Research On The Key Problems Of Three-level Power Inverter Based On VSVPWM

As a clean and efficient energy,electric energy plays an important role in the energy structure of our country.The development and utilization of new energy and the emergence of more and more precision instruments require that the inverter has the characteristics of high output power,low harmonic content,high conversion efficiency and low electromagnetic interference.The diode-clamped three-level inverter has the advantages of simple control,low stress of switching devices and low harmonic distortion of output voltage and so on,is widely used in high-voltage and high-power applications.However,there are problems of the unbalanced neutral potential and dead-time effect,which will cause distortion of the output waveform and even reduce the life of electronic components.Therefore,it is necessary to study these two problems to improve the output efficiency and the quality of the output waveform.This paper mainly studies related technologies such as VSVPWM,neutral-point potential balance and dead-time compensation of diode clamped three-level inverter.The main work of the paper is as follows:(1)In order to solve the problem of large computation in the traditional VSVPWM implementation process,the paper analyses the topology,working principle and three-level vector space of the diode-clamped three-level inverter.At the same time,the algorithm of VSVPWM is optimized based on 60 degree coordinates,and the realization process of the optimization algorithm is deduced in detail,which simplifies the judgment of size sector and the calculation of action time.The simulation model is built by MATLAB/Simulink.The results indicate that the switch driving signal,output line voltage and three-phase output waveform of the inverter are consistent with the theoretical derivation,which also proves the correctness of the optimization algorithm.(2)Aiming at the problem of large fluctuation and offset of neutral point potential,a control method based on VSVPWM is proposed,which combines distribution factor control with switching sequence selection control.For the small sector with redundant small vectors,the distribution factor is introduced to control the potential.When the reference voltage vector is located in the fifth small sector,the synthetic vectors lack redundant small vector,the offset of neutral point potential can be balanced by choosing appropriate switching sequence.The simulation results show that the proposed balance algorithm can effectively suppress the fluctuation of neutral point potential when the modulation degree is 0.9,and it also has a strong control ability to control potential offset.(3)In view of the deficiency of fixed dead-time compensation for three-level inverters and the inaccurate judgment of current polarity,the dead-time effect is discussed and analyzed in this paper.A dead-time compensation strategy based on phase voltage feedbackand current reconstruction is proposed.According to the feedback phase voltage,the error voltage caused by the dead time effect can be calculated in real time to improve the adaptability of the compensation strategy.At the same time,the Kalman filter is used in the rotating coordinate system to overcome the phase delay problem caused by the traditional filtering method,thereby improving the accuracy of current polarity judgment.The simulation results show that the polarity of the reconstructed current at zero-crossing is easier to judge.At the same time,compared with the traditional fixed dead-time compensation method,the output voltage after dead-time compensation using the proposed algorithm is closer to the target value,and the harmonic content is also significantly reduced.(4)An experimental platform of 15 KW diode clamped three-level inverter is built based on the TMS320F28335 DSP.The neutral-point potential balance and dead-time compensation algorithms of VSVPWM based on 60 degree coordinates are verified respectively,and then the whole system is debugged.The experimental results show that the proposed neutral-point potential balancing algorithm can effectively suppress the capacitor voltage offset and reduce the fluctuation of the neutral-point potential;the voltage after dead-time compensation is smoother and the harmonic content is smaller;the three-phase output of the inverter basically reaches the 220 V target value,the phase deviation between adjacent two phases is less than 6 degrees,and the harmonic of output voltage is less than 3%,which basically achieves the expected target.