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  1. #7
    Join Date
    Dec 2007

    Default Re: neutral grounding @ pole??

    Well I've got to say that you have given me some interesting reading. I must say that there is a lot of misconceptions about electricity out there, even by engineers.

    The biggest misconception is that earth is a good conductor. Its not a good conductor, its a lousy conductor in fact. If it was a good conductor, there would not be a problem with Neutral Earth Voltage (NEV). If earth was such a good conductor, there would be no need for the ground grid recommended in the ERPI reference.

    What earth is, is a giant capacitor. Its a capacitor with a lot of leakage current (lousy power factor). That is what causes the NEV. Most of the current that goes into the ground at the ground rod on one half cycle, returns on the other half cycle.

    Now for the transmission line. In a single phase circuit, if the neutral was not grounded at any point except the generator or substation, one big advantage would be reduced EMI. If the line is balanced, and I'm talking about line balance here, not the kind of balance most electricians think of with three phase circuits, line balance means that the current in the lines is equal in amplitude but opposite in polarity. When the lines are balanced, then the magnetic fields created are equal in amplitude but opposite in direction, so the magnetic fields are suppressed. When the lines become unbalanced, then the uneven currents cause uneven magnetic fields which cause radiation. The radiation that is not suppressed are called stray losses.

    Your SNCMFG reference seems to think that the stray EMI is caused by the ground currents, and of course anytime there is current flow, there is a magnetic field, but as the current flows out concentrically from the ground rod, these magnetic fields would pretty much cancel themselves, not completely but mostly. The measured EMI is coming from the unbalanced lines. But even those fields were pretty weak.

    It is clear that the writer of that reference is not an electrical engineer, but he states that "According to utility engineers, the resistance of the neutral wire causes significant voltage drops along the lines, requiring frequent voltage adjustments." That is not true. I think the author misunderstood the engineers. The resistance of the primary wire causes the voltage drop. If there were no ground rods, then the story would be different. Without ground rods, the primary wire would still cause the same voltage drops, but the return line would see the voltage rising in reference to ground, but diminishing in reference to the primary wire as the distance increases. In a grounded neutral, there is no voltage loss because the neutral is clamped to ground so voltage is always at zero volts, therefore no voltage drop.

    He also makes this statement which really scares me. "In contrast to the present multi grounded system, a totally ungrounded system has the advantage that a person could stand on the ground and touch either the neutral or the high voltage wire, but not both, and not be electrocuted." Where do you start with that one.

    The same author wrote your Mike Holt reference.

    Your Bass Engineering papers are interesting. The author seems to think that isolating the primary winding in the transformer will solve most of the problems. It is feasible, I don't know if it would accomplish anything, but it is feasible. Someone would need to do a study on circuits that were "upgraded" by using phase to phase connections instead of phase/neutral connections. This type of circuit exists where a 2400 volt circuit was increased to 4160 by using two bushing transformers connected between two phases. There is no "neutral" line here. The transformer case is still grounded and the center tap of the secondary is still connected to the case. I don't know if any of these exist on the single phase legs but on the three phase sections, this is used in three phase banks to reduce the current in the neutral line. It actually reduces the current in all the lines.

    The issue with using a two bushing pot on a normal single phase line is that the neutral is grounded somewhere. The panel is also grounded so there is a connection between the HV neutral and the LV neutral, but depending on the spacing between the ground rods, there will be at least some resistance.

    Another misconception cited in several references is that in a perfectly balanced three phase system, there would be no current on the neutral line. The only time this is true is on the secondary side of a three phase transformer that is feeding a three phase motor or motors and there are no other loads on this circuit. That cannot be true in the distribution network. Any single phase appliance on the circuit will return ALL of its current on the neutral. It has no path to the other phases.

    I still question the methodology behind the statement that 60% of the return current comes through the ground connection. There has to be a big misunderstanding here. If that were true, line losses would kill the utility. The stray losses would be astronomical. The utilities would be unhooking all those ground rods as fast as they could, except the phone companies wouldn't let them, at least not without a fight.
    Last edited by keith3267; 01-13-2012 at 11:50 PM.

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