| Energy storage system plays critical and positive function for establishing smart grid andpromoting Micro-grid to develop. The power conversion system (PCS) in energy storage is anavenue of exchanging energy between the grid and energy storage equipments, and it is one of keytechnologies.The four-leg power converter and its control strategies determine quality ofgrid-connected current and local loads’ voltage, so researching on power conversion system inenergy storage is of engineering value and important theoretical significance.The four-leg power converter in the dual stage PCS is chosen as research object.Thorough,detailed and comprehensive research with respect to theoretical modeling, simulation investigationand experimental verifications are carried out.Control strategies for the four-leg power converterunder the asymmetry grid and the asymmetry loads, the three dimensions space vector pulsemodulation(3DSVPWM), common voltage suppress methods and seamless transferencetechnologies are all researched. The main research work and novel points are as follows:①The mathematical models based on a switch function of the four-leg power converter arecreated in the three-phase stationary frame and the synchronous rotating reference frame. On thebasis, the low frequency large signal models are deduced in the three-phase stationary frame and thesynchronous rotating reference frame. To analyze and investigate the control system for the four-legpower converter, the small signal model of the four-leg power converter is explored.②The3DSVPWM algorithm for the four-leg power converter and the mechanism to producethe common mode voltage is developed, then the reasons that the common mode voltage is producedby the four-leg converter are analyzed. The four methods of suppressing common mode voltage areresearched by the mechanism. The conclusion that two zero switch status vectors cause commonmode voltage to increase is drawn, so the nonzero space vectors modulation algorithm isresearched.This modulation algorithm can decrease the common mode voltage of the four-leg powerconverter by50%; The three pairs of complementary vectors lead the common mode voltage not toappear, hereby the modulation algorithm based on neighbor space voltage vectors is researched,Thismodulation algorithm can eliminate the common mode voltage of the four-leg power converter;while the methods of suppressing common mode voltage based on common mode filters arediscussed, An active common mode filter for eliminating common mode voltage is proposed in thisdissertation.③The four sorts of current control strategies for the four-leg power converter are emphatically researched. The dual stage PCS based on a four-leg power converter usually is connected to thethree-phase four-wire system grid. The grid voltage usually is asymmetry, according to this workcondition, the two current control strategies are explored, the two current control methods are thecontrol strategy based on the TSRF-PI algorithm and the control strategy based on the PI-RCalgorithm, respectively. The current control strategy based on the TSRF-PI can affectively suppressdisturbances by negative sequence voltage and zero sequence voltage, then the quality ofgrid-connected current is improved.According to the distorted grid by harmonics，The currentcontrol strategy based on the PI-RC algorithm can eliminate the disturbances by harmonics.Thereafter, and the symmetry current is fed into the three-phase four-wire system grid by thefour-leg power converter. The current control strategies based on the SSRF-PI algorithm and thecurrent control strategy based on a proportional plus resonant algorithm are investigated, too. Thecontrol method for the DC side voltage and the PQ control strategy are researched in thisdissertation.④The PCS based on a four-leg power converter is able to supply symmetry voltages forasymmetry three-phase loads under mode of stand-alone operation.The PI-RC voltage controlmethod is discussed.The four-leg power converter can output symmetry voltages through the voltagecontrol strategy, and the THD of the loads’ voltage is decreased⑤There are a great of single phase loads, three-phase asymmetry loads and nonlinear loads inthe three-phase four-wire system grid, so the grid neutral wire carries a mount of distorted current.To decrease and compensate the current conducting in the neutral wire, the novel four-leg powerconverter and its current control strategy are proposed. The proposed four-leg power converter cancompensate the current of local loads, and the current flowing to the neutral wire is greatlydecreased.⑥The transferring between a mode of grid-connected operation and a mode of stand-aloneoperation leads to impulse on the grid voltage and the local load voltage.To reduce impulse on thegrid voltage and local load voltage and improve transference flexibility,the phase-locked loop baseda self-adaptive frequency filter is researched and the seamless transfer method based on a multi-loopback-feed control and the seamless transfer method based on a droop control algorithm areinvestigated respectively. The two seamless transfer technologies can both make the dual stage PCSrealize seamless transferring between a mode of grid-connected operation and a mode of stand-aloneoperation.To verify the effectiveness and the practicability of the3DSVPWM, the researched currentcontrol strategies and the seamless transference technology based on a multi-loop back-feed control,a prototype of the four-leg power converter is designed and developed. Detailed and thoroughly experimental investigations are carried out, and the experimental results further confirm theeffectiveness and the feasibility of the3DSVPWM and the researched three sorts of current controlstrategies including the SSRF-PI, the TSRF-PI and the PI-RC. The validity of the seamlesstransferenec technology based on a multi-loop back-feed control is verified through experimentalresults. |
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