Research On Modulation Strategies Of T-Type Three-Level Photovoltaic Inverters With Unbalanced DC-Links

As a means of realizing photovoltaic conversion in the field of new energy power generation,the performance of photovoltaic inverters directly determines the quality of power fed into the grid.With the condition of unbalanced dc-links of the T-type three-level inverter,the main goal is to improve the power quality,Carrier-Based Pulse Width Modulation(CBPWM),Virtual Space Vector Pulse Width Modulation(VSVPWM)and Discontinuous Pulse Width Modulation(DPWM)strategies are studied separately.Finally,Maximum Point of Power Trackings(MPPT)and VSVPWM strategy are combined to realize the asymmetric control of the dc-link voltages.First of all,the application of the traditional CBPWM strategy under the condition of unbalanced dc-links will have an adverse effect on the output of the inverter.Therefore,using the volt-second balance law,two CBPWM strategies applied under the condition of midpoint potential imbalance are studied: the modulation strategy for adjusting the amplitude of the triangular carrier and the modulation strategy for adjusting the amplitude of the modulation wave.In order to improve the degree of influence of unbalancing degree on modulation index,a CBPWM strategy that feeds zero-sequence component is also derived.Both theory and simulation have proved that the CBPWM strategy fed with zero sequence component can reduce the adverse effects of unbalanced degree of modulation.Secondly,the three-level space vector diagram with balanced dc-links is derived,and the mathematical relationship between the unbalanced space vector diagram and the unbalancing degree is obtained.Introduce the basic principle of VSVPWM strategy,and derive the virtual space vector diagram with unbalanced dc-links.The geometric distributions of the unbalanced virtual space vector diagram and the balanced virtual space vector diagram are consistent,which provides a theoretical basis for the application of the VSVPWM strategy with unbalanced dclinks.Simulations and experiments verify the effectiveness of the modulation strategy.Next,a DPWM strategy suitable for unbalanced dc-links is proposed.This strategy redivides the subsectors,obtain the vector action sequence of each sector,and derive the basis for judging subsectors.The traditional vector action time derivation will complicate the modulation wave expression,so an indirect method is used to derive it.The three-level state time of threephase is obtained,and the modulation wave expression is obtained by combining the carrier equivalent theorem to complete the application of the DPWM strategy with unbalanced dclinks.The relationship between switching loss and unbalancing degree is discussed,and the effectiveness of the DPWM strategy is verified by simulations and experiments.Finally,the mathematical model of photovoltaic panel and MPPT strategy which including the disturbance observation method and the conductance increment method are studied.MPPT and VSVPWM strategy are combined to realize the asymmetric control of the dc-link voltages,the effectiveness is verified by simulations.