CBCT/ZCT on solidly grounded systems:

[NickParker]

I thought CBCT/ZCT is immune to saturation, therefore can be used on solidly grounded systems. After reading this old thread, it seems like it is not.
Seems CBCT immune only for inrush currents which is less than 20x times the rated current.

Any comments!

 

[dpc]

Any CT with an iron core will saturate if the current is high enough - none are immune. If the available fault current is below the knee of the saturation curve, then that CT will not saturate in that particular application. The intended application and purpose of the ZSCT has to be considered. In most industrial applications, these are used on medium-voltage feeders. In many cases, there is no MV ground fault protection downstream of this feeder breaker, so ground fault protection can use instantaneous trip functions. For these applications, even if the CT may saturate, it doesn't do so instantly and the relay will most likely trip. If you are trying to coordinate three or four levels of ground overurrent protection and tripping times are long, then the ZSCT is probably not your best choice for sensing ground current. Generally though, industrial medium-voltage systems should not be solidly-grounded.

 

[DiscoP]

My understanding is that the CBCT will normally have an improved accuracy when compared to a residual connection because the fault current has only one iron core to magnetise vs three on the residual connection (assuming the same core materials and size).

It's benefits were more noticeable with older relays that have an increased connected burden (more voltage output required one the faulted phase CT leading to more excitation current lost in the other two CTs).

I've seen it used successfully on both instantaneous and long time delayed protection, on both solid and resistive grounded networks - provided the CT is adequately rated for the application.

A good CT supplier can design a CBCT specific to the application and even to best match the relay design, burden fault levels etc.

 

[Kiribanda]

CBCT/ZCT will be saturated if a current more than 20 times is passed. That is why we should take care of
ZCTs when used on solidly grounded systems due to considerably high GF currents.