Interpretation of Low Magnetizing Current CTs (Current Transformer) 3

[krisys]

In one of the 11kV system earthing study the final recommendation is to use the low Resistance earthing to limit the earth fault current to about 125A. In the meantime the study has also suggested to use the low magnetizing current CTs for the earth fault detection.

When specifying a CT with low magnetizing current, effectively what we are trying to achieve?

Is it another way of telling that the CT shall be of high accuracy class?

 

[cuky2000]

For low resistance grounded application, usually CT accuracy class C400 can be used.
Below is some information related to the CT for this application that hope helps.

 

[DM61850]

You want low magnetizing current so that your measurements at low current don't have a lot of error. I suspect that they want you to get a low ratio CT, something like a 50/5 so the amount of current in your secondary circuit is large in comparison to the excitation current.

 

[stevenal]

Big difference between the some of the CT locations Cuky shows above. 1 requires a CT that can handle normal balanced load and three phase fault current. The core balance and neutral CTs only see ground fault current.

 

[RRaghunath]

Low magnetising current CT are recommended if the earth fault protection when the earth fault current is sum of three phase currents.
In power systems with earth fault current limited to a low value (through neutral grounding resistor), the magnetising current of phase CTs restricts the sensitivity of protection if the magnetising current of CTs is high.
Ideal will be to provide Core balance CTs (CBCTs) in each of the outgoing feeders.
Then, the low magnetising current CT requirement can be limited to incomer and bus coupler CTs and a few large feeders which may pose challenge in installing CBCTs.

 

[krisys]

Thanks for all the participants. Raghunath's response specifically stands out.

The terms like low magnetizing current CT and low leakage reactance CT were the terms were little intriguing for me for long time. Now the application aspect of this is understood. That too after 30 years of of working.