Magnetic Properties Optimization And Modeling Methodologies For The Straight-line Type Permanent-magnet-biased Current Limiting Topology

With fast development of power grid and rapid increase of electricity load in China, the short circuit current level has been getting more and more critical, which has produced a serious threat to the security, reliability and stability of power systems. As a result, it is quite imperative to develop a new type of fault current limiter with economical investment and promising practicality in the high-voltage and large-capacity situations. As the continuous development of magnetic material and magnetic circuit topology, the fault current limiter based on permanent-magnet-biased saturation, namely PMFCL, is considered to be an effective solution to suppress the fault current. With outstanding advantages in both economic and technological aspects, the PMFCL is highly expected to find earlier applications in power systems.In order to develop the fault current limiting technology based on permanent-magnet-biased saturation, research of theoretical exploration and technological innovation for the purpose of high-voltage and large-capacity applications was investigated in this dissertation by taking a straight-line type PMFCL as example. The research contents cover several aspects as follows:a) New topology design and magnetic materials selection for PMFCL; b) Experimental research on stability of permanent magnet based on the new type of PMFCL; c) Magnetic topology modeling and experimental study of a straight-line type PMFCL considering leakage flux effect; d) Parameter design and optimization for large capacity applications of straight-line type PMFCL, including research of magnetic biased ablity on permanent magnet in straight-line type PMFCL, and parameter design and optimization methodology for large capacity applications of the PMFCL.Taken into account of the advantages and disadvantages of conventional PMFCLs, a new straight-line type PMFCL with a key feature of adjustable structural parameters was proposed. Then, with respects to normal and fault modes, the operating principles of the PMFCL were analysed by considering the relationship of the two operational points, which are in iron-cores and permanent magnets, respectively. After comprehensive analyzing the magnetic and mechanical properties of many magnetic materials in widely use, the magnetic materials selection criteria for iron-cores and permanent magnets were put forward. Theoretical basis for high-voltage large-capacity applications of straight-line type PMFCLs can be presented by all of the work in Chapter 2.The stability of permanent magnet is closely related to the reliability of the PMFCLs. The experimental research on stability of permanent magnet for Nd-Fe-B N35 which is now widely applied to PMFCLs was carried out according to the three aspects of time, temperature and external magnetic field. All of the work in Chapter 3 can be used to provide experimental basis for the application of Nd-Fe-B permanent magnet in straight-line type PMFCLs.The current limiting mechanism during the two interim stages for which a cut-off boundary of zero magnetic flux of the iron-core was adopted was analyzed for a straight-line type PMFCL. The overall leakage permeances as well as the leakage coefficients were achieved through magnetic field partition scheme, and two corresponding equivalent magnetic circuit models were further established by taking into account of leakage flux effect. A numerical solution for leakage permeance of the flux tube with quasi-torus cross section was proposed with curve fitting method as to change the integral variables. All of the work in Chapter 4 can be used to provide analytical basis for high-voltage large-capacity applications of straight-line type PMFCLs.A concept of "Saturation Depth Ratio" was proposed and defined in Chapter 5 as to indicate that, to improve the bias capability of the permanent magnet is in effect equivalent to increasing saturation depth ratio of the iron core. With regard to a straight-line type PMFCL, the mathematical relationship between the structural parameters and the saturation depth ratio is obtained through equivalent magnetic circuit analysis. Study on the Feasibility of increasing Saturation Depth Ratio of iron-cores as to improve the structural parameters of permanent magnets was carried out on the basis of the comparison of Finite Element Method (FEM) simulation and experiments. All of the work presents effective basis and guide to design of hogh-voltage and large-capacity straight-line type PMFCLs.Then, by virtue of three basic variables of saturation depth ratio, inductance ratio and inductance sum, an algorithm for structural parameters design of a straight-line type PMFCL was established on the basis of equivalent magnetic circuit method. Through analysis of the iron-core's operational points, the optimization of the structural parameters was equivalently transformed to optimization of the three independent variables, which can also be considered as the optimization of three key points on i-Be curve of iron-core. A solid foundation for optimal structural parameters was laid in view of all of the above work in Chapter 5.The result of straight-line type PMFCL in the dissertation will further enrich the basic theory, the analytical methods, and the innovative technology of the economical-type fault current limiting topology with theoretical significance and application valve.