current transformers

[powercam]

hi,
the use of class x, p, t CT's for unit protection is what is normally recommended, were we can specify what knee point is wanted etc...
what are the implications when we use class 5P or 10P ct's for unit protection. will the ct's satuarate more easily? are these types normally used for unit protection?

thanks

 

[scottf]

The 5P and 10P class also includes 2 other parameters, accuracy limit factor and burden rating.

A full protection CT rating would be something like 5P20 - 15 VA. '5P' defines the accuracy class. The accuracy limiting factor of '20' means that the CT will maintain the composite error limits of the accuracy class (in this case 5%) up to 20 times rated current, when the CT is loaded up to 15 VA burden (that's relay impedance plus round-trip lead impedance from the CT to the relay and back).

Accuracy limiting factor is directly proportional to applied burden. Therefore, if the actual connected burden is half of the rated burden, the limiting factor will double.

To put in terms of knee-point voltage, like for a Class X CT, for the example above...assuming a 1A rated secondary, the above 5P20-15 VA, would have a knee-point voltage of around 300 V or more....that's 20 x 1A x 15 ohms = 300V.