| As more and more power electronic converters are connected to the power grid,the problem of harmonic pollution and power factor in the power system has attracted increasing attention.On the one hand,in order to meet the complex and changeable power consumption occasions and to meet the standards proposed by some countries or international organizations to limit the harmonics of the power grid,the front end(connected to the power grid)of the electrical equipment connected to AC power grid must have the topology structure of power factor correction and its excellent control technology.This article mainly studies the one-cycle control technology of the three-phase three-level VIENNA rectifier,and makes beneficial improvements to the control technology.The paper first introduces the structure and working principle of VIENNA rectifier,and derives the one-cycle control strategy and the realization method of digital control.It simulates and experiments on the one-cycle control technology of VIENNA rectifier,which is used as the theoretical basis of the paper.Aiming at the problem of one-cycle control that reduces the power factor of the system due to the inductance voltage drop,the paper adopts an improved one-cycle control technology that takes into account the inductance voltage drop to compensate the input current to make the input voltage and the input current change proportionally,The core equation is derived,and the realization method is given.By considering the inductance voltage drop,the phase of the one-cycle control modulation wave is changed,and its power factor is improved.Through the analysis of the improved one-cycle control due to the modulation wave and the network The phase of the current on the side is different,which causes the distortion of the input current.The adjustable proportional constant kc is used to properly adjust the phase shift of the modulation wave to improve its power factor under the allowable input current harmonic distortion rate.The paper studies the corresponding input current distortion caused by the harmonic distortion of the input voltage,and analyzes the influence of the input current harmonics on the output voltage.The n-th harmonic of the input current will cause the DC output side to produce a pulsation of n-1 times the input fundamental frequency;the pulsation of n times the input fundamental frequency in the DC output side will cause the input current on the AC side to produce n+1 or n-1 harmonic component.In this regard,it is proposed to extract the harmonic components in the input current in the dq coordinate system and introduce repetitive control to obtain the harmonic current suppression compensation signal,which is superimposed on the modulation wave of the one-cycle control.The gain of input equivalent resistance of VIENNA rectifier at harmonic frequency point is increased significantly,which can suppress the negative impact of the harmonic distortion of the input voltage on the current,and obtain a standard sinusoidal input current waveform with low THD.The paper studies the negative impact on the input current caused by the unbalanced input voltage,and analyzes the phenomenon that the unbalanced input voltage will cause the input current to produce harmonic distortion and amplitude imbalance.In this way,the unbalance coefficient of the input voltage is introduced to modify the modulation signal of the traditional one-cycle control VIENNA rectifier.The unbalance factor is obtained by extracting the positive sequence component of the three-phase input voltage.The improved one-cycle control technology enables the input current of each phase can well track the input voltage of the corresponding phase,and while achieving high power factor,it can also obtain a balanced and low-harmonic distortion input current. |
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