Solar - Generator connection

Chamuit

Senior Member
Location
Texas
I get an email yesterday asking me if I will go out and inspect a generator/solar installation. They were not sure if it was hooked up correctly by the generator installers. The generator was installed after solar. City inspection - passed. POCO review of installation - passed. Took me a few minutes since I have hardly done any solar work to figure out what was wrong. The load center is inside behind the meter and ATS.

Not sure how they didn't have problems. Been in for about 3 years I belive.

Solar.jpg
 

GoldDigger

Moderator
Staff member
Most likely they have not had problems because there has never happened to be an outage or load test of the generator while the PV was producing and the active local load was less than the the PV output.
If there was never a reason for the GTI to try to backfeed the generator, there would be no symptoms.
That or the generator output did not meet the 5 minute stability and precision test for the GTI to lock on and start producing.
 
Where is the pv connection relative to the generator connection? I assume you are getting at that the pv is on the load side of the generator? I know it's wrong, and I wouldn't try it, but probably the inverter would shut down before any damage was done.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Where is the pv connection relative to the generator connection? I assume you are getting at that the pv is on the load side of the generator? I know it's wrong, and I wouldn't try it, but probably the inverter would shut down before any damage was done.
Not necessarily. I used to teach an off grid lab with a Sunny Boy, Sunny Island, and Generac genset. The SB ran just fine with the genny as the "grid".
 
Not necessarily. I used to teach an off grid lab with a Sunny Boy, Sunny Island, and Generac genset. The SB ran just fine with the genny as the "grid".
I remembered you said that in a previous thread - hence, I didnt say the inverter would not synch to it :D What I said was the inverter would likely shut down before any damage was done. That is, even if the inverter did synch up to the genny, if demand went below production, the inverter would drive up frequency and/or voltage and THEN shut down. That is my hypothesis anyway. Anyone want to take one for the team and try it? :D
 

jaggedben

Senior Member
I remembered you said that in a previous thread - hence, I didnt say the inverter would not synch to it :D What I said was the inverter would likely shut down before any damage was done. That is, even if the inverter did synch up to the genny, if demand went below production, the inverter would drive up frequency and/or voltage and THEN shut down. That is my hypothesis anyway. Anyone want to take one for the team and try it? :D
Just on theory, it would be the generator that would drive up frequency, and the inverter that would drive up voltage. Right?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I remembered you said that in a previous thread - hence, I didnt say the inverter would not synch to it :D What I said was the inverter would likely shut down before any damage was done. That is, even if the inverter did synch up to the genny, if demand went below production, the inverter would drive up frequency and/or voltage and THEN shut down. That is my hypothesis anyway. Anyone want to take one for the team and try it? :D
I think it would be a race. I wouldn't count on the inverter shutting down before frying control circuits in the generator.
 

Chamuit

Senior Member
Location
Texas
I am not aware of any direct code violation. If there is, I would guess it would be in 702 or 705. Note that this would be perfectly fine with the right equipment and/or controls.

Thanks. I didn't see anything that would prohibit the hook up to be arranged that way. I did call Generac and they said that the unit attach is not designed to work with solar. They do have models (controllers) that would work with solar like this.
 

jaggedben

Senior Member
Somewhere in 705 it says that interactive sources must be 'compatible'. One could make the case that is a violation here.
 

pv_n00b

Senior Member
Location
CA, USA
There's nothing in the NEC that says you can't connect a generator in parallel with a PV system, which is good because there are cases where it is the desired design. Remember 90.1(B), the purpose of the code is safety, not keeping the equipment working. That's the job of the system designer or engineer.

Having a generator and PV in parallel is not a guarantee that one will damage the other, it just creates the condition under which one can damage the other. Under the right conditions these two can get along fine, but those conditions are not usually maintained if they are not designed in.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
There's nothing in the NEC that says you can't connect a generator in parallel with a PV system, which is good because there are cases where it is the desired design. Remember 90.1(B), the purpose of the code is safety, not keeping the equipment working. That's the job of the system designer or engineer.

Having a generator and PV in parallel is not a guarantee that one will damage the other, it just creates the condition under which one can damage the other. Under the right conditions these two can get along fine, but those conditions are not usually maintained if they are not designed in.
Well said. For example, a Sunny Boy, Sunny Island, and generator system can work really well off grid because the SI arbitrates between the SB and the genny, throttling back the SB when the batteries are full and the loads are less than the available output from the SB.
 

tallgirl

Senior Member
There's nothing in the NEC that says you can't connect a generator in parallel with a PV system, which is good because there are cases where it is the desired design. Remember 90.1(B), the purpose of the code is safety, not keeping the equipment working. That's the job of the system designer or engineer.

Having a generator and PV in parallel is not a guarantee that one will damage the other, it just creates the condition under which one can damage the other. Under the right conditions these two can get along fine, but those conditions are not usually maintained if they are not designed in.
Remember that PV already operates in parallel with a generator. It just so happens to be called “The Grid”. Remember also that all grid-interactive PV is designed to disconnect when “The Grid” goes outside of specification, which is actually rather tight. Very few non-inverter generators can hold frequency and voltage well enough to keep an inverter running.
 

GoldDigger

Moderator
Staff member
But, unlike a small discrete generator, the grid can sink significant power when needed. It gets redistributed to non-local loads and/or causes the generators feeding the grid to reduce their output.

Sent from my XT1585 using Tapatalk
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Remember that PV already operates in parallel with a generator. It just so happens to be called “The Grid”. Remember also that all grid-interactive PV is designed to disconnect when “The Grid” goes outside of specification, which is actually rather tight. Very few non-inverter generators can hold frequency and voltage well enough to keep an inverter running.
That hasn't been my experience. I used to teach an off grid lab at ImagineSolar with a Sunny Boy, a Sunny Island, and a run-of-the-mill Generac generator. The SB ran just fine when the SI was in pass-through mode with the genny as the grid.
 

tallgirl

Senior Member
That hasn't been my experience. I used to teach an off grid lab at ImagineSolar with a Sunny Boy, a Sunny Island, and a run-of-the-mill Generac generator. The SB ran just fine when the SI was in pass-through mode with the genny as the grid.
If the generator is able to produce a stable enough voltage and frequency signal the anti-islanding logic in the Sunny Boy won't know it doesn't have an actual grid. It's definitely possible, especially with larger generators where the rotational mass helps to stabilize things, but extremely unlikely with crappy "alternator generators" like what you buy in a Blue or Orange store for a few hundred bucks.

I don't know how SMA's anti-islanding logic works, but the usual approach is to try changing the grid frequency. If successful, the inverter assumes it's islanded and MUST trip. If not, the inverter will continue to run so long as the generator is within voltage and frequency for the rest of the intertie requirements.
 

tallgirl

Senior Member
But, unlike a small discrete generator, the grid can sink significant power when needed. It gets redistributed to non-local loads and/or causes the generators feeding the grid to reduce their output.

Sent from my XT1585 using Tapatalk
So long as the PV system's output is less than the loads, the generator will back off the same as if the connected loads were reduced.

Automatic grid controls are actually pretty trick -- the more the instantaneous imbalance is too much supply, the faster (in very tiny amounts) the generators rotate, converting excess power into additional rotational energy, increasing frequency. The rise in frequency is detected causing some of the generators to reduce output. When the imbalance goes the other way, the deficiency is met by rotational energy in the generators being converted to electricity and a lower frequency. That frequency drop is detected and those same generators increase their output.

Each grid has its own "inertia" -- in the Texas (ERCOT) grid, a change of supply or demand of 450MW produced a frequency change of 0.1Hz under normal conditions.
 
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