Control Strategy For AC-DC-AC Converter Based On NPC Three-level With Output Stage

Three-level inverter is often used in high power applications due to high order of output voltage,low output harmonic and small voltage stress of switch,diode rectifier is regularly used in former stage of the three-level inverter which causes large amount of reactive and harmonic pollution to grid,therefore,it is important theoretical significance and practical value to developed a “green” rectifier in former stage of the inverter,which not only has high power factor and low input harmonics but also has excellent output performance of Three-level.Therefore,two kinds of three-level AC-DC-AC converters(called NPC three-level AC-DC-AC converter)with three-level inverter of NPC and rectifier of VIENNA or TSMC be studied.For the three-level AC-DC-AC converters,Modulation strategy and voltage directional current decoupling control in three-level VIENNA,Passivity Power Control in VIENNA rectifier,Over modulation strategy and model predictive control strategy in three-level matrix converter are respectively studied.The research work and results are as follows:According to the low voltage transfer ratio problem of three-level indirect matrix converter(TLIMC),A new topology from matrix converter family that can synthesis three-level voltage with improved output performance in terms of reduced harmonic content and a space vector over modulation strategy suiTab.le for TLIMC was proposed in this paper,which control the output voltage fundamental amplitude linearly.Finally,the validity and feasibility of this modulation method was tested by system model based on MATLAB/Simulink and an experimental prototype built for TLIMC;the result indicates that this over modulation strategy improves the maximum voltage transfer ratio from 0.866 to 1.053,meanwhile,make the converter own sinusoidal input currents with low harmonic content and high power factor.In addition,the algorithm are neither necessary to store a lot of data nor to do complex calculation,which is very suiTab.le for implement.Aiming to the inherent problems of midpoint voltage fluctuation and complex SVPWM of three-level VIENNA rectifier,the DC-link midpoint voltage error was introduced to the dual closed-loop control and the DC-link midpoint voltage balance control was solved by regulating the DC offset of the reference current.Combing with equivalent circuit.A simplified three-level SVPWM algorithm based on three-level principle was proposed for this topology,the working principle of VIENNA rectifier wasanalyzed,finally a prototype of VIENNA rectifier was built.Experimental results show that the adoption of this scheme ensures that the VIENNA rectifier not only owns excellent network-side performance,but also has a good static and dynamic performance and the midpoint balance characteristics.A passivity-based control method based on EL model was firstly introduced to three-phase/switch/level Rectifier,the EL model for the Rectifier was set up,the passivity characteristics of this rectifier was verified and the passivity-based controller was designed based damping injection method,meanwhile,the midpoint potential balance control was considered through the voltage error of dc-link capacitor.Finally a prototype of three-phase/switch/level Rectifier was esTab.lished to verification the proposed strategy.Experimental results show that the adoption of the algorithm ensures that this Rectifier not only own excellent input performance and dc-link voltage regulation performance,but also has a good dynamic performance and the midpoint balance characteristics.According to coupling effects and week disturbance resistibility caused by the integration of the topological of two-stage matrix converter(TSMC),The traditional space vector modulation strategy which is an open-loop control approximately needs to coordination and influence of two-stage,therefore the above problems is more likely to be generated.In this paper,a model-predictive control strategy for TSMC was proposed,according to discrete mathematical models and 72 kinds of switching states and as the control target with input reactive power and output currents,the desired switching states,which minimizes the instantaneous reactive power and the error in the output currents in normal and abnormal conditions for TSMC,are selected by predicting and controlling in each sampling period,Therefore,the proposed control strategy has a good tracking of the output current to their references and input currents with unity power factor and improves disturbance resistibility in normal and abnormal conditions for TSMC.The feasibility and validity of the proposed method has been verified by experimental results.To validate the proposed theories,The experimental prototype controlled by TMS320F2812 DSP+CPLD for three-level AC-DC-AC converter are carried out with designing sampling conditioning,drive and power circuit.Above algorithms are implemented in the experimental prototype and experimental results are consistent with the theoretical analysis,Feasibility and correctness of proposed scheme is verified.