Study On Compensation Strategy And Control Method Of Cascaded Multilevel Dynamic Voltage Restorer

With the development of grid technology,the concept of power quality has been paid more extensive attention.Dynamic voltage restorer(DVR)can effectively suppress the voltage sag and avoid under-voltage tripping for the sensitive loads,which is attracted attention by more and more researchers.For a long time,most of the research focused on the under-voltage compensation strategy,which caused low rate of equipment utilization and insufficient development value of engineering application.On the premise of further study on voltage sag compensation,the function of fault current limiting,harmonic suppression and line impedance regulation with DVR were studied theoretically and experimentally in the dissertation.Firstly,the basic topological structure and working principle of DVR is introduced,and the traditional voltage sag detection methods are analyzed.Based on the analysis of the existing detection methods,comprehensively considering the voltage sag depth,voltage sag phase,phase jump,harmonic distortion,frequency change and other factors,proposes a voltage sag detection method of half cycle Fourier transform for harmonic distortion power grid.The solution is calculating average of dq components within real-time sliding window,any variation in fundamental amplitude of grid voltage can be detected.According to simulation and experimental results under various conditions,the proposed method is verified.Advantages of the proposed method are as follow: small ripple of the calculated fundamental voltage amplitude,short detection time,no need of phased locked loop,lookup table and low-pass filter,strong adaptability to frequency fluctuation,ease of implementation and computation.Secondly,the voltage compensation and fault current limiting strategy of DVR are studied in this dissertation.Considering the combination of sag compensation and fault current limiting,a comprehensive compensation control method is proposed.According to S transformation,the fault type and fault phase is rapidly identified.By the fault location,one of the voltage sag compensation and fault current limiting strategy is selected.For the upstream fault,a discrete phase transition method combining pre-sag and minimal-energy compensation is proposed.It can ensure load waveform free of transient distortion and compensate voltage sag for a long time.For the downstream fault,the fault phase current limiting mode can be taken into action,which makes the DVR play the role of isolating fault in power distribution system.Thirdly,the auxiliary harmonic compensation and impedance regulating function with DVR are studied in this dissertation when no voltage sag occurs.A combined control method of harmonic elimination and impedance adjustment based on the H-bridge diversified portfolio strategy is proposed.Fundamental voltage component of DVR is corresponding to the line reactance control.Harmonic voltage component of DVR is determined by harmonic compensation.Meanwhile,in view of DC voltage balance of the cascaded H-bridge inverter unit,the dissertation proposes a combination control strategy of modulation wave translation,modulation index,the number of level,selection and sort of H-bridge module,harmonic round-robin elimination.The method ensures the in-phase module voltage is balanced and the loss is lowest.Finally,simulation model of cascaded multilevel DVR is built in the Matlab/Simulink environment and the proposed method is verified.Through detailed theoretical derivation and the mathematical calculation,the selection of experimental device of DVR components and the design of the physical platform are completed.The test of DVR device in simulated power grid is carried out.The proposed control methods and the rationality of the hardware and software design are verified.