neutral return current on single-phase system

[magoo2]

We have a single phase circuit 7200 V to ground in a beach area. The circuit runs 2 or 3 miles and the peak load is around 100 A on the phase. The neutral is multi-grounded.

In looking at the phase amps versus the neutral amps at different points along the circuit, I'm finding we're only getting about 1/3 of the current returning thru the neutral wire. I would have expected more like 1/2 to 1/1 split.

Any ideas?

 

[waross]

It is what it is. In our area a one wire primary system is in use and 100% of the current returns via the ground. Your grounding electrodes must be very effective.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wareagle]

You obviously have other paths being used. Does you local code allow
the water pipes to be used in the grounding system?
100 amps is a lot on a single phase line. I am surprised that you
are not receiving electrical shocking complaints.

 

[FPelec]

How far is your system from the seashore? If the ground electrodes are deep enough to reach the ground water level (maybe salt water) ground electrodes will be very effective. Thus the longer is the line (or the smaller is the cross section of the neutral conductor), the higher the current flowing into the ground return path. As wareagle said the main issue could be in touch and step voltages: the electrocution risk in beaches is a severe concern, with all those people walking barefoot...

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[magoo2]

Our distribution system is about 1/2 a mile from the coast. Grounding is pretty good. Most grounds I checked are between 20 and 30 ohms.

And to Bill's comments about using just a single phase wire for our distribution, no, we are using 2 wires.

Homes are fairly new so i wouldn't expect any conductive plumbing in the homes.

One basic question to answer is
- with a long single phase overhead circuit with distributed grounds of 25 ohms, how much neutral return would you expect?

I'm guessing it should be close to 100% of the phase value. That's why I'm looking for other paths.

 

[MKFPE]

Search with terms like Carson, mutual impedance, single phase, neutral, return, etc to find lots of information re: calculating performance of unbalanced lines including earth effects.
Good books on this subject include those by Paul Anderson and William Kersting.
It is not too difficult to set up a Matlab/Octave/Freemat etc worksheet to carry out the calculations for your particular situation.
Also, I agree with wareagle, 100 amps is kind of heavy. You may want to look at putting up a second phase and splitting the load.

 

[waross]

Have you considered an open neutral with all the neutral current past the open returning via ground??

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[magoo2]

Yes, I'm familiar with Carson and mutuals. A second phase won't help at all in terms of neutral return. Having 3 phase would.

With the grounding aspect, I think it's better to model this as a ladder network where you explicitly represent the neutral-to-earth connections.

I don't think we have a open, but we may have a high resistance connection in te neutral. Good suggestion, Bill.

 

[ghostbuster7]

We had a similar situation with the utility neutral current (40 amps) entering a large indoor community swimming pool.

People in the water were getting shocks when they touched the ladders to get out.

Also,people standing barefoot at the edge of the pool got shocks when they leaned over to touch the water.(Note: all power to this entire pool complex had been turned off when they got these shocks)

It sounds like this could be another situation just waiting to happen.