Research On Space Vector Modulation Strategy And Capacitor Voltage Balancing For Multi-level Inverter

The multi-level inverter with its outstanding advantages in terms of performance and capacity at home and abroad has become a hotspot of research and application one. In the many control strategies in a variety of multi-level inverter, the space vector pulse width modulation (SVPWM) algorithm has a big modulation ratio, to optimize the output voltage waveform and other advantages, through improved control of this algorithm can improve control performance. Three-level NPC inverter with its simple structure, low cost, etc. have been widely studied and applied, but the capacitor voltage balance is the key issue for safe operation and output performance impact study of the issue has important theory and engineering value.The thesis have studied the multi-level SVPWM modulation method under different coordinate systems by three-level NPC inverter.In order to solve the problem of computational complexity, according to the law of the basic vector’s coordinates in a’p’ two-phase coordinate plane are integers, and studied the general solution of the system of multi-level SVPWM in α’β’ coordinates, and achieved to unify and simplify the multi-level SVPWM algorithm. Take advantage of the general solution of the system of multi-level SVPWM,presents a modulation scheme of the switching times of the switching device will be fewest in a circuit, i.e., only one of each phase output voltage is changed, and only a change the switching of one level units.In order to find the right switching path to reduce the switching frequency, on α’β’ plane,advances the concept of network characteristics and features of the quadrangle, use the each vertex of sector sequence S to approximate the reference voltage vector ur. The paper found that it can be divided into two kinds of characteristics networks with the radius range of reference vector’s locus.In these two kinds of characteristics of the network,analysis of the switching route with fewest switching times based on the characteristic quadrilateral,and propose SVPWM algorithm to optimize theswitching frequency. Simulation results in Matlab shows that, the algorithm always switch in a reference voltage cycle times compared with the same phase stacking sinusoidal modulation algorithm in the same parameters is reduced to25%.Propose a self-regulating small vector line method for the capacitor voltage fluctuation problem of NPC inverter,this method can achieve the effective control of NPC capacitor voltage at all operating points. Papers from the impact angle vector and the midpoint of the current analysis of the midpoint potential balance control law in-depth, to achieve the control of the capacitor voltage through the design of balance coefficient and dynamic regulation of small vectors at different times. The simulation verifies the correctness of the analysis of the resulting conclusions and control methods. Finally, design the three-level NPC inverter experimental program, give the design of hardware and software in the perimental platform,and simulate to study.