| The traditional electromagnetic protection current transformer(CT)is a key component in the measurement chain of relay protection system,which directly affects the operation of power system.The prominent problem in its application is the significant increase of excitation current caused by core saturation,which leads to the increase of CT transmission error and the deterioration of CT performance.To study the characteristics of protective CT under the transient condition in power system,this paper takes EMTP-RV as simulation environment and builds its electromagnetic transient model.Primarily,the working principle and transient characteristics of CT theoretically are expounded in the thesis.A static model based on single-value magnetization curve and a dynamic model in view of Preisach theory of transient excitation characteristic curve are established respectively.Hence,a transient steady-state circuit that meets the needs of project is built to verify and discuss the influence of multiple factors on the dynamic response of CT.Combined with mathematical deduction and MATLAB drawing,the influence degree of these factors on CT saturation was compared,and the conclusion was drawn that the remanent flux is the most important factor causing saturation which can be controlled artificially.However,so as to demagnetize,the first thing to be solved is how to detect the remanent flux.A fast and in situ CT remanent flux detection method for protection based on fluxgate principle is proposed.The method is to connect an unbiased highfrequency voltage source to the secondary side of the CT and saturate the core slightly in the positive or negative directions.The polarity of the remanent flux is determined by the integral sign of the sampling voltage,and the magnitude is calculated according to its linear relationship with the second harmonics of the high frequency component of the voltage.This linear relationship can be measured in advance based on fluxgate theory.Subsequently,two common high-frequency and high-voltage voltage source structures are explored : a sinusoidal voltage(generated by AC voltage source VAC and AC-DC-AC AC/DC/AC converter)and a square wave voltage(composed of AC voltage source VAC and IGBT switch)are applied to excite CT’s core respectively.The feasibility of these two methods is verified by simulation in different working states of CT,different levels of remanent flux,even in the environment of voltage fluctuation and frequency offset,and the relevant errors are tested.The results illustrate that the "square wave source" is more efficient than the "sinusoidal wave source" in detecting the remanent flux of CT.Compared with other existing magnetic inspection methods in academia,they have the advantages of being "online","fast" and "accurate".Specifically,the external magnetic inspection loop will not affect the normal operation and transmission performance of CT,that is,it is not necessary to disconnect CT from the substation,avoiding the hidden danger of incorrect reconnection of CT.Its rapidity(20ms)can be used for real-time monitoring of CT remanent flux during short interruptions(e.g.the time required for automatic reclosing operation).The measurement error is tiny(<6%),and the accuracy is not affected by the voltage and frequency fluctuation in the power system.This is of great significance for the rapid,efficient and targeted development of the follow-up demagnetization work. |
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