Abstract

DC-AC power electronic converters, or inverters, are frequently used in AC motor drives, in AC grid interfaces of renewable energy sources and energy storage units and to supply AC power to loads in stand-alone systems. They are usually of the Voltage Source Inverter (VSI) type and are expected to operate either at a constant frequency and voltage magnitude or within a small range. Passive Low-Pass Filters (LPF) are employed to attenuate the switching harmonics providing outputs voltages with low Total Harmonic Distortion (THDV), in the order of 1% to 3%. This is a well-established commercial technology.
This research work concerns a three-phase 6-switch VSI that is expected to provide an extremely low THDV for the output voltage, about 0.5%, with a wide range for the voltage magnitude, 10% to 100% of rated voltage, and frequency in the range of DC to 1 kHz . In such a case, the design of the output LPF becomes more complicated, mostly considering that the load can also vary in a wide range. The target application for this inverter is the emulation of AC rotating machines. Usually linear power amplifiers are employed in this application but they are very expensive, mostly in the power range of tens to hundreds of kVA.
This Thesis proposes an approach for designing a second-order LC LPF for the demanding conditions stated above. The VSI is controlled with Sinusoidal Pulse-Width Modulation (SPWM) and an algorithm that considers the magnitudes of the voltage components of the VSI and the attenuation/amplification of the LPF is developed. It computes suitable values for L and C that allow the system to provide the required fundamental component and THDV with the VSI operating in the linear SPWM mode. Then, a voltage control loop is designed for the VSI and an assessment of the converter losses and efficiency is carried out. The performance of the proposed system is verified by means of simulation with MATLAB and PSIM.