| It is more and more popular for middle high power AC motor speed adjustment technology used in major industrial production such as steel rolling,papermaking,cement plant,coal,railway and ships.The AC motor speed regulat ing system not only can conserve energy,but also can improve the process conditions,product quality and productivity.Simultaneously the application of middle high voltage high-capacity variable frequency speed control system can reduce the emission of e xhaust gas as well,such as city subway,high-speed rail,light rail and electric vehicle.It is a most straightest and effective means to solve the environmental pollution problem.Multilevel inverter have the characteristics of high output voltage,high output power,output voltage waveform being similar to sinusoidal wave,little harmonic content and small variety rate.Due to these features,multilevel inverter is suitable to high voltage and high power field.From these,multilevel inverter has become the research hotspot of power electronic technology field along with the mass applications of multilevel inverter in middle,high frequency control and power system.The cascaded multilevel inverter is cascaded by H-bridge power cells supplied by DC sources in depended.There is no DC capacitance voltage-sharing problem,and it has the advantages such as it needs seldom elements,the output voltage harmonic content is small,it is easy to realize modularization and extend.So it becomes the one of most development prospect structure in the multilevel inverters.In the multilevel inverter modulation arithmetics the AC machine and inverter are seen as an entirety in the space vector PWM(SVPWM)control method proposed by J.Holtz,the aim of it is to get the circle flux linkage track with constant amplitude.It has the advantages of little torque ripple,low noise,high DC voltage utilization and convenient for computer real-time control.So it is applied widely.In this paper,several key issues of cascaded multilevel inverter control system were deeply and comprehensively studied.This paper included the following contents:1 General algorithm and simplified algorithm for SVPWM of N cascaded multilevel inverterA general SVPWM algorithm that is suitable to any level cascaded multilevel inverter is proposed.The sector which the reference vector vertex lies is judged and the triangle that the reference vector vertex lies is settled.The type of triangle is judged and the calculation of dwell times of base vectors which compose the reference vector is classified into two types.The dwell times of base vectors are deduced respectively by two parameters:the layer of hexagon which the reference vector vertex lies and the layer in sector vertical direction.The validity and generality are verified.So a general SVPWM algorithm is provided for any level cascaded multilevel inverter.For the phenomenon of voltage jump in the output voltage wave the dead technology is proposed to avoid the change of H parameter in the same sector.So the voltage jump is disappeared in the output voltage wave and the THD is declined at the same time.For the complexity of cascaded multilevel inverter SVPWM algorithm is increased with the increasing of voltage level,the simplified SVPWM algorithm based on mapping principle is proposed in this paper.The hexagon which the reference vector vertex lies is mapped to center hexagon.The base vectors and their dwell times are settled by the SVPWM algorithm of two-level inverter,then it is translated into the composing of multilevel inverter reference vector by reverse mapping.The judgment of the triangle that the reference vector vertex lies is avoided and the algorithm is simplified.The complexity is not increased with the increasing of voltage level.2 Over-modulation algorithm of cascaded multilevel inverterFor the realization of over-modulation of cascaded multilevel inverter superposition principle and multilevel inverter SVPWM algorithm based on mapping principle are composed.The over-modulation range was divided into two modes depending on the modulation index.Because it is only need to modify the amplitude of reference vector in over-modulation range ?,the superposition principle is utilized to modify the reference vector.The amplitude and angle are both need to modify in over-modulation range ?.In order to simplify the complexity the superposition principle is improved and the superposition coefficient is redefined,the dwell times of base vectors which compose the modified reference vector are deduced by formula.The complex Fourier transformation is avoided and the algorithm is simplified to realize.The utilization of DC voltage is further improved at the same time.For the problem of the modulation gain will decline when the modulation index increase at one value in the over-modulation range the method of compensation to modulation gain by reverse gain is proposed and the gain decline is compensated in the over-modulation range in effect.3 The SVPWM algorithm of multilevel and 30°-shift double-star winding of permanent magnet synchronous motor(PMSM)speed regulation system and the vector control systemIn order to meet the high voltage and large capacity of the speed regulating system,the SVPWM algorithm of 30 °-shift double-star winding of permanent magnet synchronous motor(PMSM)drive by multilevel inverter is proposed.The speed regulating system topology with 30°-shift double-star winding of permanent magnet synchronous motor(PMSM)is provided,which is fed by el even-level cascaded multilevel inverter.The distributions of voltage space vectors in different orthogonal spaces are analyzed in detail.Meanwhile,the four-vector space vector pulse width modulation(SVPWM)algorithm is presented based on two cascaded inverters.The harmonic loss of this method is analyzed deeply.The PMSM vector control system of 30°-shift double-star winding driven by the multilevel inverter is established.This system can realize the machine drive with 10 KV even higher voltage,and the output current harmonic and torque ripple are low.The simulation confirmed validity of these methods.4 The development of cascaded multilevel inverter deviceThis paper finally makes engineering design for the application of cascaded multilevel inverter in the field of high voltage variable frequency.The paper gives out the main circuit's hardware design,the main control circuit's hardware design and part software design based on DSP,etc.The experiments of the speed regulation system based on two modulation indexes are carried.The main waveforms are given which verifies the correctness of the proposed modulation methods.The paper can promote the very good industrial application prospect for cascaded multilevel inverter in high voltage variable frequency areas. |
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