A PLL-free control strategy for flexible DC transmission systems

Abstract

The grid-connected inverter needs to know the phase information of the grid voltage. The traditional method is to use the phase-locked loop (PLL) to capture the phase of the grid. However, the PLL takes some time to capture the phase and affects the stability of the grid-connected inverter. Based on this, this paper proposes a vector control strategy without PLL. Firstly, the virtual vector axis is constructed, and the grid voltage is transformed as the reference phase angle. Then the instantaneous power formula in this case is derived. The corresponding control block diagram is constructed. The method reduces the coordinate transformation operation and simplifies the control structure while ensuring the stability of the control system. The simulation results under MATLAB/Simulink show that the control effect of this method is better than that of the PLL vector control strategy, and the method has smaller computational complexity, less resources and more cost savings.

Keywords

• No phase-locked loop
• Virtual vector
• Grid-connected inverter control
• Vector control