Research On Regulation Strategies Of Single-Phase Three-Level NPC Half-Bridge Inverter

The topological structure of Single-phase Three-level Neutral Point Clamped(TL-NPC)Half-Bridge inverter is simple and highly reliable,which is widely used in industry and PV grid-connected systems with medium-voltage capacity.Compared with the traditional two-level topology,the three-level kind boasts the advantages of lower voltage,higher switching frequency and less electromagnetic interference for switching devices.In systems without transformer for insulation,where the electrical connection between PV panels and the grid occurs,causing leakage current to exceed the allowable range,the half-bridge inverter can compensate for the leakage current and incoming DC component.However,the Single-phase TL-NPC half-bridge inverter has the problem of double fluctuation of DC bus voltage,which will lead to third harmonic generation and waveform distortion of the output current of the Single-phase TL-NPC half-bridge inverter under closed-loop control.In addition,the unbalance of the DC-side capacitor voltage will result in the increase of the device’s voltage stress and the inverter’s output current harmonics.In this paper,the Single-phase TL-NPC half-bridge inverter is studied from the above two aspects.This article firstly described the operating principle of the Single-phase TL-NPC half-bridge inverter and deduced the mechanism of DC bus double frequency fluctuation.The difference between the double modulated wave modulation method and Sinusoidal Pulse Width Modulation(SPWM)is analyzed in detail,and the loss distribution of the switching devices of the Single-phase TL-NPC half-bridge inverter under the two modulation strategies is calculated.The double modulated wave modulation method indirectly controls the positive and negative level conduction time by adjusting the zero level conduction time to achieve the purpose of regulating the capacitor voltage.Then,the mechanism of the third harmonic generation of the output current due to the DC-side bus voltage double frequency fluctuation of the Single-phase TL-NPC half-bridge inverter is analyzed.We analyze the transfer functions of the biquad filter and biquad notch filter theoretically,and derive the Bode diagram of the transfer function.In order to suppress the third harmonic of the output current,a second-order filter or notch filter is added to the voltage outer loop path to filter the DC side capacitor voltage and indirectly suppress the third harmonic of the output current.Aiming at the problem of unbalanced DC-side capacitor voltage caused by the difference of DC-side capacitance,a control strategy of adding a feedforward voltage difference to offset the DC component of the output current is adopted to balance the DC-side capacitor voltage.Finally,through the Matlab simulation under different schemes and the research on the experimental platform,the relevant data and waveforms are obtained,and the analysis and comparison are carried out.The results show that the voltage outer loop path plus the second-order filter and the second-order notch filter are compared.When the second-order filter and second-order notch filter are not added,the third harmonic of the output current is significantly reduced;the control strategy of adding the feedforward voltage difference is faster than the ordinary closed-loop control to balance the DC side capacitor voltage,and the output voltage has a good sine.It further verified the correctness of the solution proposed in this paper to suppress the third harmonic of the output current and solve the imbalance of the DC side capacitor voltage.