How do my solar inverters know what "The grid" is?

[SpookyGhost]

I have a question for you guys. I have Solar installed as well as a generator. Everything I read warned that you CANNOT have the solar on at the same time as the generator. And it wasn't possible anyway as rapid shutdown inside the inverters would sense its not grid power, and shut down. In the end I actually got a line side tap, so its impossible for the generator and solar to be connected in any way, and it lets me run on generator power and still feed solar back to the grid.

Does anyone have any good explanations oh how the inverters know what the grid is? If I did connect up a generator and the solar at the same time, why would the solar not just "Sync up" to the generator as if it were the grid?

[trobbins]

Different PV inverter manufacturers use different anti-islanding sensing techniques, although I recall that there are typically two types in general use.  You have to do some google digging to get more detail (but still generic only descriptions).

As an example, afaik SMA use a technique that stops the generation of power for a short portion of a mains cycle at zero crossing and detects how well the grid conducts during that short portion.  That technique certainly works but can have troublesome interactions if the grid is weak where the PV inverter(s) are connected, or if there is substantial local battery inverter generation also in operation - troublesome can turn in to commercial nightmares for larger size PV and battery facilities.

[IanB]

Does anyone have any good explanations oh how the inverters know what the grid is? If I did connect up a generator and the solar at the same time, why would the solar not just "Sync up" to the generator as if it were the grid?

Related to the previous answer, I think the generic answer is that the grid should look like a sink with very low (nearly zero) impedance. That means if you push current into the grid, there should be little change in measured grid voltage. On the other hand if you were only connected to your generator, then it would have a much higher impedance than the grid and would respond much more strongly if you tried to back feed power into it.

[SpookyGhost]

Interesting, thanks guys

So is there any way to throw a device between a generator and the grid that will sync, so you could backfeed the grid safely and properly with a generator?

I'm thinking of a situation where you are signed up for a wholesale rate electrical plan, and you could make power for much cheaper than being sold. Or, if you want to load bank a portable generator, you just dial it up to whatever Kw you want to feed back to the grid

[ejeffrey]

In most places you are not allowed to feed power into the grid except for specifically allowed renewable energy sources and may have additional restrictions.  As far as I know, feeding power from a generator back into the grid is illegal everywhere in the US unless you are a power company.  So I don't think there is any way to do this "properly"

The normal way to use generators is with a transfer switch that disconnects the grid supply when the generator is on.

[Psi]

So is there any way to throw a device between a generator and the grid that will sync, so you could backfeed the grid safely and properly with a generator?
I'm thinking of a situation where you are signed up for a wholesale rate electrical plan, and you could make power for much cheaper than being sold. Or, if you want to load bank a portable generator, you just dial it up to whatever Kw you want to feed back to the grid

Generators are quite different.
A grid tie inverter can adjust it's output frequency in software to match the grid.
But a generator needs to adjust it's rotational speed to match grid frequency before connecting to the grid.
If a generator gets connected to the grid out of phase you will need a new generator and probably a new room/house, they explode violently.   

[SpookyGhost]

So when an inverter generator, it COULD be possible? Obviously you'd need control over the inverter which we do not have

Seems like the fun police are ruining it. If I want to do a 7kw load bank, why just heat the air when I could feed back to the grid! Win-win-win!

[rob77]

If a generator gets connected to the grid out of phase you will need a new generator and probably a new room/house, they explode violently.   

i would say it heavily depends on the size of the generator and the size of the distribution transformer 

[trobbins]

It's called anti-islanding protection because it saves grid servicing personal from electrocution by your diy generator.  In Australia it is very regulated, and any generator device or grid connect switch has to be fully compliant to standards and approved - so that means a significant compliance effort by the generator or switch manufacturer, and also a check and approval after installation and prior to connection to grid.  No fun police - just life saving practicality to avoid naive diyers from electrocuting innocent people.

[BradC]

If a generator gets connected to the grid out of phase you will need a new generator and probably a new room/house, they explode violently.   

Somewhere my father has a great photo of a huge generator shed with a generator sized hole in the side where one of the generators jumped off its bed and out the wall. I've seen it many moons ago but neither of us can locate it now.

[NiHaoMike]

So is there any way to throw a device between a generator and the grid that will sync, so you could backfeed the grid safely and properly with a generator?

I'm thinking of a situation where you are signed up for a wholesale rate electrical plan, and you could make power for much cheaper than being sold. Or, if you want to load bank a portable generator, you just dial it up to whatever Kw you want to feed back to the grid

Easiest would be to use an induction motor as a generator, it automatically synchronizes. A good application would be a CHP generator in a home heating system. Downside is that an induction generator will not work on its own without some source of magnetizing current, so additional circuits will be required to have it also work for backup power. (That's normally done using capacitors and some way to have at least one charged on the initial connection to get things started.)

[woody]

So is there any way to throw a device between a generator and the grid that will sync, so you could backfeed the grid safely and properly with a generator?

Yes. A solar inverter. Just use a generator with a 24V DC output, feed that into the solar inverter and connect this inverter to the grid.

You probably run into problems with the DC quality, the cost of hardware and the dramatic efficiency of the whole thing, but as a thought experiment it should work.

[trobbins]

It's likely that a PV inverter meant for grid-connect will want its PV panels to be floating, and the inverter will test for a suitably high resistance between the PV panels and mains earth.  Either a '24Vdc generator' won't show high resistance to earth, or there will be arcs and sparks in the generator if used with a grid connect.  The PV inverter will also likely have safety inhibits if it doesn't see the PV string acting like a PV string (ie. moving from OCV to an MPP to a SC current).

[free_electron]

here could be a solution : take generator output , rectify to dc , send into inverter
basically treat the generator as a solar panel.
the rotational speed and synch of the generator are now no longer an issue. you get DC out.

the existing solar inverters already solved the sync / grid disconnect problem.

[dmills]

The UK had (for a while) an insane feed in tariff for renewables (Which they paid even if YOU used the power, got to love a tory government giving a freebie to the home owning classes).

A mate of mine spotted the obvious opportunity here and had a solar system installed, then went out and bought a fat old server power supply, some judicious wiring and control gear later and he had 4kW of 'renewable' feed in running 24 hours a day!
 
During the day, 4kW from the solar, then at night 4kW from taking his 'renewable energy' back out of the grid (Remember he gets paid feed in rate for supplying it but it is net metering for usage!), stuffs it thru the power supply and back into the inverter and feeds it in all over again. Doing the efficiency sums, some of that power ran around the power supply->inverter->meter->power supply loop about 10 times.

It lasted for years before someone got wise.

[JohanH]

It's called anti-islanding protection because it saves grid servicing personal from electrocution by your diy generator. 

Reminds me of a story in electrical safety training where they told about service electricians disconnecting the grid to a smaller village and then checking the voltage (of course they would also earth the disconnected grid properly). Now lo and behold there is still voltage in the three-phase lines. Further investigations found a nearby farmer had started his tractor driven generator to keep his pigs alive/milk the cows/whatever, but at the same time powering the whole village. Here it is also very regulated. With generators, you must have a switch installed that separates it from the grid, so when one is connected it disconnects the other.

[f4eru]

Happens.
People get caught.
https://theecologist.org/2010/apr/16/spanish-nighttime-solar-energy-fraud-unlikely-uk

[Simon]

The UK had (for a while) an insane feed in tariff for renewables (Which they paid even if YOU used the power, got to love a tory government giving a freebie to the home owning classes).

A mate of mine spotted the obvious opportunity here and had a solar system installed, then went out and bought a fat old server power supply, some judicious wiring and control gear later and he had 4kW of 'renewable' feed in running 24 hours a day!
 
During the day, 4kW from the solar, then at night 4kW from taking his 'renewable energy' back out of the grid (Remember he gets paid feed in rate for supplying it but it is net metering for usage!), stuffs it thru the power supply and back into the inverter and feeds it in all over again. Doing the efficiency sums, some of that power ran around the power supply->inverter->meter->power supply loop about 10 times.

It lasted for years before someone got wise.

Hopefully he got his arse more than kicked. They should have figured out that he was exporting far too much and it's why we need smart meters. I get paid the wholesale rate through octopus.

You have also confused the generation and feed in tarrif, they are separate but you give the readings together for one payment. But they are seperate tarrifs, one to generate, one to export, I'd get 10p/kWh off the government but I renounce the feed in bit and take the octopus agile export. In my case when I switched from agile import (2x wholesale rate) to a car chaging tarrif they forgot to turn off the agile export rate and put me back on standard feed in.