Numerical Analysis And Shield Design Of Electromagnetic Field For Large Current Transformer

Large current transformer(LCT) is the interface component to help the current signal from the primary system to be really reflected by the electrical protection equipments and measuring instruments,which finds wide application in many technical areas such as detecting,protecting,controlling,wave shape recording and distance measuring in power systems.LCT plays an important role in accurate action of the control instrument and the protection equipment of relay,however,that depends on if the current signal from the primary system may be really reflected.As the building and rapid development of the modern electrical grid symbolized with large electrical capacity and high voltage,much attention has been put on the study of LCT used for measurement and protection in a large-scale powder generator system.The numerical analysis and simulation of electromagnetic field for LCT is the necessary work to make the shield winding be designed accurately and the current of the primary system be really reflected by decreasing error of the design.Accordingly,the numerical analysis and simulation of electromagnetic field for LCT is of great siginficance to keep the power generators and electrical grid in safe and steady operation.Error compensation is an important technical method,by which the quality and the measuring precision of the current transformer can be improved.Specially concerning on the method of error compensation by decreasing part of turns and part of a turn,the development and research progress of current transformer are firstly summarized in this paper.Following the desig(?)bjectives,the calculation for design of LCT with rated current of 30000A has been carried out.Based on the discussion of the application of finite element method in the analysis of electromagnetic field,the software of ANSYS has been used to simulate the magnetic field of LCT in two-dimension and three-dimension respectively.Currently,the major means to shield the LCT are shield cover and shield winding,however,the shield winding is simple and to be used easily.By the research on theory of the shield winding,the error decreasing and quality improvement has been investigated.Calculation of the error decreasing in the present investigation shows that the design of LCT meets requirement of error limit.The results from the simulation work confirm that the stray flux from the adjacent current transformer has great effect on flux density of the iron core in three-phase 30000A LCT.The effect of the stray flux on the B current transformer is most important,because the level of flux density within B-phase iron core is much higher than that within the iron cores of A-phase and C-phase.The results in this investigation give that the stray flux may be effectively decreased by correct design of the shield winding,with which the importance and possibility of the winding shield are proposed.