Dual ratio CT parameters (Knee point voltage, class, magnetizing current, resistance)

[electrical429]

I mostly see dual ratio CT specs where only one set of parameters is shown. And I was always wondering if you can make out parameters for the lower ratio with given information or not. i.e.:

1. Knee point voltage. Based on the following formula Vkp = K * I(fault)/CTratio * (RCT + RL + RR) lower ratio Vkp should be two times higher than higher ratio Vkp (the smaller the ratio the larger the voltage on the secondary side during a fault because the secondary current is higher). However, I've seen some CT specs where the same Vkp was shown for both ratios and also some specs where lower ratio had Vkp two times smaller. Can dual ratio CT be designed with any Vkp at any ratio or is there a correlation between ratio and Vkp?

2. CT resistance. I assume that if CT resistance for a 1200/600/1 CT is given as 2.4 Ohms at 1200 it would be 1.2 Ohms at 600/1?

3. Magnetizing current at knee point voltage. This depends on answer for question 1. If Vkp is proportional to ratio then I assume that magnetizing current will also be proportional to ratio.

4. 5P20 class CTs. I saw the following CT spec 1200/600/1, 5P20 at 1200/1 and 5P10 at 600/1. To me this did not make sense because 5P20 means 5% error up to 20 times rated current, which means that at 1200/1 up to 5% error will be until 1200x20= 24000A and at 600/1 up to 5% error will be until 600x10=6000A. Shouldn't it be the other way around, i.e. 5P10 for 1200/1 and 5P20 for 600/1. That way both ratios will maintain 5% error accuracy until 12000A.

 

[DiscoP]

Rightly or wrongly we re-calculate that the voltage and Rct will rise proportionally to the turns and the exciation current will lower proportionally.

If you examine test results you will find that this mostly holds true.

So a 5P20 is more like a 10P10 on half the turns, or a 2.5P40 on double the turns. This can only be treated as a rule of thumb however.

If it fails on one of the ratios where it is not explicitly stated on the nameplate, there is no way you can technically fail the CT if it tests OK on the ratio stated on the nameplate.

It doesn't happen very often, but will come up.

 

[electrical429]

Ok, so do I understand correctly, in case of Class PX CTs (assume 1200/600/1) knee point voltage on the lower ratio can only be two times smaller than knee point voltage on the higher ratio. Same with resistance. But the magnetizing current will be the opposite. Is below image what you would normally see on dual ratio test graph?

With regards to 5P20 CTs I have transmission company spec in front of me which says 1200/600/1, 5P20 on 1200/1, 5P10 on 600/1. Similarly to class PX CTs, is there a correlation between turns ratio and error? Can you make 5P20 on both turns? Or make 5P20 on 600/1 ratio and 5P10 on 1200/1 ratio?

 

[RRaghunath]

Guess you are talking of bar primary CTs where dual ratio is achieved by taps on the secondary winding of the CT!
In such a case, the number of ampere-turns and also the output will be reduced at the taps, but the short-circuit capacity remains constant. The accuracy rating applies to the full secondary winding, unless otherwise specified. At other taps it is proportionately lower.

 

[DiscoP]

Yeah - that graph looks about right.

You can only say both windings have a 5P rating if stated on the nameplate (or if you can verify it with a test).

Note that rating means the error is less than 5% at ALF and connected burden. 0.1% error or 4.9% error can still be called 5P.

I would assume the error doubles if the turns is halved if there is no test data showing otherwise

Most of the error is excitation current. If there are less turns, it takes more current to excite the core.