PV backfeed on a double throw safety switch

[BackCountry]

I have a customer that has a double throw switch for a portable generator after the meter at a panel board and before their 200a panel which is located at the house 100’ away.

We’re installing rooftop PV, and clearly I can’t backfeed that 200a panel if their generator receptacle remains up stream.

I’m wondering how to pull off essentially a three way interlock. For generator breaker to be on, main and PV must be off. For PV to be on, main must be on and generator off.

Any ideas?

 

[jaggedben]

You're gonna have to pull something 100ft. Even if it's just for a contactor to turn off the PV.

Or else do a groundmount near the meter.

 

[BackCountry]

You're gonna have to pull something 100ft. Even if it's just for a contactor to turn off the PV.

Or else do a groundmount near the meter.

I proposed both of those. We could try to pull out the feeder and pull the PV back in with it and just do a load side tap ahead of the two way switch. I just don’t want to, but that might be the reality.

 

[BackCountry]

You're gonna have to pull something 100ft. Even if it's just for a contactor to turn off the PV.

Or else do a groundmount near the meter.

Just had a new thought — I can just put the generator breaker in their 200a panel with an interlock to the main breaker, and then do a line side tap off the main breaker feeder with a fused disconnect to the PV.

 

[jaggedben]

Ha. I forgot you said the generator was portable.

 

[kwired]

I'm missing the problem here. Generator will be isolated by the transfer switch. PV typically doesn't feed power to the other source unless there is voltage from that other source. So if there is no utility and no generator voltage, the PV isn't back feeding anything. If transfer switch is in standby position and your generator is running, the PV is feeding into whatever you have for load at the time, you just have no large network connected to it to absorb any excess being produced like you do when connected to the utility.

 

[BackCountry]

I'm missing the problem here. Generator will be isolated by the transfer switch. PV typically doesn't feed power to the other source unless there is voltage from that other source. So if there is no utility and no generator voltage, the PV isn't back feeding anything. If transfer switch is in standby position and your generator is running, the PV is feeding into whatever you have for load at the time, you just have no large network connected to it to absorb any excess being produced like you do when connected to the utility.

The challenge was the generator would be feeding the 200a panel which would have a backfed PV breaker.

If I relocate the generator receptacle to the 200a panel and put it on an interlock kit off the main breaker, and then line side tap the main breaker I should be golden.

We don’t do a whole lot of line side tapping here in California, almost every panel I run into is a CSED.

 

[kwired]

The challenge was the generator would be feeding the 200a panel which would have a backfed PV breaker.

If I relocate the generator receptacle to the 200a panel and put it on an interlock kit off the main breaker, and then line side tap the main breaker I should be golden.

We don’t do a whole lot of line side tapping here in California, almost every panel I run into is a CSED.

It should have no back feed until there is voltage from the generator though. All that backfeed will do once there is voltage is reduce the loading on the generator by whatever the PV is capable of producing.

I fail to see what is a problem. Small portable generator-- might not save you much if any on fuel depending on actual loading but not a hazard for users either. You are not backfeeding the utility as transfer switch prevents that.

 

[jaggedben]

Kwired he wants to put the solar on the building with the panel powered by the generator. If not for the generator he would have just landed the PV in that panel. But if he simply does that then generator and PV are both connected to the panel when transfer switch is thrown to generator.

 

[jaggedben]

. . If transfer switch is in standby position and your generator is running, the PV is feeding into whatever you have for load at the time, you just have no large network connected to it to absorb any excess being produced like you do when connected to the utility.

If PV exceeds load then the PV tries to backfeed the generator, which at best likely shuts down the generator and defeats its purpose, and at worst may damage something.

 

[BackCountry]

Kwired he wants to put the solar on the building with the panel powered by the generator. If not for the generator he would have just landed the PV in that panel. But if he simply does that then generator and PV are both connected to the panel when transfer switch is thrown to generator.

Exactly

 

[ggunn]

It should have no back feed until there is voltage from the generator though. All that backfeed will do once there is voltage is reduce the loading on the generator by whatever the PV is capable of producing.

I fail to see what is a problem. Small portable generator-- might not save you much if any on fuel depending on actual loading but not a hazard for users either. You are not backfeeding the utility as transfer switch prevents that.

Backfeeding a generator is a very bad idea. The PV should be disconnected when the generator is running.

 

[BackCountry]

Backfeeding a generator is a very bad idea. The PV should be disconnected when the generator is running.

Precisely.

I think I’ve got it dialed. Eaton interlock kit for main breaker and generator breaker, then I’ll line side tap the panel, and we’re good.

 

[coop3339]

You could use 'kirk keys' but it would be an overkill

 

[PWDickerson]

You could use 'kirk keys' but it would be an overkill

Had to Google that one. Looks like a solution for industrial settings. I think the OP is on the right track.

 

[kwired]

If PV exceeds load then the PV tries to backfeed the generator, which at best likely shuts down the generator and defeats its purpose, and at worst may damage something.

what is it going to hurt? The PV system will synch itself to generator voltage/phasing and run parallel to it. If the load isn't there the generator is wasting fuel is the worst thing that happens. If you tie the PV to the common circuit of the transfer switch the generator isn't going to shut down. It needs to sense voltage on the utility input side of the transfer switch before it goes into shut down process.

If you don't want the PV and generator to energize the same line then supply a control relay from generator output to somehow shut down the PV supply whenever there is generator output.

 

[kwired]

Backfeeding a generator is a very bad idea. The PV should be disconnected when the generator is running.

It's not backfeeding it is running in parallel. It works the same way it does when only the PV and utility are involved. No utility voltage, no PV output. It can't see the transfer switch so to the PV system the supply to the panel it is feeding into is the utility voltage regardless what the actual source is. It will synchronize phasing once it has voltage to synch to and should be no problem. Generator quits, PV will cease output.

 

[wwhitney]

It's not backfeeding it is running in parallel.

You're talking about a microgrid, so power generated has to equal power consumed at every instant in time. The generator acts as a voltage source and will adjust its current output as required to maintain that balance. But a grid-following PV inverter, when it finds a voltage source to sink to, acts like a current source--it wants to push out as much power as it can.

Then if your microgrid loading demands more power than the PV inverter can produce, the generator can adjust its current output to make up the difference and power flow is balanced. As soon as the microgrid demand falls below the PV inverter's power output, though, there's a problem--where does the extra power go? The PV inverter isn't designed to throttle down its output, and the generator isn't designed to consume power. Best case the microgrid voltage goes out of spec and that knock off the PV inverter entirely. Worst case the generator shuts off or is damaged--no more microgrid.

So this arrangement is only workable if you have a "dump load" and associated controller (or other baseline minimum loading), something that is capable of consuming more power than the PV inverter than create. Then the dump load controller can sense that the microgrid loading is going below the PV inverter output and turn on the dump load to maintain the power balance.

Cheers, Wayne

 

[jaggedben]

what is it going to hurt? The PV system will synch itself to generator voltage/phasing and run parallel to it.

And the PV inverter(s) will likely try to push current into the generator. Because almost all interactive PV inverters are configured essentially as current sources. How the generator will react to this depends on the generator engineering but I've seen at least one example posted online where the generator shut down and its controls gave a fault error indication.

There have been other threads on this. Read up.

 

[solarken]

It's not backfeeding it is running in parallel. It works the same way it does when only the PV and utility are involved. No utility voltage, no PV output. It can't see the transfer switch so to the PV system the supply to the panel it is feeding into is the utility voltage regardless what the actual source is. It will synchronize phasing once it has voltage to synch to and should be no problem. Generator quits, PV will cease output.

As others have said, the issue is not that the PV inverter cannot sync to a generator waveform, it's that unlike when the grid is up, where there is an "infinite load" to be fed, when the grid is down, and the local building is isolated with a very small and specific load to be served, there is no coordination/intelligence between the generator and the inverter to control power output and match it to a typically constantly changing local load.