Very low Rct
Very low Rct
(OP)
I am looking at the differential protection CT's (existing) for a 390MVA generator transformer. There are two 330kV CT's of interest of the trfr nameplate, both manufactured to the same standard (AS 1675:1986).
CT1 = 2250/2400/1A, 0.05PL1925R9.5 on 1/2250T.
CT2 = 2000/4000/1 0.025PL1400R6.5 on 1/4000T
What puzzles me is the winding resistance of CT1 is 9.5/2250 = 4.22mohms/turn but for CT2 it is 6.5/4000 = 1.625mohms/turn. I'm curious as to why there is such a huge difference between them and how is it possible to achieve 1.6mohms/turn with 4000 turns. Surely it would mean a very bulky CT if a thicker conductor was used to achieve the low resistance?
CT1 = 2250/2400/1A, 0.05PL1925R9.5 on 1/2250T.
CT2 = 2000/4000/1 0.025PL1400R6.5 on 1/4000T
What puzzles me is the winding resistance of CT1 is 9.5/2250 = 4.22mohms/turn but for CT2 it is 6.5/4000 = 1.625mohms/turn. I'm curious as to why there is such a huge difference between them and how is it possible to achieve 1.6mohms/turn with 4000 turns. Surely it would mean a very bulky CT if a thicker conductor was used to achieve the low resistance?






RE: Very low Rct
RE: Very low Rct
RE: Very low Rct
Gents, thanks for the insight.
RE: Very low Rct
CT1 Overall Differential Protection
Turns Ratio - 1/2250-2400T CLASS 0.05PL1925R9.5 on 1/2250T
Rated secondary current - 0.58Amp
Thermal limit secondary current - 1.0Amp
I interpret this as a CT with a 2250A and 2400A tapping. If on the 2250A tapping then rated primary current is 2250/sqrt(3) = 1300A. The secondaries are connected in delta (checked the drawings) which means that it is overall a 1300/1A CT. Similarly if on the 2400/1 tap then it is effectively a 1385A/1A CT.
Performance wise, if Rct = 9.5ohms then if connected in delta the star equivalent Rct(star) = 9.5/3 = 3.167ohms.Since line volts equals phase volts, Ek = 1925V for the equivalent star connected CT secondaries. Thus the CT could be expressed as follows:
1300/1, 0.05PL1925R3.167
Wondering if my interpretation is correct? Anyone confirm? Thanks.