The main topic of this dissertation is harmonics control in power electronics systems, represented by switching converters in a wide range of power. The switching frequency of the converters is larger than fundamental harmonics frequency of signal being controlled and can be suppressed with passive analog filters only. To generate or to suppress the fundamental harmonic and harmonics theoretically up to the Nyquist frequency, the dedicated controllers and appropriate control algorithms must be implemented within the microcontroller.
In single-phase and three-phase systems the resonant, repetitive, DCT and dual DCT controllers are used. Additionally, rotating frame I controller is used in three-phase systems only. Based on the literature review, PI, predictive, RST (Reference Signal Tracking), hysteresis, time-discrete controller are not appropriate for harmonics control. These controllers cannot adequately follow the steady-state reference. The same applies for fuzzy logic, neural networks and genetic algorithms, therefore those algorithms have not yet been used for the harmonic control. The rotating frame I controller and the resonant controller controls only individual harmonic. To control more harmonics, multiple controllers, working in parallel, need to be used. Repetitive, DCT and dual DCT are capable of controlling several harmonics simultaneously.
Dual DCT controller is designed by myself and is a novelty that has not yet been mentioned in the relevant literature. Dual DCT controller is an upgrade of the DCT controller and it poses an alternative to the multiple resonant controllers. Unlike the repetitive and the DCT controller, it is possible to adjust the amplitude and phase lag compensation for each harmonic separately. The harmonics can be arbitrarily selected regardless of the numbers of harmonics selected.
To compare resonant, repetitive, DCT and the dual DCT controller, those algorithms are implemented into several simulations and experiments. In a bidirectional active rectifier the harmonics are mitigated, which ensures improvement of the power factor. They all attenuate harmonics well enough, however each of them has unique properties, by which the advantages and disadvantages of each controller are evaluated. The most appropriate one is selected according to the following requirements: the transient response time (dynamics), real-time computing power complexity (number of mathematical calculations within a sampling interval), memory usage requirement, complexity of implementing the controller algorithm in microcontroller, sensitivity to frequency change and limitations of the controller (i.e. exact phase lag compensation ability for each harmonic, possibility of controller parameters change during operation).
A very similar harmonics control algorithm was also tested in the output voltage control of an RLC circuit, where the frequency dependence of the phase lag is distinctly nonlinear. This system exposes the main disadvantages of the repetitive and the DCT controller. Only linear phase lag compensation is possible, which causes stability issues. The compromise between dynamics and bandwidth of those controllers must be made to ensure stable operation. Either bandwidth is satisfactory and the dynamics is worsened or the dynamics is good and bandwidth is narrowed.
When operating a three-phase permanent magnet synchronous machine, several selected current higher harmonics in the dq coordinate system are generated to eliminate torque and speed fluctuations. Electrical frequency is changing all the time according to mechanical frequency change, therefore the current phase lag is affected and stability of control loop is threatened. To solve this issue, adaptive change of controller’s parameters is implemented in control algorithm. The results prove successful and stable operation in range from practically 0 to 1200 rpm.
The physical limitations of harmonics control are also examined. The influence of converter’s parameters and RL load is analyzed. To determine whether the converter is capable of controlling the selected harmonics of the current through RL load or not, a general analytic expression has been derived, which offers fast and reliable estimation. The most important parameters limiting the operation of the inverter are: DC link voltage, load resistance, switching converter’s dead time and frequency of controlled harmonics.
In conclusion, the influence of dead time is emphasized, a concise comparison of the resonant, repetitive, DCT and self-designed dual DCT controller is made, applications in which the control of harmonics is most commonly used are given and the direction of further work is also indicated.
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