Intuitive Explanation for Window CT

[111R]

As current flows through a bus that has a window CT wrapped around it, it's stepped down by the CT ratio. 100 A through a 100:5 ratio window CT results in 5 A secondary.

If 5 A of current is applied through the secondary winding of the CT, does this cause 100 A of current to flow through the bus? If the bus is de-energized and isolated, how is this ratio maintained? Where does the current flow?

 

[waross]

This question may be better answered on Project Board.

https://projectboard.engineering.com/

Bill
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"Why not the best?"
Jimmy Carter

 

[stevenal]

I suggest you begin by imagining a 1 to 1 CT and injecting a hundred amps. This CT could be removed without changing much and then you could inject your 100 amps directly into the bus. Where does this current go when the bus is isolated? This will not give you the final answer, but perhaps begin you on the right path.

 

[jghrist]

Obviously, if the primary bus is isolated, the primary current will be zero. All of the current injected in the secondary will be magnetizing, or exciting, current. If you apply voltage to the secondary to cause current to flow, the CT will saturate before very much current flows. The CT excitation curve shows the relationship.

If you apply 20V to the 100:5 tap of a CT with the following characteristics, the exciting current will be 0.4A. Apply more than 30V (above the "knee point"), then the CT saturates and current gets very large, limited by the resistance only. All with no primary current.

 

[RRaghunath]

If you like to see 100A on bus, injecting 5A on secondary, it is possible.
You just need to create a path for the 100A to flow.
This means you need to short P1 & P2 on the busbar on either side of window CT.

 

[stevenal]

Primary current is zero as jghrist indicated, but voltage will rise and the assumption the bus is de-energized is invalid. The assumption that CTs always deliver ratio current is also incorrect. It is important to know the excitation curve above was generated with an open circuit primary while applying voltage to the secondary. I find an equivalent circuit is helpful:

The excitation curve shows the non-linear characteristic of the magnetizing reactance labeled Xm in the circuit.