METERING CORE OF CT WITH HIGHER KNEE POINT VOLTAGE

[appunni]

SIR,
WHEN I TESTED A 220KV, 800/5 CT, I GOT HIGHER KNEEPOINT VOLTAGE AND LESSER MAGNETISING CURRENT FOR ITS METERING CORE (CLASS: 0.5) THAN ITS PROTECTION CORE (CLASS: 5P10). WHICH CORE SHOULD I USE FOR PILOT WIRE PROTECTION AND OVERCURRENT PROTECTION? PLEASE NOTE THAT BOTH PROTECTIONS ARE TO BE CONNECTED TO THE SAME CORE AS THERE ARE ONLY ONE PROTECTION AND ONE METERING CORE FOR THE CT. PLEASE EXPLAIN WHAT WILL HAPPEN IF THE METERING CORE WHICH IS HAVING HIGHER KNEE POINT VOLTAGE AND LESS MAGNETISING CURRENT COMPARED TO PROTECTION CORE, IS CONNECTED TO THE PILOT WIRE AND OVERCURRENT RELAYS. THANKS.

 

[nraju]

appunni,

Beyond 120% of the rated current, metering CTs must saturate at a specific multiple of rated current (generally 5 or 10 times rated current) so that the instrument can be protected. The phase displacement limits and ratio error are also totally different between a protection and a metering CT. If I were you, I would use the protection core for the pilot wire and overcurrent protections, irrespective of what the magnetization characteristics are.

 

[scottf]

nraju-

Under IEC, the metering core does not have to saturate quickly, unless a security factor is specified, i.e. Cl 0.5 Fs5. If the Fs 5 is left off, then it will probably not saturate that quickly, as the core material used to get quick saturation is much more expensive than normal Si steel.

I also agree with your suggestion to use the core as they are rated.

 

[appunni]

THANKS FOR VALUABLE COMMENTS.