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Grid tie without battery backup question


SilverNodashi
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Hi, 

I hope someone has run into this before, since I haven't done anything like this yet. Has anyone used a grid-tie inverter without battery backup, and it still works when eskom fails? i.e. will the inverter still draw power from the PV array, or does it need a reference voltage to work? My gut is that it needs a secondary reference voltage, i.e. the grid or battery bank to work. 

2 suppliers also confirmed that it definitely needs a reference voltage of sort to work - on all brands I would have wanted to use, i.e. SMA, Victron, Infinisolar. 

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Ive always wondered about this...

If you connected your grid tie inverter to the output of your battery backup inverter would the grid tie unit operate during an eskom mains failure?

I'd be too worried to damage any equipment testing this theory.

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Far as I know, nope. To do with the sine wave IF I recall vaguely correctly, and under correction. 

I do know that you can use say a Victron Quatro, and make it the "grid" for the grid tied inverter. That works.

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5 hours ago, SilverNodashi said:

Has anyone used a grid-tie inverter without battery backup, and it still works when eskom fails?

Yes. @Chris Hobson called me the other day about that. He had a Goodwe powering loads even though the grid was down. Keep in mind though that the Goodwe has an input and an output, and this was on the output, so no feedback rules are being broken here.

I am also aware that some SMA models have a secure power feature that can power some loads when the power is out. It's meant to power smaller loads.

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The secure power feature, at least on the SMA, is meant for small loads. It's sized no more than 25% of the nominal capacity. It's for charging of phones, non critical things. I've never seen it on a South African model. It's on some of the US models, presumably for areas that get bad weather where the power might be off for a few days. Then you can at least run some essentials. This case with the Goodwe was the first time I ever heard of it happening with another brand.

So what I am saying is that it is at least possible in principle, and before you assume that it is a mistake, check the documentation. It might be a feature :-)

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So, reading all the posts, it is possible if you have the SMA inverter, not in the SA model, or Goodwe, but without batteries then clouds could be an issue, only for small loads versus No. :D

Could not resist.

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11 hours ago, plonkster said:

Yes. @Chris Hobson called me the other day about that. He had a Goodwe powering loads even though the grid was down. Keep in mind though that the Goodwe has an input and an output, and this was on the output, so no feedback rules are being broken here.

I am also aware that some SMA models have a secure power feature that can power some loads when the power is out. It's meant to power smaller loads.

Ironically, although I want to use a Goodwe, not because @Chris Hobson manage managed to do run it without batteries, but because it's a nice inverter, it's too small for this site.

I need to power 7Kw constantly so the SMA would not work in this case either. And to add battery backup to an SMA system is much more expensive, since you need to have a sunnyboy as well, than just adding a battery bank to an Infinisolar. Even Victron are battery driven inverters with MPPT as an option so that wouldn't work here either. 

Anyway, I just wanted to get my facts straight since my business partner recons it can be done, I think / thought it couldn't 

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1 hour ago, The Terrible Triplett said:

So, reading all the posts, it is possible if you have the SMA inverter, not in the SA model, or Goodwe, but without batteries then clouds could be an issue, only for small loads versus No. :D

Could not resist.

You sound as muddled as I do :-)

Just read the SMA link I posted. It is simple enough. The inverter only provides a portion of the power so that even if there is some cloud cover it can still drive smaller loads. If you attempt to draw more power than it can handle, it switches off. If the cloud cover is really heavy, it switches off. Easy as that. Subtext: Don't be a dumbass and try to run your house off this feature :-)

But.... never ever seen an actual model with this feature. Even in the US... that feature costs extra money so it's rare.

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1 hour ago, SilverNodashi said:

And to add battery backup to an SMA system is much more expensive, since you need to have a sunnyboy as well

You mean SunnyIsland.

Let's get technical. I think what those SMA inverters do (most likely) is include really big capacitors that act like small batteries, so you have a kind of self-consumption setup: The PV panels charge the capacitors, and a small inverter draws from it. Control electronics shut things off really quickly if they go out of bounds. And the PV array has to be 4x oversized.

So the question is if this recipe can be scaled to 7kw. I suspect it can... but at what cost? Might be cheaper to put in a small lithium bank then and do a proper self-consumption setup. Of which Victron remains the best... (also, could not resist :-) ).

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5 hours ago, plonkster said:

You mean SunnyIsland.

Let's get technical. I think what those SMA inverters do (most likely) is include really big capacitors that act like small batteries, so you have a kind of self-consumption setup: The PV panels charge the capacitors, and a small inverter draws from it. Control electronics shut things off really quickly if they go out of bounds. And the PV array has to be 4x oversized.

So the question is if this recipe can be scaled to 7kw. I suspect it can... but at what cost? Might be cheaper to put in a small lithium bank then and do a proper self-consumption setup. Of which Victron remains the best... (also, could not resist :-) ).

Yes, I meant that sunnyisland, not sunnyboy. 

At the scale you mention, a small lithium battery will obviously be more cost effective. 

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On 2/16/2018 at 8:36 AM, SilverNodashi said:

Anyway, I just wanted to get my facts straight since my business partner recons it can be done, I think / thought it couldn't 

Even though I have seen a Goodwe do just this I think it is anomalous and the power was coming from a internal capacitor. Remember here in the Karoo we can go the whole day without one cloud interrupting production. Essentially a inverter needs either be a GTI or have battery backup. Very small loads can either be powered by design or be a unintentional/undocumented serendipitous result  of the inverter's architecture.

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5 hours ago, Chris Hobson said:

Even though I have seen a Goodwe do just this I think it is anomalous and the power was coming from a internal capacitor. Remember here in the Karoo we can go the whole day without one cloud interrupting production. Essentially a inverter needs either be a GTI or have battery backup. Very small loads can either be powered by design or be a unintentional/undocumented serendipitous result  of the inverter's architecture.

You're lucky ;)

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