<![CDATA[Power transformers require fast and selective protection to prevent equipment damage, power interruption, and safety risks caused by internal faults. This study designs and evaluates a differential protection model for a 10 MVA, 150/20 kV, YNd11 power transformer using MATLAB/Simulink. The proposed model integrates current transformer compensation, differential current calculation, percentage differential relay characteristics, and second harmonic blocking to distinguish magnetizing inrush current from internal faults. The relay settings include a minimum operate current of 0.20 pu, two-slope characteristics of 20% and 80%, a restraint breakpoint of 1.0 pu, and a second harmonic blocking threshold of 15%. Four simulation scenarios were tested: normal operation, transformer energization inrush, phase-to-phase internal fault, and phase-to-ground internal fault on the 20 kV side. The simulation results show that the transformer model achieved a steady-state validation error below 0.25%. During normal operation, the residual differential current remained between 0.021 and 0.024 pu. During inrush, the relay was blocked because the second harmonic component reached 21.4%. For internal faults, the relay operated within 16.5 ms for phase-to-phase fault and 18.5 ms for phase-to-ground fault. These results indicate that the proposed model provides accurate, selective, and fast transformer differential protection.]]>
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