Research On Natural Frame Control Of Single-Phase Cascaded H-Bridge Converter Under Distorted Grid Voltage

As the demand for electricity continues to rise and the society’s awareness of environmental protection continues to increase,renewable energy accounts for an increasing proportion of the total power generation.Its rapid development has brought new challenges to the power system such as the grid connection of distributed generation resources and long-distance power transmission.The cascaded H-bridge multilevel converter is not only an efficient converter equipment,but also a good grid-interfaced device,it has been widely applied in medium/high voltage,high power applications due to its modularity,high reliability,and relatively few components required.The control complexity of single-phase CHB increases with the number of cascaded modules.At present,the commonly used control method is still the direct and quadrature axes decoupling control method(dq method)in the synchronous rotating frame.However,the dq method requires phase-locked loop,coordinate transformation,and construction of fictive current.There is still space for optimization in performance and algorithm complexity.Besides,as the power electronization of the power system increases,the distortion of the power grid voltage has also become common.Therefore,it is important to study the control strategy of the converter under the distorted power grid.First of all,in order to eliminate the influence of distorted signals on the synchronization link,this thesis proposes a modified single-phase cascaded delayed signal cancellation(DSC)synchronization method suitable for single-phase power grids.The DSC operator is a typical comb filter with superior harmonic elimination capability and linear phase characteristic,but the existing DSC method is mainly applied to the three-phase system.The simplified DSC proposed in this thesis based on the characteristics of the single-phase system has no feedback loop and integral term in the structure,and the filtering effect and dynamic response are better than the commonly used second-order generalized integrator(SOGI),and the response time is less than one fundamental frequency grid period in the face of phase mutation and other disturbance.And then,combing the fictive phases construction(FPC)strategy,a natural frame DSC method is discussed.Its structure is simpler and total delay is shorter under specific working conditions.In addition,compared with the dq method,the natural frame control not only has clearer physical meaning,but also has a simpler structure,which can improve the dynamic performance of the control system and reduce the calculation burden.The thesis increases the harmonic impedance of the control system by introducing harmonic voltage feedforward into the original natural frame control structure.In order to ensure the normal operation of the CHB,the DC side voltage balance strategy under the natural frame is proposed,and the generation mechanism of the double frequency ripple of dc-link voltage is analyzed.Combining all improvements aforementioned,a modified natural frame control(MNFC)method is proposed.Finally,the synchronization method and MNFC strategy proposed in this thesis are verified on the MATLAB/Simulink software environment and a downscaled experimental platform.The dynamic and steady-state performance,robustness and comparison with the traditional dq method prove the effectiveness of the proposed method.