| With wide application of nonlinear theory in circuit analysis,the bifurcation and chaos phenomena in DC-DC,DC-AC and other converters in power electronics have been studied successively.There are many kinds of power electronics topologies and the control algorithms are different,so the corresponding bifurcation behavior display more variety.The bifurcation and chaos in the circuit system are usually manifested as distortion of the current,harmonic increase,and accompanied by sub-harmonic,irregular oscillation and so on.Therefore,it is important to study the bifurcation and chaos behavior in the commonly used power electronic topology,which is important for the maintenance and optimization of the circuit system.In this thesis,a single-phase H-bridge topology is used as the basic research object.The single-phase five-level cascade H-bridge inverter,the single-phase SPWM rectifier and the single-phase SPWM inverter are modeled and analyzed respectively.The bifurcation theory is used in the power electronics topology,in order to achieve the purpose of analyzing the bifurcation mechanism and optimizing the circuit.The first step in the study of bifurcation and chaos in power electronics topology is to establish the corresponding strobe mapping model.There are many modulation methods,hence there exists complex pulse mode in a switching cycle,and common method is unable to build a precise model.Therefore,this thesis presents a virtual vector method based on the idea of ergodic,which can effectively solve the strobe mapping modeling of multi-level inverter.In this thesis,a single-phase five-level cascade H-bridge inverter is studied.The discrete model of cascaded H-bridge is built by virtual ergodic method.The model is verified by simulation.In order to improve the rapid response of the current loop,it is necessary to increase the proportional gain of the regulator,but the system will be unstable.In order to solve the contradiction,the time delayed feedback control is added on the basis of the original proportional resonant controller,which effectively suppresss the bifurcation and chaos in the current loop.Another problem in the study of bifurcation and chaos is the complexity of the topology of power electronics.The traditional research in power electronics topology is focused on non-time-varying,autonomous systems,and ignore the time-varying,systems.In this thesis,a single-phase SPWM rectifier is studied.The model is built through the variable substitution method to transfer the rectifier model into an autonomous form.Because it is difficult to solve a high-order model,a programming method is used in this thesis to obtain the strobe mapping model of rectifier.Simulation results is given.The bifurcation and chaotic behavior in the single-phase SPWM rectifier are studied by using the bifurcation diagram and the stability domain diagram.To reduce the switching losses and increase the output of the inverter,it is necessary to reduce the switching frequency,which will leads to a imprecise model and instability of the inverter.The traditional transfer function model which based on the state space averaging method will fail at low switching frequency,because this method cannot describe a switching element.In this thesis,a single-phase SPWM inverter is studied,and the reason of chaos phenomenon caused by switching frequency is given.The results show that reducing the switching frequency will cause the doubling period bifurcation,which leads to chaos phenomenon in the inverter.Moreover,the time delayed feedback controller is designed.The experimental results show that the time delay coefficient can effectively suppress the chaos caused by the low switching frequency. |
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