Steel Wired Armoured through Current Transformer 1

[jpflying]

I have an installation which is about 25 years old where there is a current transformer fitted onto a steel wired armoured cable for sensitive ground fault protection. The steel wired cable is grounded at both ends, with the earth at the CT end then passing back though the CT in the opposite direction to cancel any earth current effects in the steel wired armouring. This then leaves the current transformer to detect the out of balance of the phase currents when they do not summate to zero because of a ground fault. This I can understand. Please see attach photo.

My question is does the current transformer detect the magnetic flux produced by the out of balance of the currents not sumating to zero because of a ground fault? I would have thought the steel wired amouring around the phase conductors would act like a shield and therefore the current transformer will not detect the out of balance. We have had engineering advice that the CT will still detect the out of balance current though the armouring.

 

[PHovnanian]

I would have thought the steel wired amouring around the phase conductors would act like a shield and therefore the current transformer will not detect the out of balance.

Such shielding is effective only for E fields, not magnetic flux. So the CT will 'see' the phase current plus the shield current, minus the shield current returning through the strap. Magnetic fields don't 'care' about the configuration of the conductors passing through the CT window. The result (CT current sensed) will be the same as if these three conductors were separate wires instead of one being a shield.

 

[jpflying]

Thanks PHovnanian. I suspected the installation was OK because I have seen this type of installation of the CT over a shielded cable before, but I did not understand the exact theory behind it.

I needed to pay more attention about about Maxwell's Equations when I was at University.

 

[rbulsara]

I have difficulty udnerstanding both of your paragraphs. But the installation in the picture and engineering advice you got is correct. I presume what you have is a 3 conductor cable.

The CT's function is only to sense current and not magnetic flux or such. The core balance CT,as you have it, senses the non-zero sum of the "currents" through it during a fault, if wired correctly.

In normal conditions there is no ground current in the armor/shield. If the shield or the armor ground path is not routed back through the CT, during a ground fault the armor path would act like a neutral and net vector sum of all conductors through the CT will be zero, even during a ground fault.

By running the ground back through the CT, the CT would not see any current through the armor (as it cancels out) and the net sum of the current will not sum to zero, indicating a ground fault condition. (Just like a CT around unshielded or bare conductors/buses where no ground conductor passes though a CT)

I am not sure what you mean by out of balance current, the CT would not sense any unbalance curret in the three phases, just the net vectorial sum of the currents in the conductors passing through it and any non-zero sum would be in case of a ground fault only.

This installation method apply even to CTs on single conductor, shielded cables, where the phase CTs need to sense the currents through the phase conductor only and not through its shield or armor.



Rafiq Bulsara

http://www.srengineersct.com

 

[jpflying]

rbulsara

Sorry about my poor choice of words. Yes, I should have stated the net sum of the currents should equal zero when there is no fault instead of talking about out of balance.

My bigger issue was that I thought the armouring would have acted as a shield and the CT would not sense the resultant current when there is leakage to ground.

Thanks

 

[Pam60]

Rbulsara,

Please correct me, if I am wrong.

#1 In case of a med. voltage(13.8 or 4.16 kV) or low voltage, 3 Phase, 4 wire system, if I want to use a zero seq. CT, then the wires passing through CT will be ( Three Phases & Neutral and shield rerouted back through CT). Will imbalance operate the zero sequence CT ?


#2 In case of a 3 wire system only, Will an imbalance in phase operate the CT as well ?

Sorry for interjecting in this post.

Thanks

 

[rbulsara]

Pam60:

The answer to both of your questions is No, for a correctly wired ZCT (Zero-sequence CT or a core balance CT).

If all conductors, carrying currents going out from and returning back to the source, are contained within the window of a ZCT, they will cancel each other out and their net vectorial sum will be zero.

Even if there is an unbalance among the 3 phases of a 3 wire system, as long as there is no ground fault, the net vectorial sum of the 3 phase currents (within a ZCT) at any instant is zero. In a 3 phase system, any current going out in a phase is returning back through the other two phases at any given time. This is true for even the balanced loads. Any current that leaves a source must return back to the source to complete a circuit.

The basic idea in a ZCT is to leave the ground return path out of the ZCT 's window. So as and when a current passes through the ground path, it will be left out of the sum and be detected by the ZCT.

Rafiq Bulsara

http://www.srengineersct.com