Research On System Modeling And Stability Analyses Of Single-phase Inverter Systems In Island Mode

Power electronics technology has strongly promoted the progress of human society since its inception,and has played a crucial role in solving the problems of energy and environment in the 21 st century.As efficient enegy conversion devices,power electronic converters have been widely applied in various fields,such as industry,transportation,defense,and daily life,which has created tremendous value.In the past decades,the studies on power electronic converters have become richer and deeper.Generally,these studies contain the switching devices,topologies,control methods,parameter design,modeling methods,stability analysis,and so on.In this paper,the system modeling and stability analysis of the typical single-phase inverter systems in island mode are investigated.Three kinds of single-phase inverter systems are taken into account,including the basic single-phase H-bridge,single-phase cascaded H-bridge,and singlephase parallel H-bridge inverter system.These studied single-phase inverter systems possess several characteristics in common,since their main circuits are all based on the H-bridge topology,the synchronous reference frame proportional-integral(SRF-PI)controller is adopted in the voltage control loops,and simultaneously,the filter capacitor current feedback method is employed in the current control loops.In this paper,the system stability analyses focus on the influences of the control system parameters on the steady-state stability of the system,and the main research contents and conclusions are summarized as follows:(1)For the studied basic single-phase H-bridge inverter system in island mode,the system structure is introduced first,and the classical s-domain modeling and stability analysis methods are briefly reviewed.And then the time-domain discrete large-signal modeling and stability analysis of inverter system are discussed in detail.By using the state space averaging and stroboscopic map methods,the time-domain discrete largesignal models under both linear load and nonlinear load condition are derived.Based on the established discrete models,the Jacobian matrix method and Lyapunov exponent method are employed to analyze the stable domain of the voltage and current loop control parameters.In addition,the equivalence of the Jacobian matrix method and the Lyapnov exponent method in the perspective of state space averaging is confirmed.And the theoretical analyses are verified by the experiments results.(2)In the investigation on the asymmetric cascaded H-bridge(ACHB)multilevel inverter in island mode,the system description is presented first,in which the principle of the hybrid modulation strategy is elaborated emphatically.It shows that there is no essential difference in system modeling for the studied multilevel inverter system and the general single-phase two-level inverter in conditions of state space averaging.The sdomain model of the multilevel system under linear load is derivated.Then,the studie mainly focus on the small-signal modeling and stability analysis of the ACHB multilevel inverter system,and the small-signal model of the entire multilevel inverter system under linear load is established according to the system structure.It is found that the obtained small-signal model of the system is time-varying due to the application of the SRF-PI controllers in the voltage control loop.Therefore,the Floquet theory is adopted to analyze the stability regions of the system control parameters.Moreover,the stability region of based on the s-domain model is also figured out under the same conditions,which confirm the accuracy of the results obtained from the small-signal model and Floquet theory.Finally,a low-power multilevel inverter is used to verify the theoretical analyses.(3)For the single-phase parallel inverter system consisted of H-bridge inverter in island mode,the impedance models of the system are derived under the ideal conditions,including the scenarios where the H-bridge inverter is directly connected in parallel,and the direct current sharing control strategy is applied.By using the obtained model,the stability regions of the control parameters in voltage and current loops are calculated for a single-phase parallel inverter system composed of two H-bridge cell.And then,the small-signal models of the system with the upper droop control and secondary control are derived,and the corresponding stability regions of the control parameters are figured out to show their influence on the system stability.Finally,the theoretical results are verified by the experimental prototype of the parallel inverter system in island mode.