Utilizing PV solar power when the grid is down

[Searchman]

The recently installed solar system I have is a grid tie system and when the grid goes down, so does my solar power. I knew this would occur when designing the system.
Now I would like to utilize the PV power I am producing during a power outage. The demand is too large to make a battery backup system practical. Also the house electrical system was not designed to allow only key electrical circuits to be isolated from the house electrical system for the purpose of emergency power. So I am committed to powering the whole house. The solar array is large enough to handle the demand during PV production. It is when the panels are not producing, that I need co-generation. I know I need a transfer switch to go from grid power to co-generation power. I would like to have a soft transition from grid to PV solar if the power shuts down during PV production. I can run the whole house on the PV production during the day. No problem there.
So the issue is to go from PV power to co-generation power once the sun sets in the evening. Is there equipment available that will accomplish this?

[Naptown]

Yes you can but it will be complicated and require a battery bank and a whole bunch more inverters and a generator to keep the batteries up at night.

[billvon]

The recently installed solar system I have is a grid tie system and when the grid goes down, so does my solar power. I knew this would occur when designing the system.
Now I would like to utilize the PV power I am producing during a power outage. The demand is too large to make a battery backup system practical. Also the house electrical system was not designed to allow only key electrical circuits to be isolated from the house electrical system for the purpose of emergency power. So I am committed to powering the whole house. The solar array is large enough to handle the demand during PV production. It is when the panels are not producing, that I need co-generation. I know I need a transfer switch to go from grid power to co-generation power. I would like to have a soft transition from grid to PV solar if the power shuts down during PV production. I can run the whole house on the PV production during the day. No problem there.
So the issue is to go from PV power to co-generation power once the sun sets in the evening. Is there equipment available that will accomplish this?

Short answer - no.

Longer answer from my "top 10" list:

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Mistake #10 - "I want to get my grid tied system to keep working when the grid goes down! Can't I fool the inverter somehow into thinking that the grid is still up?"

The Big Idea - I spent a lot of money for this grid tie system and I don't want to sit there in the dark if the power goes out. I'll figure out a way to keep the inverter from shutting down; that way I can back-power my house. Just when the sun is out of course.

Reason this is a mistake: First off, grid tie inverters go off line because it's very important that they do so when the power goes out for the safety of line workers. It is critical to ensure that grid lines are de-energized during blackouts so utility workers can make repairs without getting killed. And you're not just feeding 240 volts back to the grid; under the right conditions you could backfeed local transformers and generate thousands of volts on lines that should be de-energized. Thus there are some redundant and well-engineered systems in GT inverters to look for any faults in the grid and shut down quickly when they see them.

Secondly the grid provides a stable (and very low) impedance for grid tie inverters to work into. Generating a "small AC signal" to fool it into turning on will result in a very high impedance local grid that will cause the GT inverter to shut down quickly.

Third, even if you could fool the system into starting up, there is no way to balance supply and load. The line would very quickly go overvoltage if supply exceeds loads and very quickly go undervoltage when the load exceeds the supply.

All that being said, if you are still reading this and understand the above, there IS a way to make this happen, and it's called AC coupling. You can use a good bidirectional inverter (with batteries) to stabilize the grid and provide a sink/source for power when loads do not equal supply; you essentially create an "island" of power while the grid is disconnected. Outback has a white paper on this. However this must be designed in from the beginning; it's not something you can add to a conventional GT system when the power goes out.
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Even longer answer:

In your case you have a few choices to make.

First choice - how do you switch over? Best way is a subpanel. Move your critical circuits to the subpanel then put in a transfer switch. This allows almost seamless switching from grid to backup power. With the right inverter you don't even need the transfer switch; the inverter will take care of that for you. You may also need a second subpanel for your solar arrays (see the link below.)

Second best way is a generator interlock kit. This is a mechanical interlock that allows you to use a single breaker in your panel as a backfeed breaker to backfeed power from a generator or inverter back into the panel when the power goes out. It physically prevents you from turning on both the main breaker and the backfeed breaker. When the power goes out you turn off the main breaker, slide the interlock over, then turn on the backfeed breaker.

You CANNOT just expect your inverters to just "keep your loads up" with a single panel.

Next choice - what power source do you want? You need a bidirectional inverter if you are to have any hope of using your current grid tied system for power. You'll also need a means of disconnecting the grid tie inverter to prevent overvoltages when the battery cannot handle the extra power. Some grid tie inverters have a way to disable them but more likely you will need a DPST relay capable of handling the power your inverter generates. An inverter like the Outback GTFX will give you the output needed to drive this relay.

http://www.outbackpower.com/pdf/tech_notes/AC_Coupling_WHT_Paper.pdf

[Elkhornsun]

The grid-tie inverters are designed to only output power to the grid when they sense current coming from the grid. Shut down the feed from the grid and the inverter stops feeding power - anywhere.

You would need to have a switch that feeds current from the grid-tie converter instead to a non grid-tie inverter that in turn sent current to your subpanel. For the cost of a second inverter and the switching equipment and electricians time you might consider instead either a small bank of batteries or a generator. Least expensive for occasional use of a day or two is the generator and there are tri-fuel ones that will run off propane or natural gas so if you have either at your house the generator can run for days without needing to be refueled.

[russ]

The entire line is nonsense - the OP didn't design anything and has no idea - they need to talk to their supplier and go from there.

[DonMcLRPI]

Yes, some of this is practical.... in fact some systems are commercially available .

The recently installed solar system I have is a grid tie system and when the grid goes down, so does my solar power. I knew this would occur when designing the system.
Now I would like to utilize the PV power I am producing during a power outage. The demand is too large to make a battery backup system practical. Also the house electrical system was not designed to allow only key electrical circuits to be isolated from the house electrical system for the purpose of emergency power. So I am committed to powering the whole house. The solar array is large enough to handle the demand during PV production. It is when the panels are not producing, that I need co-generation. I know I need a transfer switch to go from grid power to co-generation power. I would like to have a soft transition from grid to PV solar if the power shuts down during PV production. I can run the whole house on the PV production during the day. No problem there.
So the issue is to go from PV power to co-generation power once the sun sets in the evening. Is there equipment available that will accomplish this?

Begin with your thoughts on using solar direct. from first light until the PV output equals your load needs, either you have a battery (some storage) system to provide this deficit, or things don't work. So it is when every cloud passes by. I have explored this exact issue for our 84 panel/ Enphase system in New Mexico. No storage, no practical system.
Next ... load matching. The grid takes all you can supply. Without a grid, all you have is the time varying loads of your home. The only practical circuit I have seen uses the storage as the sump for excess power, with the specialized inverter being turned off when the storage is full and solar output exceed loads.
This system provides automatic protection preventing feed back to the grid... thus no safety concern...
However, you will need a big check book and an ability to minimize loads when the grid and your solar are down.
Dig a little .... the Enphase "off line" boards may be one source .... to find the current sources for these kits... ( Sadly, a generator is easier....)