Improved control strategy of grid connected inverter without phase locked loop on PCC voltage disturbance

To avoid the stability problem of grid connected inverter caused by the conventional phase locked loop (PLL), the direct power control of voltage source inverter in virtual synchronous reference frame without PLL is investigated considering grid voltage disturbance in the paper. Based on the small signal analysis methods, comparison between conventional PLL and no PLL is made. It can be found that the control scheme based on virtual synchronous reference introduces a virtual phase angle which is counterpart to the angle achieved by PLL. However, this virtual phase angle is directly proportional to the voltage perturbation of point of common coupling (PCC). Since PCC voltage is not stiff in the weak grid, the PCC voltage disturbance may result in instability of grid connected inverter system based on the control scheme without PLL. Therefore, this paper proposes an improved control strategy by adding two low pass filters to improve the operation performance of inverter when PCC voltage is disturbed. Simulation results show that proposed scheme can achieve the stable operation of grid connected inverter under the voltage disturbance.