Research On Waveform Control Technique For SPWM Inverter Based On IMC－PID

Uninterruptible Power Supply (UPS) system is widely used for supplying critical loads which can not afford utility failure. The CVCF-SPWM inverter is the core of a UPS system which required high quality output voltage waveform. Many waveform correction techniques have been proposed to achieve nearly sinusoidal output voltage even with nonlinear loads. This paper discusses waveform correction technique of the CVCF-SPWM inverter based on internal model control theory. The followings are the main work. With the load current treated as a disturbance, the paper establishes a linear mathematical model expressed in the forms of transfer function and state equation for a single-phase inverter. The influence of dead-time and nonlinear loads on the output voltage in a SPWM inverter is analyzed in detail. Theoretic analysis is carried out on the single-input-single-output linear internal model control system. Theory and qualities of internal model control are discussed. The internal model and IMC-PID controller design methods are respectively proposed. Adopting direct internal model control method in the inverter, the paper shows little output voltage waveform aberrance and nice static characteristics, but bad dynamic performance with linear loads, while high output voltage waveform aberrance with nonlinear loads. It also shows that the inverter has bad dynamic performance when added and cut down the load current abruptly, for the internal model controller is unable to improve dynamic performance of the response to a step signal when the load current is taken as a step disturbance. To overcome the shortcoming of internal model controller, for the first time, the paper puts forward IMC-PID control scheme with the PD instantaneous-value feedback inner loop which is designed to improve dynamic performance while the IMC-PID external loop to ensure stable performance. The paper thoroughly analyzes improvement of the dynamic performance of tracking instruction and restraining disturbance rapidly and exactly at the same time by using the instantaneous-voltage-value feed back. The inner loop with a PD controller employing pole assignment technique not only improves the ability of the dynamic tracking and disturbance restraining, but also improves the robustness stability and static characteristics. The two control methods are adopted to get the fast dynamic response and nice output voltage waveform at the same time. At the end of the paper, the result of simulation and experiments show that the control system designed has strong robustness and nice dynamic and static characteristics.