TL Inverters DC Insulation Resistance problems

confounded · Feb 18, 2014

Having followed the Power One TL inverter thread I thought I would start a thread covering TL inverters in general.

We have had 2 SMA 4000TL inverters DC Insulation Resistance problems reported this week. One we were out at yesterday and found it was possible to trick the inverter into working by starting with the "good string" and then switching on the "bad string". Having conducted our tests and provided them with our results SMA advised us it was likely to be one of our MC4 connectors so we are in the process of organising scaffolding to carry out the repair.

On to the next one today and as with the one before we were able to identify the "bad string" as our technical customer had also manged to do. He also found the only way to get the inverter to start up when in this state was to start up the "good string" and then witch on the "bad string" when up and running. Now the interesting thing is as it was a bungalow we were able to get up today to redo the MC4's to the "bad string". On coming back down we were able to start the inverter on the "bad string" with no Insulation Resistance fault reported. We then turned on the "good string" and he power increased as expected. All was fine and we felt it was a simple as everyone is suggesting re moisture ingress (despite not actually seeing any evidence of this on the old crimps removed).

Just to check all was well we shut down the inverter and started it with both strings switched on on start up. Then we got the the fault again with the original "good string" now becoming a "bad string". We changed the MC4's took panels off to check connections and then reassembled and all was ok, but from the test we were doing it looked like the DC leakage was dissipating over time as the panels heated up in the sun.

I am now not so convinced that it is the MC4's and beginning to wonder if the fault is somehow linked to the nuisance tripping that the TL's suffer from (when on an RCD protected board) in these damp conditions we have been experiencing recently. Are these same atmospheric condition causing small leakages from the panels as a whole which the TL inverters are designed not to tolerate?

Just a theory at the moment but I am finding anomalies during our repairs that make me cautious of throwing scafollding up when we were able to move the fault from one array to the other today.

Energetic · Feb 19, 2014

I am looking into this also. Have a look at this: http://www.electriciansforums.co.uk/sun-lounge/86593-low-riso-fault-2.html

I am beginning to think Parasitic Capacitance may be causing some of these problems. Is anyone taking this into account in the design stages as SMA seem to be recommending in this: http://www.sma.de/fileadmin/content..._Power_Solutions/Ableitstrom-TI-UEN114620.pdf

Trying to remember my past learning on capacitance, would earthing the array frame make a difference? Would it occur on an earthed metal roof with a connected frame ie standing seam?

I too am at the early stages of tying to understand this, but i just can't beleive that its all down to bad MC4 connections.

confounded · Feb 27, 2014

We are still having problems with one system which is a 4000TL. I still believe this to be a capacitance/whole array issue rather than some wet connectors. The problem currently is only arising after we have rain in the night. By 9:30 after the rain has gone from the panels the system generates normally. I can not see how the "wet" connectors would significantly dry in a few hours of the morning. Where as when we see the panels clearly dry out the issue goes. My theory is that due to the extend wet period certain roofs have got saturated, so when the panels are wet this increases the capacitance. In normal circumstance we have been out to inspect system after several weeks of rain and the roof remains pretty dry under the panels. With this constant rain it has led to the roof getting wet under the panels. Just a theory.

Anyone else having any luck with this issue.

Gavin A · Feb 27, 2014

it will be capacitance that's causing the issue, the inverters should be set up with the RCMU set to a high enough level to avoid this issue, and it could well be that sodden tiles will be more prone to it.

I attended the last MCS steering group meeting a few days back, and related issues are under discussions, and I've been asked to collate some evidence in relation to RCDs capacitance, leakage etc so this probably falls within that remit (ish).

I might have to start a survey form up to record the details of any instances of such failures.

if you actually look at the logs of most SMA TL inverters IME there are quite a few temporary low riso faults flag up fist thing in the mornings, presumably for the same reason - dew on the roofs and panels lowering the resistance and increasing the capacitance losses.

confounded · Feb 27, 2014

As ever Gavin thank you for your help, we would be happy to assist in any surveys. We have fault logs for both inverters if this is any assistance?

pvman · Feb 28, 2014

If the RISO reading is about 0.7 MOhm + you maybe able to alter the inverter parameters to below this with the grid guard configurator - although not sure if this is possible as not had this problem with the SMA s. The water on the panels is decreasing the capacitance hence RISO values. My theory is earthing the array is more likely to make this worse as its the capacitance dropping between the substrate and the frame which is causing the problem. As a matter of interest have you only got 2 strings running into the 4000tl.

Matt

confounded · Feb 28, 2014

Yes two strings on both the 4000TL we have had issues with. I went out today and dropped one of the inverters giving us the most problems to the lowest Insulation resistance reading allowed of 500KOhm. We have asked the customer to monitor it and see if they get any more fault readings. Interestingly we trialed a new mounting system for this job when it was installed 3 years ago and it is probably the closest to the roof we have done.

Gavin A · Feb 28, 2014

hmm - the closer to the roof, the greater the capacitance.

but having said that, I was looking at the error logs, and with it being insulation resistance faults, I would expect that there actually is some underlying issue with the insulation resistance somewhere. I don't think capacitance would cause an IR fault (but open to being corrected on that).

confounded · Mar 6, 2014

Morning Gavin, we have another system causing problems. This time it is a Fronius 3600TL, interestingly in the same vintage as the other two systems, around the 3 year mark.

The customer had reported the odd problem with it over the last few weeks and we told him to monitor it. We went out yesterday afternoon as when he got back from work the system had been shut down most of the day.

The interesting bit is with this system it worked first thing in the morning for about 10 mins from 8am, the customer had noted it was working when he left for work but also noted the panel were frosty. So it looks like as soon as the frost melted on the panels leaving the surface of the panels covered in a film of water the Capacitor effect kicked in. I can not see how this can be connector related again.

An update on the other system where we turned down the Insulation resistance, he has still been reporting faults in the morning after rain at night or when there is a heavy dew but the fault seems to clear earlier in the day so the system starts up earlier. Previously it had kept the fault until midday. I am assuming this is because once the panels are changed up it takes time for this to dissipate over the day and reach an acceptable level for the system to start up.

All very frustrating from a point of view of knowing what to fix...