Adaptive synchronization of PR controllers in grid-connected inverters

This thesis focuses on the development of control schemes for single-phase Voltage Source Inverters (VSIs) to ensure that they meet standards for injection of power into the utility grid. For instance, conventional controllers fail to accurately control power flow if there are fluctuations in the grid frequency. Also they fail to provide high quality output currents if the grid voltage is distorted. The aim of this thesis is to address the above problems while adhering to technical standards for interconnected renewable energy systems. For synchronization of the inverter, a closed-loop filter based on the Internal Model Principle (IMP) was developed and its performance was analyzed in response to frequency variations. To this end, we utilized the perturbation-based extermum seeking algorithm to minimize the error and estimate the grid frequency. The designed adaptive controller and filter can estimate grid frequency and achieve a high Power Factor (PF). The effect of harmonic distortion on the control system was investigated and the control scheme was modified to provide low Total Harmonic Distortion (THD) output current. Furthermore, the effect of the DC-link ripple on the PI voltage control loop is analyzed and the control system was modified to attenuate the unwanted third harmonic component in the output current. Simulations were performed using Matlab/Simulink and the digital controller was implemented using Matlab Embedded Coder. A power electronics prototype was built and used to validate the performance of the controller. Based on experimental results, the controller successfully regulates the output power if the grid frequency changes. Also it is able to provide high quality current if the grid is polluted with unwanted harmonic components.