| Transformer is one of the main apparatus in the power system,and its operational reliability is an important guarantee for the safety of the entire power system.The magnetizing inrush current generated as transformer no-load switching and voltage recovery after external fault clearing will flow through the differential protection relay,causing the differential protection to maloperation.Therefore,how to eliminate the influence of magnetizing inrush current has always been a research hotspot of the transformer protection.At present,the research on magnetizing inrush current mainly includes inrush current identification and inrush current suppression.Existing inrush current identification methods mostly exploit the single-phase characteristics of waveforms,ignoring the correlation between phase and phase,which may affect the correct rate of inrush current identification.When the wind farm is on low voltage ride through,the harmonic component will cause the differential protection of interconnecting transformer of the wind farm to misidentify the fault current as the magnetizing inrush current.Therefore,it is urgent to find an inrush current identification method that is not affected by harmonics.The inrush current identification method avoids the differential protection misoperation,but the inrush current still affects the life of the transformer and the safe operation of the power grid.Therefore,it is necessary to propose a method of inrush current suppression method to eliminate the adverse effects.The existing inrush current suppression method has a slow start-up time and generates a large amount of harmonic components.With the support of the National Natural Science Foundation of China(51277184),the mechanism of differential protection and magnetizing inrush current is studied.The inrush current identification methods for the conventional transformer and interconnecting transformer of wind farm are proposed respectively.Then,for the problem that the inrush current identification cannot eliminate the adverse effects of the magnetizing inrush current,a soft-start inrush current suppression method with integrated low-pass filtering is proposed.The studies are shown as follows:(1)According to the basic principle of differential protection and the generation mechanism of magnetizing inrush current,the frequency domain equation of three-phase inrush current is derived.The conventional inrush current frequency analysis is only for single-phase transformers,but a frequency-domain representation of three-phase inrush current is derived in order to further analyze the correlation between phase and phase of transformer.The characteristics of inrush current amplitude under different initial conditions(the initial phase and the saturation flux)are studied.The autocorrelation and cross-correlation properties of time and frequency domain of magnetizing inrush current under different initial conditions are studied too.Method for influencing the differential protection of transformers to find out some methods to eliminate the influence of magnetizing inrush current on transformer differential protection.(2)In order to solve the problem that the existing magnetizing inrush current identification method ignores the correlation between phase and phase,a inrush current identification method of the conventional transformer based on Multiscale Multivariate Fuzzy Entropy(MMFE)is proposed.The traditional Multiscale Multivariate Sample Entropy(MMSE)has the advantage of analyzing the dynamic coupling and complexity of multiple time scales of the signal.MMFE inherits the advantages of MMSE and softens the hard threshold criterion of MMSE by fuzzy membership function.According to the characteristics of the MMFE curve for inrush current and fault current clearly distributed in different regions,a MMFE-based inrush current visualization recognition method is proposed.This method divides the two-dimensional space composed of the scale factor and the MMFE value into an action zone and a braking zone and defines a ratio of the MMFE entropy area to the area of the given braking zone as an entropy area ratio of MMFE.The magnetizing inrush current and fault current are identified by comparing the entropy area ratio of MMFE to the constant 1.The transformer model is established on the ATP/EMTP platform and dynamic simulation system,and the correctness and effectiveness of the proposed method under different operating conditions are analyzed.(3)In order to solve the problem that the harmonic component of the short-circuit current of wind farm affects the correct identification of the inrush current during the LVRT of the wind farm,a method for identifying the inrush current of a wind farm interconnecting transformer based on MEMD-MMFE is proposed.In the low voltage ride through process of wind farm,the differential protection may be false braked by the second harmonic component of the short-circuit current of the wind farm when an internal fault occurs in the interconnecting transformer.A large amount of harmonic components in the short-circuit current of the wind farm will also affect the MMFE curve,so it is impossible to identify the inrush current of the interconnecting transformer simply by MMFE transformation.MEMD has the advantage of no basis function and adaptive synchronization analysis for multi-channel signals.Based on the advantages of MEMD,a magnetizing inrush current identification method combining MEMD with MMFE is proposed.The method firstly pre-processes the three-phase current through the MEMD transform;then performs MMFE transformation on the pre-processed original function to obtain the MEMD-MMFE entropy area;finally,the magnetizing inrush current and fault current are identified by comparing the relationship between the entropy area and the given threshold.The model of wind farm connected to the power grid is built on Matlab/Simulink platform,and the correctness and effectiveness of the proposed method under different operating conditions are analyzed.(4)In order to solve the problem that the existing inrush current suppression method has slow start-up time or large harmonic component,a soft-start based inrush current suppression method is proposed.The method adopts an inrush suppressor composed of an anti-parallel thyristor and a high-order filter circuit to ensure smooth starting of the primary side voltage of the transformer,which realizes soft start of the transformer and achieves the purpose of active suppression of the inrush current.In view of the characteristics of the odd-order harmonics generated by the anti-parallel thyristor,the filtering effects of LCLC high-order filter and high-order trap filter are compared.The high-order trap filter with RC resistance-capacitance is selected as the filter in this paper,and its parameters are optimized.The inrush suppressor model is established on Matlab/Simulink.The suppression effect of the inrush suppressor is verified by simulating the voltage,current and flux characteristics of the three-phase transformer group and the three-phase three-column transformer in different wiring modes. |
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