net current

[JohnW2]

... 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 suppose it depends upon what you mean by 'net current'. If, for example (and simplicity), you connected a load between two of the three phase, then the current drawn by the load would have to be travelling (in equal magnitude) in both directions through the cable, hence 'cancelling out', which you may call 'zero net current' - but that's no different from the situation with an L and N conductor is a single-phase circuit (and the reason why you should get a zero reading if you put a clamp meter around both L and N).

Kind Regards, John

 

[SparkyTris]

Yes, exactly as you say: the situation where, were you to put an ideal clamp meter around the whole 3-conductor cable, it would read non-zero.
we are having real trouble in reliably tracking this cable with the equipment we have. sometimes it tracks OK, mostly not.
the equipment manufacturers state that it will detect a minimum net current of 30mA.
the question that we cannot get answered from the DNO is - what is the net current in the cable.
My feeling is that it is very close to being zero.
My hunch is that the only way we are going to get net current flowing in the cable is with earth leakage.
if we need a minimum current of 30mA, at 33kV that equates to roughly a kilowatt of earth fault: not insignificant. Therefore, when it works, the equipment is probably picking up a current due to earth leakage of very much less than 30mA.

so, - under what circumstances would a net current flow in a 3-conductor 3 phase cable?

 

[JohnW2]

Yes, exactly as you say: the situation where, were you to put an ideal clamp meter around the whole 3-conductor cable, it would read non-zero. ... My hunch is that the only way we are going to get net current flowing in the cable is with earth leakage.

I think you have to be right, since otherwise Mr Kirchoff would have to start turning in his grave ...

... if current is flowing into a load through at least one of the conductors, the same current has got to come back in a conductor(s) in the cable (thereby 'cancelling', as far as a clamp meter is concerned), unless there is some other path back to the power source which does not involve the cable - e.g. a connection to earth (assuming the supply is earth-referenced) such as you mention.

Kind Regards, John

 

[ban-all-sheds]

I think you have to be right, since otherwise Mr Kirchoff would have to start turning in his grave ...

Wrap him in wire and put magnets in the sides of his coffin...

 

[jj4091]

A fascinating subject, I don't know whether this will help or not https://www.researchgate.net/public..._detection_Part_2_-_Electromagnetic_detection

 

[SimonH2]

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?)

Sort of yes and no !
The currents in the 3 phase conductors can be different - but will sum to zero when phase is taken into account. Yes they like the currents to be balanced since that also balances loads on each cable and transformer core/winding - but it isn't a requirement for the 3 currents to sum to zero. In extreme, consider a 3 wire 3 phase system and put just one load between two phases - you have one phase conductor with no current flowing, but the sum across all three is still zero.

As John says later, aside from earth leakage the sum of the 3 currents will be zero at any point in time. If you want a non-zero sum then you need to provide a current path outside of your measurement zone and induce a current in that. In practical terms, that will mean earth referencing some point at one end and inducing a current between the equivalent point at the other end.
But given that it's a 3 wire connection, the transformers at each end will mostl likely be connected in delta (so no "middle" point) - and I doubt that the operator will let you earth one of the phases
I suspect you'd need to connect a star connected transformer across the phases at each end to give you a point that you might be allowed to earth - and at one end you could put a transformer secondary (a third transformer, or simply a floating signal generator) between the star point and earth allowing you to inject a current which would be shared equally across the 3 phase lines with the earth making the return path.
An alternative might be to connect a load (resistor) between one phase and earth at one end, and a different phase and earth at the other end - this would also create an unbalanced current. Using two resistors from 2 phases to earth at one end would (if you get the values right) avoid changing the phase-earth voltages in the network - something the operator would probably insist on since unbalancing the phase-earth voltages would increase stress on the insulation of at least one phase conductor.

However you do it, you have to arrange for there to be a current which flows from one end to the other via the earth, and using one or more of the cables as the return path. This current should then be detectable.

 

[securespark]

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

When the trawler fishing for electric eel sails overhead?