PV Array problem - or not?

[forstal17]

Hi,

I installed a PV system recently on a three phase installation. Each phase had its own inverter so effectively three single phase installations. Two of the inverters were to have 12 x 250watt panels connected, the third was to have 14x 250watt panels connected to it. Until that point all inverters I had worked on had two dc inputs, one for each string, on seperate MPP trackers but the inverters for this site were single input. Because of this i installed the strings in parallel (6+6, 6+6, and 7+7) to keep the voltage within the inverters range. So two strings connected in parrallel into adaptable box with one dc pair going back to the inverter, each sytem was done this way. Everything was fully commissioned - no problems. About two weeks after installation an error code appeared on one inverter indicating a "bus undervoltage". I returned to site, read the manual and the remedy was to reset and if it continued to contact the manufacturer (NEDAP inverter). Obviously i checked and looked at each system and couldn't see anything obvious. I reported this back to the MCS company that had instructed me to do the work. Anyway I should have seen the signs but they only paid for half the work I done for them. They have now told me that they got another electrician to go there and he found a string was connected reverse polarity (I have worked in telecoms dc for a long time and am very aware of this so find it hard to beleive) They also said they swapped the inverter anyway?? They ALSO said they found reverse polarity on a second string on a different inverter and so that system was operating at 50 %?? (I checked when I had went to fault find initial fault and two working systems had approx the same amount of generation, and everything seemed fine at time of commission). Any way my theory is that the DC isolators i was using had staggered inputs across the top numbered 1,3,5,7 and on the bottom 8,6,4,2 meaning pos input on position 1 (top left) out on position 2 (bottom right) I think the "electrician" has took the cover off and it "appears" to be incorrect as normally the isolator would be in on top left out on bottom left and so on. I have done the C&G course recently but haven't done many installs so would just like to clear up the following scenarios:
1. If one string was reverse polarity on an inverter with two inputs on seperate MPP trackers, what would be the likely generation (ie would it simply be around 50%) and would it cause further problems
2. If one string was reverse polarity when you had connected two strings in parrallel into an adaptable box, then took one pair of cables to an inverter with only one tracker, what would be the likely generation,
3. Has anyone used this type of staggered isolator before (32Amp 4 pole unsure of manufacturer)
I cant find anything in my notes that answers this but strongly believe they are trying to get out of paying the remainder of what they owe. I even gave them a 5 year warranty so there was no need to pay anyone else as i would have repaired it for free if it was incorrect!

I would really appreciate any firm answers on this as i want to go to them with absolute certainty that this was not the problem. Unfortunately i cannot just go to site to check for myself. Thanks in advance

 

[screwdriver]

They are on shaky ground withholding money if they haven't given you a chance to rectify fault if there was one.
I have come across these switches before, dodgy foreign things generally designed to be used stacked as change over switches.

if polarity is wrong on one string then you would have 50% output if you had 2 mpp trackers

if you got polarity wrong in an adaptable box and were doing it live you would have a nice little flash so that is unlikely, and then plugging it into inverter you would have had no voltage to the inverter so no production.

 

[dansk]

The isolater is an IMO one, i use these and you wired it correctly.

If you had RP on the strings the inverter wouldnt fire up.

Sounds like BS to me, swapping the inverter may have been the real problem, and perhaps the chap swapping the inverter got RP on the inputs somehow and not you.

Information always gets lost in translation, try speaking direct to the guy who went in after you.

 

[forstal17]

Thanks for the replies guys.....I would go to the guy who went in after me but because of the info he has given them it is clear to me that he has dissed my work to get more for himself. It has worked for him but guess what...he hasn't been paid in full either!

 

[Gavin A]

if it had reverse polarity on one string out of 2 on a single MPPT I can't see how that inverter would have operated at all. Either it would have had reverse polarity protection that would have disconnected it all, or it would have gone bang, I'd have thought.

If it were reverse polarity on the inverter that had to be replaced then it would never have worked at all, not worked for 2 weeks, then shown up a bus undervoltage.

Could be worth emailing the manufacturer of the inverter and asking if a reverse polarity fault could possibly cause a 'bus undervoltage' message, and if their inverter would operate under reverse polarity conditions or not.

Presuming they say no to both, I'd forward this email to the client along with an invoice for the remainder of your money and tell them that this proves that their claims of reverse polarity faults can not possibly be true, and the only fault was an inverter fault which you can not be held responsible for.