A Three-Level Zero Harmonic Distortion Grid-Forming Converter For Medium Voltage Islanded Microgrids

The world has been experiencing a fast change in the production and use of electrical energy. The electric power system is fast changing from the conventional centralized generation to a more distributed generation (DG) with renewable energy sources installed near the consumers. The evolution of DG paves the way for the concept of grid-forming converters improving voltage and frequency stability in microgrids (MGs) and islanded applications, due to the growth of inverter-based resources. In islanded MG, power quality is a concern due to the presence of disturbing loads and possible resonance issues encountered with multiple electronically interfaced distributed energy resources (DERs) and loads. The present work proposes the Three-Level Zero Harmonic Distortion Converter (ZHD) grid-forming converter due to its inherently sinusoidal voltage source characteristic without the need of capacitive filtering elements that negatively impact in costs, efficiency, size and can be a source of resonances issues. These features are achieved by the proper combination of harmonic cancellation in a three-winding transformer and harmonic elimination using the Selective Harmonic Elimination Pulse Width Modulation (SHE PWM). Simulation results show the Three-Level ZHD converter setting voltage and frequency to an Medium Voltage (MV) islanded microgrid, delivering sinusoidal voltage waveforms even in the presence of nonlinear loads. The converter is able to maintain the voltage distortion below the IEEE 519 limits by respecting the current harmonic limits imposed by the same standard.