net current

[SparkyTris]

consider a subsea cable interconnector, three phase, 33kV 50hz, linking an offshore island to a mainland.

in this cable are 3 conductors only, 1 for each phase.

Under what circumstances could a net current be flowing in this cable?

 

[ban-all-sheds]

Consider telling your college tutor that you can't answer the questions he is asking you in order to judge your progress.

 

[SparkyTris]

Sorry, B-A-S, but you are - once again - wrong in your divination of what other people's ulterior motives might be. This is an internet forum. All you really have to go on is what people write in the messages that get published upon it. Sherlock Holmes was a fictional character, there is no need to emulate his insight. Whilst these leaps of faith do occasionally delight the fan base, if they fall flat, then you just end up appearing a bit of a Walter Mitty.

 

[SparkyTris]

really the most rudimentary of google stalking ought to give you a good idea about why I was asking the original question! simply phrasing a question as if it came from an academic exercise does not mean that it did. I guess it could mean that I have in the past, done quite a few such exercises, and I guess I subconsciously decided that adopting that style/phraseology was the most concise way of writing my question into the forum.

 

[ban-all-sheds]

Sorry, B-A-S, but you are - once again - wrong in your divination of what other people's ulterior motives might be.

So should I have asked what your motive was?

 

[JohnW2]

When there is uncertainty or ambiguity, asking for clarification (rather than making potentially incorrect assumptions/guesses) is always the appropriate course. Those whose assumptions/guesses are seen to 'always assume the worst' get a bad reputation, and are at risk of not being traken seriously, even when they are correct.

 

[ban-all-sheds]

So it's a case of "heads I lose, tails you win", is it?

Why do you need to know?

why did you feel the need to ask that rather fatuous question?

 

[OwainDIYer]

consider a subsea cable interconnector, three phase, 33kV 50hz, linking an offshore island to a mainland.

in this cable are 3 conductors only, 1 for each phase.

Who cares - suppliers' works innit.

If it was BS7671 there would have to be a 4th conductor or armour for cpc.

 

[flameport]

Under what circumstances could a net current be flowing in this cable?

When is has been connected in such a way that faults can be detected, and a fault has occurred, and the fault either hasn't been detected or it has been ignored.

 

[SUNRAY]

If it was BS7671 there would have to be a 4th conductor or armour for cpc.

Would there?

Edit:
Excluding the buried scenario I have worked on a few 3 phase systems using a local earth rod and without a neutral

 

[JohnW2]

No, it's not a case of "heads I lose, tails you win". Rather, it is a case of "one wins" when it is done appropriately, as in the other thread which has been quoted (although it would be an even better 'win' without the word "fatuous"), but "one loses" when it's done as in this thread.

 

[ban-all-sheds]

Err....

In the other thread I did it "appropriately". Apparently it was not a welcome question

 

[SparkyTris]

just put yourself in my position; it was not particularly unwelcome, it was just a trifle irksome. I came to an informative forum for practical guidance. I value the forum because of its practical nature. the information available within & upon it is subtly different to that available through formal academic learning and textbooks. The information here is seasoned with the actual way it is, through shared practical, personal and professional experiences of others.
I came to the forum with a decent question; I am guilty of not thoroughly searching the forum's archives for previously asked cases of the same question.
What I got, in answer to this decent question was a smug, somewhat arrogant and thoroughly unhelpful response.
Were I a student attempting to get others to do coursework for me, I agree that you would have been right to respond with the above. Others may disagree on this point.

Please trust me when I say that when I do academic coursework with tutorial style questions as a method for assessing progress, I will be upfront about asking for help with it, and only ask others as a last resort, having exhausted personal research avenues.

I am not a student at the moment though (at least, not in the academic sense), so you might forgive me for responding a tad waspishly to your prompt non-response to OP.

 

[SparkyTris]

and meantime, less is more old boy, as the actress said.
If you don't know, then that is OK! we won't think any less of you for not responding.
we might think less of you for responding unhelpfully.

 

[SparkyTris]

the ulterior motive in this case is to track an underwater cable using a triaxial fluxgate array which will detect (and even triangulate) current flow.
The accepted way of doing this is to force the cable to carry a current, by applying a "tone" to the cable ashore, from a tone generator. said genny is earthed at a suitable distance from the cable, which attempts to make the earth path relatively remote from the cable. Voila, our cable of interest is now carrying a small net current, which we can detect.
at the moment, the line operator will not countenance a tone being applied to the cable.

I am vaguely aware that one of the important parameters of running an AC distribution network is to keep the 3 phase demands balanced; effectively to try to keep zero net current in any given 3 phase transmission line. (am I wrong?)

I was wondering if there is a way that we could safely and conveniently make a net current flow in the cable, thereby to aid detection & tracking.
my first thought would be to load up one phase abnormally, but on thinking about this, it will not result in net flow. it would simply unbalance the loading but remain net zero current. (wouldnt it)
I have a feeling that the only way to do it would be to put a (controllable) earth leakage onto one phase, which may actually be feasible in this case.

Please, BAS, don't think that I am just sat there in a cable hut waiting to try any old suggestion out, I am not about to put myself, or others in any more mortal danger than they already are. Don't worry - I will talk to the grown ups before I do anything.
Or will I....