Multiple strings vs Higher voltage single string

[glenns]

Hi Guys, bit of advice from the solar experts please...

System has been up and running for a month or so, still waiting on the batteries but I'm noticing that my second string (PV2) isn't coming online as soon as it could. I'll give a brief rundown of the system to explain, I have 8 panels split into 2 strings, they are on a shallow pitch roof, one side facing SSE(PV1) the other NNW(PV2). With the lovely weather both strings are getting plenty of sun and can produce power as early as 6am but PV2 isn't coming online, just sits showing 4-5v until much later in the day. If I manually cycle the breaker the PV2 will spring into life.

I've checked the connections, all look good.... is this normal behaviour?? would I be better running all the panels in series on a single string or should I be investigating potential connection issues ?

The Inverter is RHI-6K-48ES-5G so is capable of handling the 8 panels in a single string but I had read it was better to split the load across the inputs.

TIA

 

[Andy1733]

I have 2 strings. 7 and 8 panels
1 gets a little shadowing from a palm tree early morning and it is a little while behind the other at getting upto speed. Keep them as 2 strings imo

 

[up_north]

Probably not linked but just curious as to why you have a 6kw inverter for a much smaller array which I’m guessing is no more than 3kw-ish?

 

[binky]

If they are facing different directions they will generate at different times, that's kind of the point of having split arrays. NB don't mess with the strings unless you really want to mess it up.

 

[glenns]

Probably not linked but just curious as to why you have a 6kw inverter for a much smaller array which I’m guessing is no more than 3kw-ish?

Few reasons....
1) Higher AC output, 6kVA vs 4kVA.
2) Higher battery charge and discharge rate.
3) future expansion....I've got an extension planned and will add more panels during construction.

 

[glenns]

If they are facing different directions they will generate at different times, that's kind of the point of having split arrays. NB don't mess with the strings unless you really want to mess it up.

That was my thinking when I installed it but just wondering why PV2 isn't coming online when it could. To be clear, if I cycle the DC isolator on the inverter it will shoot upto 150v immediately (so the panels are kicking out enough power) but if left it might not kick in for another hour..maybe longer.

 

[binky]

stalled voltages are different to running voltages and power, shutting off the Pv allows charge to build up, so when you switch back on it may start running, but does it stay on for any length of time?

 

[up_north]

Few reasons....
1) Higher AC output, 6kVA vs 4kVA.
2) Higher battery charge and discharge rate.
3) future expansion....I've got an extension planned and will add more panels during construction.

I get that it’s just everything I’ve read suggests matching the inverter to the array or even under sizing it a bit. But interesting that this works for you because I can only get 10 panels but want to draw more than 4kw from a battery so was also considering a bigger inverter.

 

[binky]

I get that it’s just everything I’ve read suggests matching the inverter to the array or even under sizing it a bit. But interesting that this works for you because I can only get 10 panels but want to draw more than 4kw from a battery so was also considering a bigger inverter.

Undersizing inverters is 'old hat' based on out of date wire wound transformer technology.

 

[glenns]

stalled voltages are different to running voltages and power, shutting off the Pv allows charge to build up, so when you switch back on it may start running, but does it stay on for any length of time?

Yep, stays up and running fine. The current is slightly lower than PV1 but still enough to warrant being online.

As an example, on Saturday after manually cycling the isolator the inverter output went from producing 900w on just PV1 to 1500w on the pair...this was about 8am.

I'm curious as to the potential cause, could this be the inverter logic keeping the string offline for some power optimisation I don't understand or is it more likely to be due to a fault in my installation??

 

[glenns]

I get that it’s just everything I’ve read suggests matching the inverter to the array or even under sizing it a bit. But interesting that this works for you because I can only get 10 panels but want to draw more than 4kw from a battery so was also considering a bigger inverter.

As Binky said...the old method for sizing inverters seems to be for wire wound type, that and Installers using the bare minimum to maximise profits.

The cost difference between the 3.6 and 6kw unit was negligible IMO but gives me the ability to cover the full load of the house without falling back to the grid.....at least while the batteries aren't empty.

 

[binky]

I'm curious as to the potential cause, could this be the inverter logic keeping the string offline for some power optimisation I don't understand or is it more likely to be due to a fault in my installation??

Curious indeed! Not having encountered an inverter oversized to the extent yours is, I don't have an explanation for that, but it would seem you are managing to override the algorithms. I would say you haven't got an obvious fault, but, I would double check the self fitted MC4s for any signs of arcing on the contacts. That's the most common failure point, in fact I would say about 99% of faults are poorly fitted connectors.

 

[glenns]

Curious indeed! Not having encountered an inverter oversized to the extent yours is, I don't have an explanation for that, but it would seem you are managing to override the algorithms. I would say you haven't got an obvious fault, but, I would double check the self fitted MC4s for any signs of arcing on the contacts. That's the most common failure point, in fact I would say about 99% of faults are poorly fitted connectors.

Thanks Binky, I'll do that asap, I'll also follow up with Solis to see if they have any advice. Their older inverters have an option to "disable the independent tracking on an inverter, if you want to parallel them"... which I don't believe is the equivalent to wiring the strings in parallel but could point to some logic of the system used to improve power delivery that I don't understand.

Do you think running all 8 panels in a single string could be of any benefit ? they are 460W JA Solar units, VMPP: 41.13V IMPP: 10.92A so the single string would be within the capacity of the inverter.

 

[up_north]

Undersizing inverters is 'old hat' based on out of date wire wound transformer technology.

I read that somewhere also but couldn’t find an authoritative source.

 

[up_north]

As Binky said...the old method for sizing inverters seems to be for wire wound type, that and Installers using the bare minimum to maximise profits.

The cost difference between the 3.6 and 6kw unit was negligible IMO but gives me the ability to cover the full load of the house without falling back to the grid.....at least while the batteries aren't empty.

Yep - this is exactly what I’d want a bigger inverter for also. I think I saw an advert recently with a specialty offer meaning the price of a Solis 5kw inverter was actually less than the 3.6kw one.

 

[glenns]

Yep - this is exactly what I’d want a bigger inverter for also. I think I saw an advert recently with a specialty offer meaning the price of a Solis 5kw inverter was actually less than the 3.6kw one.

Well bar this pretty minor issue I'd say it's defiantly worth it, you also have the benefit of less stress on the components (compared to an undersized unit) which will hopefully yield a longer lifespan.

 

[tomatina]

You have 4 panels on each string and each panel is about 30v ? Thats 120v ?
On your main question you can add one more panel to see if that helps

 

[glenns]

You have 4 panels on each string and each panel is about 30v ? Thats 120v ?
On your main question you can add one more panel to see if that helps

41v per panel, each string sits about 155 - 175v while producing

 

[binky]

Thanks Binky, I'll do that asap, I'll also follow up with Solis to see if they have any advice. Their older inverters have an option to "disable the independent tracking on an inverter, if you want to parallel them"... which I don't believe is the equivalent to wiring the strings in parallel but could point to some logic of the system used to improve power delivery that I don't understand.

Do you think running all 8 panels in a single string could be of any benefit ? they are 460W JA Solar units, VMPP: 41.13V IMPP: 10.92A so the single string would be within the capacity of the inverter.

View attachment 13893

So, you are struggling to meet start up voltages by the looks of it. You could try reconfiguring as a single string, but that will put pressure on the bypass diodes in the panels and is generally not recommended. Adding just one more panel to each string would help a lot.

 

[glenns]

So, you are struggling to meet start up voltages by the looks of it. You could try reconfiguring as a single string, but that will put pressure on the bypass diodes in the panels and is generally not recommended. Adding just one more panel to each string would help a lot

Thanks guys, That's probably the best solution....now I just need an extra 4 foot of roof space

Think I'll live with it for now, maybe revisit once the batteries are installed.