Repetitive current control of two-level and interleaved three-phase PWM utility connected converters

This thesis is mainly concerned with investigations into digital repetitive current control of two-level and interleaved utility connected PWM converters. The research is motivated by the relatively poor performance of classical (PI) controllers when the utility voltage harmonic distortion is high. This is due to the low gain, and poor disturbance rejection of the PI controller at the utility harmonic frequencies. Repetitive feedback controllers have the ability to track or reject periodic disturbances, such as utility harmonics, as they naturally have high gains at the utility voltage harmonic frequencies, assuming that these frequencies do not change. Repetitive controllers (RC) are known for being sensitive to variations in system parameters and disturbance frequency, which in practice renders them either ineffective or unstable. Another challenge arises from the memory requirements of RC in case of the absence of even harmonics, which can make its practical implementation difficult and expensive. In addition, another problem that has not been investigated extensively in the literature is that the effectiveness of RC is severely limited by the limited bandwidth of the plant (the utility connected converter and its filter). Theoretical analysis and simulation results presented in this thesis show that RC could not effectively reject disturbances at frequencies above the closed loop system bandwidth. The design of the converter's output filter bandwidth and the values of its components need to be selected carefully, to enable RC to be used effectively. The research in this thesis focuses on investigating the practical implementation and performance limits of two types of repetitive controllers (conventional and oddharmonics), used for current control of two-level utility connected converter with LCL output filter. The odd-harmonic repetitive controller halves the memory requirement and offers higher gains only at odd harmonic frequencies of interest. The overall control scheme consists of the traditional classical tracking controller with a dual loop feedback system and RC. The results indicate that the repetitive controller improves the steady state error and the total harmonic distortion of the output current, provided that the plant's bandwidth is sufficiently large. Finally, a repetitive controller for an interleaved utility connected converter has been designed and investigated in this study. The interleaved converter system has higher bandwidth than the two-level converter, which improves the effectiveness of RC. It provides good disturbance rejection compared to classical controllers which results in low output current THD. The RC was demonstrated to be robust despite uncertainty in utility impedance, while achieving a fast almost zero error convergence. The proposed RC has been experimentally implemented using a DSP and the results indicate that the quality of output current complies with international standards on harmonic limits and matches simulation results obtained from the Matlab/Simulink model of the system.EThOS - Electronic Theses Online ServiceGBUnited Kingdo