CT specification. 2

[ZapSib]

Hello,

I am dealing with specification of CT cores in order to ensure the correct operation of protective relays.
The specification is mainly done in accordance with IEC 44-1 (formerly IEC185) and also IEC 44-6.

A required data is Rct (internal secondary resistance of the CT).
Estimation of this value may be quite challenging.
I wonder how others members deals with this when calculating CT data.
I also welcome feedback from CT manufacturing side: how would you like to receive CT specification?

Regards.

ZapSib.

 

[scottf]

What accuracy class are you trying to specify? Since you mention IEC44-6, I assume you want to specify a TP type core?

 

[dpc]

Actually, if you know the ratio, you can generally figure out the number of secondary turns, make an educated guess at a wire size and total length based on physical size and get in the ballpark.



"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[scottf]

DPC-

That depends. If the CT is a bushing CT (meaning 1 primary turn) then possibly yes, although the Rct varies quite a bit with the diameter of the CT core.

If it's an insulated CT, specifically a wound-type primary CT, then simply knowing the ratio won't exactly tell you the number of secondary turns, since that will depend on the number of primary turns used.

With that said, when specifying CT performance ratings, the goal is not to figure out what the Rct is, but rather to calculate the maximum Rct that is allowed and/or make the Vkp or whatever parameter being used for a TP core, a function of the Rct.

 

[ZapSib]

Scottf,

I went though some all threat where you already said that it might be hard to get Rct figures, hence the "quite challenging".
Regarding CT class, most of the time I have to manage with 5P or 10P CT.
The aim is to define requirement from protective relays need.

My understanding of your reply is that I will have to specify a maximun value for Rct : the issue is how can I specify realistic value of Rct? ( Rct value with the corresponding VA).

Rgds.

 

[Ohmly]

We specify in terms of (for example)

Vk = 125(Rct+3)

If the CT has an Rct of 2 ohms, Vk is 625 Volts.

 

[slavag]

Hi.
I do calculation of CT data from relay side, per project.
I calculated a worst case ALF according to the similar CT's Rct, what I have data with some safety factor ( say + 20%).
Secondary burden is calculated according to relay perfomence
( today more or less it's same for all mnf.) and current loop according to longer cable.

As Scottf saied, we don't request Rct from CT mnf, only:
5Pxx and xxVA per core. In few cases, we asked CT mnf about estimated Rct. In the worst case possible change cross section of current wiring.

Best Regards.
Slava

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