Harmonic analysis of direct digital control of voltage inverters

Voltage inverters are the most common kind of actuator to interface with variable frequency electrical systems. They are usually driven by a modulation block that provides the gating signals for the switches. Direct digital control avoids the modulation block, being the gating signals directly produced by the controller. In such configuration, the control of the electrical system is a discrete-time and quantized-actuation problem. This paper analyzes the harmonic content in sinusoidal steady state resulting from such configuration. The analysis illustrates the higher harmonic content, providing lower bounds for such content in the case of low ratios of commutation to fundamental frequencies. Also, the dependence of harmonic content with duty cycle and with commutation losses is exposed. The findings apply to different kind of control structures, regardless of its design or tuning parameters. Some of them are experimentally compared. A two-level three-phase inverter is used in the analysis for its relevance and simplicity although the procedure can be applied to any number of phases or levels.