The right inverter for battery storage and some other Qs

[Oldgreybeard]

The source is an inverter which is inherently current limited. This is case where in my opinion the regs were not designed for this situation, things have moved on a lot in recent years regarding domestic scale inverters and their capabilities and the regs need to catch up, or be clarified. A lot of inverter cables are oversized in order to minimise volts drop (rise) to prevent the inverter volts rising too high during periods of high generation and hence tripping out on over voltage. One of my inverters has 10mm2 cable as the run is over 25m. Its a 6kW inverter, max current of 26A at 230V - less at the more normal 245V - the max current for the cable is 44A if buried , and over 60A clipped to the surface. So in my mind it all becomes a bit irrelevant as the inverter could never supply enough current to damage the cable. So the normal protection at the "grid end", in this case 32A, should suffice.

The 5kW "back up" supply on my Solis inverter just shuts down if you try to draw too much power (don't as me how I know!) - that is when the grid is down, not sure what happens when the grid is present and is fed straight through. Once "tripped" you have to then shut the unit down - disconnect batteries and PV - and restart it to get power back.

Is it "inherently current limited" in terms of never exceeding the cable fusing current, though?

That's the thing I couldn't pin down. It seems that the Sofar inverter can deliver well in excess of 3kVA for short periods, but there is no reliable data on how much current, or for how long, the inverter could sustain an overload. In my case the cable safe current limit is about 20A, due to some of it running in ducting and conduit and some of it running through insulation. I know that the cable ratings are conservative, but what I couldn't be 100% sure of is whether or not the inverter could deliver enough current to overheat the cable.

I chose to add over-current protection at the inverter end just to be safe. The cost was minimal, and at least I have some assurance that the cables are protected no matter what.

[marshman]

__

[ducabi]

I didn't say using EPS when the grid is working. I said not to draw more than 13.2A from the EPS output even when the grid is working.

The critical loads remain connected to the EPS output all the time. If the grid is working the inverter does not work in EPS mode drawing power from the battery so it is not "using EPS". But it still feeds the critical loads from the "EPS" connection as per Fig 1 which you evidently still do not know how to interpret correctly.

BTW do not be tempted to connect the ring mains, cooker or immersion heater circuits to the kettle. The sentence you have quoted is not intended to be read like that. /sarcasm

Does this mean that if I connect whole house to EPS it will mean that I'm limited to 13A always, not only when in EPS mode?
So, ideally I have 2 options (not sure 2nd is doable) in order to use it on a daily basis for night tariff:
1) by the book, as per fig1, split circuits so that EPS is not used over 13A/3000va.
2) everything connected to normal load and if anything happens reconnect to EPS and remember about limitations.

... which you evidently still do not know how to interpret correctly.

Yup, that's the trouble. Hopefully my electrician is a competent persons .

[Marcus]

...
That's the thing I couldn't pin down. It seems that the Sofar inverter can deliver well in excess of 3kVA for short periods, but there is no reliable data on how much current, or for how long, the inverter could sustain an overload. In my case the cable safe current limit is about 20A, due to some of it running in ducting and conduit and some of it running through insulation. I know that the cable ratings are conservative, but what I couldn't be 100% sure of is whether or not the inverter could deliver enough current to overheat the cable.

I chose to add over-current protection at the inverter end just to be safe. The cost was minimal, and at least I have some assurance that the cables are protected no matter what.

You probably are overthinking this:
The cable can be overloaded for a short while - as long as the overcurrent protection disconnects before the cable overheats you're ok, which is just as well as most overload devices are designed to tolerate some overloading.

For eg. A standard B16 bsen 60898 breaker will take 30A for over 3 minutes, 40A for about 20 seconds. I suspect an inverters overload tolerance would also follow some sort of inverse time/magnitude relationship.

[Oldgreybeard]

In my case yes. Firstly I only have 5.7kW pk of PV so the most I will get from that side is ..... 6kW on a really good day. The battery stack is limited to 100 A, essentially 5kW and the BMS will limit it to that and if not there are 125A fuses on the 48V side -which I accept will pass a lot more than 125A but I suspect would blow pretty quick at 200A, which is 10kW (ish) or 43A at 230V, still well within the capability of the 10mm2 cable. As always it depends on the individual situation.

I do accept what you say and no harm in belt and braces but I also think that sometimes things can be overthought and I worry about multiple protection devices on the same circuit, every one is another set of terminals and in my, admittedly not that extensive, experience more issues occur with high resistance junctions/terminals causing excess heat etc. than with situations where a correctly designed/loaded circuit causes cables to over heat.

In my case there is no real practical limit apart from the inverter protection circuitry, and that isn't at all clear as to how it limits. I have 6 batteries that can each deliver about 3.5kW for five minutes, so they could deliver a combined 21kW, way, way over the 3kVA inverter limit. They are protected by 125A NH uses, but they can carry between 6 and 8 times their rated current before they blow, they take 3,000 seconds to blow at 200A, for example. That's in excess of 10kW for 50 minutes before one or other of the DC fuses would blow.

This means there is effectively zero protection on the input side as far as limiting the current at the output goes, it all comes down to the current surge capacity of the inverter. That isn't at all clear. The inverter can definitely deliver way over its maximum rating for short periods, that's clear from looking at the way it responds to things like the cooker being turned on. What is not clear and isn't stated anywhere that I can find, is how long the inverter can deliver way over the 3kVA max continuous output. If this is only, say, 5 seconds, then it's not really an issue. If it's 5 minutes then that's a potential issue.

It all comes down to the common difficulty with a lot of this kit, and that is that the detail of the specifications is often just a bit thin.

[sharpener]

The inverter can definitely deliver way over its maximum rating for short periods, that's clear from looking at the way it responds to things like the cooker being turned on. What is not clear and isn't stated anywhere that I can find, is how long the inverter can deliver way over the 3kVA max continuous output. If this is only, say, 5 seconds, then it's not really an issue. If it's 5 minutes then that's a potential issue.

It all comes down to the common difficulty with a lot of this kit, and that is that the detail of the specifications is often just a bit thin.

Yes as we are all finding with different aspects of the specs.

Victron do actually publish their overload vs time data but ATM I can't find the details, IIRC they will supply 100% overload for 15 secs, which given the size of the heatsinks and transformer sounds about right. I would be surprised if the Chinese mfrs can achieve anything better for the price, which gives us a plausible upper bound. OTOH they also publish a max inverter output ramp rate of 400W per second, not sure how that squares with putting two rings on the cooker on in quick succession. So it's still a bit vague in theory but nothing trips in practice.

Given this and that like others' my wiring is all in 10 mm^2 I think overloading the cable is not our worst problem!

Perhaps someone else can have a go at sorting out our friend's queries this time?

[sharpener]

Does this mean that if I connect whole house to EPS it will mean that I'm limited to 13A always, not only when in EPS mode?

Yes

So, ideally I have 2 options (not sure 2nd is doable) in order to use it on a daily basis for night tariff:
1) by the book, as per fig1, split circuits so that EPS is not used over 13A/3000va.
2) everything connected to normal load and if anything happens reconnect to EPS and remember about limitations.

1. This. AFAIUI it will still supply the whole house from the PV/battery during the day up to the 3680W limit of the inverter if you programme it correctly, above that it will pull more power from the grid at peak rate.

2. No, as @marshman reported in post #118 you would then prbably need to do a "black start" which is extremely inconvenient and if e.g. it tripped because you have just turned another ring on might leave you in the dark with hot pans on the stove.

[ducabi]

I'm having really bad time finding someone to install my inverter. I can either have an electrician with a will but no experience in inverters, so he could do it if I give him full instruction or number of other options willing to sell me a proper battery system, not some "unsafe DIY chinnese toys" (pylontech us3000c + Sofar) .

[Stinsy]

I'm having really bad time finding someone to install my inverter. I can either have an electrician with a will but no experience in inverters, so he could do it if I give him full instruction or number of other options willing to sell me a proper battery system, not some "unsafe DIY chinnese toys" (pylontech us3000c + Sofar) .

"unsafe DIY chinnese [sic] toys"?

[ducabi]

I'm having really bad time finding someone to install my inverter. I can either have an electrician with a will but no experience in inverters, so he could do it if I give him full instruction or number of other options willing to sell me a proper battery system, not some "unsafe DIY chinnese toys" (pylontech us3000c + Sofar) .

"unsafe DIY chinnese [sic] toys"?

Yup, they were called chinese toys. Had another one today who sounded good but still waiting for the quote.