Understanding CT magnetization curve 5

[GavinCSY]

I know when a CT goes into saturation it starts to produce errors and problems to produce sufficient secondary current. So we do a secondary injection to determine the knee point of the CT. Ok so now i have a graph showing the excitation voltage and the excitation current.

My question is how to relate this curve to when the CT is in operation? eg. i have a knee point at 2700V@4.25mA for a protection CT with a ratio of 10 000:1. So at what current in the primary would cause the CT to saturate?

How do i know when i look at a magnetization curve that the CT is the right one for the job?

Thanks

 

[scottf]

Lionel-

Small correction, in a 0.3B0.2 metering class CT, the '0.2' stands for 0.2 ohms and not 0.2 VA.

Also, in the C800 protection rating, the '800' is the accuracy limiting voltage, which means the CT can drive an 8 ohm burden up to 800 volts based on a 5A rated secondary current at 20 times over-current:

5A x 20times x 8 ohms = 800 V

It also means if the applied burden is 4 ohms, then the CT will be linear up to 40 times overcurrent:

5A x 40times x 4 ohms = 800V

In other words, the burden rating is based on 20 times overcurrent, but the burden and overcurrent are linearly related, e.g. half the burden means twice the overcurrent.

 

[LionelHutz]

You're right Scott, 0.2 is the burden in ohms. Nice expansion on the relaying class rating.