Study Of The Control Scheme Of Single-Phase Three-Level PWM Rectifier

As an important part of AC drive system, not only does the three-level PWM rectifier keep the stability of middle DC voltage, but also achieve the power factor correction of the primary side of power transformation. The three-level PWM rectifier work as the inverter operating in regenerating mode, the phase of input current is shifted with π relative to the supply voltage, then achieving the unity power factor of energy feedback. The option of the three-level PWM rectifier control strategy has a significant impact on improving the grid power factor, reducing the harmonic of input current, keeping the DC voltage stable and balancing the neutral point voltage.Working principle of single phase three-level neutral-point-clamped (NPC) PWM rectifier is discussed and a mathematic model of the PWM rectifier on the dq reference frame is set up. On the analysis of several main current control principles, a current decoupled control strategy is presented that decline significantly complexity of the control system.The method adopts the proportion controller which can achive decoupling control for active and reactive current component. The computer simulation results demonstrate that the transient direct current control and forecast current control have good transient and stable performance, and the current decoupled control based on dq reference frame has better steady state performance and the higher power factor.The neutral point voltage affected by the rectifier work mode is analyzed for single phase three-level NPC PWM rectifier. The mathematical relationship between SVPWM and SPWM is established, thereby, balance control method based on SPWM is extended to SVPWM. The computer simulation results demonstrate that the control algorithm achieve neutral point voltage balance by directly fixing the work time of small vector. The control algorithm is easy to realize and characteristics of good stability and anti-jamming ability.With the theoretically analyzed above, the hardware and software of experiment system are designed and put the emphasis on the control circuit of TMS320F2812and its related circuits such as signal adjustment circuit, DC sampling circuit, protection circuit, phase-lock loop circuit, switch drive circuit. About the software, the control flows are designed such as main program and A/D interrupt program and phase-lock loop program especially digital PI regulator.Based on the theory and research, a single phase three level NPC PWM rectifier system is constructed with TMS320F2812. The unity power-factor simulation and feedback experiment are designed. The experimental results demonstrate the accuracy of the designed experimental system, which adopts the transient current control, predictive current control and neutral point voltage control.