current transformer sizing for relaying protection

[cmelguet]

Hi,

Can someone give a procedure to correctly size a current transformer to be use in protection ?. I guest it should be something like:

- pick ratio to be below of the nominal current of the system.
- pick accuracy limit factor to be above the maximum fault current.
- pick VA of transformer to match burden of the relay and cables

How you determine the knee point you should specify?
How you determine the correct secundary impedance you should specify?

Thanks.
Regards
Cristian

 

[Marmite]

Attached paper may be of use.
Regards
Marmite

 

[odlanor]

assume IEEE standards:

A customer needs to install a system in which CT
rated current - now called IN - is 100A.
And he knows that when a short circuit,
electrical current reaches a level of 2000A (ICC). And that
the equivalent load of the relay to the wiring more
transformer is 100 VA.

For the CT protect the electrical system of this level of
short circuit, there is a need to design a
core protection that does not saturate up to 20 times the
rated current. Assuming that the client wishes
response has an error of less than 10%,
specification will say:
• core protection 10P20 - 100 VA (10B400) for
meet a load of 100 VA relay and not to saturate
a voltage between terminals 400 secondary
Volts (for IS = 5A).

kneeVoltage see annex

 

[cmelguet]

Ok, thanks, it is clear how to size a transformer of the P type under the iec60044. But what about the PX type?. You need to specify the rated primary, secondary, ratio, rated knee point,secondary resistance, resistive burden and dimensioning factor. Where you get all this information from? and how you deside if you should use a P type or a PX type?. Cheers,

 

[slavag]

Hi.
You can find additional information about CT ( P, PX or TPS) in the next document ( see document presented by Marmite, no 194 ) next number 195,writed by Paola Fonti too.
Good Luck.

P.S.
IEC60044-6 help you too.
You can try found a good Alstom document:

http://www.scribd.com/doc/48281340/B-CT-EN-AP-C11