A Robust Phase-Locked Loop-Integrated Controller in Stationary Frame for Voltage Source Converters Connected to Weak and Unbalanced Grids

This article presents a controller in stationary frame for enhanced performance of voltage source converters (VSCs) under weak and unbalanced grid conditions. The controller incorporates a nested multiloop structure consisting of an inner phase-locked loop-integrated current controller as well as an outer dc-link voltage control loop based on integral-resonant control mechanism. The controller neutralizes double-frequency oscillations of the dc-link voltage as well as third-order harmonics in the output currents under voltage imbalance while improving system performance and stability under weak grid conditions. First, the dc-link dynamics of a VSC under unbalanced grid conditions are revisited. It is shown that in the power balance equation, there are nonlinear coupling terms between different mechanisms of the dc-link control loop. Second, this article proposes an approach to surpass these terms by separating dc voltage dynamics. Finally, using a robust multivariable control approach, system performance, and stability under weak grid conditions are significantly improved. Software simulations and hardware-in-the-loop tests are carried out to validate the enhanced performance of the proposed controller.