MCB tripping on Solar PV Circuit

[Gavin John Hyde]

Have a customer who I do a lot of work for, owns several industrial units. on top of which is a large solar pv set up.
Changed the 3ph DB for him earlier this year. the 80 panels on the roofs have until now caused no issues. Solar set up is approximately 8 years old

No issues prior to this. was then on a Triple Pole 32 Amp MCB. (exact type cant recall as it was so worn and dirty!)
SInce then the MCB for the Solar PV circuit in the landlords DB trips. this usually only becomes clear when it comes to read the meter for the FIT.
Have checked the wiring from DB to invertor - all tests okay, no loose connections etc, IR good.
MCB replaced went again this week.
It is currently on a triple pole 32Amp Type C MCB
No Rcd on this circuit. only one other circuit in landlords DB and that is a 63A submain to the split unit next door, no issues here.
I am at my wits end trying to work out why this is happening?
Is it something in the invertor or a sudden excess of PV being generated ?
Could the panels or the invertor be causing a fault that is tripping the MCB?

 

[Rockingit]

Simple things first - have you got a low enough Zs to fit a D type?

 

[Gavin John Hyde]

Simple things first - have you got a low enough Zs to fit a D type?

Yes, The Zs for the circuit to the invertor is very short, Ze for the property is excellent.

 

[Rockingit]

If you can't make out what the previous type was then maybe that's the simplest answer and it was a D type

 

[marconi]

Do some sums like these - or send me the figures.

There are 80 panels. If I assume 250W panels then the maximum pv real power is 80 x 250 = 20kW

Converted to 3 phase ac power at unity PF that is 20/3 = 7kW per phase. So 7kW x 4/5 A = 28/35A per phase.

The PF of the solar inverter is more likely to be 0.9 leading. So current per phase is now 28/0.9 = 31A or 35/0.9 = 39A.

(Aside: If a large site pv system just outputs real power ie PF=1 it actually makes the incoming site supply power factor worse - think about that. Since most sites are lagging PF the way to keep the site PF near unity is to generate from the PV inverter positive kVAr hence feed in the PV power with a leading PF thereby reducing grid lagging reactive power and real power fed from the grid together keeping it 'about the same').

So for 250W panels a phase current range of 31- 39A.

What if the panels were 300W each? We can quickly say that the range would be 300/250 = 1.2 x (31 or 39) = 37 - 47A

Irrespective of the type of mcb the 32A rating may be too low.

But why now? The inverter may have reactive power control functionality which is only now 'making its presence felt'. Has the site increased/decreased its energy consumption recently because of C19?

If you send me details of the inverter I will try to find out more.

(Check the sums - I have had a few glasses or red wine - but you get the gist of the analysis).

 

[pc1966]

I was going to suggest clamping it on a sunny day to see, but...

@marconi has an important point, but do you know if it is tripping on sunny days or randomly? If it is not obvious when it has tripped you might want to consider some monitoring arrangement, say an auxillulary contact for the MCB or changing to a 4-pole MCB so there is a spare one that can switch a mains indicator off to warn of a trip.

 

[James]

you may be able to see in the logs on the inverter,
when the mains failed (that's what the inverter will see)
if you are able to download the generation statistics, or view them on a graph, it will become clear if the breaker is tripping when the output is at its highest or if it is a more random event.

 

[Gavin John Hyde]

Do some sums like these - or send me the figures.

There are 80 panels. If I assume 250W panels then the maximum pv real power is 80 x 250 = 20kW

Converted to 3 phase ac power at unity PF that is 20/3 = 7kW per phase. So 7kW x 4/5 A = 28/35A per phase.

The PF of the solar inverter is more likely to be 0.9 leading. So current per phase is now 28/0.9 = 31A or 35/0.9 = 39A.

(Aside: If a large site pv system just outputs real power ie PF=1 it actually makes the incoming site supply power factor worse - think about that. Since most sites are lagging PF the way to keep the site PF near unity is to generate from the PV inverter positive kVAr hence feed in the PV power with a leading PF thereby reducing grid lagging reactive power and real power fed from the grid together keeping it 'about the same').

So for 250W panels a phase current range of 31- 39A.

What if the panels were 300W each? We can quickly say that the range would be 300/250 = 1.2 x (31 or 39) = 37 - 47A

Irrespective of the type of mcb the 32A rating may be too low.

But why now? The inverter may have reactive power control functionality which is only now 'making its presence felt'. Has the site increased/decreased its energy consumption recently because of C19?

If you send me details of the inverter I will try to find out more.

(Check the sums - I have had a few glasses or red wine - but you get the gist of the analysis).

As always very well explained, I was already thinking along the lines of over load. It first started tripping during the heatwave, I had naively assumed the original spec of mcb was okay for the system as it was installed and designed by a larger national solar PV contractor! I should know better than that so replaced on like for like basis without querying the panels on the roof - I didnt know there were 80 of them until recently.

I shall check the logs tomorrow on the invertor and look for any records and get the model specs.

I have just checked the old test sheets I have and the old breaker was a MEM1 32A M9 TYPE C 3PH. board was replaced to split the landlords circuits from the main unit. so why it now trips is interesting

 

[James]

remember of course that breakers trip at a lower current and a bit quicker when the ambient temperature increases.

 

[Wilko]

Keeping in mind that insolation is on the decline now, has the MCB been tripping recently? London goes from about 4.7 kWh/m2 per day in July down to about 0.5 kWh/m2 per day in December. So the output just now is unlikely to be more than 25% (say) of its max.

Since the cabling is ok, my thought is to look at the Inverter fault log for some clues.