Trafo. Differential With Summation CT

Tigerdon ,

do you have regulator of voltage and if you have where is his position ( should be zero ). Do you *gave* 2500 A in all 3 phases or only in one. If the answer is only one phase source is often poor and there is a great waste and loss.

We have practice to energise the primary of transformer with low voltage (in my country is 3x 380V) and make a short circuit first in front and the latter case behind the current transformer on secondary side of Power Transformer .Then you can measure the current on the secondary of current transformers on both side and whether a residual current ih differential relay .

Of course one should be aware of the hazards associated with work on high voltage and particular activities in open secondary circuits of current transformers.

Good luck

Tigerdone , There's something strange in your scheme . . .is that the secondary voltage 6.9 kV or 6.6 kV .For good function of differential protection both feeders at the secondary of Power transformer terminals should have current transformers transmission ratio 1500- 1 A and then ratio of Summation transformer should be 1+1 / 1 A .

In your scheme and existing current transformers of feeders ratio of summaration CT must be 2+1 / 1 A and your test result in your scheme will be OK or 0,67 A .

Good luck .

If the phases are ok and in line. I believe what you got is ok. Can you just reconfirm what you have got as results and waht is expected. By looking at the problem, seems what you got is right.

Understand your problem.

Your 750/1 CT on the high side should be matched with 3750/1 CTs on the low side to match the transformation ratio of the transformer.

Unfortunately you a 2500/1 and a 1250/1 CTs - and these need to have their ratio changed to 3750/1.

The summation transformer MUST be »3 + 1.5 / 1« . . . and NOT »1+1/1« as you indicate. The 1250/1 transformer to be connected to "3" = 3750; the 2500/1 transformer connected to "1.5" = 3750. This will provide the proper configuration for the differential relay.

NOTE, the kneepoint of the summation CT should be approximately the same as the kneepoint of the 750/1 CTs.

It is not clear to me how you are conducting the test - where you are applying your current.

Svigaty is right and results testing of current transformers and connection gave expected results.

If your transformer is 6,6 kV on the secondary side ( you wrote 6,9kV), Olehwi gave you good ratio of transformation for summation transformer .

Tigerdone , no one should not accept advice on the forum without checking, especially my ,because they sometime can be inaccurate and dangerous .