| The dual-active bridge type single-stage AC-DC converter has significant advantages such as wide voltage/power conversion range,high power density,high efficiency,and long life.It has great application prospects in AC grid-connected energy storage systems.At present,the research on this type of converter is still in its infancy,and there are problems such as large internal circulating currents,operating efficiencies to be further improved,and large harmonic content of grid current,which restrict its practical application.Therefore,the development of corresponding modulation and control strategy research work has important scientific significance and practical value for improving its theory and technical system and promoting its practical process in the grid-connected energy storage systems.This dissertation takes dual active bridge type single-phase quasi-single-stage and three-phase single-stage AC-DC converters as the research object,and conducts an in-depth study of its optimized modulation strategy,system model establishment,and high-performance grid current closed-loop control strategy,so as to significantly improve its system efficiency and grid current quality.First of all,for the single-phase quasi-single-stage topology under the classic singlephase-shift modulation strategy,the AC side transfer current distortion of the converter and the internal circulating current of the converter are large.By analyzing its current characteristics,it is pointed out that the essential reasons of the above-mentioned problems lies in the coupling of transformer current during the adjacent switching periods,which leads to current control error and waveform distortion.To this end,this paper proposes an improved three phase shifts modulation strategy.In order to eliminate the AC side transfer current control deviation and internal circulating current of the converter,based on the above analysis results,the constraint relationship between the phase shift angles that ensure the current decoupling of the transformer in adjacent switching periods is obtained.With the aid of the three-degree-of-freedom characteristic of the three phase shifts modulation strategy,the minimum RMS value of the transformer current is introduced as the optimization condition to obtain the exact relationship between the phase-shift angles.The influence of the dead band on the proposed improved modulation strategy is further analyzed,and the corresponding dead band compensation method is proposed.Detailed simulation and experimental results show that the proposed improved modulation strategy and its dead band compensation method achieve precise control of the AC side transfer current of the converter,thereby significantly reducing the total harmonic distortion of the grid current and eliminating the internal circulating current of the converter,thus effectively improving the system efficiencies.In order to achieve the excellent closed-loop control performance of the single-phase quasi-single-stage topology grid current and suppress the influence of the low-order harmonics of the grid voltage,the closed-loop control strategy is deeply studied.Based on the proposed improved three phase shifts modulation strategy,the mathematical model of the converter is established by the state-space averaging method.Based on the established model,the analysis points out that due to the existence of the grid side LC filter,when the AC side transfer current of the converter is guaranteed to be an ideal sinusoidal waveform,the low-order harmonics of the single-phase grid voltage will still cause the grid current harmonics.Furthermore,a grid current closed-loop control strategy of proportional resonant combined odd-mode repetitive controller is proposed.The proportional resonant controller is used to achieve closed-loop control of the grid current fundamental component,and the odd-mode repetitive controller is used to eliminate grid current odd harmonics caused by grid voltage.And its operating principle,parameter constraints and frequency domain characteristics are analyzed in detail.The proposed grid current closed-loop control strategy is verified by the detailed experimental results.Aiming at the problem of the AC side transfer current distortion of the converter that exists in the traditional current space vector PWM modulation strategy of the three-phase single-stage topology,a new modulation strategy is proposed.It is revealed that the essential reason for the current distortion caused by the traditional modulation strategy is that due to the coupling of various varibles of the converter,the analytical solution of the action times of the adjacent effective current space vectors cannot be obtained.The approximate calculation method adopted leads to the control errors in the AC side transfer current of the converter.Based on the established equivalent relationship between the three-phase single-stage topology and dual active bridge(DAB)DC-DC converters,a dual-switching-period decouplled space vector phase shift modulation strategy is proposed,and two equivalent DAB converters are set separately in each control period.The independent switching period is used for independent precise control of the current space vector.Further analysis points out that the equal switching period assignment method of the dual-switching-period decouplled modulation strategy limits the maximum transfer power of the system and reduces the utilization rate of the system's rated capacity.In order to increase the maximum transfer power of the system,a dynamic assignment method of switching periods is proposed,and its dynamic assignment principles are analyzed in detail.The corresponding simulation and experimental results show that the proposed modulation strategy realizes the accurate control of the AC side transfer current of the converter,thereby significantly reducing the total harmonic distortion of the grid current and effectively increasing the maximum transfer power of the system.In order to realize the excellent closed-loop control performance of the grid current of the three-phase single-stage topology and eliminate the effect of the unbalance and low-order harmonics of the three-phase voltages,the closed-loop control strategy is deeply studied.Firstly,the state space average model of the converter under the proposed variable switching period modulation strategy is established.Based on the established mathematical model,it is analyzed and revealed that the unbalance and low-order harmonics of the three-phase grid voltage will also cause the three-phase grid current unbalance and the low-order harmonics at the same frequencies as the grid voltage harmonics.In order to realize the zero-error control of the three-phase grid fundamental current while suppressing the effects of the three-phase grid voltage unbalance and loworder harmonics,and fully considering the features of the three-phase grid voltage harmonics,a grid current closed controller under the two-phase stationary frame,containing a proportional resonant compound dimensionality reduction repetitive controller,is proposed.Detailed experimental results show that the proposed grid current closed-loop control strategy achieves a good dynamic and static control performance,and effectively suppresses the effect of grid voltage unbalance and low-order harmonics. |
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