Research On The Instantaneous Feedback Control Technology Of PWM Inverters

Uninterruptible Power Supply(UPS)systems are widely used for supplying critical loads which can not afford utility power failure. A inverter is the core of a UPS system. High quality output voltage waveform is required for these inverters. To achieve nearly sinusoidal output voltage even with nonlinear loads, many waveform correction techniques have been proposed. This dissertation focuses on the research of the instantaneous feedback technology of PWM inverters. Since the instantaneous feedback technology is a real-time control according to the current error of the output waveform .Once the controller is designed properly, it can improve system dynamic response with nice static characteristics. Analysis and simulations are centered on two control approaches which are single loop controller with instantaneous voltage feedback and dual-loop control with voltage and current feedback to discuss how to improve both dynamic and static characteristics, thereby to reform the output waveform of PWM inverter.Based on the state-space averaging and linearization technique, the mathematical model is given in form of transfer function and states equations. The influence of dead-time, over- modulation and nonlinear loads on output voltage in single-phase full-bridge inverters is analyzed in detail. The method which brings output voltage feedback in the control loop to eliminate the disturbance of nonlinear load is reasonable. Of all sorts of strategies, there are repetitive control, deadbeat control, sliding-mode control and PID control. The PID control method is in favor of balancing the dynamic and static characteristics of the control system, easy to be calculated and realized. Above all it is reliably, and thus it is becoming the most universal control method. The design method of PID controller based on pole-assignment is proposed in this paper. The simulations indicate that this PID controlled inverter provides nice characteristics. A method of PI controller combined with instantaneous differential voltage is also proposed, which is equivalent to PID control in essence and can be seen as a simplified dual-loop form. Accordingly it is a simple but effective adjustment.The voltage and current dual-loop control system is divided into inductor-current feedback and capacitor-current feedback. The comparison of both is given in the paper. The characteristics of the inverter with the dual-loop control using capacitor-current feedback and voltage are obtained and analyzed. A mass of simulations are made to design both the inner and outer controller. The simulations indicate that this voltage and current dual-loop control system is superior to single loop controller with instantaneous voltage feedback in characteristics.Finally, a single-phase inverter applying output voltage and capacitor-current feedback control is researched in the paper and the experimental results accord with theoretic analyze. And it can get nice static characteristic and well dynamic response.