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bonala123
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Join Date: Sep 2009
Posts: 12

CT Magnetisation Current Ie

04/15/2013 8:34 AM

Hi

We are testing Current Transformer part of 400 kV GIS. During excitation characterstic testing, the excitation current Ie at Vk is more than rated value (on the name plate). The same current as per factory test is more than Site value. The manufacturer states that the reason is due to interferance of switchgear capacitance and Grading capacitors of Circuit breaker.

However, as per IEC the excitation current shall not be more than name plate value. Can any one help me whether the manufacturer is arguing right?
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electricalexpert65
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#1

Re: CT Magnetisation Current Ie

04/15/2013 9:00 AM

Just a thought!

Generally, the name plate excitation current for a CT is given at a fraction of the Vkp, such as Imag < x mA at VkP/2 or at VkP/4. If so, at VkP, the I mag would definitely be more. Pl. cross check for this.
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bonala123
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#2
In reply to #1

Re: CT Magnetisation Current Ie

04/15/2013 9:23 AM

Thank you for advice. However, do you think Ie will be more if measured with installed in switchgear than measured seperately?
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lyn
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#3

Re: CT Magnetisation Current Ie

04/15/2013 12:31 PM

These tests are run in a specific way for a reason. It's time to hire an engineer. Quit guessing.
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olehwi
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#4

Re: CT Magnetisation Current Ie

04/16/2013 1:41 AM

First - "what are the actual numbers"? Saying current is "more" is a meaningless expression. Lets see the specifications and the test numbers.

Secondly - switching and grading capacitors do not cause "interference" - so what is the manufacturer talking about? Perhaps he means that the capacitors will draw a bit of current if voltage is applied to them ? ? You can always test with the associated breaker OPEN and then CLOSED to what difference this will make. Also, the capacitoes are very small - in the order of 1000 picofarads - thus will draw a minute current. In addition to this - this current is "capacitive" and should reduce the "inductive" current of the CT.
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bonala123
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Join Date: Sep 2009
Posts: 12
#5
In reply to #4

Re: CT Magnetisation Current Ie

04/16/2013 10:31 AM

Thank you for your message OLEHWI. It is very relevant asnwer to the question. Adding to your view point, there would be very less voltage generated in the primary (also open circuited) due to applied voltage on the secondary of CT.

The other details for typical CTs are: 4000/1, >300V, <30 mA.

FAT values for Vk, Ie - 29.6 mA @ 300 V

At Site_ 36.68 mA @ 300V.

Also there is lot of difference in FAT values of different CTs. For example, the range of Ie at Vk varied from 17.6 mA to 29.6 mA for different CTs (Same ratio, same name plate details).
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SHOCKHISCAN
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#6

Re: CT Magnetisation Current Ie

04/16/2013 11:09 AM

Disconnect and isolate the capacitors, then run the test and see if the current value decreases below the nameplate acceptable value.

If the current readings decrease to a lower acceptable value then you know the capacitors are the culprit.

If the readings do not decrease to a lower acceptable value then there is definitely something else wrong with the CT circuit.

It may be a manufacturing fault but it could also be something inflicted during the installation/construction.

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bonala123 (2); electricalexpert65 (1); lyn (1); olehwi (1); SHOCKHISCAN (1)

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