This paper describes a new algorithm for transformer protection relays to discriminate between internal faults and inrush currents by estimating the magnetizing curve of a transformer core regardless of the ratio of the second harmonic components.
Every transformer core has its own magnetizing characteristic, called the B-H curve. Even immediately after energizing or an external fault, this curve remains unchanged. The newly developed algorithm incorpo- rates a model B-H curve as a reference, and trackes the locus of (H, B) calculated from the instantaneous values of voltage and current at each terminal of the transformer. Because of the difficulty of obtaining the initial flux density, this algorithm determines the relative values of flux density instead of the absolute values.
The magnetic flux density (B) and magnetic field intensity (H) are calculated as B=B_r+B_d=B_r+K₁ ∫ Vdt, H=K₂I_d=K₂∑I_i where B_r is the initial flux density, B_d is the flux density caused by magnetizing voltage V, I_d is the magnetizing current, I_i, the is instantaneous current of each terminal of the transformer, and K₁ and K₂ are constants. When the sum of the currents reaches a threshold I_r, this algorithm assumes B_r=0 and starts integration of the voltage to track the locus of (H, B_d). The locus follows the reference curve if there is no internal fault.