Metering CT vs. Protection CT 2

Hi everyone

On a newly buildt substation, on the 380V side, faultlevel of almost 32kA, I've noticed something. The CT's been use there is 3200/1, but it is metering CT's. When I asked one of the guys if it is correct, (in my opinion it must be 10P10 protection CT's, otherwise the CT's would saturate under a big fault) he told me that the relay been used is an electronic relay with a very low burden. So in fact is there nothing wrong with metering CT's! Is this statement true?

regards

 

[busbar]

I don't what know the contract language specifies, but I'd ask for calculations and CT manufacturer’s literature supporting that. Then see that the label information on the CT matches/supports the paper.

 

It doesn't matter if the relay has a small burden, the point is that the CTs must be protection class, such that they will transform the fault current. The metering CTs are purposely designed to saturate just above their primary ratio and hence will not provide the protection relay with a true representation of the fault current.

 

[electricpete]

I'm no expert, but it certainly sounds reasonable to me. Saturation depends on the product of current and burden impedance. If you cut the burden impedance by factor of 10 you should be able to go almost a factor of 10 higher in current without saturating.

 

[ravi2002]

In modern digital relays, when the relay is used for metering, many protection, data/event logging and many other functions, the C.T used is for the protection C.T.

Just to illustrate an example,
we are using GE Multilin relay SR489, which has multifunctions like,
Overcurent/earthfault
differential protection and all other generator protection in one relay.
metering
data logginh and communication with SCADA.
disturbance recorder etc.

Now we require to give single C.T input. which one of the following to choose?
- Class x (suitable for differential protection)
- protection C.T 5P10
- metering C.T (cl 0.5)


we have choosen C.T of class 5P10 in the line as well as neutral end C.T.

If any further technical details on the above is required, please let me know.

 

[DiscoP]

You are able to purchase hybrid Metering/Protection class CTs for modern relays. They are basically the same core material as used in metering class CTs, but designed to provide the fault current too.
I agree with busbar, you should check the claculations otherise you've got yourself a very expensive ammeter !

 

[Forward]

As we know, to protect the instrument in the secondary of the metering CT, there is a factor Fs ( instrumnet security factor) for metering CT, it means when a fault occurs in the primary, the ration of growing about the secondary current can not be as high as the primary side. if the metering CT used for protection, the growing current will not be appeared accurately. That will strongly affected protection time and so on. in one word, it is not suitable to use the metering CT instead of the protection CT.

 

[Bung]

If the CT has a high enough knee-point voltage to overcome the winding resistance and the relay burden at the maximum fault level, it will work irrespective of what it is called. Do a mag curve and resistance check and you'll soon know the answer.


Bung
Life is non-linear...

 

[pentatek]

Just want to add another real life condition. At one of our power stations the fault recorder installation was a retrofit. Not enough "protection CT" cores were available, however there was a "metering" CT core spare. With the low secondary burden measured and used in the calculations it seemed to be OK. Then we had a generator "switch on to standstill" incident. The recorded currents showed no signs of saturation with this rather severe fault. Clipping was evident in the first culsy but was due to the 12bit resolution of the recorder. (full scale not big enough vs lousy resoltion at noraml loads). It must be appreciated that the Class 0.1 CT will saturate at about 5 times at rated burden. Therfore if the connected burden is much lower than rated burden, then the accuracy limiting factor increases substantially.