WHAT HAPPENS WHEN ONE PHASE IS OPENED IN A STAR-DELTA TRANSFORMER?

This might be possible in substations while any L-G fault occurs in the transmission side.

               Let us consider a star delta transformer with a primary delta connection and secondary star connection.  The line voltage is V across the two phases.

                If any relay operates due to any fault and the phase B gets opened then, the current incoming to phase B will be zero.

                As we can see from the above diagram the B phase is disconnected.  After this we can see a closed loop formation and circulating current passes through coils in between phases RB and BY.  Thus the voltage is divided between the two phases and V/2 appears on the secondary side.  This voltage division is clearly mentioned in the above diagram.

                  This situation leads to any 3 phase equipment will continue to operate at reduced efficiency.  Motors would be noisy and would overheat leading to decreased efficiency.  Induction motor acts as single phase motor with reduced efficiency.  This problem is common in sub stations and is got rid by minimising the circuit fault clearance time.

                  This can also be cleared using single phasing protection relay.  To see more about transformer rating concept click here.

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