Solar - Generator connection

Chamuit

Senior Member
I talked to Generac. Their point of view is that the model being used is not "designed" to be used with solar.

My recommendation was to rearrange the setup to a typical arrangement with the solar separated from the generator since there was no engineering oversight or approvals.
 

iceworm

Curmudgeon still using printed IEEE Color Books
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.
Assumption:
Generator is residential grade, 10KW - 25KW, 120/240. Gen has no over-frequency, under-frequency, over-voltage, or under-voltage protective relays. Gen has over current protection.

Disclaimers:
  1. Grid tied inverters are not an area of my expertise.
  2. I don't know what happens if the gen is an inverter type. I have not seen any analysis, but inverters don't tend to sink power - unless they are designed as 4-quadrant vfd.

I absolutely agree with Chamuit's solution. However:

g -
I recall a similar thread where (I think) you explained:

  • grid-tied the inverters act similar to a constant current source.
  • They output all the power available from solar panels DC input.
  • Frequency is controlled by the paralleled source (the grid/gen).
  • Voltage is controlled by the paralleled source.
  • The inverters will shut down if the voltage or frequency get out of tolerance (I don't recall exactly, but +,- 5% Hz, and +,- 10% V sticks in my mind.)

Assuming any to this is true, here is what I am seeing:

  1. If the system load goes under the PV system output, the gen throttle pulls back, and the inverters back-feed the gen, driving it as a synchronous motor, causing it to over-speed.
  2. Frequency goes up, voltage goes up
  3. Eventually either the over-voltage or over frequency shuts down the inverters.
  4. 63Hz, 132V should not hurt the gen.

Questions:
What would be the failure mode that could "fry" the generator control circuits?
Which control circuits are you thinking are in danger?
Voltage regulator?
Start-stop controls?
Some other clearly specified control circuit?​
 

electrofelon

Senior Member
The standard frequency tolerance for a GTI is +.5hz, -3 hz for .16 seconds. Max voltage is about 264. I don't see those figures resulting in Genny damage, but that's total speculation on my part.
 

iceworm

Curmudgeon still using printed IEEE Color Books
The standard frequency tolerance for a GTI is +.5hz, -3 hz for .16 seconds. Max voltage is about 264. I don't see those figures resulting in Genny damage, but that's total speculation on my part.
So the OF is +1% and the UF is -5%?
 

jaggedben

Senior Member
I read of one case in another forum where the generator shutdown and delivered a fault message. Evidently that generator had somewhat more sophisticated protection controls than what iceworm is positing. The thing is, even if your genny doesn't get damaged, if it shuts down, you've defeated the purpose of having it.
 

ggunn

PE (Electrical), NABCEP certified
I read of one case in another forum where the generator shutdown and delivered a fault message. Evidently that generator had somewhat more sophisticated protection controls than what iceworm is positing. The thing is, even if your genny doesn't get damaged, if it shuts down, you've defeated the purpose of having it.
And of course, once the genny shuts down, so will the PV system, if it hasn't already.
 

ggunn

PE (Electrical), NABCEP certified
Questions:
What would be the failure mode that could "fry" the generator control circuits?
Which control circuits are you thinking are in danger?
Voltage regulator?
Start-stop controls?
Some other clearly specified control circuit?​
We damaged a generator in the lab I spoke of, probably due to operator error on my part in programming the Sunny Island to protect the genny from backfeed from the Sunny Boy. I was told that it took out a/the "control board" in the generator. My areas of expertise lie elsewhere, so that's just about all I know, but it seems obvious to me that if you backfeed a generator that cannot sink power with a GT PV system, Bad Things can occur. Exactly which Bad Thing would happen first is not that important, IMO. My advice is to not do it.
 

pv_n00b

Senior Member
Questions:
What would be the failure mode that could "fry" the generator control circuits?
Which control circuits are you thinking are in danger?
Voltage regulator?
Start-stop controls?
Some other clearly specified control circuit?​
Grid-tied inverters are designed to convert DC to AC and move it on. They are not designed to curtail the output based on the needs of the AC loads as an off-grid inverter will do. So if we put a grid-tied inverter in an off-grid situation and there is not enough load to absorb the inverter output that current will go somewhere it is not wanted. Where it goes and what it goes through depends on the weakest link on the AC side. If something on the AC side does not fail, eventually the inverter will raise the voltage high enough to trip itself off line as it tries to push current into the loads.
 

iceworm

Curmudgeon still using printed IEEE Color Books
gg, pv -
I appreciate the effort. But without some physics, not much help
 

tallgirl

Senior Member
I read of one case in another forum where the generator shutdown and delivered a fault message. Evidently that generator had somewhat more sophisticated protection controls than what iceworm is positing. The thing is, even if your genny doesn't get damaged, if it shuts down, you've defeated the purpose of having it.
Pretty much — unless there’s some control logic which starts disconnecting inverters as the load on the generator approaches zero, that’s the result. I tried that on a lark once, because I’m a nerd, and it’s hard to do with software. You’d have to know all the loads and always keep enough of a load on the generator to power the largest intermittent load.
 

tallgirl

Senior Member
Grid-tied inverters are designed to convert DC to AC and move it on. They are not designed to curtail the output based on the needs of the AC loads as an off-grid inverter will do. So if we put a grid-tied inverter in an off-grid situation and there is not enough load to absorb the inverter output that current will go somewhere it is not wanted. Where it goes and what it goes through depends on the weakest link on the AC side. If something on the AC side does not fail, eventually the inverter will raise the voltage high enough to trip itself off line as it tries to push current into the loads.
Most aren’t. The way the Sunny Boy / Sunny Island combination works is the Sunny Island acts like the grid both as a source and sync — a GTI doesn’t much care. When the Sunny Island “decides” the attached batteries are approaching full, it changes its output frequency. This magically signals the Sunny Boy to reduce output.
 

pv_n00b

Senior Member
Most aren’t. The way the Sunny Boy / Sunny Island combination works is the Sunny Island acts like the grid both as a source and sync — a GTI doesn’t much care. When the Sunny Island “decides” the attached batteries are approaching full, it changes its output frequency. This magically signals the Sunny Boy to reduce output.
The Sunny Boy is controlled by the Sunny Island. Left on its own it will act like every other grid connected inverter and just pump out all the power that it can and that the array can supply. There are a number of systems that use a controller to control standard grid tied inverters one way or another to make them play nice in an off grid situation.
 
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