| With the continuous expansion of human production scale,power electronics technology is widely used in high-voltage high-power inverters and AC drive systems.However,the traditional two-level converter topology is not suitable for achieving high voltage and high power output due to the increase of the harmonic distortion rate of the system network side caused by the lower switching frequency of the device and the static and dynamic uniformity problems caused by the series connection of the device.The emergence of multilevel inverter topology not only effectively improves the voltage level of the output voltage and reduces the harmonic content of the output voltage,but also greatly reduces dv/dt,and improves the performance of electromagnetic interference EMI.Therefore,it has become a research hotspot of high voltage and high power conversion technology.This article takes hybrid cascaded multilevel inverters as the research object.The main content and innovation results are as follows:Aiming at the problems of current backflow and energy feedback caused by the traditional mixed-frequency modulation strategy of a type Ⅱ hybrid cascaded multilevel inverter,a hybrid frequency-optimized modulation strategy based on carrier stacking is proposed.The method of reverse cascade,the low voltage unit adopts a method similar to the unipolar frequency multiplication,and the logic of the comparison pulse is recalculated and combined,which not only effectively solves the problems of current backflow and energy feedback generated by traditional hybrid modulation.Moreover,the harmonic characteristics of the output voltage waveform of the high-voltage unit under mixed frequency modulation are improved,thereby reducing the content of loworder harmonics of the output voltage of the hybrid cascade H-bridge multilevel inverter,and improving the quality of the inverter output power.Aiming at the problem of poor output voltage harmonic characteristics caused by the mixed frequency modulation strategy of the type Ⅱ hybrid cascade seven-level inverter,a LS-PWM based frequency doubling modulation strategy is proposed to reduce the mixed frequency.The obtained pulses have undergone a new logical operation and combination to improve the harmonic characteristics of the voltage waveforms of the high and low voltage units under mixed frequency modulation,thereby reducing the content of low-order harmonics of the output voltage of the hybrid CHB inverter and improving the inverter The quality of the output power of the converter.Aiming at the problem that the output phase voltage and line voltage of the type III hybrid cascaded multilevel inverter traditional hybrid modulation are low and the harmonic content is high,an improved hybrid modulation strategy is proposed.The high-voltage unit uses sinusoidal modulation.The method of comparing the POD of a wave with a pair of carriers located in the positive and negative half cycles.The low-voltage unit uses the difference between the output voltage of the modulation wave and the high-voltage unit as the modulation wave,and then compares the APOD with a pair of positive and negative stacked carriers.This method eliminates the 5th and 7th harmonics of the output phase voltage and line voltage,thereby improving the harmonic characteristics of the inverter output voltage.Aiming at the NPC/H bridge cascaded multilevel inverter,a hybrid modulation strategy is proposed.Among them,the H-bridge unit adopts the method of step wave control,and the diode clamp unit adopts the positive-negative inversion stacking SPWM Control method to achieve 13-level output.And optimize the proposed strategy.On the basis of ensuring the quality of the waveform,by implementing the pulse interchange cycle method within 1/4 cycle,the power equalization control of the H-bridge unit is realized.From the perspective of Fourier analysis,The detailed derivation process of the amplitude of the fundamental voltage of the output voltage of the H-bridge unit after equalization. |
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