INTERPOSING CURRENT TRANSFORMER MALFUNCTION

[vmac100]

Good day Gents,
please I need some information here. There is a differential protection system for our 33kV/11kV 12MVA transformer.However, due to the difference in the ratings of the CTs on the primary and secondary sides, an INTERPOSING CT is used used to tie in the secondary side CT to the for the comparator so that the parameters being compared are of equal magnitude. Below are the name plate parameters of the interposing CT:
gj0104070
In 5/1 A, 50Hz
Ratio: 2.899-8.66/1
GEC Measurements UK.

Recently, we got false alarms from the differential protection system and on troubleshooting, discovered that while there is power on the primary side of the interposing CT, there is no power output on the secondary side.

Does anyone have any idea what the cause of this may be?
Regards

 

[a430]

Hi-Vmac
Why dont you carry out injection test or ratio test on the faulty interposing CT,since i believe you are talking of one out of a set.
Thanks A430

 

[digitrex]

The 33/11kV transformer may be of Dyn configuration. Therefore the interposing CT is wired in such a way to reverse the phase-shift before connection to the differential protection relay.

I am not familiar with GEC's interposing CT but hopefully all it contains 3 separate transformers so that you can trace the connection of every terminals. Are you able to take the transformer off-line for trouble-shooting. If you measured the currents at the primary side of interposing CT while the main transformer is online, you can also do the same measurements(presume you use clamp-on ammeter) at all the other terminals. The secondary side of interposing CT should be connected in delta.

The problem may not be in the interposing CT. You should check the connections at the differential protection relays as well.