Research On The Cascaded H-Bridge Based On Medium Voltage APF

With the continuous advancement of industrial technology and the wide application of nonlinear devices,harmonic pollution is becoming more and more serious,which not only causes a decline in power quality,but also endangers the security and stable operation of the power grid.As a new type of power electronic device,Active Power Filter(APF)has become an effective means to dynamically suppress harmonics and compensate for reactive power in real time.The traditional APF cannot be applied to the medium voltage applications due to the resistance of the switching device voltageand the switching frequency.Therefore,cascaded H-bridge APF has gradually become a research hotspot in the field of medium voltage,and it is also the focus of this paper.Firstly,in order to solve the contradiction between switching frequency and capacity of high-power devices in medium voltage field,this paper introduces a carrier phase-shifted SPWM modulation technique for cascaded converter structure,and then explains the principle of CPS-SPWM technology and its application in cascaded APF,the half-cycle single frequency modulation method is analyzed by theory,and finally the output voltage waveform and frequency multiplication under this way are verified by simulation,and it states that this technique has an effect on APF output performance when the number of cascades and the phase shifting is different.Secondly,the ip-iq algorithm and LMS adaptive harmonic detection algorithm based on instantaneous reactive power theory are introduced in detail,and a new LMS/LMF harmonic detection algorithm is proposed.The algorithm is based on the fastest reduced mean square error theory,seeking the optimal function of the objective function,improving the step length iteration of the LMS algorithm,and adapting the threshold parameter to obtain the LMF step sizeaccording to the current error coherent average estimation,not only reducing the influence of the higher harmonic current on the step update process,but also achieves the purpose of both convergence and dynamic tracking performance.After that,the theory is verified based on the Matlab/Simulink simulation platform.Thirdly,the adaptive PI control principle and the DC-side voltage balance control principle are introduced respectively.Based on them,a voltage-stratified balance control strategy is proposed.The upper layer control improves the capability of current tracking by introducing adaptive PI outer loop control with the voltage squared difference as the input quantity,and combines the generated active command current with the improved i_p-i_q algorithm to generate the DC side main modulation wave component.To maintain the stability of the total voltage of each phase;the lower layer control is used to reconstruct the modulation wave fine adjustment amount based on the standard current,which is Superimposed on the main modulation wave generated by the upper layer control,achieving the voltage equalization in phase.Finally,the accuracy of the hierarchical control algorithm is proved by simulation,and it has certain practical value in the field of medium voltage.Finally,based on the establishment of MATLAB simulation platform,a test prototype of low-voltage H-bridge cascaded APF is built in this paper,and the main hardware circuit parameters and program design of the control system is elaborated in detail,then through debugging the program,observing and analyzing the experiment.As a result,the accuracy and practicability of the proposed method are further demonstrated.