On-grid domestic battery storage

Solarchaser

I would say it's down to the sofar me3000sp not anything to do with batteries.
Its probably worth noting that I apparently have an older version of the sofar (2017) which is why it wont communicate every parameter with the solarman app, so perhaps one of the guys with a newer model can comment if its improved.

On some overnight use the battery is fine with 50w output.
The problem is that it's not consistent.
If I was to take 10 overnights with base going from 50w to 180w (I assume its the fridge compressor cycling) then on maybe 3 of those overnight it will cover without going to standby, but the other 7, once it drops below maybe 150-200w then it will go into standby and let the grid take over.
If the load at any point goes above 200w it will take over, but when it drops below 150w (ish) again, then it will go to standby.

I purely use it as battery charging from solar, and discharging for load, no time of use, nor e7

mmmmikey

Solarchaser wrote: »

The problem is that it's not consistent.

I think that some of the inconsistency may arise from occasional high mains voltages and the effect this has on the inverter operation. Powervault fitted an additional transformer to mine to reduce the mains voltage after monitoring it for a couple of weeks after initial installation and this made it more consistent. I think it was something to do with occasional high voltages overnight causing the battery to stop discharging briefly, then not starting again due to hysteresis effect I mentioned in an earlier post. Not sure if you monitor mains voltage and have noticed any correlation?

Solarchaser

I dont monitor mains voltage, but the sofar does, it bounces about from 242 to 250 depending on time of day.
The highest voltage is when the panels are generating , and the lowest is overnight for the last week...

I'm not saying you are wrong, I've never really looked at it, but I cant see a 250v peak causing issues with the inverter, as its safety cut off is quite a bit higher.

I think there is a hysteresis issue with starting back up, but there always will be due to the fact the ac systems can only ever work reactively.

mmmmikey

Interesting that the voltage is varying depending on generation rather than just natural grid variation, I wonder where it is being measured or if there is just a correlation between the 2 where you live? Scratching my head a bit but I recall something along the lines of grid tie inverters switching off temporarily at a threshold that is more suitable to mainland European use than UK, but I could be wrong here.


As far as hysteresis is concerned, I can see that effect very clearly from last night's charts. I ate out yesterday and didn't have the normal cooking load in the evening, and without heating this time of year there's very little usage other than normal background. As generation gradually fell below usage in the evening, the shortfall was taken up by import, which gradually rose as genertaion fell. This never reached the level where the battery kicked in so my background load was met with import until about 21:00. However, when I boiled the kettle at that time it was enough to switch the battery on and start supplying, and at that point it continued to meet my background load all through the night. So the answer to getting your batteries to work overnight seems to be eating out and drinking lots of tea



(Haven't worked out how to post the charts here, happy to do so if anyone is interested and call tell me how to)


Thanks, Mike

Martyn1981

mmmmikey wrote: »

Interesting that the voltage is varying depending on generation rather than just natural grid variation, I wonder where it is being measured or if there is just a correlation between the 2 where you live? Scratching my head a bit but I recall something along the lines of grid tie inverters switching off temporarily at a threshold that is more suitable to mainland European use than UK, but I could be wrong here.

Hiya. PV inverters push up the grid voltage a little bit, think of it as having to be at a higher pressure to push the leccy out of the house, v's the mains pressure.

UK has a 'pretend' voltage of 230V, but historical pressures mean it tends to be higher.

The working range is -6% to +10% (216V to 253V), that's to say, the voltage the DNO's are required to maintain, and importantly the legislation states that it's their responsibility to stay within that range even with SSEG's (small scale electricity generators) on the network - wind, PV etc.

As PV inverters will push up grid voltage, to avoid a spiraling problem, all but the very earliest models should shutdown when grid voltage hits 253V. Mine did it once, many years ago*. They will then check on the mains voltage and restart once it is below 253V again.

*Classic example as it did it on a sunny Sunday afternoon when demand is low, so voltage can rise.

Apologies for waffle, but there was a fun article many years ago with a DNO 'complaining' that reports of overvoltage (253V+) had increased ten fold due to PV.

In reality, inverters shutting down due to overvoltage, meant homeowners where raising the issue with the DNO's. So the problem didn't really get worse, we just found out how badly some DNO's were at maintaining voltage in the 216-253V range, then they moaned that we'd caught em!

Solarchaser

It's being measured from the sofar grid connection I assume, only place it can see grid voltage.

My grid voltage has never reached cut off.
But yeah I'd 100% expect to see the grid voltage rise in my home when I am generating, makes perfect sense.
I'd reckon it's something like 1v per kw generated vs grid voltage.

Martyn1981

Solarchaser wrote: »

I'd reckon it's something like 1v per kw generated vs grid voltage.

Can I ask how you calculated/estimated that*, as it seems very similar to a figure I recall an installer giving me years ago on a different forum? I'm sure he said PV systems can push up the voltage by about 2-3V.

*Simple, general terms only please. If it's very complicated, then just say so, and I'll take your word for it, I'm just a curious soul.

Thanks.

Solarchaser

Haha I like that Martin, I'm afraid I have no complex calculations to dazzle you with, and tbh no actual proof of it either.

It's taken from looking at the voltage in the morning vs mid afternoon vs night and noting morning voltage up around a volt, say 245v, then midday around 248v and then evening about 244, suggesting the early 1kw of pv pushed up a volt, the midday 4kw gave 3-4v and when no pv went back to grid voltage.

I did it on a couple of different days, but its anecdotal at best tbh, and if quizzed on it I'd have to admit to suffering from confirmation bias.

JKenH

Interesting that this issue of grid voltages is being discussed as coincidentally on Friday one of my inverters switched off when the DC input voltage spiked at 270v. It quickly restarted. The other inverter doesn’t show as switching off but as my data only refreshes every 5 minutes or so it could have switched off and back on without me noticing. (The inverters were about 2 minutes out of sync on the day.)

The DC input voltage for one roof with 6 panels per string however is typically 30v lower at around 190v than the one which tripped out which has 7 panels per string and operates at about 220v so what exactly is the “DC input voltage” and is the issue completely unrelated to the AC grid voltage?

Solarchaser

DC input voltage is your voltage coming from your panels.
The inverter uses this to make AC power.
AC or Grid voltage is what's coming into your house from the main grid.

It sounds like your panels are too powerful for your inverter if they are causing a shut down