Power flow, grid tie system -Fundamental Question

[Designer69]

If you have a grid tie PV system that feeds into a branch breaker and a grid system that feeds into the main breaker..

what causes the house loads to take most of the power from the PV system and not the Grid?

Lower resistance since the PV system is closer?

 

[ggunn]

If you have a grid tie PV system that feeds into a branch breaker and a grid system that feeds into the main breaker..

what causes the house loads to take most of the power from the PV system and not the Grid?

Lower resistance since the PV system is closer?

A grid tied system is a virtual current source, meaning it delivers what it can irrespective of loads. It has to go somewhere. If you have active loads, that's where the power goes; if not, it goes to the grid.

 

[jaggedben]

If you have a grid tie PV system that feeds into a branch breaker and a grid system that feeds into the main breaker..

what causes the house loads to take most of the power from the PV system and not the Grid?

Lower resistance since the PV system is closer?

The house loads only take 'most' of the power if they draw it.

In some sense you are correct that if the house loads draw more power it is because they have lower resistance. (The more loads turned on, the lower their collective resistance. ) But I'm not sure this is the best way into understanding it.

The house only has one route to the utility. Power can only be going in or coming out. Since the PV is connected in parallel with the utility, and the PV outputs the max available, whatever isn't drawn by the loads exits the house to the grid. When the PV is less than the loads, whatever isn't powered by the PV is powered by the utility.

Along with understanding the difference between a current source and a voltage source,y ou might google Kirchoff's law as well.

 

[pv_n00b]

No one knows where the electron goes.

 

[ggunn]

Here's an illustrative example:

Say you have a grid tied PV system that is producing 10kW of power. If your household loads are consuming 12kW, then they will take the entire 10kW your PV system is producing plus another 2kW from the grid. If your loads are consuming only 8kW, then they won't take anything from the grid and the extra 2kW your PV is producing will be exported to the grid.

The salient point is that the 10kW production your PV system is producing is not related to what your loads are consuming.

 

[LarryFine]

Another example could be your car or truck's electrical system. At different moments, the engine and other systems are using varying amounts of power, and the alternator is generating various amounts of power.

That power is provided primarily by the charging system; if more power is being generated than consumed, the remainder charges the battery; if less is being generated than consumed, the battery provides the rest.


"But, how do it know?!" :?

 

[five.five-six]

No one knows where the electron goes.

but we know that it does come back at the speed of light.

 

[electrofelon]

but we know that it does come back at the speed of light.

The drift velocity of an electron is actually quite low, the overall effect of electricity is nearly at the speed of light however

 

[Designer69]

Here's an illustrative example:

Say you have a grid tied PV system that is producing 10kW of power. If your household loads are consuming 12kW, then they will take the entire 10kW your PV system is producing plus another 2kW from the grid. If your loads are consuming only 8kW, then they won't take anything from the grid and the extra 2kW your PV is producing will be exported to the grid.

The salient point is that the 10kW production your PV system is producing is not related to what your loads are consuming.

I was wondering why the house loads take the 10KW from the PV and not just skip it entirely and take it from the Grid?


Bc the PV is closer?

 

[ggunn]

I was wondering why the house loads take the 10KW from the PV and not just skip it entirely and take it from the Grid?


Bc the PV is closer?

No, it's because the PV is producing 10kW that has to go somewhere. If not to the house loads, then where? It cannot flow out to the grid at the same time your house is drawing current from the grid; current cannot flow two directions at once in the same wire.

Most power sources we deal with (batteries, household outlets, etc.) are voltage sources, i.e., the voltage from them stays virtually the same and the current drawn from them depends on the resistance of the load. Ohm's Law with V held constant.

Grid tied PV is a virtual constant current source (constant as long as the sunlight striking the modules is constant), i.e. Ohm's Law with I held constant instead of V. The voltage is clamped by the grid, so it is actually a constant power source. That power has to go somewhere. Where it goes depends on the loads but how much the PV system delivers does not.

It's like filling a bucket with a hole in it from a hose. If the flow through the hole is less than the flow from the hose, the bucket will overflow (exporting power). If the flow through the hole is more than the flow from the hose, the water level in the bucket will fall and you'll have to get water from somewhere else to keep your bucket full full (importing power). What doesn't change is the flow from the hose.

Flow = power, hose = PV system, bucket = your house, hole = your household loads, "somewhere else" = the grid.

 

[mivey]

The drift velocity of an electron is actually quite low, the overall effect of electricity is nearly at the speed of light however

Yes. The electrons that were in your service wire when it was built are almost all still in the wire. They mostly just wiggle back and forth.

Think of the generation supply as your body/hands, the wire/electricity as a rope saw, and the load as the limb. Power flows from your body to the limb but the saw pretty much stays in the same place with a little bit of cyclic movement.

 

[LarryFine]

I equate electron movement in wire to a garden hose filled with marbles. Push a new marble in one end and one instantly pops out of the other end. Not the same marble, but the effect is instantaneous.

 

[mivey]

I equate electron movement in wire to a garden hose filled with marbles. Push a new marble in one end and one instantly pops out of the other end. Not the same marble, but the effect is instantaneous.

For DC that's true, for AC you have people on both ends alternating marble duty.

To be pedantic, it is near the speed of light, not instantaneous. True for marbles or electrons.

 

[LarryFine]

True for marbles or electrons.

"I've lost an electron!"

"Are you sure?"

"I'm positive!"

 

[jaggedben]

I was wondering why the house loads take the 10KW from the PV and not just skip it entirely and take it from the Grid?

Bc the PV is closer?

We should probably answer the 'closer' part with a 'yes'. Wherever the solar output connects in parallel with another circuit node, it will feed that circuit first, and then the next, and then the next, and so on. At some point there will be a node where there isn't enough current to supply the load, and the solar current joins with current from the utility. If that node is on customer side of meter, then the customer is still importing. If it's on the utility side, customer is exporting.

 

[electrofelon]

No, it's because the PV is producing 10kW that has to go somewhere. If not to the house loads, then where? It cannot flow out to the grid at the same time your house is drawing current from the grid; current cannot flow two directions at once in the same wire.

I was going to use a similar argument. In math we often prove things indirectly, by contradiction. You assume the opposite of what you are trying to prove is true, and find a contradiction somewhere. Its maybe a little unsatisfying, but you can do a similar thing with this question and get to power flowing in two directions at the same time, which cant happen so its just the way it is: the PV goes to the house loads "first".

 

[ggunn]

I was going to use a similar argument. In math we often prove things indirectly, by contradiction. You assume the opposite of what you are trying to prove is true, and find a contradiction somewhere. Its maybe a little unsatisfying, but you can do a similar thing with this question and get to power flowing in two directions at the same time, which cant happen so its just the way it is: the PV goes to the house loads "first".

And even if power could flow both ways at once the net result would be the same.

 

[electrofelon]

And even if power could flow both ways at once the net result would be the same.

Right. So maybe it can flow both directions at the same time and we'll never know it

 

[PaulMmn]

but we know that it does come back at the speed of light.

But does the same electron come back? We speak of the Electron Flow, which implies that 'my' electron moves along down-stream, but another, identical, electron comes along from up-stream to take its place. Like stuffing ping-pong balls into a 2" pipe-- assuming the pipe is full, you stuff one electron in at one end, and one pops out at the other end.

Even with AC-- depending on the load, 'my' electron may end up several atoms one way or the other from where it starts-- depending on imbalances in the positive/negative AC current.

 

[Designer69]

If I put a HUGE resistor bank in front of the PV supply feeder then the house will take most of the power from the Grid and very little from the PV.

So clearly it's because of varying impedance from the 2 supplies. (much less impedance from the PV)


That's what I'm thinking