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Limiter inverter with RS485 load setting
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(04-20-2020, 09:10 AM)rtdm Wrote: Hi completelycharged

I see you are in the UK! I am looking to buy one of these inverters and was wondering if you had any problems with the utility company? i.e. if the inverters ever produced too much power and sent it to the grid....would the utility company care?

Also, do you know if these inverters have islanding protection? I'm guessing if the mains went down the limiting sensor would say the desired is 0 and and the inverter shouldn't produce......but what happens if say you disconnected the mains connection whilst there was still demand? Does it disable the inverter to prevent it producing power on potentially bare connections?

Thanks in advance! Your work on the RS485 is amazing

This is a bit long and a bit of a varied answer, to a seemingly simple question..... you need to know what your doing and fully understand what your doing. It's not just plug and play. Safety for ALL.

The inverters are only 900W (0.9kW) each and a typical house is fused at 100A (UK standard) which is 25kW BUT and here is the reality, if you were to run your house at 25kW for any significant length of time you would find that the cable or junction or joint potentially melts and breaks. I have seen this happen first hand quite a few years back when (under duress, close to getting fired) we connected a house up with about 18kW of overnight heating (Economy 7), which resulted in all of the jointing compound out of the cable/fuse housing to melt and run out onto the floor the first night. Fortunately it did not set on fire. The mains supply was 10mm2 if I remember.

Single inverters can export upto around 3.8kW without "additional" registration steps, etc.

Harmonics, the units are quiet and well within spec. The most horrendous harmonics I have are generated by a 2kW solar grid tie unit which (very amazingly) actually passed the UK testing standards and is fully approved. Just because it has or does not have a "bit of official paper" it does not mean that it is actually ideal or should be used. The specifics depend on how the output is fed in relation to the syncronised waveform as some inverter will try to push out as much as possible early and therefore end up ootentially distorting the waveform. This is way more evident when you are using the units off grid with a single master inverter that provides the "off-grid" mains syncronisation.

If the inverters send power to the grid, then the energy at the moment is not metered (in my case) an therefore effectively goes towards offsetting distribution losses (as that is where the actual metering difference is accounted for in my case) so I am effectively then subsidising the distribution company. So, technically they should care in thoery because it is an accounting issue, in reality it is not in thier interest to actually resolve the issue because their proffits would be impacted directly.

Safety. The inverter circuit is powered from the mains side (as with a lot of grid tie units, if not the vast majority) and islanding protection is actually provided for due to the loss of a stable frequency to syncronise to and the typical case of more load attached, which would also overload the unit and cause it to trip offline anyway. There are multiple ways that islanding is provided and these units do island without problem and the re-sync time can be changes so that it fits within the grid code timings.

The most difficult islanding scenario to deal with is where the inverter is supplying a resonant (of sorts) load where the inverter could potentially provide the balancing power to keep the resonance ongoing. No domestic property would have appliances or equipment that would provide this capability as you would need something like a few hp motor with a very large flywheel or a large ferroresonant transformer attached. Think of it as if your powering a flywheel the energy to keep the flywheel running is relatively little, which is where the lower output could be enough to keep things running. The critical flaw to this scenario is that induction motors have slip (they loose sync with the mains every few cycles due to the losses) and this slip cuases the frequency to go out of range and the inverter trips. The only equipment I can think of which would be an issue is a large ferroresonant transformer, which again would not be seen in a domestic property.

Multiple inverters could potentially create a resonant "cluster" if the output load is perfectly matched and no changes occur. The islanding standard specifies a maximum time for islanding, google "Experimental Evaluation of PV Inverter Anti-Islanding with Grid Support Functions in Multi-Inverter Island Scenarios" and have a read. Page 9, Figure 3. Notice the inverter(s) can still run for a few cycles before instability/excess/under voltage trips the unit off. This is a really good read as to why typically only a single inverter is connected because it can extend the islanding time long enough to cause issues with automatic breaker reset's that occur on the grid, which could result in out of phase reconnects. Chart on page 27, Figure 12.

The real safety issue is isolating your house from the grid when powering from an isolated battery powered inverter because it MUST (absolutely MUST) be double pole isolated from the grid supply cable. There is a particular scenario whereby the neutral line can back feed a live mains equivalent voltage and gets even more complicated with residual current trip breakers involved. This is where you could electrocute the cable repair guy.

"potentially bare connections" - I'm guessing this is either a scenario of the cable guy outtside repairing a fault on the grid or someone inside the house doing some work on the electrics.. Either case, isolate the inverter. NEVER rely on an inverter to not provide any output voltage that could be lethal. These units need the mains to power the switching of the battery supply to get an output, sort of like the chicken and egg, no mains, no output. That is not to say that you could still asume that you don't even get 48V out. Think of a case where the unit has shorted out in some strange way where you have the 48V passing through.

Really basic sense check, think of the scenatio and then think would I be happy/comfortable/confident if I put the bare wire in my mouth. (DONT DO IT !!!!!!)
CarlosGS likes this post
If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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RE: Limiter inverter with RS485 load setting - by completelycharged - 04-23-2020, 03:39 PM

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