Very low Rct

[veritas]

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?

 

[ScottyUK]

At a guess - and ScottF will no doubt correct me if needs be - the first CT has a much higher kneepoint voltage than the second one, and the first CT will therefore have a core of much greater cross-section area resulting in each turn being substantially longer on this CT than that with the lower kneepoint.

 

[scottf]

Yes, core cross-section being different will result in different ohms/turn. Also, the 2250/2400:1A CT could have 2 primary turns, which means the 2250:1 tap would actually have 4500 turns. And, of course, different size secondary magnet wire could have been used as well. Also, depending on the if the CTs utilize different physical designs, the cores could have different diameters (ID and OD), which will also impact the ohms/turn. There really isn't any way to tell for sure with just the ratings.

 

[veritas]

In addition CT1 amounts to 0.86 volts/turn whereas CT2 yields 0.35 volts/turn. I would tend to agree that CT2 has a substantially smaller core area. Both CT's come from same manufacturer, in same trfr, so I would tend to think same materials used, different core design.

Gents, thanks for the insight.

 

[veritas]

Reading the nameplate again, I realised that I did not quite paint the whole picture. The nameplate details read as follows:

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.