Pwer from PV systems go to the grid?

[jjhoward]

Not sure where to post this question.
But seems like there are some very in depth discussions in these parts!

My question:
When a residential PV system is generating more power than the local load (the load at the house); does the excess power acutally go onto the grid?

This assumes there is a "net metering" device in place. Here is NJ a net metering device is what one needs to get credit for any excess power that the PV system generates.

I do not understand how the current source at the house and the current source at the utility pole (the transformer) can add.

Thank you.

 

[iwire]

Yes it actually pushes current back out onto the grid in the same way multiple generators are connected together to power the grid.

The inverter takes the incoming DC from the modules converts that to an AC wave matching the grids AC, once they match up the inverter kicks on. I believe the inverter controls the amount of current going into the grid by adjusting its output voltage slightly.

 

[dbuckley]

When a residential PV system is generating more power than the local load (the load at the house); does the excess power acutally go onto the grid?

It may do.

On a full old-school PV system with batteries and grid inter-tie, on the inverter was a button marked 'Sell', and if you depressed it, then excess power was fed back into the grid. If you didn't press it, then it didn't backfeed to the grid. What usually happened was that excess power was turned into heat to stop overcooking the batteries, a/k/a shunt regulation.

Note that this has nothing to do with meetering arrangements

 

[jjhoward]

If there is a fault on the lines somewhere, will the breakers trip at the house that has the net metering device in place?

 

[iwire]

If there is a fault on the lines somewhere, will the breakers trip at the house that has the net metering device in place?

The net meter is nothing special, it is just a normal looking meter installed in a normal looking socket.

All the overcurrent devices operate just as they normally would if needed.

If the utility supply is cut off the inverter will shut down so it does not try to backfeed a dead grid and possibly hurt a lineman.

 

[jjhoward]

Hi Bob;

But if the PV system is sourcing current for loads on the supply side of the meter; a fault on the supply would be seen by the PV system. Yes, the inverter will shut down when the A/C is gone, but a fault may be seen for a few cycles before the A/C goes flat lined.

My confusion is also about transformers:
Can the secondary of transformers be tied together to increase their current capacity?

 

[iwire]

But if the PV system is sourcing current for loads on the supply side of the meter; a fault on the supply would be seen by the PV system.

I am not following you, everyting in all directions is protected by overcurrent devices. If well designed lightning aresters will also be installed on the AC and DC sides of the inverters.

My confusion is also about transformers:
Can the secondary of transformers be tied together to increase their current capacity?

Yes you can parallel a bank of transformers together for increased capacity but the transformers have to be as identical as possible, any slight differences in impedance will cause the current to divide unevenly.

 

[jjhoward]

Sure, OCPD are in place.

If my PV system is supplying a max of 40 amps at 240V while my house is presenting no load, my PV generated power is sent onto the secondary lines that feed my house.

If there is a fault on those secondary lines, will the OCPDs on the A/C output side my PV system open?

 

[iwire]

If my PV system is supplying a max of 40 amps at 240V while my house is presenting no load, my PV generated power is sent onto the secondary lines that feed my house.

Yes.

If there is a fault on those secondary lines, will the OCPDs on the A/C output side my PV system open?

Yes it might but I bet the electronics in the inverter shut it down faster than the OCPD opens.

Keep in mind the inverter can shut down immediately without the delay that opening breaker contacts creates.

 

[jjhoward]

Thanks for the information Bob.

PV systems are even better than I thought!

The excess power is actually used AND the customer can get $s for the SRECs.

Good deal, everyone should get a PV system.

 

[djohns6]

FYI , you usually get a credit from the power company , not a check .

 

[ty]

FYI , you usually get a credit from the power company , not a check .

That depends on your state, and the utility company involved.

 

[ty]

If my PV system is supplying a max of 40 amps at 240V

What size loadcenter?

 

[kingpb]

This is an interesting question, because on larger systems, there has to be an interconnection agreement with the utility, and the utility actually has control over the breaker where the generation is coming from. For residential PV systems, I'm not sure what the mechanics are to prevent backfeed when the utility doesn't want it.

Definitely going to do a little research on this.

 

[iwire]

This is an interesting question, because on larger systems, there has to be an interconnection agreement with the utility, and the utility actually has control over the breaker where the generation is coming from. For residential PV systems, I'm not sure what the mechanics are to prevent backfeed when the utility doesn't want it.

Definitely going to do a little research on this.

Every interconnection document I have seen requires us to use an inverter that has anti-islanding features.

There is a UL listing they have to meet, sorry I cannot remember the number.

 

[kingpb]

So does that mean that the system will not be able to power the load independent from grid connection?

It would be nice to operate in island mode, perhaps using a shunt trip breaker, that will not reset on a deadbus.

 

[iwire]

So does that mean that the system will not be able to power the load independent from grid connection?

That depends on the Inverter, there are some Inverters that can be grid tied and independent if the utility fails.

The only inverters I know that can do both require a battery stack as well.

I have only installed the type of inverter that will not function without the utility online.

 

[djohns6]

That depends on your state, and the utility company involved.

I said usually . There are exceptions to everything .

 

[calisparky6]

not to hijack the thread but...can the grid tie PV system be "back fed" through a sub panel, or do you need to put it at the main service panel? i don't see why it would matter? but how does this back feed work?

 

[iwire]

not to hijack the thread but...can the grid tie PV system be "back fed" through a sub panel, or do you need to put it at the main service panel? i don't see why it would matter? but how does this back feed work?

Yes you can connect at a subpanel.