High voltage CT'S ????

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parkin40

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Hello, I have a question that I can not get a good answer too. How does a high voltage CT measure amperage if it is monitoring a shielded high voltage cable, ( the shield is grounded). I do not understand how the CT can measure current because the shielded cable would not allow any inductance from that conductor. Thank you in advanced ... Jan
 
Some medium and high voltage CT's do not look like traditional CT's that we are used to seeing in the low voltage world in that they don't look like "donuts" around the wire. They look more like a series connected device and the tape shield is terminated in a stress cone before the conductor is connected to the CT. There are two HV / MV terminals, one in and one out and then the low voltage terminals where the metering conductors are connected.

Here's a good page you might want to read.
 
The shield on a cable makes no difference to a current transformer. You can measure the current flowing in a single, shielded high voltage cable using a simple amprobe type clamp-on ammeter.
 
When donut type CTs are used with shielded MV cable the shield wire is "doubled back" through the window. This allows 600V CTs to be applied because it is installed around a 0V to ground conductor but the magnetic field of any "ground current" is canceled due to the loop back, meaning the CT will only see the phase current..
 
dave_asdf said:
... can you clamp a metal conduit with cable running through it?
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Yes. My favorite supply is "impedance grounded 480". When there is a fault out in the field, usually phase to ground, there is current that does not return through the conductors in the conduit. So Sum(I) not equal to zero. So you use a big clamp-on, around the conduits one at a time, until you find the one that reads a non-zero current. That's the one with the gorunded phase. Works great.

carl
 
A donut ct's ouput is proportional to the sum of the current entering minus the sum of the current leaving. Therefore as long as the shield is not carrying the same current as the conductor it will have an output. Looping the shield through the CT as Jim mentioned is an effective way to eliminate any error due to shield current.

A shielded cable will cause the electric field to be hard to read outside of the shield.
 
coulter said:
..So you use a big clamp-on, around the conduits one at a time, until you find the one that reads a non-zero current. That's the one with the gorunded phase. Works great.
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That would be a nice way to trace harmonics. Is that clamp sensitive enough around conduit to pick up unbalances from zero-sequence neutral harmonics, or 3-wire circuits comming from a 4-wire Y xfmr?

BTW, this Peterson fault detection relay.pdf seems to use the same CT's to trip faults by detecting the direction of the same imbalance.
 
ramsy said:
...That would be a nice way to trace harmonics. Is that clamp sensitive enough around conduit to pick up unbalances from zero-sequence neutral harmonics, or 3-wire circuits comming from a 4-wire Y xfmr?....
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"zero sequence neutral harmonics" - Yikes, I'm not real sure what those are (that is?).

Are you saying that ZSNH give rise to currents that flow into one conductor and do not come back on the others of that circuit. I don't know how that could happen. Kirchoff and Norton kind of get in the road.

As for frequency response, I suspect not very good. The ones we bought last month look exactly the same as the first ones I saw 40 years ago.


carl
 
coulter said:
Are you saying that ZSNH give rise to currents that flow into one conductor and do not come back on the others of that circuit. I don't know how that could happen. Kirchoff and Norton kind of get in the road.
Click to expand...
An earlier diolog with Fluke tech. support describes how their 337 clamp probe Hz function measures neutral harmonics only if zero-sequence sums, found in the nuetral, are the predominate magnitude (ie) 180Hz. The clamp only shows 50/60Hz on phases, since pos. & neg. sequences don't add the same way.

Regarding using your clamp-on ammeter around conduit, if it has a Hz function that reads > 50/60 hz there are harmonics. A reading of 180 Hz shows the presence of triplen sums (3rd harmonics). AKA Zero-Sequence Neutral Harmonics (ZSNH) for others who read this.

If you noticed high Hz while uisng a clamp around conduit, let us know. I've never seen it done, much less from grounded-Delta secondaries, but apparantly you would know it, if the Hz show it.
 
ramsy said:
...Regarding using your clamp-on ammeter around conduit, if it has a Hz function that reads > 50/60 hz ...
Click to expand...
coulter said:
...The ones we bought last month look exactly the same as the first ones I saw 40 years ago. ...
Click to expand...

Uhhh --- No :)

carl
 
Current Transformers

Current Transformers

1) the shield is typically copper tape which does not effect the magnetic field from the center conductor UNLESS there is current on the shield.

2) HV CT are either ceramic insulated donuts for bare bus or encapsulated CTs where wire is connected to both terminals. The shield does not go through the core.

3) if the HV cable had a concentric neutral, the neutral MUST NOT go through the donut core, because the concentric neutral is intended to be a current-carrying conductor.
 
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