Power glitch when exporting to the grid?

[charlesj]

...

Perhaps reading up on current sources will help you understand this concept.

Yes, it is, especially:
"The voltage across an ideal current source is completely determined by the circuit it is connected to. When connected to a short circuit, there is zero voltage and thus zero power delivered. When connected to a load resistance, the voltage across the source approaches infinity as the load resistance approaches infinity (an open circuit)."

So, something in between those two ends there seems to be a change in voltage?

 

[power.saver]

So those statements are correct, but the grid is neither a short circuit nor an open circuit. It is a voltage source, which defines the voltage that your inverters operate at. And because that voltage is non-zero, actual real power is delivered. But because the grid is so large, and your inverters are so small, the infinitesimal voltage increase in the grid is imperceptible. Like adding water to the ocean with a garden hose, it's not going to measurably raise the sea level.

However, large scale solar farms with hundreds of MW of power, as well as millions of homes with small solar systems combined, do make a significant contribution to the grid, and have to be balanced with traditional generating stations so as not to raise the grid voltage. This is done with curtailment of either the large scale solar farms or the generating stations. This same process happens on a micro scale with your PWs when you are running off-grid. When the batteries get full, the solar inverters must be shut down, because the current they are producing has no where to go, and the voltage would quickly rise to a dangerous level.

 

[charlesj]

.. Like adding water to the ocean with a garden hose, it's not going to measurably raise the sea level.

..l.

I like your water and hose analogy.
But, I have been thinking about this analogy.
If you use a hose, it has pressure behind it to get it to the ocean, even if you are at the beach.
If it is a river flowing to the ocean, it too has kinetic energy behind it to get it to the ocean.
If that river water happens to be at sea level with no kinetic energy behind it, no elevation to push it into the ocean, it will just sit there going nowhere. And it will certainly not flow up in elevation, from a negative energy state.

As to raising the ocean, it raises by the quantity of water spread out the area of that ocean, however small it is, same as the grid. If grid is full, unlike an open ocean that can create more acreage, the grid will not accept it. Like filling up a closed system, unless there are holes in it to leak to customers sprinkling their yard excessively but then it is no longer closed.

 

[power.saver]

I like your water and hose analogy.
But, I have been thinking about this analogy.
If you use a hose, it has pressure behind it to get it to the ocean, even if you are at the beach.
If it is a river flowing to the ocean, it too has kinetic energy behind it to get it to the ocean.
If that river water happens to be at sea level with no kinetic energy behind it, no elevation to push it into the ocean, it will just sit there going nowhere. And it will certainly not flow up in elevation, from a negative energy state.

As to raising the ocean, it raises by the quantity of water spread out the area of that ocean, however small it is, same as the grid. If grid is full, unlike an open ocean that can create more acreage, the grid will not accept it. Like filling up a closed system, unless there are holes in it to leak to customers sprinkling their yard excessively but then it is no longer closed.

Think of a current source like a pump. It doesn't matter if the water source is above or below sea level... the pump will move it through the hose. That is analogous to what your inverter is doing: pushing current into the grid.

The grid will not overflow because it cannot store energy. But it must be carefully balanced so that all sources of power are canceled out by the loads on the grid. A big responsibility of grid operators is to ensure they are in balance. If they don't, the voltage won't be very stable.

 

[charlesj]

Think of a current source like a pump. It doesn't matter if the water source is above or below sea level... the pump will move it through the hose. That is analogous to what your inverter is doing: pushing current into the grid.

The grid will not overflow because it cannot store energy. But it must be carefully balanced so that all sources of power are canceled out by the loads on the grid. A big responsibility of grid operators is to ensure they are in balance. If they don't, the voltage won't be very stable.

Yes, indeed, that pump will but that pump is voltage in this case and the water is the current, not the other way around. And voltage pushes that current.
Yes, the grid is not a storage just a mechanism to transfer power to all the hungry loads.

 

[power.saver]

Yes, indeed, that pump will but that pump is voltage in this case and the water is the current, not the other way around.

No! You still don't get it. The pump is a current source, not a voltage source.

And voltage pushes that current.

A voltage source induces current flow by a difference in potential. A current source induces a voltage due to current flow.

If you have two tanks of water at the same level and a pipe between them, no water will flow because they are at the same potential.

If one tank is higher than the other, water will flow in the pipe from the higher tank to the lower tank. This is analogous to a voltage source. But the amount of water that flows is proportional to the difference in heights. If the higher tank is only 1 inch above the lower tank, there will be very little flow because there is very little difference in potential.

For the two tanks at the same level, I can install a pump in the pipe and force water to move from one tank to the other, even though they are at the same level. This is a current source. And as time goes on, the receiving tank will fill, reaching a high potential than the source tank. This is exactly what your inverters are doing, pushing current from your solar panels to the grid. But the grid is so large, like the ocean in my previous explanation, the voltage will not measurably rise.

That is why your inverters are current sources and not voltage sources. To move all the power your solar is producing with a voltage source, your inverters would need to be at a much higher potential than the grid to cause enough current to flow. But this is not possible because they are connected to your house and cannot raise the voltage. Instead, by using a current source, they are able to move the power from your solar panels to the grid, without needing to raise the voltage.

 

[miimura]

Earlier in this thread, someone said that the voltage at the inverter was displayed higher than their utility meter's display. This is due to the resistance in the wires inside the house (between those two measurement points) and a possible difference in calibration accuracy. Once you get to the utility transformer, the impedance in the distribution network is WAY lower.

 

[charlesj]

Earlier in this thread, someone said that the voltage at the inverter was displayed higher than their utility meter's display. This is due to the resistance in the wires inside the house (between those two measurement points) and a possible difference in calibration accuracy. Once you get to the utility transformer, the impedance in the distribution network is WAY lower.

I posted that.
I just checked one panel and its individual inverter performance that I can do at Enphase. This is the voltage at the inverter itself.
AC volt in the morning was 242 with very low power output. As power out increased mid morning it was 246V and 247V midday.

Yes, I am aware of IR drops but the cable size, distance is short to the sub-panel with 10ga wire to handle that .73A So, what I see is a voltage change as current increases in the output of that inverter.

Then, to the grid or almost so is a 100Ampacity cable.

 

[cali8484]

Yes, indeed, that pump will but that pump is voltage in this case and the water is the current, not the other way around. And voltage pushes that current.
Yes, the grid is not a storage just a mechanism to transfer power to all the hungry loads.

Sticking with the pump and water analogy, you can think of voltage and current sources as different types of pump. A current source pump would change its water pressure (i.e. voltage) to a level higher than the pressure it senses at its output in order to maintain a constant output rate of water flow (i.e. current). On the other hand, a voltage source pump would change its rate of water flow (i.e. current) in order to maintain a constant water pressure (i.e. voltage). Generally, grid-tied inverters are like current sources and off-grid inverters are like voltage sources.

 

[charlesj]

... A current source pump would change its water pressure (i.e. voltage) to a level higher than the pressure it senses at its output in order to maintain a constant output rate of water flow (i.e. current). ...

Yes, that I see, voltage(water pressure) changes.
That is what is reflected in that inverter picture above I posted, 242V to 247V AC.

 

[charlesj]

Another inverter performance today with a few more parameters showing AC and DC behavior; should be readable. Voltage from 241V AC to 247V AC.

 

[mongo]

For the two tanks at the same level, I can install a pump in the pipe and force water to move from one tank to the other, even though they are at the same level. This is a current source.

Depends, is it a positive displacement pump or centrifugal? Postive displacement (current source) deals in fixed volumes of water (electrons) and can generate arbitrarily high outlet pressures (infinite tank height) to move it . A centrifugal pump (voltage source) generates a fixed pressure rise across the unit and can be dead headed (max tank fill height).

Either way, the current source vs voltage source is misleading because the inverter has a digitally controlled power stage which operates in terms of power transfer. Whether the controlling variable is the output voltage, current, or power, the system is controlling one value to change the energy transfered.

Going further down the control theory hole...
Ohm's Law tells us for every voltage there is a current and for every current there is a voltage. Thevenin and Norton equivalent circuits show this also.
As you mention, the grid voltage will not be moved by one inverter, nor is the wiring a significant resistence. So, since R is small and V is sonewhat constant, the better value to control is current. The inverter ideally has a power factor of 1, so it wants to act like a negative resistance which basically means a voltage controlled current source where instantaneous_current = k*
instantaneous_voltage and k is adjusted to produced the desired RMS power output.

 

[charlesj]

... RMS power output.

Except, in a sine wave, when you multiply Vrms and Irms, you get continuous average power
Peak V X .707 is Vrms, of course and same with current

1V peak X 1A peak = 1 Watt peak

.707V X .707I = .5 Watts power

 

[power.saver]

As you mention, the grid voltage will not be moved by one inverter, nor is the wiring a significant resistence. So, since R is small and V is sonewhat constant, the better value to control is current. The inverter ideally has a power factor of 1, so it wants to act like a negative resistance which basically means a voltage controlled current source where instantaneous_current = k*instantaneous_voltage and k is adjusted to produced the desired RMS power output.

Yes, thank you. I really didn't want to bring the concept of negative resistance into this discussion, but that is indeed correct.

 

[mongo]

Except, in a sine wave, when you multiply Vrms and Irms, you get continuous average power
Peak V X .707 is Vrms, of course and same with current

1V peak X 1A peak = 1 Watt peak

.707V X .707I = .5 Watts power

True, but I'm not seeing why you say 'except'.

Yes, thank you. I really didn't want to bring the concept of negative resistance into this discussion, but that is indeed correct.

Hey, if you can't geek out with peeps on a Tesla forum during a pandemic, who can you?

Pumps are at the forefront on my mind because I just build a solar watering system for our garden and had to do parts selection. Ended up with 5 chamber displacement pump that can do 3.5 GPM and 60 psi. Came with buffer tank and pressure switch so it is hand sprayer compatible.

 

[charlesj]

True, but I'm not seeing why you say 'except'.



Hey, if you can't geek out with peeps on a Tesla forum during a pandemic, who can you?
....

Well, you did call it RMS power which is not the right word. The power is average continuous power as it is not derived from the rms of the power curve but the RMS values(effective) of V and I. That RMS voltage or current is called effective volt/current and as you know is the rms of that sine wave.

I see this mischaracterization frequently in the audio world. I thought it was limited to that sector only.
Just a nit pic on my part.

 

[mongo]

Well, you did call it RMS power which is not the right word. The power is average continuous power as it is not derived from the rms of the power curve but the RMS values(effective) of V and I. That RMS voltage or current is called effective volt/current and as you know is the rms of that sine wave.

I see this mischaracterization frequently in the audio world. I thought it was limited to that sector only.
Just a nit pic on my part.

Gotcha, yeah I was trying to tie in the RMS relationship to the k calc, but like you said, that lives on the V or I side.