The only requirement is that the power transformer winding is connected to earth in the starpoint (in case of star-connected windings) or through a separate earthing transformer (in caseof delta-connected windings).The low impedance restricted earth-fault protection REFPDIF is a winding protection function.It protects the power transformer winding against faults involving earth. Observe that singlephase-to-earth faults are the most common fault types in transformers. A sensitive earth-faultprotection is therefore desirable.A restricted earth-fault protection is the fastest and the most sensitive protection, a powertransformer winding can have and will detect faults such as:• earth faults in the transformer winding when the network is earthed through animpedance• earth faults in the transformer winding in solidly earthed network when the point of thefault is close to the winding star point.The restricted earth-fault protection is not affected, as a differential protection, with thefollowing power transformer related phenomena:• magnetizing inrush currents• overexcitation magnetizing currents• load tap changer• external and internal phase faults which do not involve earth• symmetrical overload conditionsDue to its features, REFPDIF is often used as a main protection of the transformer winding forall faults involving earth.6.2.2.1 Transformer winding, solidly earthed M13048-3 v9The most common application is on a solidly earthed transformer winding. The connection isshown in figure 31.1MRK 504 158-UEN A Section 6Differential protection79Application manual
