Abstract On the basis of the properties of transformer, A transformer-type persistent current switch(PCS) used in superconducting magnetic energy storage system is proposed. The structure of transformer-type PCS is a transformer with threecoils, the turn number of the primary coil (T1 ) is more than that of the secondary coils (T2, T3). The normal state resistance of PCS is increased by connecting bifilar coils (B1, B2 ). Two large current diodes is connected in the secondary circuit. Based on the principle...