Study On Inrush Current Identification Of Large Capacity Double-fed Wind Farm Outgoing Transformer

With the rapid development of wind power generation,more and more large-capacity wind farms are connected to the grid,and the penetration of wind power is increased.The fault characteristics of wind farms may affect the relay protection of transmission network,resulting in protection rejection or misoperation.Aiming at the influence of wind farm centralized access to power grid composed of double-fed induction Generator on inrush current identification in relay protection of main transformer of wind farm,this paper mainly does the following work:1.The composition of doubly-fed wind turbine is theoretically analyzed,including wind turbine mathematical model,generator mathematical model and converter model.The control strategies of grid-side and rotor-side converters are analyzed,and Crowbar protection circuit is built to realize low voltage crossing of wind farm.The grid-connected system of doubly-fed wind farm is built on PSCAD/EMTDC platform.2.The fault current provided by wind farm in the step-up transformer area of wind farm is analyzed theoretically.The fault current of wind farm side is mainly analyzed when Crowbar protection is put into operation under severe fault.The expression of short-circuit current is obtained,and the components of short-circuit current are quantitatively analyzed.The simulation waveforms are compared and verified.3.Program of identifying inrush current by using second harmonic content and using waveform asymmetry coefficient is compiled on Matlab platform.The adaptability of the two methods to the fault in the step-up transformer area of wind farm is analyzed.The theoretical analysis shows that both the transient DC component and the transient AC component of the short-circuit current of wind farm will make the results of inrush current identification inaccurate and may be mistaken.A simple filter algorithm is proposed to reduce the transient DC effect of short-circuit current.Finally,the recognition effect of inrush current before and after the improvement is simulated and analyzed.4.The inrush current of transformer at different closing angles is simulated on PSCAD/EMTDC platform.The improved identification method is used to analyze the inrush current.In order to reduce the influence of filtering algorithm on the accuracy of inrush current identification when the interruption angle is large,the inrush current identification criterion based on PWM waveform characteristics is added to improve sensitivity.The validity of two criteria,PWM waveform modulation and waveform asymmetry coefficient,for identifying inrush current is simulated and analyzed.