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3 x hybrid inverters - Feed to grid ?? Help


Chris-R
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Hi All

I need to pick the brains of all the clever guys out there.

I have been asked this question this morning and would like to know what you guys opinion is out there.

2 Hybrid inverters connected in parallel with a battery bank connected and 30kw panels

1 Hybrid inverter with 15kw panels and it's own battery set in a separate house about 600 meters away from the first building.

Both buildings are connected to the same Eskom connection point and both are feeding back into the grid.

Is there any danger in doing a connection like this ??

I have my own opinion, but would like to know how the guys feel out there !

3 x 3-phase inverters.jpg

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What if the unit in the second house goes down, so it becomes a consumer to the 30kw capacity on the other side. You'd need breakers to protect each leg for its own maximum capacity, assuming its not capable of the full 30kw. That's all I can think of. But I'm not a sparkie.

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Thanks Plonkie, I did the design with brakers to protect overloads from both sides.

What worries me is is the following scenario.

Pv is working both sides and everything is running of solar power, and the excess power is feeding into the grid. Beautiful !!

The grid goes down ! Will the 2 inverters realize the grid is down, or will they see each other as the " grid " and keep on feeding into the grid ? Big problem, because it could be a technician switching the grid off for technical reasons ??

 

If I am right, how do I get around this problem?

  

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4 hours ago, plonkster said:

What if the unit in the second house goes down, so it becomes a consumer to the 30kw capacity on the other side.

Depending on your definition of "goes down"

If it goes down completely it will not be a consumer of anything.

If the PV goes down, but the unit keeps running, it will become a consumer of the grid that at that stage will be made up off Eskom and the feedback from the other building. The 30 kw will just push back the same spare capacity it did before the other one went off. 

 

1 hour ago, Chris-R said:

Will the 2 inverters realize the grid is down, or will they see each other as the " grid " and keep on feeding into the grid

This will open an old can of worms if not handled with caution. If these are reputable inverters that complies with all the specifications mentioned in so many other threads, it will stop feeding back, the Grid remains the "Carier" that the inverter should sync to, and each inverter will look at and monitor the grid separately. There is still those who believe that there is a 1 out of many million chance that it can happen, but then again they are forgetting another important factor, all the buildings in that area connected to the same phase will demand so much more than the 45kw available and even if it Islands, it will not be possible to maintain that feed for more than a few seconds at most. 

In short If they used the correct invertors it wont happen, if they used the wrong ones, overall demand  the area should kill the feed.( Assuming that the breaker to this specific site hasn't tripped)

Edited by Jaco de Jongh
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44 minutes ago, Chris-R said:

Will the 2 inverters realize the grid is down, or will they see each other as the " grid " and keep on feeding into the grid

Active anti-islanding measures should handle this. PV inverters try to shift the frequency or they introduce a little squiggle into the waveform. If the grid is there, it gets absorbed. If the grid isn't there, the effect is pronounced and detected. When the grid falls out both inverters should suddenly very successfully succeed in doing one of these, and then one will shut down followed by the other.

See here around 1:40 for what it looks like.

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My concern is that when the grid goes off ( switched off by technician maybe) the inverters might see each other as the "grid" and keeps feeding into the grid making this dangerous for any worker on that line.

If I understand correctly, the anti-islanding device in the inverter should not allow this to happen?

I will be testing this shortly, but needed some assurance and input from my friends.

Thanks guys

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If the inverters are all the same brand, then all of them will attempt to perturb the grid in the same way and there would be no chance of them tying with each other. The ways they normally do it is 1) attempt shift the frequency, 2) introduce a tiny disturbance in the waveform. When you have two inverters doing that, they will tend to work together and push that thing off the cliff faster.

My visual analogy for this is a man leaning lightly against a wall. If it is a thick strong wall (aka grid connection), nothing happens. If it is a thin wood wall, it gives way and he falls over immediately. If you have two men leaning against it, it should fall over even faster. If you have two men leaning against each other... that would be the trouble. But that is unlikely even with different brands.

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Depending on ... Should ...Should not ... Would ... Unlikely ... If ... 

Where are:
YES it will work.
NO it will not work.
Do this and it is safe.
Don't know ... which by the way is also 100% acceptable.

Just wondering.

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22 minutes ago, The Terrible Triplett said:

Depending on ... Should ...Should not ... Would ... Unlikely ... If ... 

I don't do 140 character replies. I want you to know the mechanics of the thing.

I'm 95% certain it will just work. Good enough?

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16 minutes ago, plonkster said:

I'm 95% certain it will just work. Good enough?

Only if your money is on the line. :-)

Seriously. Saying I'm 95% certain it will just work, this is why I say so, and this is where I am unsure, is perfectly 100% acceptable. 

You take a firm stance in other words.

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Hi @Chris-R

Looking at your sketch I am of the opinion that the two inverters will supply each other on a grid fail. In theory they will not "know" the grid is down as there is still an external feed. 

Now for the fun part where I can be talking a hole lot of rubbish and if so Ill edit accordingly:

The way I understand it , the inverter is able to push back into the grid by raising the volts of the inverter higher than the grid and here in there might be a problem. The grid voltage can fluctuate but is fixed. The inverters can adjust so if they are pushing against each other , depending on witch voltage they sense , things could get interesting. 

I would setup some contractors between the  inverters and the grid that is powered by the grid if the grid goes down they open and there is no chance of problems. Should you worry about a contact failing have two , three ...... in series. But again this could be a hole lot of pot ... 

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1 hour ago, PaulF007 said:

two inverters will supply each other on a grid fail

Only for a really short period of time. Remember that both of them are actively trying to do something to the grid signal that it normally cannot succeed in because it is way too weak to move a strong signal like a grid connection. When the grid fails it will succeed very rapidly in moving the signal, this will instantly cause an out of bounds condition and will cause a disconnection.

The most common method used is frequency shift. Analogy time again. Ever been in a church in a small town? All the old people are singing really slowly and you always find yourself half a note ahead, especially every time the organist slows down a bit towards the end of a line, as they always do for some inexplicable reason? Well, this is like that. You sort of pull ahead but your held back by the rest of the congregation.

Similarly, if the grid falls out, the frequency would very rapidly (we're talking hundreds of milliseconds) shoot out of bounds causing all the inverters to turn off.

As far as I know though (I'm admitting a tiny bit of ignorance in case TTT tells me again to take a stand), you're supposed to have it tested before signoff anyway.

Edit: In fact, now that I think about it, in my childhood when we'd listen to church services over the radio on the farm, we would also sing along, you know, get the full active experience as much as possible. If the congregation was singing too slowly, my dad would turn the radio off and we'd finish without them, and then turn the radio back on. Invariably we'd finish about half a verse before the rest of them. So basically, that is what is going to happen :-)

Edited by plonkster
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2 hours ago, plonkster said:

... you're supposed to have it tested before signoff anyway.

That is the answer I've been waiting for ... did not want to be the one to point it out.

When the question was first posed, my thoughts where 1) If the inverters are on "The List", you are 99% there and 2) get it signed off.

You tie to the grid you must have it signed off, that legislation has been published years back, a unforgiving fact, as Eskom and NERSA is quite clear on that.

Fact that CoCT is now enforcing it for municipal feed, direct you where/what and how if you are Eskom fed, just means they are years ahead being better run.

Anyone today whom are a installer anywhere in SA, must inform their client of their obligation for their equipment.

 

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12 hours ago, PaulF007 said:

The way I understand it , the inverter is able to push back into the grid by raising the volts of the inverter higher than the grid and here in there might be a problem.

In AC and specifically in a sync operation the sync will happen when the Frequency is the same, and after sync the voltage of either the generator or the Inverter will match that of the strongest one in this case the grid. What happens in the inverter is a different story all together. Normally it will generate a waveform that will lead the grid waveform slightly and attempt to generate a voltage that is SLIGHTLY  higher than the grid to create the force to push back.  

If you measure the grid voltage before and during feedback the difference wont be detected with the inverters and generators we use,   they are just to small to make a difference. On the flip side thousands of them in the same area will raise the grid voltage, but in the case above, the SLIGHTLY higher voltage from the 3 inverters will not be a problem.  

 

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The inverters in this setup is 2 x 10kw Infini's in parallel and 1 x 10kw Infini stand alone.

They are my own and I am actually busy setting the 2 x parallel inverters up at the moment.

I think TTT and a lot of others will actually enjoy the news that my Imeon blew up 3 times now and I have actually decided to sell it as scrap metal !!!:unsure::unsure:

I saw it twice going up in flames ( And I actually mean it ! )

I managed to control it's temperature very well with external fans, but these beasts has actually got no built in protection whatsoever for any current overloads. No matter what the duration or the amount!

Well, the bright side of things are that the Infini is doing the same job 10000% better with exactly the same setup!

I appreciate all the above recommendations and will give you feedback of it as I go along

 

Thanks again

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47 minutes ago, Chris-R said:

I think TTT and a lot of others will actually enjoy the news that my Imeon blew up 3 times now and I have actually decided to sell it as scrap metal !!!:unsure::unsure:

Ouch!!! Ag nee.

No, not one iota of enjoyment. Sadness most definitely ... ok, maybe a little twitch in the corner of my mouth as I read it .. small one! ;-)

Hope you don't have any more bad experiences going forward on the Infini's though.

Just wish it was maybe a bluer make. Maybe next time ... ? :)

 

Seeing as Infini's are gird tied, Monday the sparky is coming to see me for the quote for connecting and the CoC. So far it appears we are looking at very nice figure for that, as he also deplores the fact that the others are killing the market ito ridiculous costs for CoC's for solar.

And as I was replying, here, I got a awesome call for a price for signing it off on the engineering side. If it works as engineer expects, me being a pilot site and small, that lower price for the engineering side can then be shared right here on PF if he says ok.

The smaller systems, as the engineer agrees, is where the most savings starts at, the high costs for signing them off, ridiculous! Any system for that matter, the costs should be reasonable.

 

@plonkster - each time the prices drop, I think of the debate we had on the costs for these professional fees. ;-)
Your jaw will drop when you hear what it is going to cost me to sign off.
Next step after all is said and installed ... that other idea I pitched Victron based on your astute observation on the Solis vs Fronius. I'm the eternal pessimistic optimist. 

Edited by Guest
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1 hour ago, The Terrible Triplett said:

other idea I pitched Victron based on your astute observation on the Solis vs Fronius

Well, I got a call late afternoon, we discussed it a bit... who makes it... oh someone sounds like "go long"... ah yes, "ginglong".

This is a screenshot of a video from MjLorton, circa 2012. Note that there is a PV inverter on the right with blue branding. That is a ginglong.

Selection_207.png.5d9e2478d4485c431a1f360bffff15c0.png

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54 minutes ago, plonkster said:

Well, I got a call late afternoon, ...

Sorry about that ... not. ;-)

 

54 minutes ago, plonkster said:

Note that there is a PV inverter on the right with blue branding.

So Ginlong tried, well, 6 years later, with a 5 year warranty today, I'm pretty sure there is no risk.

It there are problems, and no-one helps, I'll get quite vocal. :-) 

As long as people help to sort it, I am very patient.

Edited by Guest
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Just in case it was unclear: Back in 2012 Victron had a PV-inverter on the market. It is not a copy. It's a for real branded official thing. Like many companies, they don't do everything themselves, they farm out some of the non-core stuff to others. They still have a couple of these on the wall at R & D headquarters.

Edited by plonkster
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maybe im too simplistic but I think you should install 2 contactors

one on each incomer connected to the incomer that opens is the Eskom goes down

this means that each inverter will not see each other

3 contactors would be better to isolate 1 and 2 from each other and from 3

cheers

 

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