Analysis And Optimization Of Input Current For Threephase Three-Level Rectifier

As the key part of power transimisson between grid and load,three phase rectifiers have been widly used in wind turbine systems,battery chargers,telecommunication power system and uninterrupted power supply.The three-level converters are gaining great attention due to several advantages such as low voltage stress of the power semiconductors,low electromagnetic interference and small filter inductance.The current ripple and harmonics are most concerned performance of three phase three level rectifier.The analysis and optimization of current ripple is essential for filter inductor and EMI filter design.In addition,the current harmonics need to be reduced to meet the grid code.However,plenty of switching states of three level rectifier make it hard to analyze the input current.In addition,the widely used powder core inductor has the characteristic of soft saturation,which will affect the input current.Therefore,the main issue of this paper is the optimization of the current ripple and harmonics for three-phase three-level rectifier.The main contents of this paper are concluded as follows:1)Aiming at current ripple analysis of three-phase three-level recitifier,a ripple analysis method considering the inductance variation is proposed.Three-level converters have much more switching modes than two-level converters,which makes the current ripple complicated.However,the inductor variation makes the analysis of current ripple more complicated.Therefore,an analysis method of current ripple considering the inductance variation is proposed in this paper.A calculation method of inductor voltage with inductance variation is presented.Then,the analytical expressions of peak-to-peak current ripple are derived with reference vecter located in any sectors.Based on the expressions,the current ripple distribution of three-phase three-level rectifier is despited.Finally,the correctness of analysis results is validated by both simulation and experiment based on a three-phase three-level rectifier prototype.2)A design method and optimization method of powder core inductor for three-phase three-level rectifier is proposed.Compared with air-gaped inductor,the soft saturation of powder core will challenge the design optimization of filte inductor.On a one hand,the maximum ripple of three-phase three-level rectifier is more difficult to calculate due to the inductance variation.On the other hand,the soft saturation of powder core makes it hard to determine the core size and turns.In addition,the current ripple of powder core is more complicated,which makes it hard to estimate the core and winding losses.Therefore,a precise design method for powder core inductor is proposed,the core size and winding turns can be determined directly without iterative calculation.The powder core inductor can meet the design goal precisely using proposed design method.In addition,a loss model of inductor is established and a optimization design procedure of powder core inductor is proposed.Finally,aiming at the filter inductors of a 10 k W three-phase three-level rectifier,the design optimization of powder core inductors is implemented.Two powder core inductors of optimization results are selected and applied to a 10 k W three-phase three-level rectifier.The experimential results show that the maximum current ripple is close to designed value,the relative error is less than 6%.In addition,the total harmonics distortion with two powder core inductors are both less than 2.5%and the system efficiency are more than 98.2%.Moreover,the losses of inductors are measured and the relative error is less than 10%.3)An optimiazed modulation method for interleaved three-phase three-level rectifier is proposed to reduce the current ripple.Parallel of three-phase three-level rectifiers is an efficiency approach to increase output power and reduce filter components.The number of swiching modes is increased with phase leg paralleled,which makes it even harder to analyze the current ripple.In spite of that,the modulatioin method of paralleled rectifier is more flexible,which makes it possible to reduce the current ripple further.For this purpose,a hybrid SVM method for two-channel interleaved three-phase three-level converter is developed to reduce the current ripple.In the peak of the input current,the space vector modulation method is used and the phase-shifted angle is modified to 90°.In addition,A zero-crossing clamped modulation method with carriers 180° phase-shifted is used around the zero-crosing point of current.Compared with traditional method,the current ripple amplitude with proposed method is reduced obviously,the harmonics at switching frequency is reduced from 2.34% to 0.7%.In addition,for unidirectional three-phase three-level rectifier,the proposed method can eliminate the current distortion around zero-crosing point and the harmonics is reduced from 4.31% to2.77%.4)In order to reduce the harmonics of three-phase three-level rectifier with inductance variation,a current control method is proposed.To make best use of powder core,the powder core at peak current is saturated seriously,which leads to wide inductance variation with sinusoidal inductor current.The changing inductance will weaken the harmonics suppression ability of traditional control methods.Therefore,a harmonics suppression method for threephase three-level rectifier with wide inductance variation is proposed.In a one hand,the instantaneous inductance compensation method is proposed to eliminate the limitation on control loop caused by inducatance variation.In the other hand,a little time of digital controller is occupied since additional controllers for harmonics reduction are not introduced.Thus,the sampling frequency can be increased to twice the switching frequency,which makes the bandwidth further extended.The proposed control method keeps a wide and fixed bandwidth of current loop,which which improves the harmonics reduction ability and dynamic performance without additional control method.