Nonlinear Controller for MMC-HVdc Operating in Grid-Forming Mode

The development of the modular multilevel converter (MMC) enabled the efficient creation of high-power high-voltage direct current (HVdc) transmission systems. As a result, MMC-HVdc transmission systems became the main alternative to integrate remote renewable energy sources being deployed in accelerating rates to fight climate change. As the number of online classical synchronous generators (SGs) decreases while the one of converter-based power sources increases, power systems are suffering from lower inertia levels and from fewer providers of ancillary services. Therefore, new control strategies, such as the grid-forming (GFM) converter operation, were developed to address the ongoing power system transformation. The main contribution of this article is to propose a nonlinear (NL) control strategy compatible with GFM operation for an MMC-HVdc transmission system controlled as a virtual synchronous machine (VSM). The control strategy is developed using NL control tools, such as feedback linearization, dynamic feedback linearization, and backstepping. In addition, this article provides a rigorous mathematical stability analysis applying Lyapunov theory. The proposed control strategy is then validated by simulations using the MATLAB/Simscape Electrical package in three situations: active power tracking, converter energy tracking, and a frequency support scenario. Results show the good performance of the proposed NL controller for all situations considered, presenting a fast response and a faster disturbance rejection compared with the classical proportional integral (PI) controller.