Control and modulation of modular multilevel converters
The integration of renewable energy sources in the electrical grid is reducing our dependence on fossil fuels. However, to ensure feasibility and reliability of distributed energy generation, more efficient and higher power converters are required. The modular multilevel converter (MMC) is a modern...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2016 |
| País: | España |
| Institución: | CBUC, CESCA |
| Repositorio: | TDR. Tesis Doctorales en Red |
| OAI Identifier: | oai:www.tdx.cat:10803/404612 |
| Acceso en línea: | http://hdl.handle.net/10803/404612 https://dx.doi.org/10.5821/dissertation-2117-106490 |
| Access Level: | acceso abierto |
| Palabra clave: | Àrees temàtiques de la UPC::Enginyeria electrònica 621.3 |
| Sumario: | The integration of renewable energy sources in the electrical grid is reducing our dependence on fossil fuels. However, to ensure feasibility and reliability of distributed energy generation, more efficient and higher power converters are required. The modular multilevel converter (MMC) is a modern topology of multilevel converter that is very attractive for medium- and high-voltage/power applications, including high-voltage direct current transmission systems and high-power motor drives. The main features of the MMC are modularity, scalability to different power and voltage levels, redundancy and high quality output voltages and currents. However, the operation of the MMC is complex, and there are some issues that still have to be further investigated. One of these issues is the voltage ripples of the submodule (SM) capacitors. The voltage ripples define the minimum value of the capacitances needed for the converter, and therefore its overall size and cost. The use of a proper circulating current controller can reduce the voltage ripples. In this thesis, three techniques for calculating the circulating current reference are presented: two techniques based on optimization functions for minimizing the capacitor voltage ripples; and a fast-processing technique that provides results close to optimal. The capacitor voltage ripples can also be reduced by adding a zero-sequence signal to the modulation signals. In this thesis, the application of discontinuous modulation to the MMC is proposed for the first time. This technique is based on the injection of a discontinuous zero-sequence signal and highly reduces the switching power losses and capacitor voltage ripples. Real applications of the MMC are composed of a high number of SMs. This implies a challenge in the control system, including the data acquisition system. A new technique for measuring the capacitor voltages with only a few sensors has been presented in this thesis. From the output voltage provided by a group of SMs, the individual voltage of each one of them can be acquired. Since acquisition cannot be performed at each sampling time, the capacitor voltages are calculated between samples using an estimation algorithm. Reliability is a feature required in industrial applications. The structure of the MMC facilitates the existence of redundant SMs, but faults need to be detected and localized for deactivating the faulty component. This thesis presents a robust and fast strategy for detecting, localizing and correcting faults in SMs and voltage sensors. The technique is based on three additional sensors per arm, which measure the output voltage of a group of SMs and compare it with the expected voltage. Capacitance differences between the SMs can appear due to component tolerance or ageing of the capacitors. Capacitance mismatches cause uneven distribution of the power losses, thus increasing the thermal stress of some semiconductors, and therefore, their probability of failure. A power loss balancing technique has been proposed, equalising the losses in all the SMs and therefore avoiding the concentration of power losses in some SMs. Application of the MMC to motor drive applications has also been studied in this thesis. The operation of the MMC at low motor speeds/frequencies is still a challenge, since the capacitor voltage ripples are inversely proportional to the current frequency. In this thesis, it has been demonstrated that discontinuous modulation can help to reduce capacitor voltage ripples in motor drive applications, achieving very low speed operation. The technique is compared with other state-of-the-art methods, and it achieves similar capacitor voltage ripples and a significant reduction in power losses. All the control and modulation techniques proposed in this thesis have been studied by simulation in the MATLAB/Simulink environment and corroborated experimentally on low-power laboratory prototypes. |
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