Commissioning help please

[T0M0]

Hi All

I’ve just completed my first install but need to commission it. This install is to be used for MCS assessment so I need to clarify a couple of points. I did a BPEC course and the handbook is now causing me confusion. Here goes.

2 strings of 7, 190w panels. SMA SB2500 with 3 inputs in loft.

First question regarding dc isolators. I have installed 1 per string and connected each string to its own inverter input. When reading the commissioning section of course handbook, it says to test each string Isc with the link across the output of the string dc isolator – this I understand. It then says to repeat the step for the entire array using the MAIN dc isolator! This suggests 3 isolators! This cannot be right? Could someone clarify how many dc isolators are needed for 2 strings?

The other question is the circuit position of the generation meter. I have googled and the consensus seems to be directly after the CU and before the second ac isolator? I then look at the diagrams in my course book which indicates after the second ac isolator on the cable that goes to the first ac isolator in the loft. So could someone tell me if it is:
----------LOFT----------------------------------|---------------CU----------
Panels – dc isol – inverter – ac isol | ac isol – meter – CU
Or
----------LOFT----------------------------------|---------------CU----------
Panels – dc isol – inverter – ac isol | meter – ac isol – CU
Or does it not matter?

One last slight concern is the dc isolators my supplier has sent. They are standard 3phase & neutral isolators (basically 4 pole). The inputs have been linked out to convert to 2 pole with double isolation. So L1 and L2 are linked as are L3 and N. The diagram suggest to connect as follows using the bottom terminals
L in is connected to T1
L out is T2
N in is N out
N out is T3
Should I be concerned they are adapting components incorrectly, or is it the norm?

Hope this makes sense and thanks for reading.

 

[edexlab]

Hi All

I’ve just completed my first install but need to commission it. This install is to be used for MCS assessment so I need to clarify a couple of points. I did a BPEC course and the handbook is now causing me confusion. Here goes.

2 strings of 7, 190w panels. SMA SB2500 with 3 inputs in loft.

First question regarding dc isolators. I have installed 1 per string and connected each string to its own inverter input. When reading the commissioning section of course handbook, it says to test each string Isc with the link across the output of the string dc isolator – this I understand. It then says to repeat the step for the entire array using the MAIN dc isolator! This suggests 3 isolators! This cannot be right? Could someone clarify how many dc isolators are needed for 2 strings?

The other question is the circuit position of the generation meter. I have googled and the consensus seems to be directly after the CU and before the second ac isolator? I then look at the diagrams in my course book which indicates after the second ac isolator on the cable that goes to the first ac isolator in the loft. So could someone tell me if it is:
----------LOFT----------------------------------|---------------CU----------
Panels – dc isol – inverter – ac isol | ac isol – meter – CU
Or
----------LOFT----------------------------------|---------------CU----------
Panels – dc isol – inverter – ac isol | meter – ac isol – CU
Or does it not matter?

One last slight concern is the dc isolators my supplier has sent. They are standard 3phase & neutral isolators (basically 4 pole). The inputs have been linked out to convert to 2 pole with double isolation. So L1 and L2 are linked as are L3 and N. The diagram suggest to connect as follows using the bottom terminals
L in is connected to T1
L out is T2
N in is N out
N out is T3
Should I be concerned they are adapting components incorrectly, or is it the norm?

Hope this makes sense and thanks for reading.

I would say one isolator per string and as long as you have tested each correctly thats fine

Generation meter as long as you can isolate correctly for maintenance does it really matter I usually put them after the Ac isolator at the consumer unit position ie CU >AC isolator >Meter>cable run>ACisolator >Inverter

The DC isolator sound correct is it Kraus & Naimer and it does need the links although you can configure them to have two strings into one isolator as long as voltage rating is adequate ,I'd contact the manufacturer to be sure

 

[T0M0]

Thanks edexlab - just what I was looking for

Could I ask a couple more questions if not pushing it too much?

On the course when we were to test Isc, the clamp on had lost batteries. The instructor said it is OK to test with a multi meter on the 10A scale. This is what they did but the calcs to confirm STC were skewed. To me this is not the correct way of testing because it is not short circuited. What do you think? btw - the course was run by a local college!

Finally, on the G83 form for the load to be applied to the grid. Do you record the output of the inverter or the calcs done for STC?

Thanks again - getting lots of help off this forum which I will repay when I can.

 

[edexlab]

I have to be honest I've never used a multimeter to test current like this ,always used a clamp meter , I don't really like the idea of messing around with
croc clips etc or unfused test leads on these voltages ,on the course we did (NICEIC) the bloke recommended a £10 multimeter (not GS38 approved) ,I asked him what the voltage rating was ; 230v he replied and I said so what happens when you're testing with this at >230v he said you'll never do more than 120v dc,anyway I think most will agree he was wrong, I've not done a job yet of <120v its always >300v or if you get upto a 10kw possibly >700v per string eg 10kw 2 strings of 21 sharp 235's was 777v each string.

Inverter output but check with you're DNO ,if you read some of the threads on here you'll find that different DNO's are using either ratings or keep changing their minds