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Conceptual Question No.11 - Transient flux in the core of C.T
Why in estimating the transient flux in the core of C.T magnetizing current is neglected???
Author : Abhishek Kumar - From: India
Sat, January 29th, 2011 - 18:59
According to ampere law, the magnetizing flux is related to magnetizing current which made it, and those cannot independent together generally. But if your concern is transient flux variation can to cause high voltage spike in opened secondary condition, there are some special conditions. The peak voltage occurs when the rate-of-change of core flux is highest, which is approximately when the flux is passing through zero. The maximum flux density that may be reached does not affect the magnitude of the peak voltage. Therefore, the magnitude of the peak voltage is practically independent of the CT characteristics other than the nameplate ratio.
One series of tests on bushing CT's produced peak voltages whose magnitudes could be expressed empirically as follows:

where e = peak voltage in volts.
Z = unsaturated magnitude of CT burden impedance in ohms.
I = primary current divided by the CT's nameplate ratio. (Or, in other words, the rms magnitude of the secondary current if the ratio-correction factor were 1.0.)
The value of Z should include the unsaturated magnetizing impedance of any idle CT's that may be in parallel with the useful burden. If a tap on the secondary winding is being used, as with a bushing CT, the peak voltage across the full winding will be the calculated value for the tap multiplied by the ratio of the turns on the full winding to the turns on the tapped portion being used; in other words, the CT will step up the voltage as an autotransformer.
Author : Hamid - From: Iran- Firouzabad in Fars
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