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Grid tie inverter on separate battery inverter output


Noobie
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So I was wondering if it would be possible to connect a grid tie inverter on the output side of my Axpert inverter?

That way when the sun is shining the PV is being pushed into the homes grid and being consumed by all the other appliances that are not on the inverter output (geyser, washing machine, dishwasher etc)

In the event of a mains failure the Axpert would take over and supply power to the essential loads as well as the grid tie inverter so that the PV panels continue to supply power to the loads connected to the battery inverter only?

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The only limitation that I can see is that your available draw would determined by the Axpert (4000 va) although I haven't looked at the manual as to what the available power would be if you are in grid mode. Also this would mean that you will need to charge the batts via the grid as your panels will go through the Grid tie inverter , but since they are for backup it should not be much of an issue.

 

Unless you feed the grid tie inverter from the Axpert but I am not sure if it could be done , maybe a Multi plus could do that , almost sure it can , @plonkster what is your bias opinion? :P

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1 hour ago, Noobie said:

So I was wondering if it would be possible to connect a grid tie inverter on the output side of my Axpert inverter?

Short answer: No. For this to work there has to be a way to tell the GTI to back off. Most of them support GFPR and will reduce power linearly as you increase the frequency from 50Hz to 53Hz. A multi can do this but an Axpert cannot. You will blow something sooner or later.

Slightly longer answer: If the GTI is happy with the waveform of the Axpert (it might be), it will work as long as you never have surplus power, but do you really want to take that chance?

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Pity, I was thinking about getting a grid tie inverter to connect my PV panels to.

The only issue is that when there is a grid failure the grid tie inverter shuts down so you will have to rely on the experts battery bank only.

Has anyone built a control circuit to switch DC supply from PV panels, kind of like an automated change over switch which is operated by council power?

When there is council power the control circuit will connect the PV panels to the grid tie inverter

When there is a council power failure the control circuit will automatically disconnect the PV panels from the grid tie inverter and connect them to the Axpert battery inverter.

When council power is reinstated the PV panels are automatically disconnected from the battery inverter and reconnected to the grid tie inverter.

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If you haven't bought the GTI yet, the answer might be to go for a proper hybrid inverter instead. Not sure what kind of power you're looking at, but assuming it is around the 5kva point, a Fronius is around 25k, while an Infini is also around there (for the 4kva one). Sell the Axie and get something designed to do this instead of hacking around it :-)

Designing a changeover should not be too difficult. When the grid fails, the GTI will disconnect and your PV panels will not be under load, so switching off is not going to be a problem. When the grid returns though, that's the problematic side. You'll need a nice big (read expensive) contactor to handle the voltage and current. The GTIs often require a significantly higher voltage too (even the smaller ones, such as the Fronius Galvo, requires a good 120V to start), so this is going to take way more work than just a changeover switch.

You could throw in a better MPPT, such as the 250V units made by Victron, but in that case I'd suggest selling the Axie, skipping the GTI, and using a Multiplus with external MPPTs and ESS instead (that will cost more of course, but if you're going to buy the better MPPT anyway the rest should be about the same...).

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Thanks Plonkster, I haven't purchased anything yet, still doing homework  (saving up) 

I did some googling and I have not managed to find a fronius single phase hybrid unit? I do like the Fronius GTI's they are IP65.

The location where all my equipment is housed is outdoors so I will need a unit that is weatherproof so I dont think the Multiplus will work.

I will draw a rough sketch of what I am trying to achieve and lets see if I am on the right track...

 

 

 

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So heres my simple sketch.

What I would like to have is an inverter which feeds power from the PV panels to the essential loads (lights plugs) and the non essential loads (stove, geyser) when there is enough PV power to do so.

I would imagine that some sort of smart meter that talks to the inverter would need to be installed at location "A" to tell the inverter not to feed back power back into the grid.

Then when the grid fails I want the inverter to supply battery and PV power to the essential loads only.

 

 

SMINOLTA17081617210.pdf

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3 hours ago, Noobie said:

I did some googling and I have not managed to find a fronius single phase hybrid unit? I do like the Fronius GTI's they are IP65.

I use sonopsolar for no other reason than that they are sort-of reasonably priced and have all the pictures with prices on their site. Here is the small Galvo, but there is a 3kva galvo and a primo on the same site, all single-phase. They aren't hybrid, they are pure GTIs. I merely brought them up to make a price comparison with the hybrid that is from the same stable as your Axpert... to argue you might as well buy the Infini if you're looking at spending 20k :-)

I'm a bit out of my depth though. I know the Multiplus+Fronius setup much better, and that works perfectly, using the CCGX (or you can use a beaglebone or Rpi) to control the inverters. I don't know if the software can run a Fronius without a Multi (I can find out, though it is summer holidays in The Netherlands so it might take a while). If I were to buy a pure GTI it would be a Fronius or an SMA. I'm a bit of a snob that way :-)

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23 hours ago, Noobie said:

When there is council power the control circuit will connect the PV panels to the grid tie inverter

When there is a council power failure the control circuit will automatically disconnect the PV panels from the grid tie inverter and connect them to the Axpert battery inverter.

When council power is reinstated the PV panels are automatically disconnected from the battery inverter and reconnected to the grid tie inverter.

Hi @Noobie. If you want a GTI and a control system that can do all that, buy an Infini. It is a GTI that does not shut down when Grid power goes down. With the settings you can select whether you want to feed back excess power to the grid or not. No fancy expensive control systems required. With Grid Power available, you have a GTI. Without Grid Power, it operates the same as an Axpert. 

 

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14 hours ago, Noobie said:

What I would like to have is an inverter which feeds power from the PV panels to the essential loads (lights plugs) and the non essential loads (stove, geyser) when there is enough PV power to do so.

The Infini can do exactly that. Feed your essential loads at all times and "Export" excess power to non essential loads instead of the grid. 

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@Don - I'm looking at the Infini Super 4 for exactly that reason... no money for batteries to go off grid so get the best compromise...

Do you know what the surge value is on the Infini's?  I know the Rated Power Output is 4000w but can it surge for brief periods (ie 6000w)

 

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9 minutes ago, Mark said:

@Don - I'm looking at the Infini Super 4 for exactly that reason... no money for batteries to go off grid so get the best compromise...

Hi @Mark, without batteries you have a pure GTI. You can always add batteries later when we do load shedding again. As far as I know the surge value is at least double that of the rated power. Maybe someone with an Infini can confirm it for us. 

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I would still invest in batteries - our grid here had 4 outages in the last week or so, not due to load shedding, just old infrastructure, lack of maintenance etc, and chances of an upgrade is nil.

The longest outage was 3h. So if you are lucky to be on a stable newish grid, then OK - skip the batteries. Use EmonCMS to record power outages, you may just be surprised what you find.

I knew our grid was bad, but I see lots of small (few minutes)outages as well.

The Infini can be pushed quite hard, but when it starts too peep(or shows Error 04), better switch off, otherwise it will trip. The rest of the family is not yet trained to observe the monitor when switching on big loads. 

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9 minutes ago, Don said:

Hi @Mark, without batteries you have a pure GTI. You can always add batteries later when we do load shedding again. As far as I know the surge value is at least double that of the rated power. Maybe someone with an Infini can confirm it for us. 

I have a small bank (200ah) of "our" favorite el chepo batteries on my axpert so would go Grid Tie with backup on the Infini ;)

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7 minutes ago, Sidewinder said:

The Infini can be pushed quite hard, but when it starts too peep(or shows Error 04), better switch off, otherwise it will trip. The rest of the family is not yet trained to observe the monitor when switching on big loads. 

@SidewinderHave you done some pushing and what what the max surge you have had.  I have my systems on EMON (see signature).

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5 minutes ago, Sidewinder said:

The Infini can be pushed quite hard, but when it starts too peep(or shows Error 04), better switch off, otherwise it will trip. The rest of the family is not yet trained to observe the monitor when switching on big loads. 

It takes some time to train them. A few days ago I noticed I am drawing 5000 watts from by batteries. My daughter decided to switch on the dishwasher and tumble dryer while making some toast in the kitchen. 

I have heard of a 3kW infini handling a surge of 8kW. Not sure of the circumstances or for how long? 

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I regularly exceed 6kW on my 5kW, but for short (30 seconds or so), typically when the geyser is on, plus general house and ironing or hairdryer is used. Luckily these are cyclic in nature. Would not want permanent loads like that. 6700W has been my max in testing for few seconds, ICC records that on the daily grid max, but I need to log that on EMONCMS to get a long term view. The older my system gets, the less I tend to want to monitor it, as it is behaving well.

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17 minutes ago, Mark said:

I have a small bank (200ah) of "our" favorite el chepo batteries on my axpert so would go Grid Tie with backup on the Infini ;)

That is all you need Mark to keep the the Infini going, should the power go down during the day and prevent it from shutting down. 

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I think I might have mentioned it elsewhere, and this will likely not be enough to change anyone's minds... but there has been another 10% price decrease on the 3kva and 5kva Multi in July. The 3kva Multi is €1413 now (it used to be 1570, and before that it was around 1800). Unfortunately the exchange rate tanked again so it is still around 22k, and it still comes sans MPPT, which means I still have to concur with the others that the Infini would be your best option here, but... the gap is narrowing :-)

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8 hours ago, Don said:

The Infini can do exactly that. Feed your essential loads at all times and "Export" excess power to non essential loads instead of the grid. 

Thanks @Don If you have a look at the attached sketch you will see that my inverter is connected after the non essential loads (geyser, stove etc)

How would the Infini export power to these non essential loads but not not export it out to the grid?

I know the fronius has a Smart meter that would be installed at location A (ripple relay box) which would throttle the GTI's output to prevent exporting to the grid.

Does the Infini do something similar with its own meter? Or would it need to be connected before the non essential loads? If it had to be connected before the non essential loads the inverter would have to be large enough to carry the whole houses load - could be expensive.

@plonkster Thanks for the comments, I heard that SMA is pulling out of SA, not sure how true this is? Could a Multiplus be installed at the inverter location on the attached sketch and achieve what I need with a smart meter at location A, or would it need to be installed before the non essential loads and have the inverter sized to carry all the loads?

SMINOLTA17081617210 copy.jpg

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22 minutes ago, Noobie said:

SMA is pulling out of SA

They had a local manufacturing setup. That is closing down (or it has already, far as I know). But like the other makers, you'll still be able to buy their stuff here, it will just have to be imported.

22 minutes ago, Noobie said:

Could a Multiplus be installed at the inverter location on the attached sketch and achieve what I need with a smart meter at location A

The multi is very flexible in this regard. It can either use its own internal current meter, or an external modbus type meter. If you use the internal current meter, and you don't want to feed back current to the grid, then all loads must be on the output. If you use an external current meter, then you can install the inverter as per your diagram, with only essential loads on the output, and because the external meter (in location A) tells the inverter how much to push back, it can also feed back power via the input of the inverter and still avoid feedback. There is however another possibility, which I will cover lower down.

For current meters, there are three supported Carlo Gavazzi units, the cheapest single-phase one is €76, which is significantly cheaper than the EM24 they used previously.

But... the larger Multis have two outputs, one for essential loads and one for non-essential loads. When the power goes out, it automatically disconnects the second output. At first you may wonder what the point is of a second output if it just goes down with the grid? There are many sides to that answer:

1. These inverters have a feature called power-assist used by boats and RVs when the "shore connection" isn't capable of providing enough power, using battery power to provide the difference. Power-assist can also be used for loads on the second output (this is where you'd put things like the air conditioner for example, if you have a nice yacht).

2. They also have a feature called grid-assist, which is really the same thing, except this uses grid power to assist the inverter when the load is higher than the inverter capacity. Once again, this allows loads on output 2 that is higher than the inverter capacity, but those loads can still benefit from surplus power. They are simply disconnected if the grid fails.

3. The current meter is on the Multi's input, so it sees current going to both outputs.

In other words, if you use the larger Multi with two outputs, as long as the transfer switch is rated for the full power, you simply install the Multi before all the loads and put the non-essential ones on the second output.

For example, if you have a 48/3000/35-50, that means 48 volt, 3000VA, 35 ampere charging capability, 50A transfer switch. Since most houses have only a 50A feed, that is perfectly suitable.

But it might be that the way your loads are laid out, perhaps your meter is far away from the main DB, or you have other buildings... well then you'd fit the CG meter at location A.

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remember that gti feeds back to house and then grid . your gti would feed surplus power to the inverter damaging it .most  inverters are made to give out power not to receive . unless you have a grid tie limiter do not do this . you could place your gti before your inverter  to feed the inverter and surplus  power could be fed to geysers and the like and  with clever timing you could avoid sending power back to the grid

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Hi Guys

if you want a machine that can do all  this, I can recommend the infini/axpert v series which really comes in at a fair price and has three features that I like.

1 it  can be used for off grid

2 it has a self consumption mode (it uses Eskom and subsidises this with solar , my Eskom usage  usually sits around 0 to 20 w during the day)( it also doesn't cycle the bats)

3 it has a gti function if you want it and that can be used to supply the rest of house  the geyser or what ever

if the power goes down during the day I barely notice it

also if you place a contactor in you circuit that is supplied by Eskom but controls some inverter circuits then it will auto switch off high usage loads during a power failure . this contactor could also be controlled by a the inverters  dry contact ( usually used to switch the genie on ) under the battery settings

where's my two cents?

cheers

M

Ps I have had a gti and a  inverter   . the problem with switching the solar panels from inverter to gti is usually they run and two completely  different voltages gti high and inverter lower

Edited by maxomill
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