Research On The Key Technologies Of The Novel Solid State Fault Current Limiter

With the increase of the voltage level and capacity of the power electronic devices, the solid state fault current limiters based on the power electronic devices have more and more extensive application prospect, of which the novel solid state fault current limiters (SSFCL) with the following advantages:negligible impact on the power system during the normal operating state, automatic response to limiting the fault current without time delay and without voltage and current oscillation and strike upon the fault inception, has been paid extensive attention in the field of the investigation of the fault current limiting technology both at home and abroad. Based on the achievements in the past, the key technologies of the SSFCL have been focused and studied in this paper, including:The design methodology of the saturated coupling transformer (SCT) in the novel SSFCL is proposed. Firstly, the three operating stages are analyzed, and the equivalent circuit is setup during each stage, based on which, the main objective of the design of the SCT is obtained, which is to reduce the leakage inductance during the normal operation stage(the secondary winding is equivalent to be short-circuited) and the secondary voltage of the SCT during the fault current limiting stage (the secondary winding is equivalent to be open-circuited)as soon as possible on the premise of the satisfying the requirement of the fault current limiting. Secondly, to achieve the design objective, the detailed design steps of the SCT are given:1) choose the turns ratio between the primary and secondary windings and the cross section of the wire of the SCT;2) based on the parameters obtained from the design of the air-core transformer, a series of the combinations of the parameters of the turns of the winding and the radius of the steel core of the SCT are obtained, by applying the field-circuit coupling FEA method;3) in addition, the change law of the leakage reactance voltage drop in the normal state and the secondary voltage of the SCT in the fault current limiting state with the core radius is achieved, and the optimal result is obtained by curve fitting and optimization. Lastly, the validity of the design methodology proposed is verified by the comparison between the results of the simulation and the experiment of the small-scaled prototype employed in the100V/1A system. Based on the operating principles of the three-phase saturated transformer coupling SSFCL, the equivalent magnetic and electrical circuits are obtained, and the working process of the SSFCL is analyzed applying the fundamental magnetic and electrical circuits principles combined with the subsection linear magnetization curve. Then the3-D FEA model of the three-phase saturated transformer and the topology model of the SSFCL are built up in the Anosft/Maxwell and the Ansoft/Simplorer software respectively. Applying the field-circuit coupling FEA method, and combining the two software packages above, the magnetic field inside the three-phase saturated transformer and the whole working process of the SSFCL is studied, during which, the emphasis is paid on the research of the relationships between the primary and secondary phase voltages, the primary and secondary line voltages, and the secondary phase and line voltages of three-phase saturated transformer during the fault current stage. Lastly, in order to research the fault current limiting effect of the SSFCL in the ulti-machine power system, employing the results of the FEA simulation achieved above, the nonlinear inductance model of the SSFCL is obtained, which is simulated in the3-machine12-node system. The analysis and simulation results show that the novel SSFCL has the following advantages:negligible impact on the power system during the normal operating stage, automatic response to the short-circuit fault, and during the fault current limiting stage the secondary phase and line voltages are far less than the values of the primary winding, further more, the ratio of peak values of the secondary line and phase voltage is far less than√3, which are of great significance to the reduction of the bridge voltage level, the DC reac, as well as the volume and cost of the whole equipment.Two topologies of the novel autotransformer coupling SSFCL employed in the high voltage level power system—two-winding and three-winding autotransformer coupling SSFCL(ATCSSFCL) are proposed, meanwhile, the fully controlled bridge topology with the combination switch is introduced to speed up the shutting off of the bridge and reduce the DC inductance. Then the operating principles of the two schemes of the ATCSSFCL are analyzed, in addition, both of the conventional and the novel bridge topologies are comparative studied, highlighting the advantages of the novel fully controlled bridge topology. According to the principles proposed above, the schemes of the two-winding and three-winding ATCSSFCL employed in the220kV/1.5kA system are designed, which are simulated in the environment of the Matlab/Simulink, and the respective characteristics of the two schemes are obtained by the comparative analysis of the simulation results. In order to research the autotransformer as the key fault current limiting device, its FEA model is set up in the environment of the Anosft/Maxwell, utilized to analyze the magnetic field distribution of the autotransformer employed in the220kV/1.5kA system during each operating stage combined with the circuit model built in the Ansoft/Simplorer software, based on which, the whole performance of the ATCSSFCL is obtained employing the field-circuit FEA method. Lastly, the feasibility of the proposed schemes are verified by the comparison between the results of simulation and experiment of the small-scaled prototype.To solve the problem of the protection for the FACTS equipments upon the short-circuit fault, the application of the SSFCL for the protection of the FACTS is studied, the single and three phase topology of the FACTS equipments with the fault current limiting function, including the topologies with the saturated transformer and the fault shunt module, are proposed. Based on the equivalent circuit of the secondary side during the fault current limiting, the change law of the DC inductance current and the voltage across the capacitor is obtained as well as the guidance for the key parameters of the scheme. Then the three-phase topology employed in the lOkV/1kA system is taken as the case study, the protection principle for the over voltage and over current is studied by simulation, including the comparative study between the schemes with the conventional and saturated transformer, which highlights the advantage of the saturated transformer in the fault protection, meanwhile, the scheme with the fault shunt module is also simulated to show its positive effect on suppressing the capacitor voltage rise. Lastly, the feasibility is verified by the380V/3kVA prototype experiment.