neutral current in multi-grounded system

[alski]

Will there be neutral current of significant magnitude in a multi-grounded secondary system? At present, we have a line-to-line 240V system and I'm trying to justify the use of line-to-ground 240V system. At first, I assumed that the line losses for L-G would be half of that for L-L. But when I made the circuit model with distributed loads along the sec. line, then I thought I should consider the neutral current.

 

[ScottyUK]

For a single-phase system the neutral current should be exactly the same a the line current, all the way back to the origin of the circuit. One of the few situations where this might not be true is when the neutral uses PME - Protective Multiple Earthing - which is a distributed earthing method. This earthing technique is firmly in the domain of the electricity distributors - RECs or DNOs in UK parlance. Its use is prohibited for installation work.

Assuming the current and conductor sizes are the same for a line-line and line-neutral circuit, the losses will be exactly the same. There are other reasons for using a line-ground system, but reducing losses is a new one to me. You might argue that it is possible to use an undersized neutral in a well balanced three-phase, 4 wire system, but with so many non-linear loads creating triplen harmonics which sum in the neutral, there is an increasingly strong case for putting in an oversize neutral conductor.


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If we learn from our mistakes,
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[tinfoil]

There WILL be a marginal reduction in losses when comparing a Line-to-Isolated Neutral system vs. a Line-to-Multigrounded Neutral system, as the impedance of the return path is a bit lower in the latter case.

However, this reduction is not very significant, even for single-phase systems, and is negligible for 3-phase systems.

AFAIK, in practice, this difference is ignored.

 

[jghrist]

If you have 240V loads connected Ø-n, and they are not balanced (same load on each phase everywhere), then there will be current in the neutral and there will be losses in the neutral. If these same loads were connected Ø-Ø and were not balanced, the phase currents would be unequal and there would be more loss than if they were balanced.

At one extreme, if all the load were connected Ø-n on one phase, the current in the neutral would be the same as the current in the phase conductor. If the load were connected Ø-Ø between the same two phases, the current in the two phases would be the same as the phase and neutral current in the Ø-n case. If all conductors were the same size, the losses would be identical. As Tinfoil notes, with a multigrounded neutral, some of the current would flow through the earth and the effective neutral resistance would be somewhat lower, so losses would be lower.

Overall, I doubt that there would be enough difference in losses to matter.

 

[weh3]

I have used multi-grounding on long (> 500m) underground runs of medium-voltage shielded cable, mainly because of cable heating from shield currents that. Multi-grounding minimizes potential differences on the grounded conductor between terminations. I would ground the neutral shield in every manhole, and I can see the same argument holding for low voltage, too.

William

 

[mc5w]

You also need to consider the consequences of an open circuit in the grounded side of an end grounded system. If your grounded conductor opens you will have a NASTY tingle voltage at each end of say 80 volts or so. Our older 480 volt streetlighting systems are usually 480 volts end grounded and the newer ones 240/480 volts single phase. The 3-wire single phase version also has more or less balanced capacitance to ground which makes troubleshooting for an underground wire that is leaking electricity a lot easier.

For U.S. National Electrical Code there are some instances where a premises electrical system is allowed to have the neutral regrounded at each building, but then some of the neutral current will flow back through the soil. Since cow udders are sensitive to shocks as small as 1.5 volts the voltage drop in soil from this method is unacceptable. Therfore, for livestock buildings we now use separate neutral and equipment grounding conductors running from the service point to each building.

One thing that I learned when I was a child was that if I let enough perspiration salts build up on my skin picking up a 1.5 volt dry cell by the ends would really get my attention. Cow udders are like this too and cows definitely do not like the Cow Voltmeter Method.

On the other hand, how we found out that grandmother cannot let go of 120 volts is how we found out that rabbits think that 120 volts is delicious. Rottweilers also like to chew on 120 volts. A lot of old timers liked to use the Tongue Voltmeter Method on 120 volts to 277Y480 volts - saliva is semiconductive enough that if you have not eaten a salty snack lately you can stick pretty hot stuff in your mouth if your voltmeter is broken. If you stick 120 volts or 240 volts up your armpit, then the story is different.

Now you know how mice get away with chewing on the wires! Be very careful when working in a handhole that contains ANY voltage particularly 480 volt streetlighting power.

 

[ScottyUK]

"A lot of old timers liked to use the Tongue Voltmeter Method on 120 volts to 277Y480 volts - saliva is semiconductive enough that if you have not eaten a salty snack lately you can stick pretty hot stuff in your mouth if your voltmeter is broken."

You have done this? You are insane. No offence.



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If we learn from our mistakes,
I'm getting a great education!