An LQ controller with disturbance feedforward for the 3-phase 4-leg true sine wave inverter

In this paper we develop a new approach to combined full-state and disturbance feedforward control system synthesis for a three-phase four-leg true sine wave inverter. Proposed scheme is an extension of the augmented state feedback controller with multi-oscillatory terms. Multi-resonant controllers are well-established solution for this kind of converters if high quality output voltage is needed in the presence of nonlinear loads. They take advantage of repetitiveness of the control process under the steady load conditions (this includes nonlinear and unbalanced loads). No presumption on load type entails introduction of oscillatory terms for all three voltage components with resonant frequencies up to available bandwidth dictated by technically justified switching/sampling frequency. This makes the practical implementation challenging. Our idea is to reduce number of needed oscillatory terms by adding disturbance feedforward. In the discussed system the disturbance signal is easily accessible/measurable and hence this feedback can be very effective because of lack of estimation errors. The control system is designed in dq0 reference frame rotating with the fundamental output frequency. Extensive numerical experiments indicate that the number of oscillatory terms can be reduced significantly and high quality of output voltage can be maintained.