| With the vigorous development of China’s power electronics technology and the continuous update of power electronic devices,three-level inverters have been widely used in new energy industries,electric vehicles,motor drives,and other occasions due to their advantages of small voltage stress,high withstand voltage level and high output efficiency.Among all kinds of threelevel inverter topologies,Neutral Point Clamped(NPC)inverter is the most widely used,but NPC inverters have unbalanced losses between different switching tubes and unbalanced capacitor voltages on the DC side due to their own structure.Therefore,this thesis studies the three-level Active Neutral Point Clamped(ANPC)inverter,and probes into its capacitor-voltage balance control strategy and loss balance control strategy.This thesis introduces the topology of three-level ANPC inverters and compares and analyzes the advantages and disadvantages of traditional Space Vector Pulse Width Modulation(SVPWM)methods and carrier cascade Pulse Width Modulation(PWM)methods.Aiming at the low DC voltage utilization rate of the carrier stacked PWM method and the complex algorithm of the SVPWM method,the change law of the three-phase output state in the space vector graph is analyzed,and an improved carrier PWM strategy is proposed.Firstly,the switching sequence of the synthesized reference vector is re-selected,and the duty ratio of each phase output state is deduced.A triangular wave is taken as the carrier,the output duty ratio is substituted into the carrier,and the modulated wave expression is calculated.The modulation wave is compared with the carrier wave,and the driving signals of each switching tube are obtained through the corresponding logical relationship.The effectiveness of this strategy has been verified by MATLAB/Simulink simulation platform.The mechanism of midpoint potential unbalance is briefly analyzed,and the principle of controlling midpoint potential balance by traditional SVPWM modulation is expounded.Considering that the SVPWM strategy does not consider the influence of the middle vector on midpoint potential balance,the influence of the middle vector on midpoint potential is still uncontrollable,and a zero-sequence component injection method based on the above improved PWM strategy is proposed.Based on the three-phase modulated wave,the zero sequence component is injected,and the output component that makes the midpoint potential fluctuate is eliminated through the zero sequence component,so as to realize the midpoint voltage balance.The simulation results show that the proposed modulation strategy is effective in the midpoint potential balance.The unbalanced switching tube loss caused by different converter modes of three-level ANPC inverters is analyzed in detail and the power component loss model is established.Considering the strong positive correlation between switching frequency and switching loss,a modulation strategy of switching frequency cycle rotation to select a zero-level switching state is proposed to equalize the switching loss of devices.Firstly,the modulation period is divided according to the different switching frequency characteristics of the two commutation modes,and the zero states is fixed for each region to optimize the switching sequence.The switching sequence is applied to the modulated wave derived from the above content.By comparing with the carrier wave,the average switching frequency of each switching device within a modulation cycle is equal,and the loss balance effect is achieved.The verification was completed in the co-simulation platform of MATLAB/Simulink and PLECS.Finally,the topology structure of a three-level ANPC inverter is built on the hardware-in-loop platform and the experiment is carried out.Experimental results verify the correctness of the proposed modulation strategy. |
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