Variable Switching Frequency PWM Technology Research On NPC Three-level Inverters

Multilevel converters have become the widely used power electronic conversion devices in the field of medium & high voltage and high power,due to their high power quality and flexibility in modulation.The neutral point clamped(NPC)three-level topology was first proposed in the 1980 s.It has the characteristics of simple structure and fewer components.Although it has been developed for nearly 40 years,it still receives extensive attention.In this paper,a three-phase NPC three-level inverter is taken as the research object,and the variable switching frequency(VSF)modulation technology based on the time-domain model prediction is researched to improve the conducted EMI.Based on the outline of the working principle and basic PWM,the equivalent principles of SVPWM and carrier-based PWM(CBPWM)are established.And the influence of PWM on switching loss,harmonics and EMI is analyzed.Since the current / voltage ripple determined by pulse width modulation is synchronized with the pulse sequence,it can be predicted in real time according to the equivalent circuit model,which provides the possibility to change the switching frequency in real time to achieve current / voltage ripple control.There is the problem that the EMI spikes are high because the conventional PWM fixes the switching frequency and the energy is too focused on the switching frequency and its multiples of narrow band frequency range.To solve the problem,active suppression can be achieved by changing the switching frequency.First,according to the current ripple requirement of the NPC three-level inverter for AC measurement,a VSF modulation based on the AC-side current ripple prediction is proposed.This method analyzes the characteristics of the current ripple based on the current ripple time-domain model,and can accurately predict the current ripple.Then it controls the peak value of the ripple to clamp to the limit value to achieve the switching frequency change.The simulation and experiment prove that the method does not change the peak value of the ripple.Furthermore,the average switching frequency is effectively reduced and the conducted EMI is effectively attenuated.Corresponding to the AC current ripple,the voltage ripple caused by the charging and discharging of the clamp capacitor by the DC side switching current of the NPC three-level inverter is another object that needs attention.A neutral point voltage(NPV)balancing VSFPWM strategy is proposed to solve the NPV imbalance problem of the NPC three-level topology.The strategy controls the low & high frequency ripple of the NPV,respectively.This method uses a zero-sequence injection CBPWM scheme to eliminate the low-frequency fluctuations of the NPV.Then the NPV high-frequency ripple prediction scheme is proposed based on the time domain model for high-frequency ripple.VSFPWM controls the predicted ripple peak value to clamp to the limit to change the switching frequency.This method effectively suppresses the NPV fluctuation,reduces the average switching frequency and attenuates the conducted EMI on the DC & AC sides.Experiments prove the effectiveness and feasibility.The thesis work proves that the switching frequency can be effectively changed in real time,based on the establishment of an accurate current / voltage ripple prediction model.The methods can achieve the switching loss and EMI suppression of NPC three-level inverter while controlling the ripple.