| The Modular Multilevel Converter (MMC) is a voltage source inverter topology currently undergoing increased attention in the industry and academia as one of the preferred choices of power conversion for medium to high power applications. This is due to its promising features over the other multilevel converter topologies such as modularity, voltage scalability, multilevel waveform capability, fault tolerant capacity, redundancy, dispensability of DC-link capacitor and transform-less nature. The converter concept has already been introduced in the market for the High Voltage Direct Current (HVDC) transmission applications. In this work, the principle of operation, dynamics and structure of the MMC are studied to deduce its modeling equations. The different multicarrier sinusoidal PWM schemes are also evaluated based on the intrinsic features of the MMC configuration.;The basic building block of the MMC topology is based on the submodule, which mainly consists of two complementary switches and DC storage capacitor. In this topology, the energy stored in the converter is distributed among the capacitor of submodules, instead of a single DC-link capacitor as in the conventional two-level converter topologies. However, an important aspect of a stable and safe operation of the MMC is to maintain the total sum of the capacitor voltages equal to the DC-link voltage and ensure an equal voltage distribution among the individual submodules. Thus, the voltage balancing control is required to avoid the flow of large circulating current in the phase leg of the converter and distort output voltage waveform. The capacitor voltage balancing control scheme is one of the control algorithms that proposed for the MMC. An important prerequisite of the MMC control schemes is the ability to provide the submodular capacitor voltages. The majority of voltage balance control algorithms assume the knowledge of the submodular capacitor voltages, while in practice these measurements are obtained using voltage transducers or sensor combinations. However, for a high number of submodules a large number of voltage transducers are required, which increases the complexity of the system adds cost to the design and reduces the reliability. The main contribution of this thesis is proposing a new approach that provides a soft-sensing technique to obtain an accurate measurement of the capacitor voltages of the MMC, which are not directly measured. The proposed methodology provides a soft sensing of the submodular capacitor voltages by utilizing a sliding mode observer.;The observer is an artificial dynamical system that derived with a structure similar as the MMC system with an extra correction term to reduce the discrepancy between real and estimated current. Moreover, the observer can replace the voltage transducers to obtain the measurement of the capacitor voltages based on the knowledge of the arm currents and DC-link voltage. The design aspects of the traditional sliding mode observer and sliding mode observer using an equivalent control method have been studied to develop an observer model for the MMC. The stability and convergence of the observer estimates to the intersection of discontinuity surfaces is examined based on the Lyapunov analysis. The sliding mode observation based on the equivalent control method has been chosen to complement the operation of the capacitor voltage balancing control scheme. This is due to its robustness against the variation of the capacitor voltages. Finally, simulations have been performed using MATLAB software to validate the sliding mode observer estimation. |
