The reason why Star Delta CT configuration asre used?

What type of PLC? What type of input? Is the program accessible? I smell a scaling problem. Has it worked before? If so, what has changed recently? Help us to help you!

The PLC is a ABB type, 5 Amp through Transducer, No program not accessible. It did work before the guys started changing wires and "upgrade" die panel. My thought is that some of the wiring has been swapped, There is No drawing or wire labeling, all wire colors are the same, so the possibility of a wire swap is 99%. I thought of doing a Vector check with a Fluke 435 to see if there is not a Phase rotation or phase shift, the same Voltage for the same CT? I still do not understand the reason for a Delta connection of the on the Interposing CT's and do not know what die wire config, should be like?

Guest,

you should provide us connection and vector group of your power transformator . It is nesesery somethime make connection of the current transformer in Delta configuration to proprely compesate

1. primary current magnitude difference on different sides of the protected transformer (i.e. current magnitude compensation)
2. power transformer phase angle shift (i.e. phase angle shift compensation);

If your Differential protection of power transformer work good without tripping you do not have problem with wiring of current transformers .You probably have problem with measuring current inside and outside delta connection of CT"s second winding . Difference of current is 1,73 or √3.

Good hunting

RDGRNR and Panta SD, Thanks for the advice, I did do the hunting, was quite a hunting trip, no wire number and the locations of the series circuits was quite some thing. At the end of the day I measured all the Delta currents and simultaneously measured the "second" current circuit that came from a Multi tap CT located in the Tap changer of each transformer. Well, as these multi tap Ct's are very old and the "Wiper" (to select the taps) is mechanically driven, I found that 2 of the Ct's readings is not a exponential output (as the Delta" Ct's output. these 2 multi taps were very linear in comparison to the 3rd Ct's output. we replaced the 2 multi taps and everything is 100% again, never to old to learn. Pantsa SD, you were right, the Delta configuration of the metering Ct's is for compensation.