Low RISO fault

[pvman]

Have a problem with a customers array as both the Power One 27.6tl and the 20.0tl are registering low Riso faults. The array has been earthed on the roof and is made up of Renesola 255s. When tested across the +ve and -ve of each string to earth the same voltages were obtained for all strings on the different trackers. Also the same voltage across the -ve and earth was found to be the same on all strings.

I am wondering if anyone has any ideas
Thanks
Matt

 

[Gavin A]

are those results with the DC isolators all switched off?

 

[BruceB]

I'll have a stab. It is likely to be a MC4 cable connector somewhere out on the array which is drooping, staying damp, touching the roof or array framework and has green slime growing on it.

You need to measure the insulation resistance to earth of the various strings, not the voltages. Testing can all be done from the dc isolators, as Gavin is suggesting, if the system is designed with each string having its own isolator.

 

[pvman]

Thanks had thought of that Bruce but the error points for the strings are all over the roof. The other problem is that the job is 3.5 hrs away so going up next week and want to have as much ammo as possible. I have also considered damaged cables. All the DC switches were off.

 

[BruceB]

........but the error points for the strings are all over the roof......

Not sure what that means.
Sorry if teaching you to suck eggs.

 

[pvman]

probably just my english. There are nine error points and they are nt in the same area of roof

 

[pvman]

here is an image of the string layout the voltages show that the errors are at the border of the red/ blue panels. There are two seperate arrays on 2 roofs

 

[Gavin A]

missing component here - what was the voltage that was found between + to earth and - to earth on each string?

if it'[s on all strings equally, it looks to me as if the culprit could be capacitance in wet weather, with the roof having very low resistance to earth, particularly if the panels are very close to the roof.

Either that or you've run all the strings into a combiner box which has filled with water, or you somehow did something to create a fault at the middle panel on every single string at the same time, but that seems extremely unlikely.

I'm thinking capacitance is the most likely candidate, but would be pretty concerned about why it's happening to this degree, though high voltages, very low resistance to earth from the roof, and a big array with all rails connected, I guess could have this level of impact.

SMA explain it here.

not 100% on this, but that'd be my chief suspicion.... good luck.

 

[BruceB]

The capacitive coupling causes leakage currents during operation.
Riso is measured before operation starts and if too low then the inverter will not start.

So I see Gavin's SMA document and raise him this one.
http://files.sma.de/dl/7418/Riso-UEN123622.pdf

I would still put my money on compromised connector

 

[SRE]

If it's any help Power One has said this has happened a lot since the wet and very windy weather. It's been damp connectors in most cases.

 

[pvman]

The +ve / -ve to earth voltages are:

27.6tl Tracker 1 255V/319V (22 panels), Tracker 2 290V/319V (24 panels) ;
20.0tl Tracker 2 195V/319V (20 panels), Tracker 2 230V/ 320V (21 panels)

I have been looking at the sma documents but was n't sure if this could be capacitance because of the different +ve / -ve voltages
There is no combiner box the strings just go back to the inverters.

Thanks for you advice

Matt

 

[pvman]

Riso values are between 1.6M and 1.3M

 

[BruceB]

With the smaller Power One TL inverters, then once the Riso gets below a figure, from memory somewhere around 1Mohm, then they will not start up. If those figures above are paralleled up then they will fall below the critical figure. The normal reading without wet connectors should be 20 Mohm (on the inverter display).

I noticed in the PowerOne manual (for the smaller inverters):

Riso: Measured insulation resistance. Unlike the parameters discussed above, this
is not an instant value but a one-off measurement taken upon inverter start-up.
Low Riso value could depend by DC side varistor. Please check ( inside and
outside of the inverter ) before to call assistance.

The Italian/English translation looks a bit dodgy, but I think it is saying check the varistors on the DC side as a potential cause. I have no experience of that.

 

[BruceB]

Have had a look at the manual for the larger inverters now and it says pretty much the same thing.

 

[pvman]

It was connectors after all thanks