Solar PV fault 'isolation failure'

[MeldrewsMate]

Dear Learned colleagues,

my dad's 10 years old solar PV array has started playing up. It comprises 14 panels of 250Wp each fed in two strings to a Solis (?) So;arRiver 3600 inverter.
Here's the chronology:

30th December 2023: Noticed no output for the previous 2 days. Weather had been awful, raining constantly for 4 days!
No time to investigate so a Microsoft reset was instigated (OFF-ON at both DC and AC isolators).
System booted up, producing 350W.....still she-ite weather.

1st January 2024: Back to 'isolation failure'. Cover off isolator, noticed both strings paralleled into single DC input to inverter (that's part of the inverter design, not a mod)
I isolated one of the strings, a 50:50 guess, and now it's reliably working, albeit producing less than it should; never mind, it's January.

What this indicates (I think) is that one of the strings has an insulation fault to earth, and probably because of the constant battering from the elements. In spring I aim to go on the roof to investigate and cure the fault, and this is where you experienced chaps come in:

1. Is it OK to use a Megger on the string leads (at night!) to determine/confirm an insulation breakdown? 500V? 1000V?
2. How does one disconnect an MC4 connector? Special tool or just the right technique?
3. Is the best method to half-split the string by taking out the middle panel, then re-test with the Megger?

Luckily it's a bungalow!

Kind regards, MM

 

[DetlefSchmitz]

Have you considered the possibility that there was nothing wrong and that PV not working well when it's really dark is caused by lack of light?

If you're determined, why not try the other string. You might find that works too.

 

[ajstoneservices]

Most iso faults are due to poorly made connections Usually mc4)
Some are due to cable damage (mostly squirrels) I had a system once where water found its way into the panels (Perlight Deltas) during prolonged wet weather.

I wouldn't want to be working on the roof at night bungalow or not. I think Chris Kitcher has a video showing you how to do an IR test on solar panels. You will test at 500v.

If the connectors are mc4 you won't need a special tool. Almost every other make and you will.

See if you can find the aurora power (ABB) iso fault finding guide on the net.

Growatt's fault finding guide here

 

[MeldrewsMate]

Have you considered the possibility that there was nothing wrong and that PV not working well when it's really dark is caused by lack of light?

If you're determined, why not try the other string. You might find that works too.

No, I hadn't considered that nighttime could cause the inverter to report an isolation fault. There have only been around 3800 nighttimes since the system was installed. I hadn't considered it because it's irrelevant. Please read the original post.

 

[MeldrewsMate]

Most iso faults are due to poorly made connections Usually mc4)....

If the connectors are mc4 you won't need a special tool. Almost every other make and you will.

Thanks for that, but how do I disconnect an MC4?
Simply pull?
Twist and pull?
I'm asking this because there are a four connectors under my loft mounted inverter which look like MC4 but I couldn't easily disconnect them, and it's awkward to see them in their recessed location. I also asked because I won't be going on the roof until spring, and wanted to be sure I had all I needed (information and tools) before that expedition.

MM

 

[SpecialK]

 

[ajstoneservices]

Thanks for that, but how do I disconnect an MC4?
Simply pull?
Twist and pull?
I'm asking this because there are a four connectors under my loft mounted inverter which look like MC4 but I couldn't easily disconnect them, and it's awkward to see them in their recessed location. I also asked because I won't be going on the roof until spring, and wanted to be sure I had all I needed (information and tools) before that expedition.

MM

You need to push the 2 tabs in and pull down at the same time. If you cant push the tabs in with your fingers try using a small flat bladed screwdriver. Push 1 tab in while pulling down and while still pulling down move to the second tab.
You might be lucky enough to have room for a pair of long nosed pliers instead of a screwdriver.
Some inverters make it harder than other to disconnect the strings even with a tool.