Comparison of Output Harmonics between Two- and Three-Level Three-Phase Space Vector PWM Inverters
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Kaksi- ja kolmitasoisten kolmivaiheisten avaruusvektorimoduloitujen PWM vaihtosuuntaajien lähdön harmonisten vertailu
The usage of renewable energy sources is growing. These sources, such as photovoltaic devices and wind turbines, are connected to the power grids via inverters. Power electronic inverters produce non-sinusoidal voltages and increase the amount of unwanted distortion in the grids. Standards define the quality of the electricity in the power grids. Therefore, it is sensible to study the phenomena and methods to reduce the harmonic distortion in the outputs of different inverter topologies. LCL filters are used to suppress the harmonic components in the output waveforms of the grid connected inverters. The filter is one of the most expensive component in drive systems. The amount of harmonics can also be decreased by adding voltage levels to the inverter’s DC bus, optimizing the switching sequences and increasing the switching frequency. Therefore, better output harmonic performance can be achieved with the same LCL filter by using three-level inverter instead of two-level inverter. Three-level inverter also enables higher switching frequency. However, some additional costs are created, such as more semiconductor switches are needed, and the device requires more complex control system. The main issue in this thesis is to compare output harmonics between two- and three-level inverters. The studied topologies are two-level voltage source inverter and three-level neutral-point-clamped inverter. A detailed space vector modulation method is explained for these inverters. Requirements for a well implemented space vector modulator are also discussed. Space vector pulse width modulators are created using MATLAB® and Simulink®. Output harmonic performance is compared by simulations with different switching frequencies and modulation indexes under linear modulation region. The simulations show that increasing the switching frequency reduces the phase current’s harmonics. The switching frequency doesn’t have similar effect on the line-to-line voltage’s total harmonic distortion value. It only increase the frequencies where the voltage’s harmonic components occur. It’s also shown, that the voltage’s and current’s THD values in the NPC inverter are half of the THD values in the VSI. The modulation index has an effect on both, the line-to-line voltage’s and phase current’s THD values. When the modulation index is reduced, the voltage’s and current’s THD values are increased. Small modulation index value reduces the difference between output performance of the VSI and NPC inverters.