Inrush Current And Its Influence To Current Differential Protection Of Transformer

The security and stability of the power system is very important to our normal life. The relay protection is responsible for cutting off fault rapidly and restoring power timely. As an indispensable electrical equipment,the transformer occupys an extremly important position in the power system,it plays the role of conversion in the whole power system,its fault will have serious impact on the reliability and security of the system.Two parts are included in this paper.One is principle analysis,the other one is simulation assessment.Based on the requirement of security of electric power system,transformer protection most widely used method is still the differential protection who is simple in principle and has a high sensitivity,but the inrush current is one of the reasons which affect the correct operation of transformer differential protection. In this text, firstly,analyze the generation mechanism of the transformer inrush current and the waveform characteristics of inrush current when single-phase and three-phase.In addition,this paper discusses the two important factors which influence the inrush current such as closing angle and the remanence.The effects of current compensation methods on the inrush current are researched.According to the characteristics of inrush current,analyze and compare various identification methods and point out the defects.Then,this paper analyzes the basic principle of transformer longitudinal differential protection and analyzes the unbalance current.Set a model to simulate the fault current when transformer happen internal failure or external failure or no load switching in internal fault,as well as respectively using the differential quick-break protection principle and ratio braking protection principle, the action behavior of protection devices.These years,there are instances of sympathetic inrush resulting in the malfunction of transformers’differential protection and back-up protection.In the thesis,this paper carefully study the sympathetic inrush between transformers, analyze the generation mechanism of the sympathetic inrush and the influencing factors,and analyze the causes of the malfunction of protection.Several relevant methods for the operation are presented to enhance the reliability of the power supply.In simulation assessment part,a Matlab and Pscad based simulation system is designed,on this basis,using the protection operation demo module of VC++development,fault data which from Pscad simulation is read into the module,the data processing of protection devices is presented in the front of the user.