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CT measuring conditions

CT measuring conditions

CT measuring conditions

(OP)
Hi all,
Since this is my first thread, couple of words for introduction. I'm a junior project engineer at consulting/engineering European company, in the field of water treatment plants and substations. I'll appreciate ur tips/experiences/links about current transformers:
1. Can I transfer a ring type transformer from 0,4kV outgoing feeder to 10kV supply cable for similar current value? Inner diameter is ok for both cables. Would it work and how?
2. How reliable are measuring results in high current THD conditions? Is there a way to assess a influence of THD to current error, security factor, accuracy limit factor, thermal rating and avoid unwanted current paths/loops?
Thank you.

MikiBg

RE: CT measuring conditions

If the the 10KV system cable is shielded you can install a 600 volt class CT without a problem.  All the other aspects of your question depend on the ratings of the CT,  what you are connecting it to and what the function is.
 
Power/harmonic measurements
Control
Protection
 


 As for unwanted current paths or loops; a properly connected CT circuit will have niether regardless of the CT characteristics.

 

RE: CT measuring conditions

(OP)
I'm assuming a "standard" CT, for energy measurement or protection, no current probes for power quality measurements, no special construction for transients, and CT rating close to the nominal feeder current. Let's say that, by time, current THD on measuring feeder has significantly increased. What would be the consequences and what action should be taken?

RE: CT measuring conditions

You will find two classes of CTs. Metering and protection.
Metering CTs are optimized for accuracy and will typically maintain their accuracy with currents up to 200% of nominal.
Protection CTs have a much higher saturation voltage so as to accurately reflect fault currents.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: CT measuring conditions

(OP)
Yes, by "for energy measurement or protection" I meant one type or another, same scenario for both: low THD and moderate/high THD. You say up to 200% of nominal, and that's ok if we compare RMS under the same (low) THD, but what if it increase? Maybe true RMS (including harmonics) is then valid, or it isn't that simple?

RE: CT measuring conditions

It's not the magnitude of harmonics so much as their frequency spectrum.  CTs are pretty good up to maybe a few thousand Hertz.  I have some old papers where testing was done on frequency response of CTs and PTs.  

Any dc components in the waveform are death to the CT output, since it will saturate for any appreciable dc current.  

For short term faults or events, the CTs can handle a lot of rms current.  For longer term events, you need to consider the CT's Thermal Rating Factor.   

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

RE: CT measuring conditions

(OP)
Suppose u don't have those papers in e-form, dpc? Any link perhaps?  

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