# Transformer differential protection - testing with earth faults

**Vector group of the transformer is YyO, the differential protection function trips by √3 times higher value than expected. What is the reason for it?**

Protecta uses delta-transformation matrices for zero-sequence current elimination. It has the consequence that if you are injecting current only in one phase, the differential function will calculate differential currents higher than 0A in two phases and these differential currents will be √3 times lower than the injected current. That does not come up only with Yy0 group, similar phenomena can be observed by other vector groups. More details can be find in the detailed description of the differential protection function: http://www.protecta.hu/epp-english/Function_blocks/DIF87_V1.0.pdf

The point is that you have to observe the on-line data of the differential function block when injecting, as the set characteristic touches the calculated differential currents. Injecting in 3 phases will cause no misunderstanding.