MV / Substation grounding

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bnewport

bnewport

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Indiana
On a medium voltage (4160/2400V) system, is it normal to have current flow on the ground? I measured about 5 amps flowing on the ground from the case of a pothead. I also have to troubleshoot a substation ground that has about 7 amps flowing on it. Is this current normal, and if not, what could be causing it? I have to troubleshoot the latter problem for another contractor, and am not real familiar with grounding systems over 1 kV. Does anyone know of any good websites with this sort of info?

Bryan
 
Re: MV / Substation grounding

Bryan, are you talking about a primary distribution substation? If so, the substation grounding grid will be collecting neutral amps from the earth and bringing them back to the transformer. In a primary run, neutral flows into the earth the far half of the run, then back from earth to neutral the half nearest the substation, with the greatest return through the substation ground grid. This is a normal consequence of the multi-grounded neutral system we use.

Karl
 
Re: MV / Substation grounding

Karl-

I am not sure I understand the neutral amps flowing through the earth. Could you please explain?

Thanks
Bryan
 
Re: MV / Substation grounding

Bryan the neutral of these systems is grounded to the earth many times. That multiple grounding puts the earth itself in parallel with the neutral conductor.

With the earth and the neutral in parallel with each other the unbalanced neutral current splits (not equally) between traveling through the neutral conductor and the earth.

[ May 15, 2005, 08:56 AM: Message edited by: iwire ]
 
Re: MV / Substation grounding

Bryan, consider the way you bond in front of the service equipment (to the grounded conductor); that is the way we bond everything. Every location where we have equipment, we install grounding (we use ground rods). We are also required by the NESC to have a minimum of 4 grounds per mile not including our customer's services. Add to that, every customer will have grounding set up for their service. All of these grounding locations are connected to our system neutral and it is called a MGN or multi-grounded neutral. Some electric utilities do not use this system but they are the exception. The neutral is connected to the earth in so many places that it is in parallel with the earth.

Now, let's address your installation. I am assuming that this is an industrial substation or you would not be measuring the current. Part X in Article 250 (begins with 250.180) permits the same multi-grounded neutral system to be used for your primary distribution from the substation. :D
 
Re: MV / Substation grounding

Thanks to all who have responded. This is an industrial substation. I have not been to the site yet, (going Tuesday), but I am trying to learn what's happening ahead of time. They tell me they have 7 amps of current on the ground. At first they said it was the cap bank ground, with the caps on or offline, but now I am told it is the substation ground. So... if I understand correctly, the neutral is in essence a parallel path with the earth ground, which allows some neutral current to flow through the earth? This is similar to an incorrect N-G bond at subpanels, which lets neutral current flow on the ground. Am I understanding this correctly?

Bryan
 
Re: MV / Substation grounding

Thanks guys- you are what makes this trade great! I was told today that there is 7 amps with the caps online, and mA flow with them offline, so I think I will be checking for a bad (shorted) capacitor. Does this sound correct? On another project, we have MV distribution in a campus tunnel. We measured amperage on an external ground to a pothead. Any ideas on this one?

Bryan
 
Re: MV / Substation grounding

Current in substation ground grids and industrial ground systems is very common. Some of it is induced current - the ground grid, building steel and bonding jumpers can form low impedance ground loops. If a high ampacity circuit is nearby, currents can be induced in the loops.

Also, the complete high voltage system has some capacitance to ground, supposedly about equally balanced between phases. All of the cable, bus, and transformer insulation on the system is just like a big capacitor with the hot phase wire as one lead and ground as the other. If the phases are not perfectly equal, some charging current will flow in the neutral to ground connection.

Current can be measured in most capacitor bank neutral/ground connections. Caps are usually wired in a wye, phase to ground with the capacitor case connected to ground. If there is any third harmonic noise in the system, 3rd harmonic current has a path to flow to ground through the capacitors. The amp meter may not differentiate between 60 Hz & 180 Hz currents. Also any difference in voltage across the caps or in the capacitance values will make unequal phase currents flow. If curents are not equal, they can't add up to zero in the wye connection and end up flowing in the ground connection.

Cable shield grounds can have currents induced by the cable current. The power current in the cable induces a voltage in the shield which forces currents to flow if both ends of the shield are ground. The Okonite website had some good articles on this. One solution is to only ground the cables at one end.
 
Re: MV / Substation grounding

You'd be amazed at how regularly this subject comes up in audio and audio/video forums. Ground loops and other noise induction is a constant concern. Connecting the shield of a coaxial interconnect to chassis at only one end is a common solution.
 
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