Robust Sensorless Virtual-Flax Direct Power Control of Weak Grid Connected Converters

In renewable energy sources applications, the system line equivalent impedance is affected by many factors, such as the saturation of the core of low-frequency transformers and the overload of the cable. In addition, changing environmental conditions such as variable temperature and humidity along the line can affect its parameters. The mismatch between the actual values of the system parameters and their values in the control system causes the control system based on the model to choose inappropriate voltage vectors and resulting in system instability. This is especially more important in a weak grid condition. When a converter is connected to a weak grid, the system may become unstable due to voltage fluctuations at the point of common coupling (PCC). Therefore, maintaining the stability of such systems is an important concern for the power industry. In this paper, a virtual flux-based DPC of VSCs is presented that enjoys a better performance over the conventional VF-DPC the elimination of the dependence of the active power on the line inductance changes. Therefore, in comparison with the conventional VF-DPC, the proposed method has advantages such as lower power ripples and better current THD under weak grid condition. Also, the stability of proposed control system is studied using a small-signal model, where the model is directly extracted from a look-up switching table and enables the stability analysis of concurrent variations in the system parameters. These findings are verified through extensive simulation and experimental results.

Keywords:

direct power control , grid connected converters , renewable energy sources , stability analysis , weak grid.