CR4® - The Engineer's Place for News and Discussion®

 Previous in Forum: Motor With High Starting Current And Long Starting Time Next in Forum: Synchronization of Alternator
Associate

Join Date: Aug 2008
Posts: 41

# Current Transformer Burden Calculation

04/10/2011 4:10 PM

Dear Engineers,

The rating of the CT is 600 - 300 - 150/1 A. The burden of the CT is 15 VA (1S1 - 1S4). Class 5P20.

1S1 - 1S4 = 15 VA

1S1 - 1S3 = ? VA

1S1 - 1S2 = ? VA

Kindly tell me, how to calculate the VA for the CT tappings?

Pathfinder Tags: CT Burden
Interested in this topic? By joining CR4 you can "subscribe" to
Guru

Join Date: Feb 2009
Location: Houston, USA
Posts: 973
#1

### Re: Current Transformer Burden calculation

04/10/2011 7:40 PM

- MS

__________________
Associate

Join Date: Aug 2008
Posts: 41
#2

### Re: Current Transformer Burden calculation

04/10/2011 8:10 PM

Dear Engineer,

Thank you for your response. Actually, the rated burden of the CT is 15 VA for the tapping 1S1 - 1S4. But I want to calculate, the Burden for the tapping 1S1 - 1S2 AND 1S1 - 1S3.

Kindly clarify.

Commentator

Join Date: Feb 2010
Location: India
Posts: 93
#3

### Re: Current Transformer Burden Calculation

04/11/2011 12:40 AM

I feel the VA rating of the CT is same for all combinations. Because at each combination, the secondary current is same. So the VA remain same.

Participant

Join Date: Nov 2009
Posts: 3
#4

### Re: Current Transformer Burden Calculation

04/11/2011 2:32 AM

As I suppose, the CT has secondary taps, and the primary winding is always the same (it number of wires remains constant).

In this case, in each tap, the Ampere-Turns are different, and also the burden will be different. (To maintain the same burden in different ratios, the Ampere-Turn has to be the same, and this implies changes in the primary winding, not in the secondary winding).

When you purchase the CT, if you ask for 15 VA in all ratios, then you will have 15 VA in the lower ratio 150/1 (1S1-1S2), and in the other ratios, you can have a bigger burden. An approach could be:

150/1 (1S1-1S2): 15 VA

300/1 (1S1-1S3): 25 VA (or 15 VA 5P30)

600/1 (1S1-1S4): 37 VA (or 15 VA 5P 37)

But when you purchase the CT, you do not say anything about the 15 VA in all ratios, then probably, the 15 VA will be only in the bigger ratio, or all winding, in 600/1 (1S1-1S4), and in the other taps, the burden will be smaller. An approach could be:

150/1 (1S1-1S2): 3.5VA (or 15 VA 5P5)

300/1 (1S1-1S3): 7 VA (or 15 VA 5P10)

600/1 (1S1-1S4): 15 VA

To know the exact burden, you have to know the internal resistance in each tap and the section of the core.

Best Regards

Off Topic (Score 5)
Guru

Join Date: Oct 2009
Posts: 2029
#5

### Re: Current Transformer Burden Calculation

04/11/2011 7:55 AM

The rated Burden of any CT is the sum of its internal burden and the external burden. And the internal (VA) burden would remain constant as at lower tap ratios the number of secondary turns would also change, thereby changing the Ohmic burden of the CT Secondary winding.

For example, assuming a bar primary (single turn) in your case, at a ratio of 600/1A, the primary ampere-turns would be 600AT and to offset this, the CT secondary of 1A should have about 600 turns.

If the Primary Current is chosen as 300A, then the Primary AT would be 300AT and to balance out the CT Secondary (of the same 1A) would now have only 300 Turns. So, also in 150A Tap.

As you can see at lower tap ratios, the number of turns and hence the internal burden of the CT would reduce.