Research On Asymmetric Three-Level Neutral-Point-Clamped Converter In SRM System

Switched Reluctance Motor(SRM)has advantages of a simple and reliable structure,low cost,large starting torque,wide speed range and strong fault-tolerant ability,making it widely used in textile,electrical appliances,new energy vehicles drives and other industrial areas.However,with the continuous expansion of the application scope,the performance requirements of SRM system are gradually improved.As the core of the system,the power converter plays an important role in the system performance.Therefore,in order to improve the performance of SRM system,this paper focuses on the theoretical analysis and research on the asymmetric three-level neutral-point-clamped(NPC)converter.Firstly,the topological structure of the asymmetric three-level NPC converter and control strategy of the SRM system are studied.Compared with the asymmetric half-bridge converter,the new converter adds two power transistors and two clamping diodes in each phase,which can realize the multilevel control of the SRM system and reduce the maximum withstand voltage of power transistors.In addition,for a SRM system based on the new converter,a speed-current double-loop pulse width modulation(PWM)strategy is presented.Compared with the traditional PWM control strategy,this strategy has a better control performance of current,and can effectively suppress the midpoint potential offset of the DC side capacitor.The simulation and experimental results show that the asymmetric three-level NPC converter combined with the speed-current double-loop PWM strategy can effectively reduce the current ripple of the system,optimize the control performance of the system,and improve the competitiveness of the system in high-voltage and high-power situation.Secondly,in order to reduce the cost and volume of the new SRM system,this paper presents a phase current reconstruction method based on two current sensors.By optimizing the position of current sensors,the equations of sensor currents with respect to phase currents are established.According to the equations,the conditions for constructing the phase currents by two sensors are studied,and then a unified phase current reconstruction method is proposed,which considers the existence and the non-existence of overlapping areas of phase currents.Compared with the traditional phase current detection method using one current sensor in each phase,this method reduces the number of sensors,leading to a reduction in the cost and volume of the system.Moreover,the method does not require pulse injection,and there is no detection dead zone.Simulation and experimental results verify the feasibility and accuracy of the phase current reconstruction method.Finally,in order to improve the reliability of the SRM system,an online fault diagnosis method for the asymmetric three-level NPC converter is presented.By analyzing the differences of winding voltage before and after failure of the converter,the inconsistency of actual and theoretical phase voltage after the fault is used as the characteristic value for fault diagnosis,and the fast fault location can be realized by injecting working mode.After the open-circuit,short-circuit fault of power transistor or open-circuit fault of clamping diode occurs,the fault detection and location can be completed quickly according to the given working mode and winding voltage.Then,the measures will be taken to limit the further development of the fault in time,so the system reliability can be effectively improved.Simulation and experimental results verify the validity and reliability of the fault diagnosis method.There are 55 figures,14 tables and 109 references in the thesis.