Influenced by the complex environment on the AC/DC hybrid power system, converter transformer is susceptible to inrush current or DC bias interference, resulting in false blocking of differential protection during internal fault. In order to distinguish inrush current and internal fault accurately, this paper proposes an inrush current identification scheme for converter transformers based on the flux-current characteristic model matching method. Firstly, the magnetization curve model is selected to fit the nonlinear magnetization characteristics of the transformer, and a model matching scheme based on voltage comparison is designed. Then the model matching degree index (referred to as matching per unit value M in this paper) is constructed to quantitatively describe the model matching degree, and the inrush current identification criterion based on the model matching degree M is proposed. Finally, the action reliability of the proposed identification criterion is verified by simulation experiments.
The operation of the disconnector in the gas insulated substation (GIS) may produce very fast transient overvoltage (VFTO), which has the characteristics of short rise time, short duration, high amplitude and rich frequency components. VFTO can cause damage to GIS and secondary equipment, and the frequency components contained in the VFTO can cause resonance overvoltage inside the transformer, so it is necessary to study the spectral characteristics of the VFTO. From the perspective of signal processing, VFTO is a kind of non-stationary signal, the traditional Fourier transform is difficult to describe its frequency which changes with time, so it is necessary to use time-frequency analysis to analyze VFTO spectral characteristics. In this paper, we analyze the performance of short time Fourier transform (STFT), Wigner-Ville distribution (WVD), pseudo Wigner-Ville distribution (PWVD) and smooth pseudo Wigner-Ville distribution (SPWVD). The results show that SPWVD transform is the best. The time-frequency aggregation of SPWVD is higher than STFT, and it does not have cross-interference terms, which can meet the requirements of VFTO spectrum analysis.
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