CT Question

[Elec81]

I have a quick question about current transformers.

I have a relay that requires a CT with a burden of 2.5VA (1A secondary CT) or 7.5VA (5A secondary CT).

I only currently have access to a 400:5 (2.5VA burden) 5P20 CT. However, as my fault and load current is low (less than 80A and 1600A respectively) I was wondering if I could set the relay up as 80:1 and use the 400:5 CT. To my mind this then satisfies the burden and ratio requirements as 80A through my 400:5 CT = 1A and the CT burden is 2.5VA.

Any thoughts welcome.

 

[buzzp]

Those burdens are likely the max burden so I think you are fine. If the 1A and 5A are wired to different points on the relay, you can't wire up a 5A secondary to a 1A terminal. However, usually, they are the same terminal and its just a programming change.

 

[Elec81]

Hi

Thanks for the reply. Just checked the relay and in does have separate 1A and 5A inputs.

Why couldn't I connect the 5A secondary to the 1A input? As I say, I'm confident the secondary normal running load won't exceed 1A.

Thanks again in advance.

 

[che12345]

Dear Mr. Elec81

Q. '...I have a relay that requires a CT with a burden of 2.5VA (1A secondary CT) or 7.5VA (5A secondary CT)...."
A1. Your CT is 400/5 (2.5VA. 5P20). Your load is 80A. The output with 80A load would be 80/400 x5 =1A, fine. Your short-circuit current at 1600A, it would be 1600A/400A = 4 times, fine.
A2. With the existing CT 400/5 (2.5VA. 5P20) would be fine when connection [is made on the 1A input] connection (if the relay is with 1A and 5A input connections. If it is with only one 5A input, you are also fine.
A3. BTW: Your load 80A is 20% of CT primary 400A . It would be "better" to have a CT say < two times the load current. This is beside the point, as your are already having the CT in hand. There is NO necessity to replace your existing CT.
Che Kuan Yau (Singapore)

 

[Elec81]

Thanks for the replies.

Can someone explain why connection to the 5A input would be ok? As the CT is only rated at 2.5VA and the relay manual states a 7.5VA burden is needed when using the 5A connection.

Also, I have since found some 1A secondary CTs with a rated burden of 3VA. Would there be any advantage of using these? I understood it wasn't good to have a burden exceeding the relay rating, as the CT output voltage would be excessive. However, I was wondering if a resistance (dummy burden) could be fitted in series with the relay. Any thoughts welcome.

Any literature you can point me towards, books etc. would also help.

Thanks again for peoples help with this.

 

[che12345]

Dear Mr.
Q1. "... Can someone explain why connection to the 5A input would be ok? As the CT is only rated at 2.5VA and the relay manual states a 7.5VA burden is needed when using the 5A connection."
A1. a) Relay internal burden is 2.5VA for 1A input, i.e. Isq x R = 2.5VA , where Isq = 1A x 1A =1 . Therefore internal resistance is R= 2.5/1 = 2.5 Ohm.
b) Relay internal burden is 7.5VA for 5A input, i.e. Isq x R = 7.5VA , where Isq = 5A x 5A =25 . Therefore internal resistance is R= 7.5/25 = 0.3 Ohm.
c) When with 1A CT secondary but connected to the relay 5A input terminal, the AV burden = 1A x 1A x 0.3 Ohm = 0.3 VA. Therefore it is fine.
Q2. " ... I found some 1A secondary CTs with a rated burden of 3VA. Would there be any advantage of using these?
A2 Fine, there is no significant advantage or disadvantage. Use the ones that is [lower in cost and ensure that there is sufficient room] for it.
Q3. "... I understood it wasn't good to have a burden exceeding the relay rating, as the CT output voltage would be excessive. However, I was wondering if a resistance (dummy burden) could be fitted in series with the relay. Any thoughts welcome ".
A3. In general, it is not "recommended" to select a CT VA that is >> than the relay VA. This is in view of the [size and cost], [including what you had mentioned]. In normal cases, it is NOT necessary to insert any resistor in series to match the CT VA to that of the relay.
A4. BTW As for your case, the short-circuit current is 1600A. I see no problem with your 5P20 CT. You may use a lower say 5P5 or 5P10, which would be at a [lower cost and also smaller in physical size]. There is NO necessity to replace your existing CT. Use what is available at a lowest cost and size that would fit the installation.
Che Kuan Yau (Singapore)

 

[buzzp]

You can hook up the 5A to the 1A input with some caution. Your short circuit current is only 1600A so the relay should be able to handle 4A on its input without causing damage. If you want to be sure, ask the relay manufacturer but you should be fine. selinc.com has some good papers on this subject.

 

[LionelHutz]

400:5 CT with 1600A of current will put 20A on the CT secondary. Check the relay 1A input specifications to see what it can handle. At 20X rated current, you will likely be into the time limited range of the input.

I really don't see the point though with a modern digital relay. It will read the 5A CT connected to the 5A input just fine.

 

[LionelHutz]

Also, got a link to the 400:5 CT with 2.5VA?