Model-based control methods to improve the power qualify of grid-connected single-phase inverters.

Abstract

Power electronic converters are commonly used for interfacing distributing generation sources (DGs) to the electrical power system networks. This is necessary because these DGs usually have different output characteristics and cannot be connected directly to the local load and/or the grid. The power electronic front-end converter is an inverter whose dc link is fed by an ac/dc converter or by a dc/dc converter, according to the DG source type. The commercial front-end inverters are designed to operate either in grid-connected (GC) mode or in stand-alone (SA) mode. In the SA mode, the inverter is connected to local load, but in the GC mode the inverter must be connected to the utility grid and a local load could be connected to this system as well. Based on this, any designed or proposed controller for such systems should work well in both operation modes. The control objective in SA mode is to improve the quality of the local load voltage, and the control objective in GC mode is to inject clean current to the grid with low total harmonic distortion (THD). Most of the control schemes in the literature have been designed to work in one of these operation modes and ensure low THD either for the local load voltage or for the injected grid current. However, some of the existing control schemes in the literature proposed different control architectures for each operation mode. Moreover, there are a few researches have been reported in the literature based on the cascaded control theory to obtain low THD for both the local load voltage simultaneously with the injected current to the grid in the grid-connected mode. Due to the growing penetration of the DG sources in the residential applications, single-phase grid-connected inverters have gained much attention. For this reason, the single-phase grid-connected inverter systems have been chosen in our study. Since such systems have nonlinearity in its behavior, different nonlinear model-based control schemes have been designed in order to improve the quality of the local load voltage while injecting clean current to the grid for single-phase grid-connected inverter systems by using single structure control scheme. Furthermore, the proposed control schemes ensure the seamless transfer between GC and SA operation modes without adjusting the controller structure and with self-synchronization ability.