CT selection for a generator line

[bettoni]

I´m investigating about CT selection, from all the papers about current transformers can´t found any method for TC selection on a generator line. The ANSI/IEEE rules do not apply because of the dc offset of the generator contribution to the fault. My interest is to obtain good measured signals for line relay protection (differential squemes, etc). Please send some information for this case.

 

[scottf]

What is your specific concern?

Is it the measurement of the current during the fault, or the ability of the CT itself to withstand the fault current? I assume since you mention DC offset, you are concerned with the CT saturating during a fault. If you don't think the ANSI/IEEE 'C' class protection rating will work, you may want to check into the IEC or CSA special transient protection class CTs (normally called TPY, TPZ, or TPX). This rating allows one to specify system parameters such as primary time constant, percentage offset of initial peak and then the manufacturer can determine what is called the transient dimensioning factor to drive the CT core design. Normally this is done with gapped core design. In my experience, though it is generally overkill and will cost a good bit extra, but if you think you need it, then...


You may want to contact Ritz, as they make just about any kind of IT to any standard and have pretty good technical folks. Try

http://www.ritzusa.com

for starters. I think they have contact info there.

 

[electricpete]

DC offset is not unique to generating stations, although it is more severe there.

Maybe this GE tech note on dimensioning ct's will help:

http://www.geindustrial.com/pm/notes/ger3973.pdf

 

[dpc]

All of the generator applications that I have been involved with used standard ANSI Relaying Accuracy CTs. This includes generators up to 900 MVA. The dc offset and possible saturation issues need to be considered, but if applied properly, there should not be a problem.