Overhead Distribution Lines - Neutral Conductor and Grounding

[mgtrp]

Hi all,

After moving to Canada 6 years ago, I have been walking for a small utility that predominantly works with generation and transmission but also dabbles in small amounts of distribution. One significant difference between Canada and my native country is that the medium-voltage distribution system typically includes a neutral conductor, which is generally connected to source and destination substation earth grids as well as being connected to earth every so many poles outside of the substation.

From what I gather, this neutral conductor has quite a number of uses:
1/ Canadian (and US) practice is to use a large number of small step-down single-phase transformers connected phase-neutral, as compared with using larger three-phase transformers connected in delta, or, where single phase power is needed, single-phase transformers connected phase-to-phase. This practice requires a neutral conductor.
2/ The neutral conductor is often used by line crews as an earthing point while establishing their earthing and equipotential zones when working on lines and equipment.
3/ The interconnection of the earth grids for all substations (via the neutral conductor running between each substation) combined with the earthing at many poles creates a much lower impedance to the general mass of earth, reducing GPR in substations.
4/ In the event of a downed conductor, there is some chance that the conductor may contact the neutral resulting in a readily detectable fault current (especially in areas prone to high impedance faults).

From my own observations and "discussions" with various colleagues, I have a several questions:
A/ Apparently in some areas in Canada, the practice is not to connect the neutral conductor into the substation earth grid, to ensure that GPR is not transferred from the substation out onto the distribution system (especially bearing in mind that pole ground locations etc won't use crushed rock etc to limit step and touch potentials). While I can appreciate the desire, surely this results in the general mass of earth being used as the return path for the various phase-to-ground connected transformers on the distribution system?
B/ How is the issue of GPR being transferred to the distribution system handled? Is it simply assumed that the GPR will be low enough, and the impedance of the neutral conductor relative to the many pole-grounds such that the GPR is not going to be a concern? Do utilities sometimes include switch in the connection from neutral conductor to substation earth to allow this to be isolated while line crews are undertaking work?
C/ How significant is the improvement in detection of downed conductors? It seems to me that, unless the conductor contacts the neutral conductor or lands very close to a pole ground, most faults would be similar regardless of the presence of a neutral conductor.
D/ In addition to a 25 kV distribution system, we also have a 35 kV system that is effectively a subtransmission system about town. It has no phase-to-ground connected loads, with all loads being three-phase transformers with delta HV connections. This negates the first reason above to have the neutral conductor, but I note that a neutral conductor is still present for each line. I assume that this is for the other three reasons listed, but are these really sufficient to justify the additional cost? I ask this because we occasionally run short sections of line to connect our generation into their system, and could probably quite easily get away with omitting the neutral conductor.

Any comments on the above would be appreciated.

Thanks,
mgtrp

 

[waross]

Although the 35 kV line is at present a sub-transmission line, 35 kV is an accepted distribution voltage. The neutral leaves the option open to connect single phse distribution loads in the future.
Safety: In Canada more and more decisions are being decided on the basis of safety rather than savings as a result of Bill C-45.
What is Bill C-45?

Bill C-45 is federal legislation that amended the Canadian Criminal Code and became law on March 31, 2004. The Bill established new legal duties for workplace health and safety, and imposed serious penalties for violations that result in injuries or death. The Bill provided new rules for attributing criminal liability to organizations, including corporations, their representatives and those who direct the work of others.
New Sections of the Criminal Code

Bill C-45 added Section 217.1 to the Criminal Code which reads:

"217.1 Every one who undertakes, or has the authority, to direct how another person does work or performs a task is under a legal duty to take reasonable steps to prevent bodily harm to that person, or any other person, arising from that work or task."

Bill C-45 also added Sections 22.1 and 22.2 to the Criminal Code imposing criminal liability on organizations and its representatives for negligence (22.1) and other offences (22.2).

I would imagine that work is made safe by ground chains from each phase to the neutral. I would not like to face an inquiry and the possibility of criminal charges because I implemented a system without a neutral conductor.
As CEOs and managers become aware of their responsibilities under Bill C-45, safety is becoming King as managers strive to show a chain of due diligence in the protection of workers.



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

 

[cranky108]

On transmission systems the neutral conductor, may not be all that conductive. We typically use steel wire over our transmission lines for lightning protection. The steel is conductive but not as much as the aluminum conductors. In some places on a 35kV systems they would also use a steel wite for lightning protection, but that dosen't work if you have single phase loads, like you said. But if you have a joint circuit with both 35kV and a distribution under it, you want a conductive neutral. So it is possible that there is a standard to use a conductive neutral so that joint circuits can have a conductive neutral.

Because of the area of the pole grounds, there may not be that much GPR due to the soil conditions. But there may be enough to direct a lightning stroke to ground, which in many places the utilities are not held lible for because the utility did not generate it.

The other issue is the transformer configurations between voltage levels, are typically such that single phase loads are not transfered to the transmission system as zero sequence currents. Which maybe why there are no 35kV single phase taps. However they can be added if there is no other alternitive, because of the carried neutral.

No I don't know the specifics of the Canada systems, but well enough bursed in the typicals of the North American electric grids.

 

[mgtrp]

Bill - I think the gist of the one part of my query is whether or not the neutral conductor adds appreciably to safety.

Cranky - I am referring to the neutral of a four wire system used as a return path for normal load currents, not an overhead shield wire used for lightning protection. The conductor is similar to that used for the phase conductors, and conductivity shouldn't be a concern.


My queries above are motivated by my belief that it is important for an engineer to have an understanding as to why specific standards and practices were developed, partly to ensure that the standard is applied correctly, and partly to be better able to identify problems that are not addressed by standard practices.

 

[dpc]

In a typical US distribution feeder, the neutral conductor is there to provide a low impedance path for both load current and fault current. Line-to-neutral loads rely on the neutral for normal load current. These feeders are often not well balanced and neutral currents in the 10 to 15% range are common. Of course, for single-phase taps, the neutral is even more essential.

For line-to-ground faults, if there were no neutral, the ground fault current would generally be drastically reduced and much harder to detect. High impedance ground faults are a major concern even with a neutral. Removing the neutral would make matters much worse. More ground fault current is better than less.

But to be honest, I'm not sure I really understand your question. Maybe you can simplify it for a someone with a short attention span - like me.

 

[waross]

Safety may be more than a simple matter of ground fault current levels. It may also have to do with similar systems with different safety procedures and possible confusion.
In the absence of a grounded neutral conductor it may be difficult and ineffective to drive a ground rod into frozen ground for the attachment of grounding chains when work is being performed. Frozen ground may have a fairly high impedance.

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

 

[crshears]

Hi Bill,

Hopefully this will be just a slight divertissement without detracting from the thread...

'Grounding chains'? I've never heard this terminology used before except in reference to vandalism or other deliberate intent to trip equipment out of service or cause damage to it.

Our utility, and I suspect this is virtually universal, uses working grounds constructed with jacketed, stranded copper conductor of such gauge as stipulated by Engineering Services based on the fault current infeed capability that prevails in the instnce under consideration.

CR

 

[davidbeach]

Ground chains would be an archaic term for "grounds". Deliberately applied grounds (and shorts) for protection of personnel working on what is supposed to be "dead" equipment as a protection against inadvertent energization.

 

[waross]

Yes David, exactly.
The company I am with now has some senior high voltage people who still use the term "Chains" for the working grounds.
And, back in the day when I was young, the safety of switch gear to be worked on was assured by actually wrapping steel link chains around the bus bars or around the load terminals of a large breaker. A process called "Chaining the bus".

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