Phase Control to Eliminate Inrush Current of Single-phase Transformer by Using Approximate Calculation of Residual flux

The residual flux of transformer ferromagnetic-core is not random when it enters into power system without load form last cutting off. Firstly, the relationship between residual fluxes and the last exciting current was discussed, then hysteresis loop was divided into three antisymmetric sections according to reversibility of magnetic domains; methods were proposed to calculate residual flux approximately section by section based on the principle of similar-shape and ferromagnetic characters. Combined with time-character of ferromagnetic material, a phase control tactic to eliminate inrush current was developed, finally EMTP simulation was taken to testify its rationality, and in this simulation, according to inrush current overshoot limited to 115%, the ranges of phase at different ratios of residual flux was measured, which is useful in device design.