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

Abstract

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.

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Identifiers

ORCID for S.M. Sharkh: orcid.org/0000-0001-7335-8503

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