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Can a 4kw generator be used to supply a grid-tied 7.5kw Inverter's signal voltage?

I would first isolate the house from the Grid. Then I'd fire up the 4kw generator and plug it into the 220 volt receptacle for the whole house. The Fronius Inverter would sense the sign wave and start pumping juice into the house panel.

Would the generator and the Inverter play together?

Would the 7.5kw solar array overpower the generator and burn it out?

Stan

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In this situation, they will not play together nicely at all.  It can result in backfeeding either the generator or the inverter.  Now, let's say that you had a an automatic transfer switch between a dedicated loads panel that the grid or the generator can supply and the PV inverter is not connected to the dedicated loads panel.  In this setup your generator can supply the loads that you consider important whilist the PV inverter is non-functional until the grid returns.

 

Well, I basically do that now.  I throw the house (service panel) switch, as well as the PV breaker (to keep the generator power from activating the Fronius) and I run the whole house from the 4kw generator using the 220volt single phase outlet.  But....if the sun is shining, I lose ALL that electricity that the 7.5kw array on my house COULD have been sending throughout the house.   This is the point.  Why don't they allow us homeowners to isolate the house from the grid  (when the grid is down) and then have the PV inverter, if the sun is shining, power the house?  If the inverter needs a sign wave, have the generator supply it...or a small battery with a signwave generator on it.

I know, it seems like this should be an option for all grid-tied PV systems on its face.  The Fronius inverter, like most grid-tied inverters, are listed UL 1741 which means that they are equipped to detect line voltage from the grid and to shut down when this voltage is lost to prevent them from backfeeding into the grid.  This called anti-islanding and it is meant to protect linesman who could be servicing the utility from harm.  If a grid-tied inverter were feeding into the grid while the linesman is working on the power lines that person could be seriously injured or worse.

 

Now, there are grid-tied inverters, UL 1741 listed, which posses the qualities you are looking for.  They detect line voltage from the grid, but instead of shutting down, they isolate themselves from the grid and power your loads while continually monitoring the line voltage in case it comes back.  This function allows the inverter to feed your critical loads until the grid comes back.  Some of these inverters can also incorporate an automatic generator start module (AGS) which controls the generator and assists or provides backup power to the inverter while the grid is down. This function is important because it prevents the generator and the inverter from forcing back power to each other. 

 

In a system like yours with an existing grid-tied inverter, the next best step would be to incorporate a battery back-up system and battery inverter.  This is called an AC coupled system, where the grid-tied inverter's output is installed downstream of the battery inverter.  The battery inverter acts as the gatekeeper for all the power that the grid-tied inverter and/or the generator supply.  So, when the grid does go down this battery inverter will allow the PV system to continue to supply power where in a normal setup it would just be offline.

If you want to know more about how to get your existing system set up to function this way, we are here to get you up and running.

You must remember that your inverter is programmed to provide voltage slightly higher than the grid voltage which it is monitoring.  This higher voltage, usually on the order of .1 volt, is necessary to provide sufficient electromotive force to cause the power to flow back to the grid.  The grid can handle this, but your generator most likely cannot, and using it in this fashion will certainly void the generator warranty.  

The only good way to use your grid-tied inverter during a utility outage is to create a mini-grid using batteries.  There are many ways to accomplish this, and most off-grid inverter manufacturers address their methodology on their web sites.  IMO SMA is doing it best with their Sunny Island/Sunny Boy grid-tied battery backup system.  Schneider Electric, Outback Power Systems, and Magnum Energy also make systems that will accomplish the task.

Stan, I use the Outback Radian GS8048 and it will do exactly as you describe.  It's a 'grid interactive' system with a battery bank as opposed to a 'grid-tied' system which has no battery bank.  The inverters will power loads independent of the grid or when the grid is down, and will auto-start the generator when there is insufficient input from solar and the batteries are low.  The only other way to accomplish what you're looking to do would be with an AC coupled system - which can also operate a generator as a slave device, but the AC coupled solution is usually more expensive than a grid interactive system (because you're doubling the number of inverters).  Hope that helps!

Except that this is an existing Fronius installation, why not use an 7.7kW or less TL inverter?  It has a built-in "Secure Power" feature, that's basically an isolated critical circuit panel that can be used when the grid is disconnected and you have sunshine. Here's a couple links for the TL series with "Secure Power."

https://www.greentechrenewables.com/manufacturers/sma

http://www.sma-america.com/products/solarinverters/sunny-boy-3000tl-us-…

 

Stan, You have hit on a common problem people have with grid tied PV, why can't I use it when the grid is down? Let's look at a couple of examples. When the grid goes down and you hook up your generator to power your house the generator is designed to only provide as much power as you are currently using. Turn on a light and the generator output goes up by the amount of power the light uses. Turn the light off and the generator output goes down. Turn all the loads off and the generator does not output any power. Now let's look at a grid tied inverter. It does not sense the load on it, it does one thing and that is take all the power being generated by the array, convert it to AC, and send it out. So what happens if you use it to power your house in parallel with your generator? It depends on how much load you have and how much power the inverter is generating. If the inverter is generating 5kW and the load is 8kW then your backup generator will make up the 3kW difference and everything is fine. If the inverter is generating 5kW and the load is 3kW then you have a problem. The grid tied inverter does not reduce its output to 3kW like the backup generator would, it just keeps trying to output 5kW. That extra 2kW is going to go somewhere you don't want it to and damage something, most likely either the inverter or the backup generator. This is basically why parallel backup generator and inverter systems don't work. You can make a combination generator and inverter system work on a larger scale where the expensive control systems are justified but not on a small scale. The less expensive solution is a grid tied backup system with batteries.

Quite a bit of fantastic content here that illustrates how frustrating it is to employ a standard UL listed grid-tied inverter for advanced use cases! Here's the latest info for US-based systems to consider for this topic:

UL 1741 had, at its core, one mission: Safely push power to the grid and then stop and don't do ANYTHING if the grid is gone or looks funny. This worked fine for a few years, but when PV started to take a significant portion of the generation market, it began to not work so well as it was either on or off - not an elegant way to control anything.

Now we have UL 1741 SA. Instead of merely turning on and off, we can now offer the grid:

  • Anti-islanding
  • Low/High Voltage Ride Through
  • Low/High-Frequency Ride Through
  • Must Trip Test
  • Ramp Rate (Normal & Soft-Start)
  • Specified Power Factor
  • Volt/Var Mode
  • Frequency Watt
  • Volt Watt

And guess what? Some of this funtionality can offer interesting new use cases. For example, Using Volt-Watt control, we can now parallel a PV inverter and a generator. To reliably parallel with a generator, the PV inverter provides a Volt-Watt mode in which line voltage rise is constantly monitored by each inverter and power will be linearly decreased between preset start and stop voltage thresholds. This negative feedback approach prevents an over-voltage condition from developing on an islanded generator controlled system and allows the microinverter array to co-generate (decreasing fuel consumption) without need for a transfer switch between the generator and the array (Chilicon Power).

As Jim Turner pointed out earlier, some inverters also now have the abiity to add a current sensor, which could also possible set 'export' (to the generator) to 0W by monitoring current at the connection to the generator.

Of course, this is still new territory, so you need to have a high tolerance for risk, or be able to work closely with both the generator manufacturer, or both...

Submitted
10 years ago
Asked by
Stan Graff