Model Predictive Control For T-type Three-level Photovoltaic Grid-connected Inverter

In recent years,the frequency conversion speed control technology in the fan,pump and other load AC motor applications,has achieved very significant economic and social benefits,high-voltage and high-power multi-level inverter has become a research hotspot.Because of its low voltage requirements,no dynamic pressure,the high equivalent switching frequency and the good output waveform,it is widely used in PWM reversible rectification,flexible transmission system and active filter system.In this thesis,the most widely used diode-clamped(NPC)three-level inverter is used as the research object,and the research is carried out in depth.Focusing on the existence of the midpoint potential fluctuations,dead zone compensation of two issues,in the virtual space vector pulse width modulation(VSVPWM)control strategy for research.Firstly,this thesis reference relevant literature both at home and abroad,reviews the background and development of multilevel inverters and the key problems of three-level inverters,and introduces the topology of diode-clamped three-level inverters the working principle of the main circuit is analyzed in detail,and the corresponding mathematical model is established.Secondly,aiming at the problem of midpoint potential imbalance in NPC three-level inverter,an accurate compensation control strategy based on virtual space vector is proposed in the traditional space vector modulation strategy.By detecting the DC side point current direction,introducing the voltage adjustment coefficient,the different small vector to set a different adjustment factor,increasing the small vector control of the midpoint of the current capacity,when the sum of the output current is zero,to achieve full control of the midpoint voltage.Using Matlab/Simulink to simulate,the simulation results demonstrate that the deviation range of the midpoint potential is smaller,the output line voltage and phase voltage waveform of the inverter are better,and the harmonic distortion rate is lower.Finally,the dead-zone compensation problem of three-level is discussed.In order to reduce the output voltage distortion caused by dead time,the dead-zone compensation strategy of AC inverter is analyzed and optimized.A half-cycle dead-zone compensation strategy isproposed to combine the current acquisition time.This strategy first obtains the corresponding compensation time by calculating the dead-band pressure difference in the control period(half PWM period)and using the volt-second equivalent principle.By increasing the output pulse width offset,effectively increase the modulation duty cycle,so that the compensation strategy is simplified,the current sampling time of each control cycle is constant and the effect of current harmonics on the control system is reduced.Using Matlab/Simulink to simulate.The simulation results demonstrate that the proposed method is effective and can improve the current harmonics more effectively than the one-cycle dead-zone compensation algorithm,the output current waveform is better and the harmonic distortion rate of the current is greatly reduced.