Research On Dynamic Modeling And Control Of Hybrid Cascaded Multilevel Converter

With the increasing demand for renewable energy sources such as solar,wind power and tidal power,multilevel inverters have become the enabling power conversion technology for high power applications in today's power systems and large motor drives.Among three kinds of classical multilevel topologies,the H bridge cascaded topology has prominent advantages.Asymmetrical cascaded multilevel inverters with unequal DC bus voltage are main studied in this paper,and it is proved that if the bus voltage ratio is 1:2:6,the output voltage level can be maximized with same devices.The grid-connected current control strategies,the filter design method,and the multilevel inverter system which are operate in grid-connected and islanding modes with linear and nonlinear loads are mainly investigated in this paper.This paper is organized as follows:(1)The topology of asymmetrical cascaded multilevel inverter is investigated.The topology of the cell multilevel inverter with DC bus voltage of 1:2:6 ratio is established in the simulation model.The correctness of the multilevel hybrid carrier modulation strategy is verified through the Simulink.In addition,the stability problem of three typical current feedback control schemes in multilevel grid-connected system are analyzed and compared systematically.Including the inverter side current feedback control,inverter side current plus grid current feedback control and capacitor current plus grid current control.The stability analysis are presented by root locus in the discrete domain,the optimal values of the controller and filter with different feedback configurations are provided.On this basis,the improved WAC damping method is proposed with an additional capacitor current feedback loop.A systematic parameter design guideline for the optimal selection of the current loop PR controller and the additional capacitor current feedback coefficient are presented for the improved WAC feedback control strategy.The improved WAC method enhances system robustness under weak grid scenarios,and the stability and robustness can be enhanced under control delay.(2)In view of a new type of LCL-LC filter,the effects of various damping methods for LCL-LC filter are compared and investigated.It is verified that the resistor in series with the filter capacitor provides the best passive damping effects.With the consideration of system delay,this paper presents a systematic design method for the LCL-LC filtered grid-connected photovoltaic(PV)system.The simulation results are presented to validate the proposed design method and the control scheme.(3)The hardware circuits of a single-phase three cell H bridge asymmetrical cascaded multilevel inverter is designed and completed in this paper.Including the multilevel main circuit,power supply module,DC power input module,A/D sampling circuit and DSP interface circuit etc.On this basis,the multilevel inverter is tested and investigated under the grid-connected and islanding modes with linear and nonlinear loads,the tests including the steady state response and the dynamic performance.The operating conditions under different control parameters are compared.The effectiveness of the control strategies are verified through the experimental tests.