Novel control structure for grid connected and islanding inverters

In this paper, a novel control structure is proposed for the single phase inverter either operating in grid connecting mode or in islanding mode. The system is designed in such a way that the same control structure can work both in grid connected and in islanding mode without islanding detection. Adaptive virtual impedance is introduced to minimize the circulating currents during parallel operation and to avoid any damages during the short circuits. The Phase Locked Loop (PLL) is based on a Second Order Generalized Integrator (SOGI) because of its very good frequency adaptive characteristics and the same SOGI can further be used for the calculations of various grid parameters. Virtual impedance is introduced in both grid connected and islanding mode. Low Voltage Ride Through (LVRT) tests are conducted successfully while using adaptive virtual impedance in grid connected mode.