Donut and Bushing CT's 1

[NicolasK]

Does anyone know how the metering accuracy will change depending on the location of a cable or bus through a CT? I know it is recommended a cable or bus be as closely centered as possible in a CT. Also, the metering accuracy decreases when the cable or bus is not centered. But, I'm looking for a report or test that shows how the accuracy changes as the position changes.

Thank you in advance.

 

[busbar]

Section 8 of IEEE Std C57.13-1993 states that with respect to accuracy, bushing- [or window-]type current transformers should have ‘secondary windings appropriately distributed around the core, and… “through” primary conductor(s) symmetrically located in the opening.’

Uniformly distributed secondary windings are potentially as critical, but possibly not as obvious in field/switchgear applications.

Aside from metering applications, most critical may be zero-sequence sensing with three phase conductors positioned mutually symmetrical for equalization—limiting false operation of associated low-range/sensitive overcurrent relaying. [In this case there are effectively three primary ‘windings’ to accommodate.]

 

[jbartos]

Suggestion: The cable or bus position inside the CT core and its impact on the CT accuracy is imbedded in CT overall accuracy. There may also be an impact of the electromagnetic field of the neighboring buses that may distort the field of the bus where the window CT is located. Details might be obtained from the CT manufacturer, e.g.

https://www.kuhlman.com/clientdata/Kuhlman_General_Catalog1_1.pdf

or by calculations.

 

[scottf]

As mentioned above, the primary conductor centering's affect on accuracy is a function of the CT design (secondary winding design) and also the magnitude of the magnetic field. For most normal metering applications, if the primary conductor is off slightly, then the affect on accuracy is typically negligable. For generator applications, where the primary current is very high >6,000A, then local core saturation can be caused by mis-aligned primaries. Local core saturation is also a function of the secondary winding design, in that for generation CTs, most secondaries are made from segmented windings.

To answer your question in short, there is not too much general data available for the primary conductor affect on accuracy, mainly because it depends on so many factors. If you really want to be sure, then you have to test the actual CT under simulated conditions, which is normally not too hard to do.