The LQ controller for the 3-phase 4-leg inverter with an LC output filter — Choosing the right reference frame

In this paper three different implementations of the full-state feedback linear-quadratic (LQ) controller for a 3-phase 4-leg sine wave inverter are compared. The LQ controller is designed in stationary reference frames abc and αβγ as well as in a dq0 reference frame rotating with the fundamental output frequency, and is not cascaded with any inner controller. In the proposed control schemes integral and multi-oscillatory terms are added to the input of the LQ controller to ensure high quality output voltage waveforms for unbalanced and nonlinear loads. All three approaches are tested in an experimental inverter (a physical model at the level 10kVA). The experimental results for linear and nonlinear loads are presented, discussed and some remarks on choosing the best reference frame are then given.