300A Supply Side Interconection

wwstrick

Member
I am going to install a Photovoltaic system for an orphanage here in Guatemala. There will be 7 inverters with a maximum current of 42A and 240 Volts. The area is quite large with 150 children and 50 adults living on the grounds. The energy used has a minimum current of 65A 24 hours a day. The maximum is 135A mostly between 6PM and 10PM. They have a large generator with an automatic cutoff switch. I would like to connect the inverters on the line side of the inverter between the cut off switch and the meter. I need to find a clean way to connect large cables basically like a tap. The existing connections are bolted on compression lugs. Can someone recommend junction blocks to join together three large compression lugs? I will combine the inverter outputs before the supply side connection. I really don't work with that type of connection much and local suppliers are not much help.
 

electrofelon

Senior Member
I am going to install a Photovoltaic system for an orphanage here in Guatemala. There will be 7 inverters with a maximum current of 42A and 240 Volts. The area is quite large with 150 children and 50 adults living on the grounds. The energy used has a minimum current of 65A 24 hours a day. The maximum is 135A mostly between 6PM and 10PM. They have a large generator with an automatic cutoff switch. I would like to connect the inverters on the line side of the inverter between the cut off switch and the meter. I need to find a clean way to connect large cables basically like a tap. The existing connections are bolted on compression lugs. Can someone recommend junction blocks to join together three large compression lugs? I will combine the inverter outputs before the supply side connection. I really don't work with that type of connection much and local suppliers are not much help.
I would look at either a triple stack lug or a Polaris type connector. You can also stack compression lugs.
 

ggunn

PE (Electrical), NABCEP certified
I am going to install a Photovoltaic system for an orphanage here in Guatemala. There will be 7 inverters with a maximum current of 42A and 240 Volts. The area is quite large with 150 children and 50 adults living on the grounds. The energy used has a minimum current of 65A 24 hours a day. The maximum is 135A mostly between 6PM and 10PM. They have a large generator with an automatic cutoff switch. I would like to connect the inverters on the line side of the inverter between the cut off switch and the meter. I need to find a clean way to connect large cables basically like a tap. The existing connections are bolted on compression lugs. Can someone recommend junction blocks to join together three large compression lugs? I will combine the inverter outputs before the supply side connection. I really don't work with that type of connection much and local suppliers are not much help.
Just to be clear, you are making the line side connection between the utility meter and the transfer switch, correct? The PV system and the generator should never be connected to the loads at the same time.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Just to be clear, you are making the line side connection between the utility meter and the transfer switch, correct? The PV system and the generator should never be connected to the loads at the same time.
So, you are saying the PV inverters are always connected to the utility. The transfer switch disconnects the utility and PV inverters when the generator is on line.

Question: I'm not familiar with PV inverter systems. So why not allow the PV inverters to be on-line when the generator is running? I can see that one would have to deal with some sync issues.
 

electrofelon

Senior Member
So, you are saying the PV inverters are always connected to the utility. The transfer switch disconnects the utility and PV inverters when the generator is on line.

Question: I'm not familiar with PV inverter systems. So why not allow the PV inverters to be on-line when the generator is running? I can see that one would have to deal with some sync issues.
Probably the inverters will just shut down due to the generator not being "clean" enough. I am not exactly sure what would happen if they didn't shut down and production exceeded usage. I am guessing voltage and or frequency would rise until the inverters shut down, possibly damaging the generator before that happened.
 

jaggedben

Senior Member
If the inverters total output is 292A and the maximum load is 135A then it must be a grid tied system. (He didn't mention a huge expensive set of batteries.)
 

iceworm

Curmudgeon still using printed IEEE Color Books
If the inverters total output is 292A and the maximum load is 135A then it must be a grid tied system. (He didn't mention a huge expensive set of batteries.)
.... There will be 7 inverters with a maximum current of 42A and 240 Volts. ....
jag -
you lost me

AH - you're saying it is 7 inverters at 42A each.

Okay, I was thinking 42A total. I'm guessing 6A per inverter is smaller than normal.
 

ggunn

PE (Electrical), NABCEP certified
So, you are saying the PV inverters are always connected to the utility. The transfer switch disconnects the utility and PV inverters when the generator is on line.

Question: I'm not familiar with PV inverter systems. So why not allow the PV inverters to be on-line when the generator is running? I can see that one would have to deal with some sync issues.
Yes, that is what I am saying. If the generator and PV are both serving off grid loads and the loads are demanding less than what the PV is delivering, and there are no batteries to absorb the excess, the PV will try to backfeed the generator. That would be bad news for the generator.
 

ggunn

PE (Electrical), NABCEP certified
Probably the inverters will just shut down due to the generator not being "clean" enough.
Not necessarily. I have seen Sunny Boy/Sunny Island/battery/generator systems where the SB runs just fine with a generator acting as the grid.
 

jaggedben

Senior Member
jag -
you lost me

AH - you're saying it is 7 inverters at 42A each.

Okay, I was thinking 42A total. I'm guessing 6A per inverter is smaller than normal.
Other than for microinverters, yes, outputs are bigger. But the main clue is the subject line of the thread.
 

wwstrick

Member
Just to be clear, you are making the line side connection between the utility meter and the transfer switch, correct? The PV system and the generator should never be connected to the loads at the same time.
That is why I am not connecting in individual distribution panels around the site. In fact I am getting the job because another solar guy did not take this into account and the customer figured it out. Thanks
 

iceworm

Curmudgeon still using printed IEEE Color Books
Probably the inverters will just shut down due to the generator not being "clean" enough. I am not exactly sure what would happen if they didn't shut down and production exceeded usage. I am guessing voltage and or frequency would rise until the inverters shut down, possibly damaging the generator before that happened.
Yes, that is what I am saying. If the generator and PV are both serving off grid loads and the loads are demanding less than what the PV is delivering, and there are no batteries to absorb the excess, the PV will try to backfeed the generator. That would be bad news for the generator.
Not necessarily. I have seen Sunny Boy/Sunny Island/battery/generator systems where the SB runs just fine with a generator acting as the grid.
I need a physics lesson specific to this type of inverters. Here is my confusion:

The inverters I work with are battery fed ( or rectifier fed) 5kw - 15kw, 60HZ, 120V or 120/240V, inverter output is never paralleled - and not designed to parallel. The inverter output appears to be a voltage source. Regardless of load, the inverter will hold 60hz, 120/240V- - at least until the semiconductors overload. Yes, I understand that grid-tied inverters are a different animal.

My translation of what you are saying is the grid-tied inverters operate as a current source. As in, they will output 42A each (as long as there is sufficient sun to supply the DC input) The grid voltage could go up a bit, or down a bit, and it wouldn't matter. The inverter will still output 42A. The inverter does not have any voltage control. And since they are designed to always operate paralleled to a grid, they likely don't have any frequency control either. Is that correct?
 

jaggedben

Senior Member
I need a physics lesson specific to this type of inverters. Here is my confusion:

The inverters I work with are battery fed ( or rectifier fed) 5kw - 15kw, 60HZ, 120V or 120/240V, inverter output is never paralleled - and not designed to parallel. The inverter output appears to be a voltage source. Regardless of load, the inverter will hold 60hz, 120/240V- - at least until the semiconductors overload. Yes, I understand that grid-tied inverters are a different animal.

My translation of what you are saying is the grid-tied inverters operate as a current source. As in, they will output 42A each (as long as there is sufficient sun to supply the DC input) The grid voltage could go up a bit, or down a bit, and it wouldn't matter. The inverter will still output 42A. The inverter does not have any voltage control. And since they are designed to always operate paralleled to a grid, they likely don't have any frequency control either. Is that correct?
You basically have it correct. Grid tied inverters are designed to maximize energy harvest, and will thus output whatever is available from the solar array. (Note, they won't output their maximum if that power isn't available.) Per UL standards, they will max out at their nameplate maximum output. They will also stop outputting if the grid voltage or frequency deviates out of proscribed windows. These are basically firmware settings and they could be configured differently, and sometimes are for special off grid situations.

If a grid tied inverter tried to backfeed a generator with excess output beyond what the load required, presumably the voltage would quickly rise to the point that the inverter would shut down. But as noted, it's anyone's guess if the generator would be damaged more quickly.
 

iceworm

Curmudgeon still using printed IEEE Color Books
... If a grid tied inverter tried to backfeed a generator with excess output beyond what the load required, presumably the voltage would quickly rise to the point that the inverter would shut down. But as noted, it's anyone's guess if the generator would be damaged more quickly.
Generators I do know about. Although less about smaller ones than bigger ones * For the OP's case, 70KW solar inverters, likely 50KW gen, and a load of 15KW to 30KW:

Just guessing, it would be a frequency (overspeed) issue, not a voltage issue. But I really don't know that - that is why it is a guess.

Generators are pretty tough - no, I'm not talking about hardware store grade. Over-speeding them a bit doesn't hurt much. Damage to the generator would depend on the Inverter over frequency shutdown. The worst one could do is to open the grid connection, open the load and leave the constant power solar inverters tied to the gen. If the inverter shutdown was 66HZ or less, the gen/driver should be fine. (SWAG - one step up from a WAG)

Any more than that the gen needs a reverse power relay and a shunt trip disconnect - not common on 50KW.

Given my limited understanding, one could trick up the inverter controls, add some gen protective relaying and do fine. However, not something I would ever recommend - non-standard controls that only one person understands is rarely a good idea.

Just idle rambling. Thanks for the physics lesson.

the worm
* "small" is < 250 KVA
 

ggunn

PE (Electrical), NABCEP certified
My translation of what you are saying is the grid-tied inverters operate as a current source. As in, they will output 42A each (as long as there is sufficient sun to supply the DC input) The grid voltage could go up a bit, or down a bit, and it wouldn't matter. The inverter will still output 42A. The inverter does not have any voltage control. And since they are designed to always operate paralleled to a grid, they likely don't have any frequency control either. Is that correct?
Essentially, yes.
 

ggunn

PE (Electrical), NABCEP certified
I assume those off grid system inverters have looser tolerances?
No, the Sunny Boy is a standard grid tied inverter. Also, many modern generators are DC generators coupled with an inverter; there's no reason for the power from them to be "dirty".

Most places that have PV and a generator use the genny for power during grid outages and let the PV shut down. Given that grid outages are infrequent and of short duration it doesn't make sense to spend a lot of money on a system which would let the PV run during outages.
 

electrofelon

Senior Member
No, the Sunny Boy is a standard grid tied inverter. Also, many modern generators are DC generators coupled with an inverter; there's no reason for the power from them to be "dirty".

Most places that have PV and a generator use the genny for power during grid outages and let the PV shut down. Given that grid outages are infrequent and of short duration it doesn't make sense to spend a lot of money on a system which would let the PV run during outages.
Good point about inverter generators. I'm sure they have near perfect frequency. Iirc standard GTI frequency range is +/- .3 hz. Not sure I agree that "many" out there used to back up a house are inverter based however.
 

ggunn

PE (Electrical), NABCEP certified
Good point about inverter generators. I'm sure they have near perfect frequency. Iirc standard GTI frequency range is +/- .3 hz. Not sure I agree that "many" out there used to back up a house are inverter based however.
Well, what kicked off this discussion (300A on the supply side) is hardly a residential PV system. :D
 
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