Fooling a grid-tie inverter to provide power without grid.

[rin67630]

Did someone manage to fool grid-tie inverters to provide power without grid with a small inverter?
If yes, what must be considered?
I have noticed a pure sine inverter claiming "omnidirectional stability", is that what is required not to burn through if the grid-tie converter feeds-back?

 

[newbostonconst]

The problem is the inverter can likely make more power from solar then is used and then where is that extra power going to go? Answer: It fry's things...

You need a hybrid inverter that can AC couple to do this....but yes you can do it with the right equipment.

Here is a list and many in here can do it.

Hybrid Inverter | Hybrid Solar Inverter | altE

Hybrid inverters, mostly used in grid-tie solar systems, can provide backup power when the electric grid fails. Call 877-878-4060 to size your system today.

www.altestore.com

 

[rin67630]

I think about not feeding directly power into the grid-tie inverter but over a current limiting light bulb.
I suppose the grid-tie inverters sense the grid power for some microseconds at no-load to determine where to adjust their contribution.

 

[solar8484]

Did someone manage to fool grid-tie inverters to provide power without grid with a small inverter?
If yes, what must be considered?
I have noticed a pure sine inverter claiming "omnidirectional stability", is that what is required not to burn through if the grid-tie converter feeds-back?

Just about any pure sine inverter with stable frequency output can activate grid-tied inverters (GTI's) but to keep it running safely the minimum requirements include:

1. The inverter power capacity must be large enough relative to the total power capacity of the GTI's. Typical conservative practice is to have inverter capacity be 10% larger than the GTI's.
2. The GTI's are connected via switch to allow for disconnect in case of unsafe conditions. Typically a relay is used and it's controlled by some monitoring/protection circuit.
3. There must be enough loads at all times to absorb the power generated by the GTI's. Typical approach is have a battery bank large enough that can absorb all power from GTI's if necessary via battery charging and when battery bank is near fully charged state (e.g. 90% SOC) the protection relay to the GTI's is opened to disconnect the GTI's.

I doubt "omnidirectional stability" has anything to do with backfeed protection.

 

[solar8484]

I think about not feeding directly power into the grid-tie inverter but over a current limiting light bulb.
I suppose the grid-tie inverters sense the grid power for some microseconds at no-load to determine where to adjust their contribution.

Grid-tied inverters are not like typical off-grid inverters as they pump out as much power as possible at all times based on available power input from solar panels.

 

[newbostonconst]

Just about any pure sine inverter with stable frequency output can activate grid-tied inverters (GTI's) but to keep it running safely the minimum requirements include:

1. The inverter power capacity must be large enough relative to the total power capacity of the GTI's. Typical conservative practice is to have inverter capacity be 10% larger than the GTI's.
2. The GTI's are connected via switch to allow for disconnect in case of unsafe conditions. Typically a relay is used and it's controlled by some monitoring/protection circuit.
3. There must be enough loads at all times to absorb the power generated by the GTI's. Typical approach is have a battery bank large enough that can absorb all power from GTI's if necessary via battery charging and when battery bank is near fully charged state (e.g. 90% SOC) the protection relay to the GTI's is opened to disconnect the GTI's.

I doubt "omnidirectional stability" has anything to do with backfeed protection.

And if the stars align it might work....sorry your inverter fried....and your house burnt down.

 

[HateGrid]

If this is a new system, just buy a hybrid inverter with off-grid capability. It will have a disconnect relay to disconnect from the grid side when the power fails and then operate in off-grid mode. You will want batteries if you do this, otherwise the inverter will shut down if the PV output is not enough to satisfy your load.

 

[rin67630]

I have no real need for an hybrid inverter with off-grid capability, my grid never had a single outage in 15 years.
I just want to explore and learn, I want to understand how devices operate a bit more than "and it will fry and your house will burn down"
Thank you for your opinion anyway...

 

[time2roll]

I think about not feeding directly power into the grid-tie inverter but over a current limiting light bulb.
I suppose the grid-tie inverters sense the grid power for some microseconds at no-load to determine where to adjust their contribution.

I believe the grid tie needs to see the grid power for 1 to 10 minutes before the power flows. This is no microsecond hand shake.

 

[rin67630]

I believe the grid tie needs to see the grid power for 1 to 10 minutes before the power flows. This is no microsecond hand shake.

There is more than just sensing voltage a the beginning.
The GT inverter MUST detect a power outage while it is running and disconnect instantly.
So the only way to do it while feeding power is to stop feeding for a very short time and look if there is a power continuity.

But i think it would have been better to discuss that further in the "Danger zone".

 

[solar8484]

There is more than just sensing voltage a the beginning.
The GT inverter MUST detect a power outage while it is running and disconnect instantly.
So the only way to do it while feeding power is to stop feeding for a very short time and look if there is a power continuity.

It's much more sophisticated than that in GTI at least for the ones that you can legally connect to the grid in the US. You can research anti-islanding mechanisms to learn more.

 

[solar8484]

And if the stars align it might work....sorry your inverter fried....and your house burnt down.

No need to align stars just need to know what you are doing. Otherwise, anything can happen. Hybrid inverters with batteries follow the same requirements.

 

[newbostonconst]

No need to align stars just need to know what you are doing. Otherwise, anything can happen. Hybrid inverters with batteries follow the same requirements.

The only reason hybrid inverters work when AC coupled is they frequency shift to shut inverters down when they are making too much power...

Many hybrid inverters can take solar directly in and step back the solar production when needed...

Just faking out your solar inverter isn't going to work because when it make to much power the power has no place to go...

 

[sunshine]

where is that extra power going to go? Answer: It fry's things.

Is this true?
Where does the extra power go when I turn appliances off when running from a inverter? ....no demand no output??

 

[Tecnodave]

off grid inverter......no demand no output
grid tie inverter....generated as much power as available and assumes that the grid can use it all

Grid tie .............grid tie inverters must monitor the grid for 5 minutes and watch voltage and frequency. EDIT: and not output any power until the 5 minute clock is up. END EDIT. If there is one cycle where frequency or voltage is out of limits then the 5 minute clock restarts again. This is to prevent a grid collision where the grid has shut down and then comes back up again, sometimes the grid will come up for 8-15 seconds, determine that all is not well, shut down and then restart. It’s coded into the National energy policy set by the federal government the DOE.......U.S. Department of Energy.


Rule number 1 in doing this.....your loads must always be greater than the grid tie inverter can output. I have done this but it is not for newbie’s....you can do massive damage if you mess it up.


Some smart hybrid off grid inverters have a way of dealing with this for instance the MagnaSine MS4048PAE when paired with a grid tie inverter will “bump” its frequency up to 66 hz for a cycle or two when the output voltage goes out of range which will cause the grid tie inverter to shut down.

Not recommended unless you have a big system with MagnaSine, Schneider, or Outback or some such “hi-end” inverters

 

[solar8484]

The only reason hybrid inverters work when AC coupled is they frequency shift to shut inverters down when they are making too much power...

Many hybrid inverters can take solar directly in and step back the solar production when needed...

Just faking out your solar inverter isn't going to work because when it make to much power the power has no place to go...

It's true that typical hybrid inverters use frequency shift to turn off/modulate GTI but that's the only way. GTI can also be turned off/modulated by voltage shift. So, voltage monitoring switches or other mechanisms can also work.

Also, as explained there must be enough load capacity to absorb max power from GTI at all times. Battery charging is typically used for this purpose as explained and it's also used by typical hybrid inverters. There are other ways like diversion loads.

Anyway, typical hybrid inverters with batteries certainly can work but they are not the only option. Regardless of the option, it's only safe if you know what you are doing.

 

[svetz]

Is this true?
Where does the extra power go when I turn appliances off when running from a inverter? ....no demand no output??

Appliances run on "voltage sources" (e.g., the grid, a battery, a capacitor, a non-GT inverter). That is they provide current to try and keep the voltage constant.

So, when you switch on a blender, for example, the resistance drops from infinite and current rushes through the device. V=IR and the a voltage source wants to keep V constant so by knowing R you can calculate the current. Turn it off and the current stops as R increases to infinity.

Grid-tied inverters are "current sources". That is they try to push as much power as they can, raising the voltage to do so. This is how they push current uphill to the grid. So, a blender might have a fixed R when switched on, but a current source is only interested in pushing power, so it'll keep jacking up V to increase current to push everything available. The grid is big so pushing a few kilowatts uphill doesn't change the voltage much.

Take a look at the AC coupling release notes and you'll see most want you to have a battery sized to the number of panels, the way they work is they toggle the panels off until the batteries are discharged enough that sudden power shifts (e.g., Air Conditioning going off) can be funneled safely into the battery (or caps big enough to absorb it while they cut the relay to the panels).

 

[Tecnodave]

Solar8484,

only partially true.....if your grid tie unit is putting out more power than your mini grid can consume then the hybrid inverter shuts down and can not drop the voltage. Of course when the hybrid shuts down the grid tie should see that as a grid shutdown......

 

[rin67630]

...because when it make to much power the power has no place to go...
...there must be enough load capacity to absorb max power from GTI at all times...

Why? Aren't GTIs regulating AC voltage as well, if the load is not demanding they just produce less and let the panel voltage increase?
In this video the guy starts with zero load:

Others are pushing just enough power to the home to never export energy, so they can't be pure current sources.

 

[solar8484]

Solar8484,

only partially true.....if your grid tie unit is putting out more power than your mini grid can consume then the hybrid inverter shuts down and can not drop the voltage. Of course when the hybrid shuts down the grid tie should see that as a grid shutdown......

That's one way. When there is excess power the island grid voltage will naturally rise and if you have appropriate monitoring and control mechanisms you can switch off GTI when voltage rises above a configurable threshold even before inverter shutdown.