Patent ID: 7340355

Claim:
A compensated current differential relaying method for transformer protection comprising the steps of: sampling a primary current, a secondary current, and a primary voltage for each phase; obtaining a primary induced voltage using said primary current and said primary voltage; obtaining a core-loss current using said primary induced voltage; before a core saturation of said transformer, obtaining a differential current considering the ratio of numbers of transformer primary and secondary windings, deriving a first compensated differential current which compensates for said core-loss current from said differential current, and detecting a core saturation instant when a value of said first compensated current exceeds a predetermined threshold; at the time of said core saturation instant of said transformer, obtaining a core flux of said core saturation instant by comparing said first compensated differential current with a magnetization curve; after said core saturation instant of said transformer, obtaining a core flux at a corresponding instant using said core flux of said core saturation instant as an initial value, deriving a magnetizing current of said corresponding instant by comparing said obtained core flux at said corresponding instant with said magnetization curve, and obtaining a second differential current which compensates for said magnetizing current and said core-loss current from said differential current considering the ratio of numbers of windings; and detecting an internal fault of said transformer when the value of said second differential current exceeds a predetermined value.