Novel Principle of Transformer Protection Based on Flux Linkage Balance Equations

A novel principle of transformer protection based on the flux linkage balance equations, which is not influenced by the magnetizing inrush current, is proposed in this paper. According to the flux linkage equations of the primary and secondary windings, the saturation fluxes are eliminated by the two equations, based on which the internal fault and external fault discriminative expressions are established. However, the coefficients of equivalent resistance and leakage inductance should be precisely estimated instantaneously. The least square method can accurately realize the parameter estimation, at the same time the errors of estimated parameters are dependant on the width of data window. Because there are 4 unknown variables for a two-winding transformer, at least 4 sampling points are needed, so that the time window should be more than 4 sampling points. With the increasing of data window constantly, the error of estimation is decreased first and then increases because the errors are accumulated and increases rapidly, which can lead to the overreach of the protective discrimination. Moreover it can lead to large amount of calculations, hence, a novel variable window least square estimation method is proposed in the paper as well, which can self-adaptively select the optimal data window width. Not only the error of the estimation is minimized, but also the calculation quantities are also decreased. The ATP and Mat lab simulations associated with single-phase transformers and three-phase transformers showed that the principle is correct and quite efficient.