On performance advances of flying capacitor multilevel converter topologies

Abstract

The thesis is focus on multilevel topologies based on flying capacitors such as the flying capacitor (FC) multilevel converter and the stacked multicell converter (SMC). Two modulation techniques are investigated, phase-shifted pulse-width modulation (PS-PWM) and phase-disposition pulse-width modulation (PD-PWM), and improved voltage balancing methods have been developed and incorporated to these modulation techniques. The analysis of the converter is conducted in terms of switching frequency reduction, total harmonic distortion (THD) and capacitor voltage ripples. The performance of the converters is validated by simulation and experimental results.

Multilevel FC converters provide natural capacitor voltage balance under PS-PWM. However, natural balancing may not be robust enough to maintain the capacitor voltages at the reference values, especially under certain transient conditions. A closed loop voltage balancing method for the multilevel FC converters using PS-PWM is developed. The proposed method balances the voltages of the FCs by modifying the duty cycle of each switch of the FC converter using a proportional controller. The crossed effect between FC currents and duty cycles is considered and is used for optimal FC voltage balancing. The proposed method is applied to a five-level FC converter and seven-level SMC. Simulation and experimental results verify that the proposed voltage balancing method is very robust to different operating conditions, including unbalanced and non-linear loads.

PD-PWM produces line-to-line voltages that are spectrally better than those achieved by PS-PWM. However, capacitor voltage balance cannot be achieved if PD-PWM is applied straightforward to converters based on FC topologies, unless the carrier waveforms are properly reshaped. A new capacitor voltage balancing method under PD-PWM is developed in this thesis. The method uses the redundant switching states of the converter, which are properly selected by means of the continuous evaluation and minimization of a cost function. The proposed voltage balancing method can achieve excellent results. However, the use of standard triangular carriers in PD-PWM leads to additional switching’s due to transitions within the same voltage level, thus increasing the switching frequency of the power devices. Such drawback can be avoided by using sawtooth carrier waveforms. A second capacitor voltage balancing method based on PD-PWM is also proposed. In this second method, the transitions between consecutive voltage levels are evaluated instead of considering each of the switching states independently. The cost function is redefined to evaluate the switching transitions. Furthermore, a reduction in the switching frequencies of the power devices is achieved if the non-optimal transitions between consecutive states are avoided. A significant characteristic of both voltage balancing methods is that they do not require tuning of parameters in order to achieve optimal performance. These methods are applied to a five-level FC converter and a seven-level SMC. Simulation and experimental results confirm the robustness of the proposed methods operating with different loads and transient conditions.

Finally, the thesis addresses the initial charging of FCs in grid-connected applications. A novel method for pre-charging the capacitors in the FC multilevel converter is presented. The method can charge the capacitors in a controlled way besides producing low voltage and current stress to the power devices and the passive components. The method is simple to implement and can be applied to FC multilevel converters with any number of levels. Experimental results demonstrate the effectiveness of the proposed method.