7/8/2013 6:29 AM | |
Posts: 47 Rating: (8) |
Hi, The selection of CTs for a particular fault level is critical... refer the attached formula used for calculating the suitability of Protection CTs i.e. 5P10 or so... The voltage across the secondary terminals of a CT can be calcualted:- U2eff= S/ I2 In the even of short circuit:- U2 = 50 x U2eff with the secondary terminals open circuited : U2 =Us x AfexW2 Where: U2eff: effective rms value of CT secondary voltage U2: peak value of secondary voltage Us: specific magnetizing voltage which depends on the core materila and the field strength Afe: Cross sex. area of core W2: number of CT secondary turns S: Power consumption of connected load I2: Secondary Current The above formula shows when U2eff uncreases only slightly in the saturation region ( current more than overload zone of IDMT characteristic curve) of the CT, the peak value U2 can increase substantially, i.e. the limiting effect of eddy currents has a little effect on it. Thus the peak of set voltage is not merely 2 times the effective value, but substantially higher. It can be typically approx 50 tumes of effective value. U2 genrally attains values above 50V in the event of s/c in the primary. Hope it clear your doubt and selection easier. AttachmentCT @ Short circuit.pdf (388 Downloads) |
Last edited by: Scorpio31 at: 7/11/2013 12:35 PM |
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