AC Current Flow Question

[crossman]

Cause the majority will not rule here. This is a math model discussion. Opinion really doesn't matter.

You are right and I respect your opinion on that.

 

[dbuckley]

Ok, worked example.

Assume a 220KV three phase line has a capacitance to ground of 8nF/KM. At 60Hz, the impedance of the 8nF capacitor so formed is about 331Kohms, so 331K/KM.

So, how much line do we need for a current of 60mA to flow? Well, ohms law tells us that at 127KV (the phase voltage with respect to ground of a 220KV circuit), to have 60mA flowing we need an impedance of about 2Mohms. So to have 331KOhms, we need a length of about 165 meters. Yes, only 180 yards of cable suspended on pylons connected to nothing, and we have 60mA of line charging current.

So now we have our 60mA, back to the problem: given that the meters look like meters and look like they are not capable of measuring with sub-milliamp resolution, we're probably going to have about 60mA flowing in both meters. Kirchoff would be annoyed if it were something else.

 

[ronaldrc]

I would think amp. meter #2 would be a little higher since where dealing with high voltage and some of the charge that is leaking from the wire surface is being loss in making a ion field before it reaches the upper amp. meter .I think there are to many varibles for any body to give a accurate answer.

 

[coulter]

...I respect your opinion on that.

cute:roll:

 

[coulter]

.... Kirchoff would be annoyed if it were something else.

yep - probably Norton as well

carl

 

[crossman]

cute:roll:

Err, thanks!

 

[steved]

How high is the voltage? Is it high enough to produce a corona discharge?

 

[charlie b]

. . . some of the charge that is leaking from the wire surface is being loss in making a ion field before it reaches the upper ammeter. I think there are to many variables for any body to give a accurate answer.

If the motion of one or more electrons within the wire creates enough of a magnetic field to ionize the air surrounding the wire, the charge itself does not leave any electron, and the electrons still move down the wire. There are no variables. Every electron that passes through ammeter #1 has no place on earth to go except through ammeter #2. The currents must be equal.

Keep in mind that I am basing my comments solely on the image shown in post #1 of this thread. I do not know what other sketches, experiments, or opinions were presented in "the other thread." If you look only at the sketch that started this thread, all the components are in series, and current passing through one item must necessarily pass through all others.

 

[charlie b]

How high is the voltage? Is it high enough to produce a corona discharge?

That would not change anything. The corona is formed within the air surrounding the wire, and no charge within the wire passes through the insulation into the surrounding air. All current still passes through every component in the series connection.

 

[ronaldrc]

Charle the variables I was speaking of where the resistance of the insulation of the wire and the atmosphere or air around it. But of course I know you could figure all this out.

 

[crossman]

Gentlemen, thank you for your replies and thoughts.

I also believe meter 2 will read close to 60 mA.

To the diagram above, I added the electrostatic field near the conductive body and earth. Now, the direction of the field is actually changing because this is an AC source. Of course I have not drawn all the field lines in. There are many more extending in many directions between other components. But I have drawn in the lines which I feel are most important to allow current to flow in the meters.

And I would say that without this field, that very little, if any, current could flow. If we had some type of material which could completely block the field, then no current (?or an extremely small amount from inductive coupling?) would flow at all.

The current flow in the circuit could be changed by changing the size of the conductive body, the current could be changed by changing the distance between the conductive body and earth, and, if we could, the current could also be changed by changing the size of the earth. My point is that the current flow would be controlled by the exact laws of physics which determine capacitance and capacitive current flow. But let me make it clear that any change in the circuit current flow would be seen at BOTH meters.

This is all very straight-forward and is supported by all Physics texts and electrical theory books I have ever studied.

Thanks again for the replies.

Edit: I want to be clear that the red lines are NOT actual electrons flowing. There is no arc in this case between the body and earth. This is an electrostatic field.

 

[zog]

But some of the current in the field could return to the transformer directly instead of through ground, in fact based on your drawing I bet the R of the air to the transformer is close to or less than the resistance of the air to ground+ground (Entry point then back to the transformer).

Either way. as I stated earlier, this is not a simple series circuit, there are several different paths, therefore the ammeter #2 would read less than #1 just as it did when I actually performed this experiment.

 

[crossman]

But some of the current in the field could return to the transformer directly instead of through ground

There is no current flow in the electrostatic field. At a given point in the sine wave, electrons will collect on the bottom of the conductive body. These electrons will repel electrons in the earth via the electrostatic field. These repeled electrons in the earth will be attracted to the positive terminal of the secondary which is connected to earth.

You may ask why the electrons would collect mostly on the bottom of the conductive body. The reason is that at the very same time that electrons are flowing onto the body, the source is pulling electrons out of the earth underneath the conductive body. The positive and negative charging of the body and the earth is occurring at the same time or extremely close to it, as the effects of current flow occur at near the speed of light. These effects go hand-in-hand anytime you have a voltage source like a xfmr secondary.

So, considering both the positive and negative charging of the body and earth, we have a complete cirucit. Obviously, electrons do not flow completely through the circuit because there is a gap between the conductive body and the earth. But, every physics text and electrical theory book I have studied says that this is still a completed circuit through the capacitance.

in fact based on your drawing I bet the R of the air to the transformer is close to or less than the resistance of the air to ground+ground (Entry point then back to the transformer).

Actual electron flow through the air is going to have a negligible affect on the meters. Because of the symetrical placement of components in my idealized experiment, and because the meters are placed as close to the source terminals as possible, any current flow through the air is essentially equalized through each meter. For example, if an electron were to flow from point A to the opposite side of the source, it is likely that an electron will also flow from point B to the other side of the source. This same argument will hold for any other stray currents or capacitances or inductances which would cause the meters to read differently.

Either way. as I stated earlier, this is not a simple series circuit, there are several different paths, therefore the ammeter #2 would read less than #1 just as it did when I actually performed this experiment.

This "thought" experiment could be designed in a manner that all extraneous currents precisely canceled and the meters would read EXACTLY the same amount.

In the real world, I readily admit there may be a difference of current in the meters. But this is ABSOLUTELY not due to some speculated theory about current flow to the conductive body with no other path. It is due to shortcomings in the design of your real-world experiment. And I specualte that the real-world high-pot/lab/building/wiring arrangement that you have is very much NOT an ideal situation and I certainly would expect some currents bypassing the meters. Indeed, you actually don't even have two meters in your experiment. if you could place two meters, one at each terminal of the high-voltage secondary , and then go from there, you would find the meter readings to be amazingly close.

Again, all of this is entirely explained by the normal laws of electricity and current flow in capacitors, resistors, and inductors. Other explanations lead to absurdities.

 

[zog]

You are right, my experiment did not have perfect conditions, they rarely do, but at least I tried something instead of reading books that think electrons actually "flow" near the speed of light.

Have you ever actually observed this arcing between 2 different potentials with no path for current? Ever racked in/out a 30kV breaker or done any transmission line/substation work?

 

[coulter]

...Have you ever actually observed this arcing between 2 different potentials with no path for current? ...

Nope. Anytime I have observed "arcing" there was definitely a path - generally ionized air.

... Ever racked in/out a 30kV breaker or done any transmission line/substation work?

yes (and ...?)

carl

 

[crossman]

If you were to reserch my posts on Mike Holt's forum, you would find that I am one of the few people who actually DOES perform experiments, and I even post photos of the experiments and plenty of diagrams and attempt to give lucid explanations. Go ahead, check the forum.

For you to deride me on that subject without even knowing who I am is entirely without basis and is poor form.

No, I don't do high voltage. But does racking in a 30kV breaker make one an expert in electrical theory? Please do not deride me and consider yourself to somehow be "more knowledgable" than me.

Also, you deride me because I mentioned the speed of light. Go back and re-read what I wrote. The AFFECTS of electricity occur at or near the speed of light. This is entirely different than saying that the actual current is flowing at the speed of light.

I applaud you for your efforts in your experiments.

However, I am highly dissappointed in your latest post.

 

[zog]

First off, I didnt mean to deride you, i was just asking a question. I quit smoking this week and have been "in poor form" all week.

Second, when you rack in/out a MV breaker you hear a hissing noise, in the rare case you can observe the source you see an arc occur from each phase bus to the line side of the (Open) breaker, ther is no "path" for current and is an occurence that I would assume would be more familiar with many people in this discussion.

Third, I never claimed to be more knowledgeable than you or anyone else, those claims (Or insuations) have been made by enough people in this thread.

 

[nakulak]

disclaimer: I am no expert on this subject.


however, the hissing sound you heard and the arcing are commonly known as ionization. When the air around the medium or high voltage electricity is ionized due to the intense field, it becomes what is commonly referred to as plasma. plasma being the ionization state of the material - not gas, not solid, not liquid - but an ionized state which conducts electricity. the arcing is conservation of potential energy, and may occur in one or both directions. if there was no difference in potential, and no path available, then no arcing would occur. (imsho)

 

[weressl]

Gentlemen, thank you for your replies and thoughts.

I also believe meter 2 will read close to 60 mA.

To the diagram above, I added the electrostatic field near the conductive body and earth. Now, the direction of the field is actually changing because this is an AC source. Of course I have not drawn all the field lines in. There are many more extending in many directions between other components. But I have drawn in the lines which I feel are most important to allow current to flow in the meters.

And I would say that without this field, that very little, if any, current could flow. If we had some type of material which could completely block the field, then no current (?or an extremely small amount from inductive coupling?) would flow at all.

The current flow in the circuit could be changed by changing the size of the conductive body, the current could be changed by changing the distance between the conductive body and earth, and, if we could, the current could also be changed by changing the size of the earth. My point is that the current flow would be controlled by the exact laws of physics which determine capacitance and capacitive current flow. But let me make it clear that any change in the circuit current flow would be seen at BOTH meters.

This is all very straight-forward and is supported by all Physics texts and electrical theory books I have ever studied.

Thanks again for the replies.

Edit: I want to be clear that the red lines are NOT actual electrons flowing. There is no arc in this case between the body and earth. This is an electrostatic field.

Call it a capacitor. Electrostatic only applies to DC and I don't think that is a DC transformer

 

[ronaldrc]

I would think amp. meter #2 would be a little higher since where dealing with high voltage and some of the charge that is leaking from the wire surface is being loss in making a ion field before it reaches the upper amp. meter .I think there are to many varibles for any body to give a accurate answer.

I changed my mind, I was thinking along the lines of a DC current if that makes any sense.But its AC.

I really don't know,"its above my head"