Neutral conductor on a 3 wire line
Neutral conductor on a 3 wire line
(OP)
IF you have a 3 phase 3 wire line with all loads connected in delta, is it better to run the neutral along the poles and ground/bond the system exactly like an MGN or simply skip the 4th conductor all together? What are the pros and cons of both?






RE: Neutral conductor on a 3 wire line
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Neutral conductor on a 3 wire line
Also a shield wire has been shown to reduce lightning strikes, so a 4th somewhat conductor is good for that.
RE: Neutral conductor on a 3 wire line
Bill
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"Why not the best?"
Jimmy Carter
RE: Neutral conductor on a 3 wire line
According to the NESC, don't you ground the can and arrestors to one ground, and then connect the LV neutral to another ground 8 feet away when there is no MGN? Or just tie all of them together? What does the IEC say?
You do at the substation, you can either draw it out or leave it there.
RE: Neutral conductor on a 3 wire line
Unfortunately the problems with skipping the neutral are hard to quantify compared to the obvious costs of wire and taller poles.
For a line to ground faults, fault current may flow in and damage adjacent objects such as communication wiring or fencing. If any of the circuit has tape shielded underground cable, a bare neutral is needed in parallel with the UG sections. I am not quite clear on what happens eventually there are sections alternating between 3 wire overhead and 4 wire underground. Also, line regulators have to be connected in delta, which can cause unbalanced phase angles.
RE: Neutral conductor on a 3 wire line
Yes, a 115kv to 12kv substation serving an over head distribution. The 12kv over head is what is in question. The 115kv to 12kv transformer is low impedance earthed, though solidly in cases where the power transformer is small.
This a legit concern, and one where I am at a loss. Relying on the earth alone will put inadvertent bonding (grounding) conductors like TELCO shields at risk for high current through faults. That is of course, unless, the transformer is high impedance earthed which in theory is also doable.
Would these not be treated like any other ungrounded legacy system found in most US POCOs? My understanding (though limited) is the shields are bonded together at each end and then connected to ground electrodes. Perhaps a California or IEC member could chime in (where 3 wire is common)
How would these phase angle shifts compare to a wye grounded system where neutral shift is another concern? My understanding is that with delta regulators one pf the phases can be higher in voltage relative to gorund than the other two, however 15kv class equipment being run at 12kv mitigates this concern.
RE: Neutral conductor on a 3 wire line
RE: Neutral conductor on a 3 wire line
None to odd ball for POCOs which use them exclusively (California for example) to which relay manufacturers are aware. However, in my mind I am imagining a greatly reduced zero sequence setting to take this into account. Would I be correct?
RE: Neutral conductor on a 3 wire line
RE: Neutral conductor on a 3 wire line
What would really be fun is the transition between assumptions. I grew up in an area with a 12kV delta distribution system. All three wire distribution, no neutrals; no phase-neutral/ground connected loads. Faced with capacity issues, that company converted from 12kV delta to 21kV wye. Some stations had one, other, perhaps adjacent, had the other. Worked in that area for a while, years ago, and traced out feeders to see what a particular site might have. I'd follow a circuit that was clearly 21kV with three phases on fully rated insulators and a neutral on pole clamps into an area where one of the phases dropped out and the neutral moved up into one of the phase positions. Didn't think too much of it at the time, all of my work then was on the customer side of the meter, but now that I'm wholly on the other side of the meter it gives me the willys. So the 12kV needed to follow one set of assumptions and the 21kV a different set of assumptions, but to me, at the time, the boundary was entirely random. I hope they got it right, and I hope, 25 years later, that they're long done with the transition.
RE: Neutral conductor on a 3 wire line
Z0 = (0.301 +j 1.890) Z1 = (0.131 +j 0.594) Multi grounded with 100 ohm-m soil
Z0 = (0.417 +j 2.940) Z1 = (0.131 +j 0.594) substation only grounded with 100 ohm-m soil
I assume under different soil conditions, the individual pole or transformer grounds might be high enough resistance to prevent adequate sensitivity with overcurrent relays.
I can't seem to find the reference about phase angles on delta regulators. I might have been thinking of open delta regulators. We had looked into adding some to our 3 wire system, but decided to not to install regulators until we add enough of the neutral to allow wye regulators.
RE: Neutral conductor on a 3 wire line
Thanks. Any idea where you re getting 100ohm meter soil from? My understanding (yet I could be very wrong) is that the bulk of the impedance comes from the electrodes and not the soil itself as the earth has a near zero ohms resistive impedance. The substation will usually be a good value (say under 25 ohms) and the primary concern is the grounding at the 12kv/480 volt transformer. However, in most case, a zero sequence pickup value of 5 amps is sufficient for a 3 wire line in regards to tripping out the feeder itself in nearly all cases?
I can see a 50K or 25T fuse being hit or miss for a line to ground fault, however would not a 5 amp ground setting on the feeder relay (or transformer relay if protected by such) be adequate in most cases? Ignore selective coordination for a moment.
You might be, but still worth pointing out. Better to check list everything before hand then have a surprise down the road "Oh man, how did we miss that" lol.
RE: Neutral conductor on a 3 wire line
RE: Neutral conductor on a 3 wire line
RE: Neutral conductor on a 3 wire line