[GUEST]

If the star point of a transformer is earthed and one phase develops a fault to earth can anyone provide a link with a detailed explanation to the resultant effect on the healthy phases with respect to earth ?

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[Hamid]

When a phase is grounded, the voltage across this phase will become zero. The other two phasors will remain the same as in a healthy system and add to give the residual voltage. In healthy condition: V_R=V_A1+V_A2+V_A0 V_Y=a²V_A1+a V_A2+V_A0 V_B=a V_A1+a²V_A2+V_A0 V_A0 =0 (residual voltage) In faulty condition: V’_R=0 V’_Y=V_Y+V_A0 V’_B=V_B+V_A0 V_A0>0

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