Research On Modulation Strategy For NPC/H-bridge Five-level Inverter

Multilevel inverters has the merits of low harmonic content in output waveforms, low voltage stress of switching device, low swithing loss and so on, so it become a research hotspot of high voltage and high power. This thesis takes NPC/H-bridge five-level inverter as the research object. In order to improve the output performance of the multilevel inverter, it analyzes and researches on modulation strategies.Firstly, this thesis analyzed the working principle of NPC/H-bridge five-level inverter and proved the carrier disposition SPWM strategy which is fitted for this topology. Aming at the swithing loss imbalance in carrier disposition algorithm, this thesis adopted a hybrid SPWM algorithm based on carrier disposition and carrier phase shift and analyzed the output voltage harmonics of this algorithm.Secondly, this thesis adopted traditional space vector pulse width modulation(SVPWM) in NPC/H-bridge five-level inverter that simulated and analyzed it. In view of the problems of large calculation amount, poor real-time and complexity of reference voltage vector positioning, it adopted a novel reference voltage vector decomposition algorithm. This algorithm can quickly locate the reference voltage vector, avoiding level transforms and voltage modification. Meanwhile, basic voltage vector function time is the same as two-level inverter time calculation, so it reduces the amount of calculation and improves the real-time performance. In order to improve the utilization rate of voltage further, the thesis researched on over-modulation strategies based on reference voltage vector decomposition algorithm. It adopted two-mode overmodulation that with no need for solving nonlinear equation and lookup table. The simulation verified that overmodulation strategy can smoothly transition from a linear area to modulation area. The thesis analyzed inherent correlation between seven-segment SVPWM and SPWM, then it researched on a SPWM algorithm which can be equivalent to arbitrary number of segments SVPWM sequence, achieving the the unity of SPWM and arbitrary number of segments SVPWM.Furthermore, the power spectral density of random pulse sequence and a random frequence PWM strategy was studied. The simulation results proved the effectiveness of the modulation strategy. Aiming at unfitness of random frequence PWM strategy in motor for vector control, a fixed switching frequency random SVPWM strategy was proposed which apply to NPC/H-bridge five-level inverter. This strategy can effectively extended harmonic distribution range based on unchanged switching frequency.Finally, the thesis introduced NPC/H-bridge five-level inverter experiment platform based on DSP and FPGA controller. Then the modulation strategy experiment was completed on the platform and the results verified the effectiveness of modulation strategy in this thesis.