Research On Control Technology Of Three-level NPC Inverter Based On DSP

With the development of power electronics technology,main circuits and driver circuits of three-level inverter in continuous improvement; all kinds of control fusion of confucianism in control area, coupled with more advanced control chips constantly upgrading,make the three-level inverter control technology constantly optimized, higher control accuracy and reliability stronger, playing a huge advantage in the low-voltage control high-pressure field. In this paper, the main control technology of three-level NPC inverter is studied refer to the development status and experimental platform is designed by TMS320F2812primarily controller.The main contents as follows:In this paper main topology and basic working principle of three-level NPC inverter is studied on the basis of summing up development situationat home and abroad, two basic control methods (SPWM and SVPWM) are analyzed and simulation experiments are designed. Both SPWM and SVPWM are determined as control scheme.The mechanism of neutral-point fluctuation for three-level NPC inverter is studied. A mathematical model of neutral-point current is established on the basis of analysis of neutral-point fluctuation factors and the balance control of the neutral-point voltage is achieved by a simple closed-loop control. The dead-time effect by joined the dead-time between PWMs is studied. The software and hardware dead-time compensation strategies are proposed on the basis of analysis of work process of three-level inverter and dead-time effect. Software dead-time compensation is achieved by a reasonable adjustment of reactive power switches turn-on and turn-off time, while the hardware dead-time compensation by appropriate correction cutting-edge or trailing edge of pulses. VVVF speed control of induction motors is also studied. Smooth speed control is realized by software control program.The balance control model of neutral-point voltage is established. Simulation results show the validity of the method used. The control model of software and hardware dead-time compensation strategies is established. Simulation results show the correctness and feasibility of the two kinds of compensation strategies.Hardware circuits of system control are designed including the main circuit and control circuit:the main circuit including three-level NPC inverter bridge of power MOSFET and the associated driver circuits,achieve the conversion of electrical energy; the control circuit including TMS320F2812control chip as the core of the control panel, the hardware dead-time compensation circuit, the corresponding sampling circuit,protection circuit, the current polarity detection circuit, keyboard and display circuit and current limit start-up circuit, etc, achieve a smooth and reliable operation of the system.Various parts of the software are designed based on C, including the initialization process, the main program and interrupt subroutine. The main program designs keyboard and display routines and VVVF speed control subroutine; interrupt subroutine mainly designs SPWM generated subroutine and the dead-time compensation subroutine. Keyboard and display program has ample room for expansion and practicality for the system parameter setting and display; Algorithms of VVVF speed control program are reasonable to achieve the purpose of smooth speed control; SPWM generator program is simple,easy to implement; dead-time compensation program in the DSP is implemented simplly, to achieve a good compensation effect.Finally, build experimental platform of three-level NPC inverter based on DSP.Dead-time compensation and VVVF speed control are experimented on that.The experimental results demonstrate that two dead-time compensation strategies decrease5,7times harmonic of the output current, the total harmonic distortion and increase effective value of the output current.U/F control system is simple to implemented and stable highly, the motor has speed adjust smoothly and less torque ripples.