Study On Control Strategy Of Cascaded Multilevel Inverter

By cascading several low-voltage H-bridge power units, cascaded multilevel inverter achieves the power conversion in high-voltage field, at the same time, reduces the rate of voltage change effectively, improves the electromagnetic environment significantly. So it has been widely concerned in applications such as medium voltage drive and flexible AC transmission system. As an important factor in deciding the effect of inverter control, multilevel pulse-width modulation (PWM) method is received extensive attentions from scholars at home and abroad. The modulation method can be divided into triangular-carrier modulation and space vector modulation types. Cascaded Multilevel Inverter usually use a triangular-carrier modulation method currently. It is easy to be computed and implemented. However, because it is based on the average theory, and has a low DC voltage utilization rate. So it causes poor performance of inverter output, and difficultly mixes with high-performance motor control strategy.?Space vector modulation has a high DC voltage utilization and a low Current ripple. And its output voltage form is diversiform. However, its calculation is very complicated.?And when levels are more, it is difficult to achieve real-time control. So that its applicability is limited. In this respect, doing optimizing improvement for PWM control techniques of multlevel cascaded inverter will be able to effectively improve its output performance.Firstly, the current control mode of cascaded multilevel inverter is discussed, mainly including triangular-carrier modulation and space vector modulation.?Moreover, their respective characteristics and basic principles are analyzed. And to three-level inverter as an example, the nature contact of the triangular-carrier modulation method and space vector method is derived.For this issue that the traditional triangular-carrier PWM techniques has a low-voltage modulation depth and small fundamental amplitude. The respective advantages of single-carrier modulation method and trapezoidal wave modulation is combined to propose a new type PWM control method, which is Trapezoidal wave PWM based on single-carrier. And its modulation principle is analyzed.?On the basis of double Fourier analyzing three-phase three-level cascaded inverter output line voltage, the best values of trapezoidal modulation wave is obtained. Analyzing the spectral characteristics of three-phase 13-level cascaded inverter output voltage waveform under different modulation method,?superiority of trapezoidal wave PWM control method is verified.General multilevel SVPWM based on two-level SVPWM is proposed in this paper to solve the problems that traditional algorithms become complex, and difficult to achieve. The reference vector corresponding to basic vector and its role time under linear modulation mode is decided. As well as redundant switches are optimization selected by extreme value method to reduce the switching frequency. To further enhance the utilization of inverter DC side voltage,? this algorithm will be extended to overmodulation mode and discussed in two ranges according to the classical theory dual-mode overmodulation of Joachim Holtz. From the algorithm flow chart we can see that the algorithm is very simple. And its complexity is not increasing following the addition of level.?Simulation confirms the correctness of the algorithm.Finally, after systematic analyzing the basic principle of induction motor vector control and deriving the mathematical model of asynchronous motor under the stationary coordinate system and synchronously rotating coordinate system, the basic equation of vector control is summarized, at the same time, voltage decoupling control equations are derived. So the decoupling control of motor stator current excitation and torque is achieved. In accordance with engineering design methods, the parameters of PI controller in the control system is designed Then the general SVPWM algorithm proposed in this paper is used in the rotor-flux-oriented control of a voltage-source inverter-fed induction motor drive. Simulation shows the good performance of the three-level inverter fed drive with vector control under this algorithm.