Standby Generator Install with Solar

[mkgrady]

I have installed numerous automatic standby generator on homes and businesses but never where the service also had a PV system.

I know almost nothing about solar systems (except that I never want to be on a roof installing them) so I am having trouble understanding what effect they have on an automatic standby generator system.

Can a standby generator be connected to a whole panel that also has a PV system connected to it? On this theoretical job let's assume I am connecting a 20KW generator to the main service panel utilizing a service rated ATS. I'm pretty clear on how to do the job including the need for load shedding I just don't know the effect that the PV system has on this. I'm guessing it can't be done and that I will need to put in a sub panel for the circuits I need to power during an outage which seems like the ATS will isolate it from the PV. Do I have this right?

 

[ggunn]

I have installed numerous automatic standby generator on homes and businesses but never where the service also had a PV system.

I know almost nothing about solar systems (except that I never want to be on a roof installing them) so I am having trouble understanding what effect they have on an automatic standby generator system.

Can a standby generator be connected to a whole panel that also has a PV system connected to it? On this theoretical job let's assume I am connecting a 20KW generator to the main service panel utilizing a service rated ATS. I'm pretty clear on how to do the job including the need for load shedding I just don't know the effect that the PV system has on this. I'm guessing it can't be done and that I will need to put in a sub panel for the circuits I need to power during an outage which seems like the ATS will isolate it from the PV. Do I have this right?

Assuming that the power from the generator is "clean" enough that the inverter sees it as grid power and turns on, the problem arises when the generator is running and the grid is down, and the local loads are demanding less than the output of the PV system. Being essentially a current source, the PV will backfeed the generator which is bad news for the generator. Usually the best solution is to connect the PV on the grid side of the ATS.

 

[mkgrady]

Assuming that the power from the generator is "clean" enough that the inverter sees it as grid power and turns on, the problem arises when the generator is running and the grid is down, and the local loads are demanding less than the output of the PV system. Being essentially a current source, the PV will backfeed the generator which is bad news for the generator. Usually the best solution is to connect the PV on the grid side of the ATS.

I don't know enough about PV systems to know if it can be changed to grid side connection but I assume it can't easily be done. I also assume connecting a subpanel downstream of the ats is the way to go in order to isolate the PV system from the generator output.

 

[GoldDigger]

I don't know enough about PV systems to know if it can be changed to grid side connection but I assume it can't easily be done. I also assume connecting a subpanel downstream of the ats is the way to go in order to isolate the PV system from the generator output.

Actually, since the generator must be upstream of the ATS, you cannot isolate the PV from the generator by connecting it downstream of the ATS.
The proper connection would be upstream of the ATS, on the generator side. But if the ATS is also the service disconnect that would make it a line side connection.
So the ATS must be downstream of the service disconnect and the PV will then be connected to the feeder from the service disconnect to the ATS. At the ATS end or at the service disconnect end will both work, but be careful about how your AHJ enforces the 120% rule.

 

[jaggedben]

I don't know enough about PV systems to know if it can be changed to grid side connection but I assume it can't easily be done. I also assume connecting a subpanel downstream of the ats is the way to go in order to isolate the PV system from the generator output.

If you've "installed numerous automatic standby generator on homes and businesses" then you'll probably have enough general electrical knowledge to assess if the PV connection can be easily relocated. Simply put, the PV needs to be connected on the utility side of the ATS as Golddigger said. If the current PV connection is far away from the planned ATS location then I'd guess you're as qualified as anyone to assess the difficulty of relocating the wiring to a totally different location.

The PV connection can be via a breaker in a panelboard, or a feeder or service conductor tap to a fused disco, as long as it's on the utility side or the ATS. Article 705 will be your guide to the rules as far as ratings of conductors and such.

In case your customer believes that they're going to have the solar available when the grid is down, you'll have to disabuse them of that notion. Or alternatively, they can look at some cutting edge storage solutions designed to work with solar. But that will be more expensive and require you to educate yourself on some different technology, or hand over the job to someone more qualified.

BTW, we're assuming the solar is a grid-tie-only system with no energy storage, typical of 99% of PV systems out there. If it does happen to have storage then everything I've said could be different.