Four-Leg Inverter Based on Three-Dimensional Space Vector Modulation Method for Desired Output Frequency

Conventional six-switch inverters, designed for balanced three-phase systems, face challenges in low-voltage networks with mixed single-phase and three-phase loads, leading to voltage imbalances. This imbalance adversely affects equipment, especially protection devices and power electronic converters. The increasing use of power electronic devices in industries with nonlinear loads has driven the adoption of four-leg inverters to handle neutral currents during unbalanced load connections in three-phase networks. This shift introduces challenges in pulse width modulation, particularly in the application of Space Vector Modulation (SVM). This work proposes a novel control strategy using a direct 3D-SVM approach, effectively determining switch states for voltage capacitor balancing and precise output current control. The study's main focus is presenting an advanced control strategy for a four-leg inverter utilizing the 3D-SVM technique to operate efficiently under highly unbalanced loads in distribution networks. The goal is to achieve balanced output voltages and currents under varying load conditions while maintaining the desired variable output frequency. Simulation results validate the proposed control strategy, showing a Total Harmonic Distortion (THD) of output current is 2.06% at 50 Hz and 2.55% at 25 Hz under balanced load, same thing in second case under unbalanced load, the THD is 1.65% at 50 Hz and 2.10% at 25 Hz. with similar under nonlinear load, the THD is 1.94% at 50 Hz and 2.46% at 25 Hz. Importantly, all outcomes fall within IEEE Standard 519-1992 for harmonic limits, highlighting the robustness and reliability of the proposed control approach.