Announcement

Collapse
No announcement yet.

More fallacious grounding

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    #76
    Originally posted by mivey View Post
    Yes. As you move out from the ground point on a uniground system the voltage during a fault approaches line-line voltage. The fault current drops because of the added impedance.
    In full agreement 100%


    Originally posted by mivey View Post
    Theory does not say that. Theory says, and correctly models, that the Earth path has impedance. We model the soil resistivity and can indeed model the current in the Earth path. We do this using Carson's equations.

    If we could run current in the whole of the Earth, we could perhaps parallel enough dirt to negate soil resistivity but it does not work that way. It tends to be a more localized phenomenon as far as Earth scale is concerned.

    But why the impedance? The earth is so massive that the resistance should be near zero.

    Comment


      #77
      Originally posted by mbrooke View Post
      But why the impedance? The earth is so massive that the resistance should be near zero.
      Dirt is matter and has electrical properties. Our currents travel in a limited amount of the earth's total matter.
      BB+/BB=?

      Comment


        #78
        mbrooke,

        Do you think if we had a copper wire as big as the earth we would get full benefit of all of that paralleled copper mass? We don't at a smaller scale do we?

        The fact is the currents travel in a peculiar manner in the wire and in the earth.
        BB+/BB=?

        Comment


          #79
          Originally posted by mivey View Post
          If you parallel enough dirt you can drive the net resistance to zero but the dirt has resistivity and the net impedance we see is not zero.
          The net resistance is indeed not zero, but most if not all of that resistance is the term associated with the earth electrode resistance rather than the distance to the uniground point. If the two electrodes are within each others' sphere of influence the overall resistance will be more complicated. That can lead to the appearance of the net resistance depending on the earth path length.


          Sent from my XT1585 using Tapatalk

          Comment


            #80
            Originally posted by mivey View Post
            mbrooke,

            Do you think if we had a copper wire as big as the earth we would get full benefit of all of that paralleled copper mass? We don't at a smaller scale do we?

            The fact is the currents travel in a peculiar manner in the wire and in the earth.
            Probably not, but it would be low.


            Is it exceptionally high reactance that does it?

            Comment


              #81
              Originally posted by mivey View Post
              Dirt is matter and has electrical properties. Our currents travel in a limited amount of the earth's total matter.
              Why though? Does it not take all paths? I've heard people measuring 50 cycle power here in 60 over seas in earth driven electrodes.

              Comment


                #82
                Originally posted by mivey View Post
                #1:
                At the sub the N conductor is at the neutral point. A A-N fault drags A to the neutral point and the B-N and C-N arrestors see P/sqrt(3) volts (line-neutral).

                Far away the N conductor has a large impedance that allows it to float from the neutral point. An A-N fault drags N to A and the arrestors see B-A and C-A voltage (line-line).

                #2:
                The earth path has impedance.
                True.

                Though much less so for an MGN, right?

                Comment


                  #83
                  I'll find you a better vid, but notice the wire from the 4th MV bushing going down into conduit which disappears into the soil:


                  https://youtu.be/l53NrBvlorQ?t=180
                  Last edited by mbrooke; 06-16-19, 07:07 PM.

                  Comment


                    #84
                    Originally posted by romex jockey View Post
                    so....i just gotta ask...am I looking at one huge poco MBJ?~RJ~
                    Notice the wire from the bushing, into the CT, into some flexible conduit and down into the rock- same for the primary:


                    https://youtu.be/bV2pMeKdkQM?t=142
                    Last edited by mbrooke; 06-16-19, 07:09 PM.

                    Comment


                      #85
                      Originally posted by mbrooke View Post
                      Probably not, but it would be low.


                      Is it exceptionally high reactance that does it?
                      They are not perfect conductors. Internal reactance influences the path.
                      BB+/BB=?

                      Comment


                        #86
                        Originally posted by mivey View Post
                        They are not perfect conductors. Internal reactance influences the path.
                        By how much though?

                        Comment


                          #87
                          Originally posted by GoldDigger View Post
                          The net resistance is indeed not zero, but most if not all of that resistance is the term associated with the earth electrode resistance rather than the distance to the uniground point. If the two electrodes are within each others' sphere of influence the overall resistance will be more complicated. That can lead to the appearance of the net resistance depending on the earth path length.


                          Sent from my XT1585 using Tapatalk
                          Not quite. If we could parallel enough dirt at no cost then we would have a perfect earth conductor. The real circuit does not behave that way. Nature tends to the lowest energy state and simply will not reach the whole earth for the transmission line. The current tends to maximize nearer the transmission line rather than going through China. This limits the parallel dirt and the net dirt resistivity becomes a significant factor.
                          BB+/BB=?

                          Comment


                            #88
                            Originally posted by mbrooke View Post
                            By how much though?
                            You have to run the numbers.

                            I ran a model for a 1 mile 115 kV transmission line with a 0.0001 ohm N-E connection to remote earth on the source end. Beyond that, I ran a 20 mile transmission line with a 0.0001 ohm N-E connection to remote earth at the source and load end. In the 20 mile section, I paralleled the earth with a giant conductor to short the earth resistance during a fault for case #1. For case #2 I then opened the parallel conductor to run fault current through earth only.

                            If the earth is a perfect conductor, then we would expect most of the current to be in the earth instead of in the parallel path. But that is not the reality of how currents run through the transmission line and earth.

                            The fault I created was a A-N fault at the end of the 21 miles. Clearly the earth path is resisting the current even though we have a negligible connection resistance to remote earth.



                            With parallel path:

                            In the 20 mile section:
                            Phase A: 590 amps
                            parallel path: 395 amps
                            earth: 196 amps

                            In the 1 mile section:
                            Phase A: 590 amps
                            earth: 590 amps



                            Without parallel path:

                            In the 20 mile section:
                            Phase A: 430 amps
                            earth: 430 amps

                            In the 1 mile section:
                            Phase A: 430 amps
                            earth: 430 amps
                            BB+/BB=?

                            Comment


                              #89
                              Originally posted by mivey View Post
                              Not quite. If we could parallel enough dirt at no cost then we would have a perfect earth conductor. The real circuit does not behave that way. Nature tends to the lowest energy state and simply will not reach the whole earth for the transmission line. The current tends to maximize nearer the transmission line rather than going through China. This limits the parallel dirt and the net dirt resistivity becomes a significant factor.
                              Not to say nothing runs in the China dirt but it might not even be measurable. i.e., The closer dirt gets much more current than the far dirt so soil resistivity is a significant factor.

                              This is modeled using Carson's Equations.
                              BB+/BB=?

                              Comment


                                #90
                                Originally posted by mivey View Post
                                You have to run the numbers.

                                I ran a model for a 1 mile 115 kV transmission line with a 0.0001 ohm N-E connection to remote earth on the source end. Beyond that, I ran a 20 mile transmission line with a 0.0001 ohm N-E connection to remote earth at the source and load end. In the 20 mile section, I paralleled the earth with a giant conductor to short the earth resistance during a fault for case #1. For case #2 I then opened the parallel conductor to run fault current through earth only.

                                If the earth is a perfect conductor, then we would expect most of the current to be in the earth instead of in the parallel path. But that is not the reality of how currents run through the transmission line and earth.

                                The fault I created was a A-N fault at the end of the 21 miles. Clearly the earth path is resisting the current even though we have a negligible connection resistance to remote earth.



                                With parallel path:

                                In the 20 mile section:
                                Phase A: 590 amps
                                parallel path: 395 amps
                                earth: 196 amps

                                In the 1 mile section:
                                Phase A: 590 amps
                                earth: 590 amps



                                Without parallel path:

                                In the 20 mile section:
                                Phase A: 430 amps
                                earth: 430 amps

                                In the 1 mile section:
                                Phase A: 430 amps
                                earth: 430 amps
                                Thank you!

                                Really like your method and choice of numbers.


                                In what program did you model this? Just curious.

                                Comment

                                Working...
                                X