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We have a problem with one of our transformers. As soon as the load crosses 15% of the rated current, the transformer trips on differential protection. The transformer is Dyn11, 13.8/4.16 kv. Primary CTs are connected in star (1000/1A). The secondary CTs are also star connected (3000/1A). The primary CTs are wired to a Siemens 7UT513 differential relay, which in turn is wired to the secondary CTs through a 1:1 matching transformer which is connected as star-star.
We have done a complete check of the transformer itself, the wiring between the relays and CTs, polarity checks, secondary injection tests, etc., and all of them are okay. But the moment we put the transformer on load, it trips when the load crosses more than 15%.
I am not familiar with the Siemens relay you describe, but in order for the connection that you describe to work, the relay must include ratio matching, a zero sequence filter and vector shift compensation. If it does not, or if ratio matching and vector shift compensation are not correctly set, the relay will not function correctly. If your relay does provide ratio matching, a zero sequence filter and vector shift compensation then I expect it would be OK to have primary and secondary CT’s both in star. Check this from the relay technical data. Certainly, with an Alstom KBCH, the correct CT connection is star-star irrespective of transformer vector group.
If the relay does not have the above functions, it could can never work properly.
The best test for a differential protection scheme is always primary injection, but often the actual testing is easier said than done. In the case of a transformer, you need to be able to isolate the transformer in such a way that you can apply voltage and circulate current through primary and secondary CT’s. Isolate the transformer and apply a 3-phase short immediately downstream of the secondary CT’s then energize the transformer at much reduced voltage to circulate current. Then you can see and measure and find out exactly what is going on. The reduced voltage to apply for the test can be less than the transformer impedance voltage. If you apply the impedance voltage to a shorted transformer you will circulate one per unit current but often you can see what is happening at well less than full load current.
In a recent installation the above test resulted in the transformer differential tripping out at about the same current as yours. We found the secondary side CT’s were connected the wrong way around. After the connections were reversed we got zero differential current when the tests were repeated.
You could possibly have a dud relay but I doubt it.