PV - The magic of it all

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
Electricity is still and always will be a mystery to me. Sure I can wire pretty much anything but sometimes understanding the flow gets me.

Take a simple scenario--

Solar Panel---to---Inverter----to---main panel---to Meter. Nothing is fed from the main panel so the panels are just feeding back to the utilities. How does the utility know how much kw they are getting back from the solar panels? Does the meter turn backwards without a load? And if there are circuits in the panel how does electricity know to give power to the loads rather than back to the power company.

iT IS MAGIC TO ME.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Does the meter turn backwards without a load?
Yes.

And if there are circuits in the panel how does electricity know to give power to the loads rather than back to the power company.
The loads take whatever power they need. If the solar provides more, the excess goes to the grid; if less, the extra needed comes from the grid.

Similar in your car: The car needs what it needs to run; any leftover goes to charging the battery. Also same as using your cellphone while it's charging.
 
Does the meter turn backwards without a load?
Just to expand on Larry's answer a bit: I have heard there are some meters that do not spin backwards, as they are just not "expecting" to see power go the other way. But your typical older meter with the wheel yes that will spin backwards. Usually most POCO's will install a more complex meter though for distributed generation, which separately keeps track of reverse power flow. Then of course you can get into time of use rates in some areas so its even more complex.

Regarding power flowing to different loads, it just comes down to impedance and differences in potential. You you can control one of these, you can make the electrons do what you want.
 

jaggedben

Senior Member
Electricity is still and always will be a mystery to me. Sure I can wire pretty much anything but sometimes understanding the flow gets me.

Take a simple scenario--

Solar Panel---to---Inverter----to---main panel---to Meter. Nothing is fed from the main panel so the panels are just feeding back to the utilities. How does the utility know how much kw they are getting back from the solar panels? Does the meter turn backwards without a load? And if there are circuits in the panel how does electricity know to give power to the loads rather than back to the power company.

iT IS MAGIC TO ME.
They are two sources in parallel. It's a grander version of putting two batteries or generators in parallel to feed loads. Imagine a DC bus with positive and negative conductors. Two batteries are tapped off the busses. Also the loads are tapped off the busses. The arrangement doesn't really matter from the loads point of view. They do not know or care which battery the power comes from or which direction it flows on the bus to get to them. In a building with a service and PV, the 'bus' would be the service conductors and whatever feeders are used in series along the way to the inverter. The loads are branch circuits and whatever feeders are in parallel.

The real magic is that digital signal processors these days can allow inverters to match the voltage and frequency of the AC waveform from the utility in real time, and even tell if the utility stops being there while the inverter is outputting power.

A key point is that the inverter does not try to set the voltage in the grid-tied scenario. It simply matches the utility voltage and then (to my understanding) raises it just enough to output all the power it can from the solar panels. It is a current source. As long as the grid is gigantic compared to the solar, it doesn't matter if the inverter outputs its max.
 

Carultch

Senior Member
Location
Massachusetts
Just to expand on Larry's answer a bit: I have heard there are some meters that do not spin backwards, as they are just not "expecting" to see power go the other way. But your typical older meter with the wheel yes that will spin backwards. Usually most POCO's will install a more complex meter though for distributed generation, which separately keeps track of reverse power flow. Then of course you can get into time of use rates in some areas so its even more complex.

Regarding power flowing to different loads, it just comes down to impedance and differences in potential. You you can control one of these, you can make the electrons do what you want.
You can make the NET FLOW of electrons (or rather the power associated with them) do what you want, but you can't really control where any given electron goes. The meters keep track of the net flow of the power/energy, and in general, sources will most likely feed the nearest load. The occasional electron might actually feed a load 20 miles away, instead of your load or your nearest neighbor's load, but that won't be relevant in the big picture. In the big picture, it is the net flow of power/energy that matters.

Electrons don't actually make it from source to load, due to it being AC. It is the "pressure wave" of inter-electron repulsion that flows among electrons in the conductor, that carries the energy.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
They are two sources in parallel. It's a grander version of putting two batteries or generators in parallel to feed loads. Imagine a DC bus with positive and negative conductors. Two batteries are tapped off the busses. Also the loads are tapped off the busses. The arrangement doesn't really matter from the loads point of view. They do not know or care which battery the power comes from or which direction it flows on the bus to get to them. In a building with a service and PV, the 'bus' would be the service conductors and whatever feeders are used in series along the way to the inverter. The loads are branch circuits and whatever feeders are in parallel.
It's a little more complicated in that it's a parallel circuit of sources and loads, but the sources are a current source (the PV inverter) in parallel with a voltage source (the grid). This is what gives rise to the notion that your loads "preferentially" receive your PV output over being serviced by the grid. The PV pushes out whatever it can irrespective of the loading and the grid either makes up the difference or takes the surplus.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
I knew it was magic....:D

So if I have a loop circuit, ie, a circuit that comes back to the panel and ties to another breaker of the same phase, then why doesn't that affect the meter. I realize that the circuit is fed from both breakers but why is it not back feeding the panel also?
 

Carultch

Senior Member
Location
Massachusetts
I knew it was magic....:D

So if I have a loop circuit, ie, a circuit that comes back to the panel and ties to another breaker of the same phase, then why doesn't that affect the meter. I realize that the circuit is fed from both breakers but why is it not back feeding the panel also?
Because on net, the current through the main supply due to that configuration would add up to zero. As much current as would leave breaker 1, would return back through breaker 2. What you'd really be doing, is assisting the busbar by creating another parallel path for current to get to bypass the busbar and get to the opposite end.
 

pv_n00b

Senior Member
Location
CA, USA
Electrons don't actually make it from source to load, due to it being AC. It is the "pressure wave" of inter-electron repulsion that flows among electrons in the conductor, that carries the energy.
This one gets a lot of people. "If the electrons just move back and forth how does that power anything?" I describe it this way, say you have a water pump with a handle on it that moves back and forth to pump the water. If you connect a long stick to that handle and then go to the end of the stick and move it back and forth you are pumping water. The stick I am touching never leaves my hand yet the lever on the pump moves.
Now think of the stick as a string of electrons in a wire and you move the electrons back and forth inside the wire to move a lever at the end that pumps water. The electrons on your end just move back and forth but the pump handle moves and pumps water.
 

PWDickerson

Senior Member
Location
Clinton, WA
The OP's original question is an excellent one. How the heck does the electricity know which way to flow???

The water analogy is a great way to think about how it all works. Consider the following:

You have a large body of water, say a lake. (think zero volts, or ground)
You have a swimming pool next to the lake.
It is one of many similar pools, and all the pools are connected by pipes so their water levels are all the same. (think grid voltage)
The water level in the pools is maintained by a large pump. (think utility power plant)
The pump sucks water from the lake, and it enters each pool at the elevation of the water level in the pool.
There are several pipes just under the water level of each pool that draw water off to water wheels. (think loads)
The water that passes over the water wheels is returned to the lake via a pipe that lies on the ground. (think neutral wire)
The pump is run at a speed that fills the pools at the same rate as the water drawn off the pools by the water wheels.
In this way, the water level in the pools always remains the same.
Another small pump is installed that pumps water into one of the pools at a height just above the water level. This is an inverter.
If the water wheels are drawing less water from that pool than is supplied by the small pump, the water level in the pool will rise.
Which will result in water flowing backward through the pipe that normally fills the pool.
The water that flows backward will fill the adjacent pools.

How does the water know which way to flow? Simple. It doesn't know squat. It just runs downhill. Electricity works the same way.

Hope that helps.
 
Backfeed

Backfeed

We use ct’s Connected to a modbus meter. Ct’s Connected either line or load side of main breaker. The modbus meter connects via rs485 connection to the pv inverter. We can then program the modbus meter to export a max kw, kWh, infinit export, or non export, or a combination of either. The modbus meter tells the pv inverter to produce energy, curtail energy, or shut down.
 
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