Any CT, in fact, develops only a voltage at its secondary terminals, when a current is flowing in its primary. This is following Faraday's Laws of Electromagnetic Induction that whenever a current carrying conductor cuts a magnetic flux, an e.m.f. is induced in the conductor. When a current is flowing in the primary of a CT, a flux is set up in the core by the current. This flux, when it cuts the CT Secondary winding conductor, an e.m.f. is induced in the secondary winding conductor.
The magnitude of this e.m.f. is directly proportional to the rate of change of flux. So also, in PS Class CTs, when a primary current is flowing, a voltage is induced in the secondary terminals. Since, in a balanced protection scheme, many CT secondaries are connected in parallel, all the other CTs act as loads to the CT that is seeing the fault. Now this CT has to supply - apart from the relay operating current - the magnetising currents of all the other CT cores connected in parallel.
This has to happen when the maximum fault current is flowing. Hence, the Imag requirement will be dependent upon the number of CTs connected in parallel. For example, in a 4 CT REF Scheme, the CT on the faulty phase, should, at relay operating voltage, drive enough current to supply the magnetising losses of all the 4 CTs plus the relay operating current.
Hence, it is all the more imperative that the CTs, in such schemes draw only as little a magnetising current as possible so that maximum current is available for relay operation during a fault. That's the reason why the magnetising current is kept at a nominal value of 30mA at Vk/2.
Thanks for giving ur time,
Dont treat it as disrespect but i must say that for usual fault currents around 5-10KAs, such a small addition of magnetising current of the order of mA wud not be a problem. Thus i genuinly think that this can not be the reason for specifying magnetising current at Vk/2 or Vk/4.
U have good knowledge, please think about it....
I think you are shifting gears now! Is your question "why magnetising current is less than 30mA?" or is it "why it is less than 30mA @ Vk/2?"
If it is the second one then I think I can answer! You see, when you calculate the Vkp of a Class PS Ct, you multiply the anticipated maximum faut current by a factor of safety, which, conventionally is taken as 2. If this is taken as 2, then at the maximum anticipated fault current the CT would develop a secondary voltage of Vk/2. We are interested in knowing what would be magnetising current of the CTs connected in parallel, at this fault voltage. Hence, it is specified at Vk/2.
Your answer really gives a clear view of why it is specified at Vk/2 and not at Vk or Vk/10, it gives clear sense of HALF the question.
I think i am not able to clearly ask what i wish.
Let me ask the same in other way:
If Imag = 200mA at Vk/2 then wud it bring a difference than being <30mA, as for a CT in fault condition which is already catering KA's of current an addition of some milli amperes wud not be a problem.
Thus i genuinly think that this statement "Imag<30mA@Vk/2" has some other significance on some other important issue
Okay! It needs a better expert than me to clarify your doubts. I am sending by another mail to your mail ID: <email removed> - a technical paper on CT Spcifictions authored by an authority on the subject, Dr. K. Rajamani. Please go thro the same and lt me know if your doubts are clarified.
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Oh i was busy somewhere else and not having net access..
Thank you very much for the detailed paper...
That really clarified so many doubts.
Thanks again
CT Sizing
Re: CT Sizing
