Direct Power Control For Three-level Voltage Source PWM Rectifier

Multilevel converters, widely used in the field of frequency converters, not also have the advantages of bidirectional energy flow, controllable power factor and low current harmonics, but also achieve low voltage drop of devices and low switching frequency. Therefore, multilevel converters are promising in high-power industrical areas, such as reactive power compensation of power systems, high voltage transmission and distribution, frequency control of speed for high voltage and large motor. Direct power control (DTC), which controls the converters'instantaneous power in a closed-loop directly, has the advantage of simple algorithm, fast dynamic response and high efficiency. In this paper the DTC technology for diode-clamped three-level voltage source PWM rectifier is focused on.The paper establishes the high-frequency mathematical model of three-level voltage source PWM rectifier based on Kirchhoff's law, conducts the transformations of the model from static abc coordinates to staticαβcoordinates and rotating dq coordinates, respectively, and analyzes the characteristics of the mathematical model in different coordinates. Moreover, the rectifier's power control model according to the instantaneous power theory of three-phase circuit is established, and the power control model is analyzed using the traditional method and a new method proposed in this paper, which provides a theoretical basis of DTC for the three-level PWM rectifier, respectively.Based on the space voltage vector model of the three-level voltage source PWM rectifier, the effects of basic vectors on neutral-point potential and the reason of neutral-point potential imbalance are analyzed in detail, which is to provide the possibility of software implementation to balance the neutral-point potential. And based on the rectifier's power control model,the paper analyzes the structure of the power grid voltage oriented switching table DTC system of the rectifier, briefly introduces the tuning method of PI parameters for the voltage-loop and improves the switching table using the novel method. However, the proposed method leads to irregular rectifier switching frequencies caused by hysteresis power comparator in the inner loop, since which the design of grid filter is limited. In order to solve this problem, the traditional power bang-bang control in the inner loop is taken the place by the power feedforward control strategy of PI regulator for decoupling, Also, the space vector pulse width modulation (SVPWM) technology is combined to fix the switching frequency of the rectifier and effectively control the balance of neutral-point potential. In addition, the stator flux concept is introduced from AC motor to three-phase PWM rectifier, through which vector orientation and the computation of instantaneous power based on virtual flux linkage can be achieved. Meanwhile, the virtual flux linkage observer is improved and the initial value of integration is resolved. Therefore, the grid voltage sensors are saved and the system stability is improved.The DTC strategies of three-level voltage source PWM rectifier are simulated and analyzed respectively based on grid voltage orientation and virtual grid flux linkage orientation in Matlab/Simulink. The design of the main circuit and its control system of the three-level voltage source PWM rectifier is conducted, the experimental platform is established and the experiments of switching table DTC based on grid voltage orientation. The experimental results indicate that the defects of the traditional method in reactive power control can be overcome by the proposed method, and better steady-state and dynamic performances can be achieved.