Research On The Magnetizing Inrush Current And Its Suppression Of Large Capacity AC Transformer

Transformer is the key equipment of UHV AC transmission system,and it is an important link to ensure the safe operation of power system.With the increase of voltage level and the capacity of the transformer,the amplitude of magnetizing inrush current becomes larger in the serious condition,which may cause more serious problems like harmonics,voltage distortion,voltage sag and so on.Considering the demand of power grid development,the characteristics of magnetizing inrush current of EHV/UHV large capacity transformers are studied in this thesis.Through analysis,a new method to identify magnetizing inrush current has been found,so as to study the effective suppression method.The main research work and conclusions of this paper are as follows.Research on the induced mechanism,electrical characteristics and influence factors of magnetizing inrush current of UHV large capacity transformer in different conditions is carried out.Various conditions of the study include inrush for transformer energization,sympathetic inrush and recovery inrush after fault clearance.On the basis of this,the most serious condition of magnetizing inrush for transformer energization is evaluated in order to lay the foundation for the research on the identification method and suppression measure of transformer magnetizing inrush current.This paper proposes two new methods to identify the inrush current of transformer,which include the identification method based on the attenuation characteristic of DC component and the identification method based on the attenuation characteristic of the harmonic distortion.Methods choose the current from the high voltage and medium voltage side of the transformer as the signal source to discriminate inrush current and fault current.The CTs of three-phase are respectively connected with the protection device,and the amplitude ratio of DC component and fundamental component or the harmonic distortion rate of the current can be set as the criterion.Then logic judgment is made based on the current data from CTs,thus the inrush current and fault current can be discriminated.The effectiveness of the two methods is verified by simulation and the field measured data.This method can effectively identify not only the magnetizing inrush current during energizing,but also the sympathetic inrush and fault recovery inrush.In this paper,three types of inrush current of UHV AC transformer are studied.Based on the analysis of the electric characteristics and influencing factors of inrush current,suppression measures of magnetizing inrush current are simulated in various situations including transformer demagnetization,circuit breaker with closing resistors and synchronous switching.Through investigation of the selecting principle of the optimum switching time in the phase-control switching strategy,the sensitive relationship has been found that the effect of phase-control is affected by the discreteness of circuit breaker.At last the optimized phase-control strategy and suppression measure of the recovery inrush after clearance are proposed.