Photovoltaic Grid Backfeed

[caryj]

I am working on a project where i am installing a group of photovoltaic cells that feed DC to an inverter. The AC from the inverter goes through a meter and then backfeeds a branch panel through a breaker. The way I understand it is that if there is a load on the braches that are served by this panel that it is less than or equal to the PV systems contribution, the service that feeds the panel (not the PV system) will not see any of this load and the service meter will not see those kW/h's. Am I thinking about this correctly? How is this achieved with no controls?

-Confused

 

[iwire]

Assuming you are using the correct inverter the invert handles it all.

There is really no trick to how it works.

If the inverter is putting out 100 amps and the building needs 75 amps to run the remaining 25 amps heads out to the utility.

If the building load increases to 125 amps the utility will supply the balance that is above the inverters output.

 

[caryj]

So you have two sources utility and pv system. Why would the pv system supply and the utility supliment rather than the other way around.

On a side note. Suppose the main breaker is 200A and the backfed breaker is 30A, wouldnt there be potential for 230A to feed that bussing? would that bussing have to be rated for 230A?

 

[iwire]

So you have two sources utility and pv system. Why would the pv system supply and the utility supplement rather than the other way around.

The pv system is on the customer side of the meter, think of it as just reducing the current being drawn through the meter.

On a side note. Suppose the main breaker is 200A and the backfed breaker is 30A, wouldnt there be potential for 230A to feed that bussing? would that busing have to be rated for 230A?

Yes that is remotely possible under the right conditions and you should account for that.

 

[kingpb]

It has to do with the elements of Power System Analysis and load flow. Modeling the system, even though small would show you what/how it works.

 

[broadgage]

On a side note. Suppose the main breaker is 200A and the backfed breaker is 30A, wouldnt there be potential for 230A to feed that bussing? would that bussing have to be rated for 230A?

IMHO it depends on the physical location of the backfeed breaker in the panel.
If the backfeed breaker is located next to the main, then the total current that could flow on part of the bus would indeed be 230 amps, and it could be argued that this would be a violation if the equipment was only listed for 200 amps. (Though it could also be argued, that the installed load in the building should have been less than 200 amps, and that the load has not been increased by the presence of the backfeed breaker)

If however the backfeed breaker was located at or near the end of the bus remote from the main, then it cant increase the current on any part of the bus.

As backfeeding becomes more popular,I suspect that loadcenters may soon have an instruction that backfeed breakers are only to be installed in certain spaces (the ones at the other end from the main)

 

[iwire]

See 690.64(B)(2) 2008, it directly relates to the issue of overloaded bus bars and the placement of the back fed breaker

 

[jaraujo1]

back feeding to the utility service

back feeding to the utility service

I have some elementary questions in backfeeding the uitility that might be appropiate for this issue.

You normally feed a static load from the utility, in the case of backfeeding the utility you are feeding an active source which much higher current capability.
How can you make the current flow up from the alternative source to the utility?
I am thinking you need some kind of interface such as a transformer IGT diode to feed the utility at the point where the sine wave voltage is at the zero degree point?
Is my line of thinking wrong? any feedback will be appreciated.

 

[weressl]

So you have two sources utility and pv system. Why would the pv system supply and the utility supliment rather than the other way around.

The PV system controls take care of that it will supply as much as it can. It tries to run with 'open throttle' all the times and that is one of the reasons why it is designed to shut down when disconnected from the Utility.

On a side note. Suppose the main breaker is 200A and the backfed breaker is 30A, wouldnt there be potential for 230A to feed that bussing? would that bussing have to be rated for 230A?

Absolutely.