Stabilization of voltages on capacitors of cells in modular multilevel inverters with space-vector PWM

Abstract

Over the two past decades, a new type of semiconductor power converter has appeared – modular multilevel converter. This topology consists of several transistor-capacitive modules, allowing to realize multilevel rectifiers and invertors with various number of levels and phases. Currently, the modular multilevel converters are widely used in high-voltage direct-current power supply systems (HVDC), produced by Siemens and ABB companies, with a rated power 400-2000 MVA. Modular multi-level converters in comparison with other types of semiconductor converters have a number of significant advantages. The presence of a large number of levels (100 and higher) makes it possible to use relatively low-voltage switches and realize a low switching frequency, which leads to a high efficiency of the converter. The declared efficiency of the power transmission system of the HVDC power supply system with modular multi-level converters manufactured by Siemens is 99.5%.In addition, modular multi-level converters have increased reliability. This is due to the fact that modular converters can function even if they are output from individual transistor-capacitive modules. Constructive modularity and scalability of the converter is convenient in production. However, there is the problem of stabilizing the voltage on the capacitors of the cells. In the course of the research it was found that when using classical control algorithms for multi-level inverters (space-vector and sinusoidal pulse-width modulation) there is an imbalance of voltages on capacitors. Tension in one voice pair doubles. This leads to a significant deterioration in the shape of the output voltage. The article uses improved space-vector modulation algorithms for a modular multi-level converter, which provide the ability to stabilize the voltage in the converter cells, as well as the ability to control the amplitude and frequency of the output voltage. The Matlab / Simulink program has created a model of a modular five-level modular inverter. In the Matlab / Simulink program, a modular five-level inverter model was created. Studies have shown that the classical spatial vector modulation algorithms in modular multi-level inverters lead to an imbalance of tension on the capacitors and in modular multi-level inverters, and as a result, a voltage arises, which leads to a significant increase in the shape of the output voltage. The article uses an improved space-vector modulation algorithm for a modular multi-level converter, which allows stabilizing the voltage in cells. The simulation results confirm the proposed theoretical aspects.