets of 6 series connected panels. I've had a problem with the junction boxes on the back of the panels blowing (Diodes completely melting and cracking). Can anyone advise what causes the diodes to blow? Could it occur if the MPPT were faulty or can that only occur from lighting or when a panel is shaded? Mike

My guess: from what I've seen, bypass diodes are rather marginal for the current possible. So the right combination of
intense sun, higher temps, and hard shade could overheat them. I'd try replacing the diodes with higher current types,
possibly even heat sinked (insulated case style) to a panel frame. You don't have lightning issues? good luck, Bruce Roe

[michaelniszl]

My guess: from what I've seen, bypass diodes are rather marginal for the current possible. So the right combination of
intense sun, higher temps, and hard shade could overheat them. I'd try replacing the diodes with higher current types,
possibly even heat sinked (insulated case style) to a panel frame. You don't have lightning issues? good luck, Bruce Roe

There is a lot of lighting in my area but this occurred when there was no lighting, so I don't believe it was that. I've replaced the diodes with new ones which are slightly up rated. I've decided I am rather going to add another panel and connect them in 5 sets of 5 to reduce the series voltage of the strings as well as the voltage across the MPPT as it is fairly close to that 240V limit.

Does anyone have any further recommendations I should implement in order to avoid blowing panels or the MPPT?

Thank you very much for all of the assistance

[Mike90250]

..... reduce the series voltage of the strings as well as the voltage across the MPPT as it is fairly close to that 240V limit...

What 240V limit ? What is your charge controller ? This sounds quite strange.

[wes@SH]

Based off the info you supplied about your CC, it looks like you might have a Generic Brand MPS405. I wouldn't begin to know how to make a compatible system with it from what I could see on their spec table. I would question the ability of one120v 40a MPPT to handle the production of a 6kw system, where you need two MPPT 60-150s. If that is the controller you are sticking with, what kind of battery bank are you hitting with the 126-140V charging and is the inverter compatible....not that it has anything to do with the blown diodes. I just thought you might have more problems than just those.

Do you have properly rated DC breakers on your strings at the combiner box?

[ncs55]

What actually are all of the components that you are using? Inverter brand, type. Module brand, type, Battery arrays size brand and type, complete system information and products used? We need to see the specs of the products in order to get the full picture of what is happening.

[michaelniszl]

The specs of the system are as follows:

Solar Panels

25 x Jinko JKM-255M-60 (255W)

Connected in 5 sets of 5

Producing 189V at 44.8A using Voc and Isc
Producing 154V at 41,4A using Vmp and Imp

PV Array Box

10 x PV string inputs with max current of 20A (Each string has a 20A fuse)
Includes lighting protection

MPPT Charge Controller Will be changed from 40A to 50A

Made by Foshan Ouyad (MPS405)

Untitled.png

Inverter

Made by Foshan Ouyad

Off grid 120v - 6kw
Pure sine wave output 220v/50Hz

Batteries

Made by Foshan Ouyad

20 x 12v 200ah Gel batteries
Connected in 2 parallel strings of 10 batteries
Battery output = 120v

[ncs55]

Well seeing that all of your equipment is made in China, I can only say that from my experience, maybe you should buy better equipment or expect the failures you are seeing. I cannot help you with this equipment other than to recommend to send it back for a refund or throw it away and start over.

[wes@SH]

You should look at the max fuse rating for your panel, I think you will find it to be around 16 amps. I'm guessing your CC can only really accept strings of 3 or 4, and you are melting your blocking diodes before your 20 amp fuse would trigger.

[sensij]

You should look at the max fuse rating for your panel, I think you will find it to be around 16 amps. I'm guessing your CC can only really accept strings of 3 or 4, and you are melting your blocking diodes before your 20 amp fuse would trigger.

It does look like the panels should have a 15 A fuse per string, not 20 A. Strings of 3 or 4 would not be enough voltage for the system to function... there is nothing in the data sheets that suggests a 5 panel string is not appropriate.

[wes@SH]

Right, I was only looking for that fuse information, as I suspected that was the cause combined with strings that were too large. Even if you are upgrading to their 50A CC, you should still downsize your line fuses.