Sensorless robust flatness-based control with nonlinear observer for non-ideal parallel DC-AC inverters

Parallel operation of multiple AC/DC inverters is favorable in hybrid AC/DC microgrids to avert consecutive conversion stages and increase the system's efficacy. Yet, several emerging technical challenges impede the expansion of such layout such as the circulating currents, sensitiveness to the input voltage disturbances, load variations and complexity of the control structures caused by using several required measurements. This paper proposes a one-loop sensorless controller which is based on the flatness technique for a non-isolated power supply consisting of n-parallel inverters. The proposed control scheme primarily relies on employing a nonlinear online observer to estimate the line inductor currents and the dc link voltage via information from the input voltage, output voltage, and load conditions to avoid using excessive sensors. In this way, the system reliability is improved by reducing burdens of the communication delays and/or the failures, signal noise, thus the system is featured by simple control. Besides, the system entire losses are modeled by equivalent voltage sources and one current source which implicitly represent all types of the losses by using an online nonlinear estimator for the control purposes. The proposed controller not only has high dynamic performance, wide-bandwidth, low voltage THD but also robust to the abrupt variations in the load and the input voltage. To validate the applicability of the proposed control method and the observer, both simulations and experimental investigations are performed for two paralleled three-phase inverters setup. The obtained results assure the effectiveness of the proposed control method in regulating the output voltage of the parallel DC/AC inverters with fewer number of the sensors against the fluctuations of the input dc voltage and the load perturbations.