How does anti-islanding work?

PVfarmer

Senior Member
the PV will backfeed the generator with a possibly catastrophic result for the generator.
Usually, sure. But some inverters can do it.

This is from the files.sma.de link from the previous comment.

The purpose of the DIGIN or Digital Input is to signal to the Sunny Island that there is a backup generator available when the public utility power is lost. This allows the Sunny Island to call for a generator with a two wire start. Along with calling for a remote start generator, it will provide reverse power protection in order to prevent any renewable energy current from back feeding to the generator.
 

GoldDigger

Moderator
Staff member
Usually, sure. But some inverters can do it.

This is from the files.sma.de link from the previous comment.

The purpose of the DIGIN or Digital Input is to signal to the Sunny Island that there is a backup generator available when the public utility power is lost. This allows the Sunny Island to call for a generator with a two wire start. Along with calling for a remote start generator, it will provide reverse power protection in order to prevent any renewable energy current from back feeding to the generator.
And the only way it can do that is if the loads are connected to the output of the SI while the generator is connected to an AC input. That means that the SI can monitor the direction of power flow from the generator.
 

ggunn

PE (Electrical), NABCEP certified
Usually, sure. But some inverters can do it.

This is from the files.sma.de link from the previous comment.

The purpose of the DIGIN or Digital Input is to signal to the Sunny Island that there is a backup generator available when the public utility power is lost. This allows the Sunny Island to call for a generator with a two wire start. Along with calling for a remote start generator, it will provide reverse power protection in order to prevent any renewable energy current from back feeding to the generator.
Of course, but a Sunny Island is a battery inverter, not a PV inverter, and you have to buy batteries for it because the Sunny Island cannot run without them. A Sunny Island can indeed stand between the grid and a Sunny Boy and regulate its output so that it doesn't backfeed the generator. But now you are talking about at least two inverters (maybe more - Sunny islands are only 5kW and you may need more of them because they are 120V single phase and they cannot pass through more than 5kW each) plus batteries ($$$), and you have to set up a protected loads panel, and only the loads in that panel have access to the PV/battery/generator power when the grid is down.

Now the customer has to ask himself if it is really worth it to pay another $10-20k or more (maybe way more) just to have the solar run when there is a power outage when he is powering his loads from the generator. Do the math. Is it possible? Yes. Is it worth the expense? In most cases, no.

The math is simple. Say there is a one hour grid outage five times a year. Say you have a 10kW AC PV system running full out every time the outage happens. The PV system is going to make 50kWh of energy during the outages for a year, which sells for about five bucks in most markets. How much did you spend to make the PV run during outages? And what if those outages happen at night or during a storm when the irradiance is way down so the PV won't be producing anything?

It's usually nowhere near worth it.
 

jaggedben

Senior Member
The purpose of the DIGIN or Digital Input is to signal to the Sunny Island that there is a backup generator available when the public utility power is lost. This allows the Sunny Island to call for a generator with a two wire start. Along with calling for a remote start generator, it will provide reverse power protection in order to prevent any renewable energy current from back feeding to the generator.
If I recall correctly, in a Sunny Island system the SI starts the generator if its batteries are drained and/or there isn't enough power available from Sunny Boys. It does not ever, I believe, use the generator as a grid reference for interactive inverters.
 

ggunn

PE (Electrical), NABCEP certified
If I recall correctly, in a Sunny Island system the SI starts the generator if its batteries are drained and/or there isn't enough power available from Sunny Boys. It does not ever, I believe, use the generator as a grid reference for interactive inverters.
That is correct. It never synchs with the generator; it is either inverting from the batteries with the generator off or passing through power from the generator with its inverter off. I believe that it does, however, allow the Sunny Boy(s) connected to the protected loads panel to synch with the generator if the loads and the requirements of the battery charging circuit together are sufficient to use all the power from the PV.
 

PVfarmer

Senior Member
Of course, but a Sunny Island is a battery inverter, not a PV inverter, and 1you have to buy batteries for it because the Sunny Island cannot run without them. A Sunny Island can indeed stand between the grid and a Sunny Boy and regulate its output so that it doesn't backfeed the generator. But 2now you are talking about at least two inverters (maybe more - Sunny islands are only 5kW and you may need more of them because they are 120V single phase and they cannot pass through more than 5kW each) plus batteries ($$$), and you have to set up a protected loads panel, and only the loads in that panel have access to the PV/battery/generator power when the grid is down.

Now the customer has to ask himself if 3it is really worth it to pay another $10-20k or more (maybe way more) just to have the solar run when there is a power outage when he is powering his loads from the generator. Do the math. Is it possible? Yes. Is it worth the expense? In most cases, no.

4And what if those outages happen at night or during a storm when the irradiance is way down so the PV won't be producing anything?

It's usually nowhere near worth it.
Stuff in bold-
1-You'll need 4 to 8 (12V or 6V) batteries- that's $1000-2500 or so.
2-I'm not sure how many inverters we're talking about!
3-Of course it isn't worth it if it costs that much! I wouldn't personally pay more than an inverter + $1000 BOS if that was all it was for, for my own house.
4-The batteries would kick in, then when they got low, the gen would kick in and charge them while powering loads if it was the right size?

You could start from scratch, or add back up to an existing system, but he was talking about existing systems with gen and grid tie PV.

Just so we're on the same comment...romex said:
But i do know many grid tie folks also have a genny . So my Q would be, does the inverter know the dif between the poco & the genny? Or would it simply continue to 'help the genny out' .....?

So...all inverters know when the grid goes down, but only hybrids will keep running and "help the gen out". But isn't it really the gen helping the inverter by the gen making a voltage for inverter to follow?

There are a few ways to have grid tie PV and a gen
You could:
1. use switches. Open the PV disconnect switch and have a double throw main switch, so with no grid you'd have just the gen into the main load panel.
2. use a SMA TL-US-22 and set up switches to separate the 22 and the gen- like an emergency/backup/protected panel with <1500w of loads. Maybe if all <1500w was going to a hybrid/battery inverter grid input from that panel the TL-22 power could charge batteries?
3. use the hybrid inverter for the grid connection. With an Outback 8048, you'd have to use a DC charge controller instead of an AC inverter for PV panels, with the gen into gen input and grid into grid input.
So you wouldn't need the 2 inverters with the 8048 setup like you would with the SMA 6048s.
 

ggunn

PE (Electrical), NABCEP certified
Stuff in bold-
1-You'll need 4 to 8 (12V or 6V) batteries- that's $1000-2500 or so.
2-I'm not sure how many inverters we're talking about!
3-Of course it isn't worth it if it costs that much! I wouldn't personally pay more than an inverter + $1000 BOS if that was all it was for, for my own house.
4-The batteries would kick in, then when they got low, the gen would kick in and charge them while powering loads if it was the right size?

You could start from scratch, or add back up to an existing system, but he was talking about existing systems with gen and grid tie PV.

Just so we're on the same comment...romex said:
But i do know many grid tie folks also have a genny . So my Q would be, does the inverter know the dif between the poco & the genny? Or would it simply continue to 'help the genny out' .....?

So...all inverters know when the grid goes down, but only hybrids will keep running and "help the gen out". But isn't it really the gen helping the inverter by the gen making a voltage for inverter to follow?

There are a few ways to have grid tie PV and a gen
You could:
1. use switches. Open the PV disconnect switch and have a double throw main switch, so with no grid you'd have just the gen into the main load panel.
2. use a SMA TL-US-22 and set up switches to separate the 22 and the gen- like an emergency/backup/protected panel with <1500w of loads. Maybe if all <1500w was going to a hybrid/battery inverter grid input from that panel the TL-22 power could charge batteries?
3. use the hybrid inverter for the grid connection. With an Outback 8048, you'd have to use a DC charge controller instead of an AC inverter for PV panels, with the gen into gen input and grid into grid input.
So you wouldn't need the 2 inverters with the 8048 setup like you would with the SMA 6048s.
Be that as it may, when confronted with the actual numbers of dollars they would have to spend in order to have their PV operate when the grid is down, and then they compare that with the amount of energy they would be buying with those dollars over the lifetime of their system, most people that have a fairly reliable grid connection opt out.

A viable alternative is the line of SMA inverters that provide a live outlet with up to 1500W capacity when the grid is down and the sun is shining. You can't run your AC or clothes dryer on that, but you can charge your mobile devices, power a radio, or run a computer with it. No batteries necessary. We sell a lot of them.
 

jaggedben

Senior Member
A viable alternative is the line of SMA inverters that provide a live outlet with up to 1500W capacity when the grid is down and the sun is shining. You can't run your AC or clothes dryer on that, but you can charge your mobile devices, power a radio, or run a computer with it. ...
Or keep your beer chilled in your man cave mini fridge!:thumbsup::thumbsup:
 
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