Research On The Modulation Strategy Of Random SVPWM

The space vector pulse width modulation(SVPWM)has become the basic control mode of voltage source inverter because of its advantages of low switching loss and high bus voltage utilization.In general,inverters are operated at fixed switch frequency,which makes narrowband harmonic clusters arise at switch frequency and its multiples in the power spectrum of output voltages and currents,resulting in distortion of voltages waveforms,electromagnetic interference(EMI)and serious switching noise.Switch noise can be reduced with more than 20 kHz switch frequency,But the higher frequency greatly increases the switching loss of the inverter.In view of this problem,the dissertation take the three-phase three leg inverter and three-phase four leg inverter as the research models,and the random SVPWM modulation strategy was studied.The dissertation describes the basic principle of random PWM technique,and analysis the research status of three kinds of random PWM in china and abroad(i.e.random carrier frequency modulation,random pulse position modulation and hybrid random PWM).A theoretical model of power spectral density of those modulation strategies was established using the statistical communication theory.Based on the spectrum dispersion effect of random PWM,and the advantages and disadvantages of those modulation strategies were compared and analyzed in the article.A novel dual randomized modulation strategy was proposed for three-phase three-leg inverters,the modulation strategy was based on dual zero vector random pulse position and randomized switching frequency.According to the modulation ratio M,zero vector is selected as V(1 1 1)when M?0.7 and as V(0 0 0)when M<0.7,by increasing switch frequency randomization to realize random variation of the pulse position and the switch frequency.To insure better random variation degree of random SVPWM,WELL algorithm was introduced to generate random number.The simulation results show that the modulation strategy can eliminate the discrete components in the spectrum effectively,and harmonics in the switching frequency and its multiples have disappeared dramatically,which show a good harmonic dispersion effect.Two randomized pulse position modulation strategies are proposed for three-phase four-leg inverters.Based on the volt-second balance law and the fixed switching frequency,the first proposed strategy places the center randomly during one carrier cycle,and the second proposed strategy randomly assign all vectors action time between the ascent and descent stage of each triangle carrier.The simulation results show that the proposed strategies have dispersed the discrete harmonics to a wider sideband.Compared with the first strategy,a larger degree of freedom can be achieved in the second proposed strategy with lower harmonic spikes,however,it has a relative high THD.The modulation strategies are implemented with dSPACE real-time operating system for experimental verification.The experimental results verify the effectiveness of the two random pulse position 3D-SVPWM strategies and the WELL algorithm.