PX class CT burden? 2

[homeco]

I've received specifications from a supplier for a PX class CT, but it doesn't have any information about a burden. When I queried this, they said that PX class CTs do not have a burden? Is this correct?

Not very experienced with specifying CTs.

 

[RRaghunath]

Class PX CTs are specified - Ratio, Knee point voltage (Vk), CT secondary Resistance (Rct) and magnetising current (Im).
Vk will dictate indirectly what burden can be connected to the CT.

 

[Parchie]

homeco (Electrical)(OP)

If you know the accuracy limit factor ALF, you can compute for the CT VA.
Here's the formula:
Vk = ALF x (VA/In + Rct x In); e.g. ALF = 20; Vk = 130 volts; CTR = 2500/5; Rct = 0.65 ohms
The VA = (5) x [(130/20) - (0.65 x 5)] = 16.25 VA

 

[edison123]

Any reason why the burden is not explicitly mentioned in PX class?

Muthu

http://www.edison.co.in

 

[Parchie]

@edison123,
I cannot answer in behalf of the manufacturer but I guess when you are given the details of a specific CT such as the internal resistance and the knee-voltage, you can be guided as to what through fault current will the CT behave properly. We will know the limit of the load resistance allowed to connect to the said CTs for a desired accuracy limit, IMHO.

 

[FreddyNurk]

edison123, I'm sure I've read it in other sources (such as Cahier's Techniques) but can't find the correct reference.
This has an excellent description of the differences, put better than I can.

EDMS Australia

 

[RRaghunath]

IEC doesn't ask for indicating VA for Class PX CTs.
However, almost all the relay technical literature indicates formulas for CT sizing with Class P as well as Class PX CTs.
It is only high impedance circulating current differential protection where Class PX is made mandatory.

 

[edison123]

Thanks, guys.

Muthu

http://www.edison.co.in