CT demagnetization

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I am sorry if this is a long one but as i read through this topic i had some doubts to be cleared.... This is a topic related to Current Transformers

""""If the current which is interrupted is high, or if it contains a large
dc component and is interrupted when total flux is high, remanence will be substantial, perhaps being above the flux equivalent of the knee point.
When the current transformer is next energized, the flux changes required will start from the remanent value, and if the required change is in the direction to add to the remanent flux, a large part of the cycle may find the current transformer saturated. When this occurs, much of the primary current is required for excitation, and secondary output is significantly reduced and distorted on alternate half cycles. This condition can be corrected by demagnetizing the current transformer. It is accomplished by applying a suitable variable alternating voltage to the secondary, with initial magnitude sufficient to force the flux density above thesaturation point, and then decreasing the applied voltage slowly and continuously to zero."""""""


Question: as i read this i thought of an example... now say a certain situation occurs like this for the first time a fault occurs and the breaker trips(say CB of outgoing feeders) and after 5 minutes again a fault occurs and again another occurs.. so by now the above said instance has already occured i.e (""""When this occurs, much of the primary current is required for excitation, and secondary output is significantly reduced and distorted on alternate half cycles."") so does this mean that the personal must go to the substation to apply a suitable variable alternating voltage to the secondary, with initial magnitude sufficient to force the flux density above the saturation point, and then decreasing the applied voltage slowly and continuously to zero so the the CT is demagnetized???


****************relay***********************
 

ohmhead

Senior Member
Location
ORLANDO FLA
Well nice reading US PATENT 6160697 methods of demag of ct transformers . Its nice reading patents they really get into the how and why and its save lots of writing just thought you would be interested in this one .take care
 

richxtlc

Senior Member
Location
Tampa Florida
I am sorry if this is a long one but as i read through this topic i had some doubts to be cleared.... This is a topic related to Current Transformers

""""If the current which is interrupted is high, or if it contains a large
dc component and is interrupted when total flux is high, remanence will be substantial, perhaps being above the flux equivalent of the knee point.
When the current transformer is next energized, the flux changes required will start from the remanent value, and if the required change is in the direction to add to the remanent flux, a large part of the cycle may find the current transformer saturated. When this occurs, much of the primary current is required for excitation, and secondary output is significantly reduced and distorted on alternate half cycles. This condition can be corrected by demagnetizing the current transformer. It is accomplished by applying a suitable variable alternating voltage to the secondary, with initial magnitude sufficient to force the flux density above thesaturation point, and then decreasing the applied voltage slowly and continuously to zero."""""""


Question: as i read this i thought of an example... now say a certain situation occurs like this for the first time a fault occurs and the breaker trips(say CB of outgoing feeders) and after 5 minutes again a fault occurs and again another occurs.. so by now the above said instance has already occured i.e (""""When this occurs, much of the primary current is required for excitation, and secondary output is significantly reduced and distorted on alternate half cycles."") so does this mean that the personal must go to the substation to apply a suitable variable alternating voltage to the secondary, with initial magnitude sufficient to force the flux density above the saturation point, and then decreasing the applied voltage slowly and continuously to zero so the the CT is demagnetized???


****************relay***********************

CT saturation can cause relay misoperation if not corrected. An example of this is a differential operation for a fault outside the differential's zone of protection. If it is suspected that the CT is magnetized, then the procedure you described must be performed to restore the hysterisis curve back to normal. When we installed CTs for the first time we applied the same test to see if we could reach the saturation point (waiting for the ammeter to indicate no change in current, or little change in current for a corresponding change in voltage. We then slowly brought it back to zero.) We did this to check the impedance of each CT prior to installing them in a circuit.
 
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