Multivariable phase-locked loop free strategy for power control of grid-connected voltage source converters

A multivariable control strategy in a dq reference frame for grid-connected voltage source converters (VSCs) without using a phase-locked loop (PLL) is presented in this paper. First, common VSC controls such as vector current control (VCC) and grid voltage modulated direct power control (GVM-DPC) are analyzed and their main drawbacks are identified. Then, the multivariable control strategy is presented, including the implementation of saturation and anti-windup mechanisms, and limitation of overcurrents. Next, different uncertainty channels that may lead to instability in each approach are analyzed, showing the drawbacks related to the use of a PLL, which are avoided in our proposal. The effectiveness of the three strategies is compared by means of MATLAB/Simulink simulations, showing that the proposal presents a more robust behavior, specially in weak grids.