Digital Resonant Current Controllers for Voltage Source Converters
Supervisor: Jesús Doval-Gandoy
Abstract
Sinusoidal current regulation of voltage source converters is an aspect of paramount importance to achieve a high level of performance in a lot of different applications, such as ac motor drives, active power filters, wind turbines, static synchronous compensators, photovoltaic inverters or active rectifiers.
One of the most extended types of current regulators are resonant controllers, which achieve zero steady-state error at selected frequencies, while providing a good combination of simplicity and high performance. Nevertheless, there are certain aspects with regard to these controllers that have not been approached in the technical literature on the matter, and that should be investigated in order to take advantage of their actual potential.
Most studies devoted to resonant controllers have been carried out in the continuous domain; however, their observations and conclusions cannot be directly applied to digital devices, which work in the discrete-time domain. In nowadays scenarios, most current controllers are implemented in digital platforms, so the influence of the discretization process should not be ignored. Several discrete-time implementations of resonant controllers have been proposed, but a comparison among the performance obtained by a wide variety of discretization techniques applied to resonant controllers has not been presented at this point. One of the contributions of this thesis consists in an in-depth comparison among the effects of discretization strategies when applied to resonant controllers. The discretization process is proved to be of great importance in these regulators, mainly because of their resonant characteristics. The optimum discrete-time implementation alternatives are assessed, in terms of their influence on the resonant peak location and the phase versus frequency response.
The implementations of resonant controllers based on two interconnected integrators are widely employed due to their simplicity regarding frequency adaptation. However, it is proved in this thesis that these schemes require lower resource consumption, but at the expense of important inaccuracies that significantly worsen the performance, except for very low resonant frequencies and sampling periods. Alternative implementations based on two integrators are proposed in this dissertation, which achieve higher performance by means of more accurate resonant peak locations and delay compensation, while maintaining the advantage on low computational burden and good frequency adaptation of the original schemes.
Finally, the analysis and design of resonant controllers is approached. The existing methods, which are mainly based on the phase margin criterion, present some limitations, specially when there are multiple 0 dB crossings in the gain versus frequency response. This situation arises in cases such as selective control and when relatively high resonant frequencies with respect to the switching frequency are required (e.g., in high power converters, where the switching frequency should be low in order to reduce the commutation losses). In this thesis, resonant controllers are analyzed by means of Nyquist diagrams. It is proved that the minimization of the sensitivity peak permits to achieve a greater performance and stability rather than by maximizing the gain or phase margins. A systematic method, supported by closed-form analytical expressions, is proposed to obtain the highest stability and performance, even when there are multiple 0 dB crossings.
Contributions of this dissertation have been published in three JCR-indexed journal papers and presented at two international conferences.