Study On New Type Three-phase Five-level Grid-connected Inverter And Its Control Strategy

With the increasingly serious of energy shortages, renewable clean energy has beenpaid more and more attention. Photovoltaic (pv) is the important form of solar energyutilization. For distributed pv grid-connected generation system, two-stage-structure canrealize decoupling control of the objects. During them, preceding stage realizesmaximum power point tracking and dc voltage rising while backward stage realizesgrid-connected controlling and power regulation. This paper studies the backward stagecircuit topology and puts forward grid-connected control strategy to improveperformance of the system.Firstly, multi-level inverter topologies are analyzed. At the same time, advantagesand disadvantages of the traditional multi-level inverters are summarized. Then, a newtype multi-level inverter topology used in this paper is given. At last, its workingprinciple and modulation method are introduced. There are simple structure and highreliability characteristics of this type of topology structure.Secondly, pv grid-connected inverter mathematical model is established. Tofacilitate the current controller design, the model from a, b, c three-phase stationaryframe is transformed into α-β stationary frame. Because there is no coupling betweentwo phases, current controller design is more convenient. In order to curb harmoniccomponent in the grid current, T filter is used in the main circuit. The filtering effectsbetween T type filter and L type filter are compared. The addition of T type filter bringsinstability to the system, so the system damping control is required. After Comparingwith the passive damping method and active damping method, this paper uses a hybriddamping method to eliminate the unstable factors brought by the T type filter. At last, aneffective hybrid damping parameter design method is given.In order to realize low THD and unity power factor of the grid current as well as tosatisfy the requirement of dynamic and static performance, hybrid controller is appliedin the grid current loop. Repetitive controller in parallel with proportional resonantcontroller is used in the system. During them, PR controller’s main mission is toguarantee the stability of the system and achieve zero static error of grid current. It canmake up for the inadequacy of repetitive controller cycle control delay. Repetitivecontroller’s main mission is to suppress the harmonic interference brought by gridvoltage. It can make up for the shortage of PR controller’s limited to suppress the harmonic interference in grid current. In addition, some scholars put forward withQuasi-PR of harmonic compensation controller. This controller can suppress theinterference from grid voltage harmonic. This paper analyses the repetitive controllerand Quasi-PR of harmonic compensation controller. The results show that the repetitivecontroller has better control effect.Finally, in MATLAB/Simulink, the new type three-phase five-level grid-connectedinverter and its grid-connected control strategy are simulated. The results of thesimulation verify the correctness and effectiveness of the theoretical analysis.