Research on Economical-Type Fault Current Limiting Topology and Applied Technology

Research on Economical-Type Fault Current Limiting Topology and Applied Technology
　　With rapid increase of the power loads as well as continuous incorporation of the new-built large capacity generators, the short circuit current level of HV power systems is getting more and more critical. As a result, the operational safety, reliability and stability of the power systems are unprecedentedly threatened. To account for this arduous issue, some traditional measures to limit fault current have been utilized, such as system reconfiguration, operating mode modification and so on. Though these solutions definitely suppress the fault current level to a certain extent, they also render negative impacts on the flexibility, economy and reliability of the power systems. Hence, investigation Neodymium Magnets on developing new types of fault current limiters with preferable performance both technically and economically is an indispensible and irresistible trend for the time being, which is extremely significant and invaluable for development of nowadays power grids in China.Based on conventional electrical devices and components, some novel fault current limiting technologies without superconductors or power electronic equipments were systematically investigated in this dissertation. The principal research work focused on the key fundamentals with a view to facilitating their applications in the high voltage power systems. The research contents cover several aspects as follows:a)Optimization of the economic-type fault current limiting topology based on ZnO Arresters and a Discharge Gap, as well as development of a digital on-line monitoring & comprehensive protection scheme; b)Exploratory research on an economic-type fault current limiter based on permanent-magnet-biased saturation, including material selection, operating principle analysis, equivalent modeling and topological optimization; c)Comprehensive analysis on the impact and interaction of the economic-type fault current limiters with the high voltage power systems, such as transient stability, interrupting characteristics of circuit breakers, single-phase autoreclosure and so on. Taken into account of the advantages of different series resonant FCLs, a new fault current limiter http://www.everbeenmagnet.com/en/products/110-sintered-neodymium-magnets topology based on ZnO Arresters and a Parallel Discharge Gap was proposed, and its fault current limiting characteristics was verified by laboratory experiments. Furthermore, a detailed design scheme for 110kV FCLs was presented and thereby a digital on-line monitoring, diagnosis, protection, triggering and control scheme was established accordingly.
　　The research results provide fundamental theory and technological reference for development of the ZnO Arrester-based FCLs targeted for extra-high voltage transmission lines.The economic-type fault current limiting technology based on permanent magnet biased saturation, noted as PMFCL, was explored in this dissertation to provide governing principles. With respects to normal and fault modes, the cooperation criterion for operating points of the permanent magnets and the magnetic cores was investigated. After comprehensive analysis in terms of magnetic characteristics, mechanical performance and economic property of different kinds of magnetic materials, a material selection guideline for magnetic cores and permanent magnets was proposed. Further, the equivalent magnetic circuit models were established based on the static and dynamic magnetic field-circuit characteristics of different PMFCL topologies. Some rationale experiments were carried out as to find effective ways to improve the permanent magnet biased performance and fault current limiting characteristics of the PMFCLs, which also presents the foundation to develop high voltage and large capacity PMFCLs.Power-angle transient oscillation or instability of the power grids caused by short-circuit fault will threaten the system safety. When a fault occurs, the FCLs will function to limit the fault current and also influence the system’s transient process, which needs further investigation and verification. With respect to a typical one-machine infinite-bus installed with economic-type FCLs, the transient physical process was analyzed in details by using the Equal Area Criterion. The theory analyses were verified by MATLAB simulations for both one-machine and multiple-machine systems. Both theoretical analysis and simulation results show that, the installation of economic-type FCLs can effectively suppress the oscillatory amplitudes of swing curves, reduce the decaying period, extend the critical clearing time and hence improve the transient stability of the power systems.Installation of FCLs in the power systems can effectively suppress the short-circuit current level and thus reduce the breaking capacity requirement from HV circuit breakers, however, it may also result in more severe breaking conditions for circuit breakers. With regards to FCL terminal fault and short-line fault in power transmission lines installed with economic-type FCLs, mathematical formulas were presented, based on theoretical analysis and deduction, as to properly describe the relationship between the rate of rise of recovery voltage (RRRV) of circuit breakers and the current limit factor, the stray capacitance as well as http://www.everbeenmagnet.com/en/products/110-sintered-neodymium-magnets the short-line fault distance. According to these formulas, detailed computations were carried out to fully account for the concrete impacts on the RRRV from every principal parameter, and were correlated with the conclusions drawn by simulations in previous publications available, which accordingly well verified the proposed analyzing methodology.
　　The research results present analytical basis and theoretical reference for parameter optimization of the inductive FCLs and also for reliable selection of the breaking characteristics of HV circuit breakers.Especially for the out-phase-faults, the impact of FCLs on the interruption of out-phase-fault current was analyzed and the methods for computing the critical length of the transmission lines were presented. The conception of interrupting severity of circuit breakers was proposed and the influence of the current limit factor and the stray capacitance on the interrupting severity of circuit breakers was investigated in details. The above research results show that, the design of current limit factor should avoid around 0.5 so as to reduce the interrupting severity of circuit breakers, and it is better to effectively increase the stray capacitance of the FCLs by optimizing the structure design. For the extra-high voltage transmission lines operational with ZnO arrester-based FCLs, the capacitor of the FCLs may contribute to the low frequency current components of the secondary current with big amplitude and slow attenuation. With regard to a 500 kV long-distance transmission line installed with ZnO arresterbased FCLs, the influence of both the FCL bypass switch operation and the different arc resistances on the characteristics of the secondary arc current was analyzed by EMTP (electromagnetic transients program) simulations. The decay factor and the oscillation frequency of the secondary arc current were computed based on the equivalent impedance network and Laplace transformation, with a view to elucidating the physical nature of the low frequency oscillation of the secondary arc current. A time-sequence coordination scheme between the single-phase autoreclosure (SPAR) and the FCL operation was proposed and verified by concrete simulations. The results indicate that, the proposed scheme can effectively eliminate the side impact caused by the low frequency current components on the SPAR, and can guarantee the normal operational sequence of a SPAR as well as the self-restoration of a FCL.The research achievements in the dissertation have further developed the fundamental theory and analyzing methodologies of the fault current limiting technology, and present great significance for both the scientific research and engineering application of the high voltage and large capacity economic-type fault current limiters.