Study On New Method And New Principle Of Transformer Magnetizing Inrush Current Identification

Although the identification methods of inrush current are various, but their perfect degree and applicability are yet to be improved; Transformer protection principle with comprehensive utilization of voltage and current has richer fault information, and therefore has more advantages. Based on deep analysis and research, new inrush current identification algorithms and new principles of transformer protection are put forward.The major contuibutions of this dissertation are as follows.1. A new algorithm based on sine fitting method to distinguish between the magnetizing inrush and internal faults is presented. After fault steady state, internal current match up standard sine wave, but inrush current does not.2. A novel approach using fractal dimension for the discrimination between inrush current and fault current is proposed.This approach focuses on the properties of fault differential current waveform is smooth, symmetry and no singular point after half cycle.3. A new algorithm based on time-frequency characteristics to identify between inrush current and internal faults is put forward. This algorithm use wavelet decomposition to depict the differences of time-frequency characteristics between inrush current and internal fault current in sliding time-window.The above three kinds of inrush current identification approach are not simply using single feature of differential current, but the integration of multiple information, such as amplitude, phase, singularity, frequency distribution and so on.These algorithms have clear principle, obvious characteristics.4. A novel principle of transformer protection based on simulation after test is put forward: when the transformer operates normally, enmerging external fault or magnetizing inrush, it’s parameters has not changed and its equivalent circuit model is steady, there will has high correlation between measured signal and analog signal; when internal fault happened, it’s parameters has been changed and the equivalent circuit model will be destroyed, now the correlation not only low but also negative.The voltage is brought in this principle, which makes the information more complete. Different from the traditional differential protection, it gets rid of the influence of inrush current. Compared with magnetic flux characteristics identification method, transformer model identification method, parameter identification of transformer winding, this principle has distinct advantage because of avoiding the complex parameter identification process.