Protection CT - Choosing a CT based on your Max Fault Current
Protection CT - Choosing a CT based on your Max Fault Current
(OP)
Good Afternoon,
I need help in determining whether the CT we have meets the fault requirements.
The CT is a 1000:5 5P20 30VA Protection CT - I have a saturation curve with a knee point at 140V. Rct (Secondary Winding res.) = 0.245 @75C
The max fault I need to design for is 32 kA.
I understand that the correct CT will need to operate at 32kA without saturating i.e. before the knee point.
How can I determine this ?
I appreciate all the help.
Thanks Again
Coren
I need help in determining whether the CT we have meets the fault requirements.
The CT is a 1000:5 5P20 30VA Protection CT - I have a saturation curve with a knee point at 140V. Rct (Secondary Winding res.) = 0.245 @75C
The max fault I need to design for is 32 kA.
I understand that the correct CT will need to operate at 32kA without saturating i.e. before the knee point.
How can I determine this ?
I appreciate all the help.
Thanks Again
Coren






RE: Protection CT - Choosing a CT based on your Max Fault Current
Vsec = 32 x 5A x (Rct + Rb) < 140V
Works out to being ok if Rb < 0.63 ohms.
RE: Protection CT - Choosing a CT based on your Max Fault Current
If the knee point is measured @140V with I=0.124A giving the measured CT burden at 17.36VA.
As the CT is rated at a 30VA burden - is it safe to estimate that the CT can handle a current 34.56 times the Primary (20 * 30/17.36 = 34.56) =~34kA prior to saturation ?
Unfortunately at this time I dont yet know Rb - would the above derive a good estimate/indication ?
Thanks Again
Coren
RE: Protection CT - Choosing a CT based on your Max Fault Current
The rating of 5P20 - 30 VA means that the error won't exceed 5% at 20 time over-current when loaded at 30 VA.
20 x 5A = 100A
5% of 100A is 5A, which means excitation current can't exceed 5A up to a secondary voltage of (30/(5^2)) * 100A = 120 V
The CT was measured with an Iexc=0.124A at 140V, so it meets the class by a good margin.
With all that said, you still need to know your Rb to confirm what over-current the CT can handle without saturation. There really isn't any other way to do it correctly. You can certainly approximate it for a good estimate. You need the lead wire size, the round-trip lead length, and the relay input impedance. If it's a modern electronic relay, then it can really be ignored, since the leads will dominate the burden calculation.