GE Multilin D30 Line Distance Protection System 8-118 THEORY OF OPERATION 8.2 PHASE DISTANCE APPLIED TO POWER TRANSFORMERS88.2PHASE DISTANCE APPLIED TO POWER TRANSFORMERS 8.2.1 DESCRIPTIONAs a Wye-Delta transformer introduces discontinuity for the zero-sequence circuit, the ground distance protection cannotbe applied, except special circumstances, to respond to faults behind the transformer.The phase distance elements, however, could be developed so that both accurate reach and correct fault phase identifica-tion is retained for faults behind the power transformer as seen from the relaying point. Without appropriate compensation,the relay's reach would depend on a type of fault, creating considerable difficulties in applying the relay.The D30 provides for any location of the VTs and CTs with respect to the involved power transformer and the direction ofany given zone.In the following equations, the VT and CT locations are referenced as “None” if the transformer is not present between theCT/VT and the intended reach point. Otherwise, the location is to be selected as a type of a transformer as seen from theVT/CT position towards the intended reach point. The following figure explains the adopted rules.Figure 8–4: APPLICATIONS OF THE “PHS DIST XFMR VOL/CUR CONNECTION” SETTINGS830717A1.CDRZ1Z3Z3 XFRM VOL CONNECTION = NoneZ3 XFRM CUR CONNECTION = NoneZ1 XFRM VOL CONNECTION = Dy11Z1 XFRM CUR CONNECTION = Dy11delta wye, 330o lag(a)Z1Z3Z3 XFRM VOL CONNECTION = Yd1Z3 XFRM CUR CONNECTION = NoneZ1 XFRM VOL CONNECTION = NoneZ1 XFRM CUR CONNECTION = Dy11delta wye, 330o lag(b)Z1Z3Z3 XFRM VOL CONNECTION = NoneZ3 XFRM CUR CONNECTION = Yd1Z1 XFRM VOL CONNECTION = Dy11Z1 XFRM CUR CONNECTION = Nonedelta wye, 330o lag(c)Zone 1Zone 3(e)L1 L2ZL1 ZT ZL2
