Simulation, control and validation of a low voltage Modular Multilevel Converter

The project deals with the design of a Modular Multilevel Converter (MMC) with one cell per arm. The one cell per arm configuration maintains the MMC structure with the minimum num- ber of semiconductors per arm, which is the simplest possible system, allowing several studies without requiring a more complex system. The prototype presented behave as an actual high- voltage MMC, but it cannot implement the Nearest Level Modulation (NLM) neither the inner capacitors arm balance since it only has one cell per arm, instead the Pulse-Width Modulation (PWM) is used. The PWM injects more harmonic content but it is competitive with another state-of-the-art low voltage MMC testing platforms. The first part introduces the MMC. The principal characteristics of the MMC as its structure, the used modulation in the High Voltage Direct Current (HVDC) MMC, the different models normally used in simulations, and the analytical description are explained in this part. The second part describes the low voltage MMC design and operation. There, the converter parameters, the MMC state machine and the operating limits are defined. Then, the design and implementation of the control is described. Finally, some experimental and simulation validation are presented. The tests developed during the project confirms that the controls designed and implementation are adequate. The presen- ted modulation is needed to reduce short circuit current and harmonics. The experimental tests and the simulations validates the control structure and design. The MMC with one cell per arm shares the control with a normal MMC and have the same performance. The presented converter reduces to one cell per arm the cells of an MMC, which implies a reduc- tion of cost since less semiconductors are needed. This cost reduction does not imply a several reduction in the voltage quality compared to other state-of-the-art MMC testing platforms.