Research On Repetitive Control Technique With Pole-assignment For SPWM Inverters

Among the power electronics facilities, the uninterruptible power supply (UPS) systems are widely used whose core is a CVCF SPWM inverter. The technical characters of the output voltage waveform consist of low total harmonic distortion (THD) at steady state and rapid transient response. In this paper a repetitive controller is adapted to improve the steady state performance and a pole-assignment controller with integral control is proposed to improve the dynamics of the PWM inverter. The followings are the main work: Illuminated the working characters of the SPWM half-bridge inverter. Treated the load current as a disturbance input, the continuous and discrete models of single-phase inverter are established. Using coordinate transformation, the model of three-phase PWM inverter is converted to two single-phase inverters. The causes of the output voltage waveform distortion are analyzed,which include the influence of nonlinear loads, dead-time and voltage imbalance of the split capacitor. A precise simulation model taking into account dead-time effect and switching process is established with MATLAB software. Comprehensive theoretic analyze is carried out on repetitive control system. System stability, error convergence and steady-state error are discussed. A repetitive controller is designed to control a special inverter, which used a notch filter to eliminate the high resonant peak of the SPWM inverter and a time advance function to achieve phase cancellation. Simulation results show that the repetitive controller is able to nearly cancel the output voltage waveform distortion. To improve the dynamics of the inverter, the state feedback pole assignment control is used . The integral control is added in the pole-assignment to eliminate the error caused by the sudden change of the load. This modified State feedback control used as an inner loop, the repetitive controller used as an out loop. Simulation results show that the combination of these two strategies gives excellent dynamic response as well as high quality voltage waveform. Experiments were finished with a 50Hz single-phase half-bridge inverter, the experimental results proved the theoretic analyze and the simulation results: the output voltage waveform THD is low at steady state, the transient response is rapid with no error.