It is found that certain 110 k V dry-type air core reactor has hidden trouble in operation.Single unit has such problem as severe inter-turn short circuit

complete burn and no operation of its protection.Excessive protection dead zone is a common problem for shunt dry-type reactor over-current protection in system.The influence factor of short circuit current in case of inter-turn fault is analyzed

and the relationship among short circuit current factor only and residual reactance ratio and operation voltage is defined.A kind of over current protection based on residual reactance ratio with small protection dead zone and reflecting slight inter-turn short circuit.The convenience of upgrade and innovation can be applicable for air core reactor at various voltages.Before innovation

corresponding limit can be set in the substation monitoring system.Alarm information can be sent out in time to remind the operator to check and handle in case of slight inter-turn short circuit.