Advantages of using a CT having 1A Secondary

Dear Sir,

Normally for MV,EHV (11 and above) CT secondary current is 1 A and and for LV (below 11 kV) CTs are chosen for Secondary Current 5 A is used. All the Protectioncircuits are designed for these rating only.

Best Regards,

N.Suresh

Hello lightning.arrester,

The main advantage in selecting the CT of 1A is reducing the ohmic burden which can be seen from following equation.

Rated VA burden = I^2 * rated external ohmic burden where I = rated secondary current.

Since the external ohmic burden remains constant during operation, the CT burden keeps on varying as per the variation in I.

I leave this answer here and ask you to please calculate the Rated burden with I= 1 A and I = 5 A. You will find the main advantage.

Wish you All Merry Christmas & Prosporous 2009.

Yes I agree with the reply given by Mr. DD, I would also like to throw some lights in this topics in details.

Secondary Current of CT

The standard CT secondary current ratings are 1A & 5A,The selection is based on the lead burden used for connecting the CT to meters/Relays.5A CT can be used where Current Transformer & protectives device are located within same Switchgear Panel.

1A CT is preferred if CT leads goes out of the Switchgear.

For Example if CT is located in Switch Yard & CT leads have to be taken to relay panels located in control room which can be away.1A CT is preferred to reduce the load burden.For CT with very High lead length,CT with Secondary current rating of 0.5 Amp can be used.

In large Generator Circuits,where primary rated current is of the order of few kilo-amperes only,5A CTs are used, 1A CTs are not preferred since the turns rations becomes very high & CT becomes unwieldy.

thanks

SJ