Research On Selective Harmonic Optimization PWM Strategy Of Three-level NPC Converter

In the field of high voltage and high power,the three-level converter has the advantages of low voltage stress,low switching loss and low electromagnetic interference compared with the two-level converter.Among commonly used three-level converters,the use of Neutral Point Clamped(NPC)is more widely used.In the three-level NPC converter control system,its inherent midpoint voltage imbalance problem will make the voltage stress of the power switching dcevice in the circuit unbalanced,which will affect the normal operation of the device and even destroy the device;reduce the use of DC side capacitors.Life and reduce converter output efficiency.Therefore,the study of the point voltage imbalance in the three-level NPC converter is of great significance.The Selective Harmonic Elimination Pulse Width Modulation(SHEPWM)strategy is a Fourier decomposition that predetermines the switching moment of the switching angle to achieve the switching of a specific switching angle,and finally eliminates the selected low frequency harmonics.technology.In this paper,the three-level NPC converter is taken as the research object.Based on the SHEPWM strategy,the online control of the switching angle in the SHEPWM strategy is studied.The midpoint voltage imbalance is optimized and the SHEPWM strategy is improved.Firstly,the working principle of the SHEPWM strategy for the three-level NPC converter is analyzed,and the nonlinear transcendental equations for solving the switching angle are established and solved.The SHEPWM strategy is used to solve the nonlinear equation of the switch angle into a specific control system,using the look-up table,integral controller and decoupling controller to help the system converge to zero error steady state;introducing the internal instantaneous observer to directly extract the harmonic of the output voltage The spectrum,i instead of using the FFT algorithm to output the step voltage waveform,eliminates the phase delay generated by the traditional spectrum extraction algorithm,and can achieve accurate control of the switching angle and fast response.Secondly,the reasons for the unbalance of the point potential in the NPC converter are analyzed.Based on the SHEPWM strategy,two schemes are studied to control the midpoint voltage of the DC side.Scheme 1 derives the relationship between the fundamental component and the 3rd harmonic at the midpoint potential offset,calculates the optimal 3rd harmonic content,and adds the optimal 3rd harmonic amplitude in the nonlinear transcendental equations of the SHEPWM strategy.The equation suppresses the midpoint voltage fluctuation;the scheme 2 introduces a small change of the switch angle based on the positive and negative polarity of the converter output current and the voltage across the DC side capacitor,and does not affect the output voltage harmonic performance,and changes the zero level switch state.The duty cycle,thus controlling the midpoint voltage.Finally,a three-level NPC converter simulation and experimental platform is built to verify the specific harmonic real-time cancellation and midpoint voltage control techniques studied in this paper.The results prove the correctness and effectiveness of the SHEPWM strategy switch angle online control technology and midpoint voltage control technology studied in this paper.