Shielding from magnetic fields?

[veritas]

Hi

We are considering ways of minimising the effects of induced voltages in control cables in substation HV yards due to fault currents in overhead lines or transients on the lines.

A rather fundamental question - is it possible to shield from magnetic fields? If so how?

Many thanks.

 

[GamezBeCJ]

Hi veritas:
A way of shielding a magnetic field is to use a material that would "trap" the magnetic field before it reaches the point that you want to shield it from. This can be achieved by placing a low reluctance material (one with low "resistance" to magnetic flux, i.e. silicon steel, like the one used to build transformer cores) betweeen the magnetic flux source and the shielded point. As an example imagine a bar magnet, this type of magnet has a magnetic field around it, which you can easily see by placing metal filings in a paper sheet on top of the magnet. If you totally enclose the magnet with a box of steel and repeat the metal filings experiment, you would see a much weaker field or not one at all out side the box. This is explained because most of the magnetic flux lines get traped by the magnetically soft material (the steel box) before they reach the outside of the box. However, there is a precaution you must have with alternating currents. Since the shielding material is also a conductor, if you form a closed circuit with it around an energized source, it would induce circulating currents within the shielding material. To avoid this you just have to close the magnetic circtuit without closing the electrical one, i.e. by just traslaping the closing ends of the shield, this would sourround the source without closing an electrical circuit.

 

[corrosivo]

Hi Veritas

You say that you want to minimize the effects of the fault current magnetic fields in you control wiring.

Are you are having troubles with induced transients in you control wires? Protective relays mistripping? Burned instruments? Burned circuit gound bonding wires?

How do you know it is a magnetic field causing your troubles?

Is the control wiring is laid along power cables?

Generally the control and power wiring are laid in different trays and conduits, its a bad practice to run power cables and control cables together as you might have induced voltages due to sudden changes in current(fault).

Steel conduit is a sufficient shield for magnetic field and it will not heat as long you keep the phase wires and neutral together in the conduit because the mag. fields cancel each other.

For a magnetic field from the overhead lines induce a significant voltage and create a destructive current in a control wire there must be a BIG loop cutting the field such as when you bring a neutral wire in a path different a phase wire.

But you might have ground loops in the control cables due to improper grounding. If the neutral wire in a cable such as the ones conenected to a voltage or current transformer is grounded at the outside terminals and also at the control board somewhere, there will be a ground loop, when lighting strikes a line or a guard wire a lighting arrestor will divert the overvoltage to the structure. The structure and the control board might be subject to different potentials and you will have a very high current flowing thru the neutral cable to your equipment inside.

This is one reason why the control ac circuit is bonded to ground only in one point. Also you might have parallel current paths which under certain conditions cause misoperations.

If you are getting induced current flow tru your control circuit wiring try using a clamp on anmmeter in the ground bonding wire, this might help you to identify the problem.

Also see thread 248-99158

 

[veritas]

Hi GamezBeCJ

Thank you for a very good reply. What you say makes sense. My problem is now applying it in a substation environment where there are LV control cables with 110V DC on them in a concrete trench. I have to come up with a way of either "shielding" these cables from induction due to switching or fault transients in overhead lines or finding some other way of guarding against the inductive effects.

A trench with a built in Faraday cage was suggested but I see two problems with this :

1) A Faraday cage is there for electrostatic shielding (capacitive effects).

2)What if there is an HV power cable in the trench itself?

Another suggestion was shielded cable. But this is usually a thin copper sheath which again will ensure electric but not magnetic shielding.

Comments?

 

[veritas]

Hi Corrosivo

Your suggestions and comments are also very good ones. Thank you. Maybe a bit of background - the utility I work for would like me to quantify the effects of inductive and capacitive coupling between power circuits and control cabling. Basically we want to formulate a policy as to whether screened or unscreened control cables should be used for protection purposes. There have been a few age old practices in the past that are not consistant with each other.

Based on what you have said I surmise then that a steel wired armoured cable will give good electrostatic as well as magnetic shielding?

We have not had specific incidences where equipment was damaged - it is a generic study I am conducting, though there have been unexplained trips and incidences in the past. I have calculated that an 11kV overhead line carrying 20kA, 6m above ground can induce 46V in a control cable running parallel to it for 50m at ground level.

I am yet to investigate as to what the implications of all this is for DC and AC circuits.

But at the risk of reinventing the wheel I'd like to hear what others have to say.

Bye for now

 

[sreid]

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