The Control Strategies Of Cascaded Multilevel Inverters

With the development of power electronics technology, Multi-level power conversion circuit has been concerned widespread, because of its good performance in the medium voltage (1KV-10KV) high-power (200KW-20MW) applications. the main advantages of Multi-level converter which is relatively traditional two-level converter: a single device under voltage stress is easier to achieve large-capacity high-pressure; the same switching frequency of the output voltage is closer to sinusoidal current waveform, harmonic content is low; EMI electromagnetic interference problems are greatly reduced, to use the appropriate algorithm can guarantee a more secure operating system.In this paper, focusing on several multi-level inverter topology typical of the theory, and analysis of its strengths and weaknesses, focusing on analysis of multi-level cascade inverter structure of principle, because of its smaller harmonic pollutionl, high input power factor, and good output waveform, which is used in this article and the study of the structure and up to three units connected in series for cascade multi-level topology of the experimental platform.In the control strategy, we introduced the carrier PWM and SVPWM, detailed account of their classification, control theory. In the study for the control methods, we found that two of the internal relations of control, so got the carrier PWM technique based on optimized PWM method - third harmonic injection method. In this paper, a detailed analysis of the modulation wave and third harmonic transform into multi-level inverter principle, as well as after injection of third harmonic modulation inverter input DC voltage signal and the relationship. In addition, analyses of multi-level in the Cascade open-loop PWM inverter control the general and the third harmonic injection SPWM different. Third harmonic injection method proposed can not be affected in the case of load current and voltage to achieve optimal control of carrier modulation to improve the utilization voltage. In Matlab/Simulink simulation environment are given in three modules in series of multi-level cascade inverter structure to carrier-based phase-shifting into the third harmonic of the control strategy of the simulation waveforms, confirming the theoretical correctness.Through theoretical analysis, we identified a three-unit series of cascading multi-level inverter main circuit and the third harmonic injection method for the control strategy, set up the experimental platform. And the control circuit is a digital signal processor DSP and CPLD. In front of theory analysis and simulation, we introduced this experimental platform of the system hardware and software related to the relevant functions and design principles, and a carrier phase-shift and third harmonic injection modulation methods of the two experiments.