Pwm And Nonlinear Control Method Of Multilevel Converters

With the extensive application of power electronic devices, a large number of harmonics current and reactive power is injected into power grid, resulting in a serious power "pollution", thus eliminating the harmonic pollution and improving the power factor has become a hot research direction in the field of power electronics. Voltage source rectifier (VSR) is used increasingly in a wide diversity of applications in recent years, as it can provide constant DC bus voltage, low harmonic distortion of the utility currents, bidirectional power flow, and controllable power factor. And multi-level technology has made rapid development, as it can reduce the switch device voltage stress, the output voltage would be improved, low switching frequency, the rate of power will be increased and so on. In this paper, the major objective of the research is to investigate the pulse-width-modulation (PWM) method and its nonlinear control method of single-phase and three-phase multi-level converter topology, and provide the research basis for its application in high-voltage, high-power occasions.View of the single-phase multilevel inverter almost used carrier modulation method, it is difficult to control the mid-point potential precisely, and hard to implement digitally. In this paper, a space vector PWM (SVPWM) algorithm is proposed for the single-phase three-level neutral-point-clamped (NPC) inverter. In order to reduce the power device switching frequency and balancing the capacitor voltage, a optimal PWM method which positive or negative small vectors are adopted as first active vector is proposed. The cause of the imbalance of the capacitor voltage in DC-side is analyzed and the strategy of changing the voltage adjusting coefficient to adjust the effective time of the positive and the negative small vectors to control the neutral point potential balance is proposed, which is based on the detection of the direction of the load current and the capacitor voltage, and this modulation method is also applied to the cascaded H-bridge, simulation and experimental results are presented to validate the proposed modulation method.In this paper, a simplified SVPWM based on the virtual voltage vector is proposed for the three-phase three-level NPC inverter. It makes the calculation to be simplified and solve the problem of capacitor voltage low frequency oscillation effectively, and simulation and experimental results are carried out to verify the theoretical analyses.The SVPWM which is proposed for single-phase diode clamped three-level inverter is applied to several hybrid multilevel topologies. The single-phase asymmetry diode clamped three-level topology, single-phase three units cascaded H-bridge topology, single-DC two units H-bridge topology and three-phase hybrid cascaded H-bridge topology are analyzed in detail, and some simulation experiments which are based on the proposed modulation method are carried out. Based on the modulation method, the single-phase diode clamped asymmetric three-level topology can combine the advantage of different switching devices, it can permits faster devices and high voltage devices operation in synergism. The single-DC two units H-bridge topology only needs a single DC power supply, and it can reduce the size of the device. The output voltage of single-phase three units H-bridge and three-phase hybrid cascaded H-bridge topology also achieves good results under the proposed PWM method proposed in this paper, and they have good application value.Based on the Euler-Lagrange (EL) mathematical model of the single-phase three-level voltage source inverter, energy function of the system is selected as the Lyapunov function. The passivity-based (PB) method is derived and the stability of the system is proved. This method has clear physical meaning, can maintain a wide range of stability, and the whole system has good static and dynamic performance. A single-phase diode clamped three-level experimental prototype is built, and the experimental results verify the validity of theoretical analyses.In this paper, the topology of sing-phase diode clamped three-level, asymmetric diode clamped three-level and cascade H-bridge three-level are analyzed in detail, including its working principle and mathematical model. The reasons of capacitor voltage imbalance in DC side are also analyzed, and the methods are given for the midpoint voltage balance respectively. The energy function of the system is selected as a Lyapunov function, and a nonlinear control strategy based on the Lyapunov's direct method is deduced for single-phase diode clamped three-level rectifier topology. The converter can be stabilized globally to handle large signal disturbances, and exhibits high-quality output voltage and unity power factor.A new type of passive power control strategy is derived for three-phase three-level NPC rectifier, which combined with the instantaneous power theory and the passivity-based control methods. The proposed strategy which is based on the Lyapunov's direct method can achieve decoupling control of active and reactive power. The whole system has a fixed switching frequency, and has strong robustness about parameter variations, external perturbation, as well as no modeling error. The method has clear physical meaning, can maintain a wide range of system stable and has good static and dynamic performance.Finally, low power diode clamped NPC three-level converter experimental prototype is made based on the microprocessor TMS320F2812, extensive simulation and experiment are carried out in order to validate above theories.