PV Inverters and batteries

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jonescg
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PV Inverters and batteries

Post by jonescg » Tue, 22 Aug 2017, 14:42

Quick question for the off-grid / partial on-grid types.

Is it possible to connect a 260 volt battery to the input of a common grid-tie inverter? I realise PV arrays are quite high impedance, so a short circuit would barely muster 12 amps in most cases, but a battery could deliver many thousands. Are they able to tell the difference between a high impedance voltage source like a PV array and a large HV battery?
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Re: PV Inverters and batteries

Post by weber » Tue, 22 Aug 2017, 19:51

AFAIK they aren't able to tell the difference. But they will limit the current they draw from the battery, to that which allows them to push their maximum rated AC output power into the grid.
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Re: PV Inverters and batteries

Post by coulomb » Tue, 22 Aug 2017, 21:39

At a former job, we needed a small 3-phase inverter to detect the mains dropping out. It was easiest to use an off-the-shelf inverter that had approval; it shutting down would trigger other inverters to shut down and not feed power into a dead circuit.

The smallest 3-phase inverter was 10 kW. We fed it with a 100W transformer and rectifier. (No overunity jokes please. ;) ) To prevent it from drawing too much power, we had to insert an impedance between the power supply and the DC input. I think we used a MOSFET in linear mode as a kind of constant current source. You may need to do something similar if you don't want the inverter to run flat out.

I'm intrigued as to why you might want to do this.
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Re: PV Inverters and batteries

Post by jonescg » Tue, 22 Aug 2017, 22:24

coulomb wrote:
Tue, 22 Aug 2017, 21:39
I'm intrigued as to why you might want to do this.
More of a general question really.

I'm also wondering if the likes of the Powerwall, with it's high voltage battery does exactly this. Just seems like a way of cutting out the inefficient middle steps - Solar DC to battery DC via some basic charge regulator, then battery DC to grid AC. The battery would never charge from the grid, but would be able to provide power well into the night provided the daytime charge was sufficient. As you say though, the PV inverter would be delivering full current unless throttled to some extent.

The shopping list would be pretty simple:

PV array
MPPT solar regulator
High voltage battery (260 V+)
Grid tie inverter

The other option is to have a grid-tie inverter running as per usual, but charge a battery from mains power during daylight hours. Then use a battery to AC inverter to run selected appliances. It means the shopping list becomes

PV array
Grid tie inverter
Appliance timer
Battery charger
Low voltage battery (50 V)
Power inverter - and you can still only use it for some appliances, not all, lest you double up on household wiring.
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Re: PV Inverters and batteries

Post by antiscab » Wed, 23 Aug 2017, 00:03

You would want the battery to be above 380v, for the inverter to be most efficient. That way the mppt stage doesn't have to boost the battery voltage above thr nominal rectified mains.

Depending upon topology of the inverter, if you are above 380v you could connect directly to the internal dc bus after the mppt stage.
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Re: PV Inverters and batteries

Post by coulomb » Wed, 23 Aug 2017, 06:59

@jonescg, I think that what you want to do may be provided by hybrid inverters. I've had these explained to me, but I never got interested enough to fully investigate them (and hence I've forgotten the details).
They seem to have two inverters in one box, usually with an MPPT as well. [ Edit: I've since retracted this idea; see this post. ] I suspect that you really need the second inverter because when the mains goes down, usually you will have a large part of the grid's load in parallel with your household loads, which will cause a black out or at least a brown out before you can disconnect the dead mains. Of course you also need the usual test for the case where the mains drops out but also enough of the load that you can still supply that island of load (i.e. you need the usual anti-islanding). So you supply essential and/or sensitive loads from a stand alone inverter not connected to the mains, but larger or insensitive loads from the mains with the grid tie inverter in parallel. So if the mains drops out, some loads see the brown out, but still get powered from the battery via the grid tie inverter. If the outage is more than fleeting, you might shed some low priority loads to conserve battery energy for essential loads. But now the grid tie inverter is not wasted, and you get power during a blackout. If it's day time, you can continue to supply most or all loads and perhaps continue charging the battery. So PV isnt wasted during a blackout either. You might even be able to operate without a battery under some conditions.
The Infini inverters from Voltronic Power are like this, though still with an ELV battery. There may be models with higher battery voltage, but I suspect that the advantages might not outweigh the risks.
I think I read somewhere that the powerwall actually has a 48 V battery inside, but with a DC-DC converter to feed a standard grid tie inverter.
I hope I haven't screwed up the details too badly.
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Re: PV Inverters and batteries

Post by Richo » Wed, 23 Aug 2017, 12:33

You could always buy a smaller GTI that it's maximum continuous output is about your average night time consumption.
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Re: PV Inverters and batteries

Post by Richo » Wed, 23 Aug 2017, 12:35

antiscab wrote:
Wed, 23 Aug 2017, 00:03
You would want the battery to be above 380v
I'd go 380V when Flat.
Even then I found some need a marginal overhead so 400V flat.
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Re: PV Inverters and batteries

Post by Richo » Wed, 23 Aug 2017, 12:40

As a bit of a kick in the pants the microgrid tie inverters are usually less than 50V dc input.
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Re: PV Inverters and batteries

Post by Richo » Wed, 23 Aug 2017, 12:45

coulomb wrote:
Tue, 22 Aug 2017, 21:39
...a MOSFET in linear mode as a kind of constant current source. You may need to do something similar if you don't want the inverter to run flat out.
Yeah that'd work.
Normally the current draw on a GTI is updated every 10-20ms.
So a fat cap and a chopper cct could be enough to trick it without going into linear mode.
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Re: PV Inverters and batteries

Post by Richo » Wed, 23 Aug 2017, 12:54

What would be really cool is if you could put a clip on current sensor on the mains line at the meter box.
Then feed back the current sensor to the GTI/solar chopper cct to end up with net 0.
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Re: PV Inverters and batteries

Post by coulomb » Wed, 23 Aug 2017, 20:02

Richo wrote:
Wed, 23 Aug 2017, 12:54
What would be really cool is if you could put a clip on current sensor on the mains line at the meter box.
Then feed back the current sensor to the GTI/solar chopper cct to end up with net 0.
Actually, I think that one of the modes of the hybrid inverters is to aim for net zero export. The South Africans optionally have a thing called a "pre-paid meter", which is strictly zero feed-in (export) to the grid, and some of them attempt to use hybrid inverters with such an arrangement. But of course, the inverters struggle to maintain zero export immediately after a large load suddenly comes off (e.g. jug has boiled). So they have to be careful to get the meter that is most forgiving of brief periods of export. I think some of them average the import/export over a decent time period, say 15 seconds, so if the inverter overshoots then undershoots and the average is still zero, you're all good. But my understanding is that if it decides you've exported energy (shock horror :roll: ) then you get disconnected. I think it would still be a problem, as you'd never know when the 15 second averaging period starts or ends. So if you accidentally export at the end of one period, and import it the next, then you've still had a period where you net exported.

I believe that they can export if they have a different account and therefore a different type of meter.

Perhaps I shouldn't give our regulators any crazy ideas... :evil:
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Re: PV Inverters and batteries

Post by jonescg » Wed, 23 Aug 2017, 20:54

I'm thinking the latter option - a 2.2 kW array, grid tie inverter, battery charger plugged into mains, 6 kWh battery (cheap salvage LiFePO4). I can then use the battery for anything I like, including a fast charger for my scooter :) Alternatively I can power a 2 kW 48 VDC to 240 VAC inverter which the scooter charger can plug into. The trick will be to ensure the battery is only able to charge from 9 am to 3 pm.
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Re: PV Inverters and batteries

Post by antiscab » Sat, 26 Aug 2017, 20:43

I have long thought about feeding some of the circuits in my house from a 320v nominal battery. Pretty much only loads with motors running direct online can't run from dc, everything else can. No having to much around with inverters and loses.
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Re: PV Inverters and batteries

Post by weber » Sun, 27 Aug 2017, 06:39

antiscab wrote:
Sat, 26 Aug 2017, 20:43
I have long thought about feeding some of the circuits in my house from a 320v nominal battery. Pretty much only loads with motors running direct online can't run from dc, everything else can. No having to muck around with inverters and losses.
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Re: PV Inverters and batteries

Post by jonescg » Sun, 27 Aug 2017, 22:09

Yes DC is a different beast. Great video!

I see the Powerwall will be a bolt-on option for any existing solar system - Reecho had some cool videos...
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Re: PV Inverters and batteries

Post by weber » Tue, 29 Aug 2017, 16:57

jonescg wrote:
Wed, 23 Aug 2017, 20:54
I'm thinking the latter option - a 2.2 kW array, grid tie inverter, battery charger plugged into mains, 6 kWh battery (cheap salvage LiFePO4). I can then use the battery for anything I like, including a fast charger for my scooter :) Alternatively I can power a 2 kW 48 VDC to 240 VAC inverter which the scooter charger can plug into. The trick will be to ensure the battery is only able to charge from 9 am to 3 pm.
Your posts about an AC changeover contactor today, prompted me to revisit this thread to make sure you're aware that an ordinary grid feed inverter cannot invert unless it is connected to the grid, or something very much like it. It relies on the grid to determine the frequency and the voltage waveform. It operates only in "current control mode".
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Re: PV Inverters and batteries

Post by jonescg » Tue, 29 Aug 2017, 17:17

Well aware of it, thanks Dave :)

Just thinking further down the track should we ever win lotto and get a mortgage (since a lotto win would roughly equal most of a deposit). Off-grid with the grid as a backup generator might be an option, provided the battery can be secured cheaply enough.
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Re: PV Inverters and batteries

Post by coulomb » Wed, 27 Sep 2017, 10:27

coulomb wrote:
Wed, 23 Aug 2017, 06:59
... hybrid inverters. I've had these explained to me, but I never got interested enough to fully investigate them (and hence I've forgotten the details).
They seem to have two inverters in one box, usually with an MPPT as well.
After discussion with Weber, I'm going to capitulate on this. I can't find the photo that suggested to me that an Infini inverter had more power electronics than the similar powered PIP/Axpert, and I don't have time at present to investigate further.

I thought that brief black-outs or brown-outs were such a bad thing that it would be worth adding the extra stand alone inverter inside one box. But presumably, it should be possible to change over after detecting half a cycle of bad mains, and it takes another half cycle or so to switch over.

There is also the fact that hybrid inverters cost a lot more than stand alone models of about the same power. I assumed that this was because of the extra inverter, but it could be merely the extra regulatory costs associated with anything grid tied.

Finally, there is the observation that the electronics for a hybrid inverter and a bypassing stand alone inverter like the PIP/Axpert could (it seems to be) be the same. Why not use the same case and electronics, and merely different firmware, model number, and case colour? Most likely, I'm overlooking some important factor.

So: most likely the hybrids don't have a separate stand alone inverter. I don't even see how this was relevant to the original question, but I prefer not to have misinformation posted in my name.
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