| The environmental pollution caused by fossil energy consumption is becoming increasingly serious.In order to solve this problem,the proportion of photovoltaic power generation has increased year by year.Grid-connected inverters are the interface for photovoltaic power generation to connect to the power grid.As a type of inverter,the current source grid-connected inverter has short circuit protection,and the DC side does not need to be boosted.The characteristics have attracted more and more attention.Among them,the multilevel current source grid-connected inverter Because of its high efficiency,low harmonic current content,low electromagnetic interference,and small output filter volume,it has become a research hotspot.In this paper,a five-level current source grid-connected inverter is taken as the research object,and its DC-side current sharing strategy and full feedforward control strategy are studied.The topology structure of the five-level current source grid-connected inverter is established by using the three-phase module combination type,and the five-level current source grid-connected inverter is established in the three-phase stationary coordinate system and the two-phase stationary coordinate system.Mathematical model.The harmonics of five-level current-source grid-connected inverters with two multilevel space vector modulation techniques,DC-side inductor current sharing strategy,main circuit parameter design,and full feed-forward strategy in conventional power grids and weak power grids are studied.Suppression strategy.Combining carrier phase shifting SPWM and SVPWM,a carrier phase shifting SVPWM(Carrier Phase Shifting SVPWM)is proposed,and the modal analysis of the logic switching state and action vector of carrier phase shifting SVPWM is analyzed.Drawing on the simplified SVPWM of a three-level voltage source inverter,a simplified SVPWM applied to a five-level current source inverter is proposed,and the modulation process of the simplified SVPWM is analyzed.Aiming at the DC-side inductor shunt imbalance of a five-level current source inverter,the influence of each type of redundant vector on the DC-side inductor current is analyzed,and the magnitude of the DC-side inductor current and the three-phase capacitor voltage in the actual system are analyzed.Relationship to achieve dynamic balance of the DC-side inductor current.Among them,when a modulation strategy based on carrier phase shifting SVPWM is adopted,the redundant vectors of the zero vector and the middle vector are reasonably selected to adjust the current dynamic balance.When a modulation strategy based on simplified SVPWM is adopted,a small vector and a medium vector are reasonably selected to adjust the current dynamic balance.A mathematical model of the full-feedforward strategy for the grid voltage is established.The feedforward function consists of a primary differential term and a proportional term with a lead link.In this paper,a repeated predictor is used to approximate the one-shot advance link,and a backward-order Euler-based first-order phase compensation differentiator is used to implement the differential link,and then the harmonic suppression effects of the full feed-forward function are analyzed.Simulation and experimental results verify the correctness of the harmonic suppression effects of the proposed full feedforward.A continuous domain mathematical model of the full feedforward strategy for weak grids is established.In the discrete domain,the relationship between the stability of the closed-loop system under the weak grid and the grid impedance,controller parameters,and differential functions is analyzed.A control strategy using a combination of FIR low-pass filters and repeated predictors is proposed to improve the weak current.The stability of the offline system realizes the trade-off between the effect of harmonic suppression and the stability of the system. |
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