Interposing CTs

[Cerkit]

Hi,

I am trying to calculate what interposing CT I will require. For a delta/star transformer with the star side solidly earthed, the transformer 600MVA 22/400kV. I am proposing to have Ct ratios of 200/1 on HV side and 4000/1 on LV side assuming a 5A rated relay with the following:

HV side full load current of transformer
(600×103) / (400×√3) = 866A
CT secondary current corresponding to HV side full load current
=866/200 = 4.33A

LV side full load current of transformer
(600×103) / (22×√3) = 15745.9A
CT secondary current corresponding to LV side full load current
=15745.9/4000 = 3.94A

I am proposing to use a star/delta interposing CT on the HV side to account for the phase shift, the ratio to be (4.33A/3.94):1/√3

Please can you advise on if my approach is correct?

Thanks

 

[thermionic1]

Why not use a digital relay and eliminate the interposing CT's by having a (Wye) Star connection for all CT's? What advantage is there to using interposing CT's and a transformer protection package that isn't digital? The list of the advantages to the IED route are numerous. For a transformer of that size, I would expect 2 relays (Pri/BU (or similar). Will you have to duplicate the interposing Ct's for the BU relay? Do you have additional regulatory requirements for testing and maintenance on the the additional interposing CT's?