Integration of a Microgrid Consisting of Hybrid Energy Sources to a Weak AC Grid

The conventional vector current controlled Voltage Source Inverters (VSI) are commonly used to integrate renewable energy sources to ac grids. However, the presence of a Phase Locked Loop (PLL) results in stability problems when the VSI is integrated to a weak ac grid. The inner current controller may interact with the PLL dynamics leading to instability. To overcome this limitation a PLL-less control technique like Voltage Modulated Direct Power Control (VMDPC) may be used. In this paper the effectiveness of VMDPC technique is validated by using it to integrate a microgrid with both photovoltaic and wind energy generation units to a weak ac grid. The results shows that the weak grid connected inverter can track the real and reactive power references with the VMDPC technique. The Total Harmonic Distortion (THD) of the current injected into grid is found to be within acceptable limits and the performance of the controller under a sag in grid voltage is also found to be satisfactory.