Nonlinear Model Predictive Control for Droop-Based Grid Forming Converters Providing Fast Frequency Support

With the increase of renewable generation in the power system, inverter-based resources will dominate the future grid. It is expected that droop-based grid forming converters (GFCs) are going to play an important role in providing frequency support in future grids. However, in the presence of the limitation on the power from the renewable source connected to the dc-side of the inverter, the dc-link voltage is prone to collapse after a significant active power demand from the GFC. A nonlinear model predictive control (NMPC) is developed to control this dc-link voltage and to supply frequency support from GFC to the grid when needed. The NMPC control senses the frequency of the neighboring buses of the GFC to identify the requirement for active power support. It tracks a predefined reference of the dc-link voltage to control the active power output of the GFC, and ensures quantifiable frequency support after the contingency. The design of the NMPC incorporates all the constraints of the GFC, including the current limits and control input, such that it can provide maximum output power in a short time after the contingency while avoiding dc-voltage collapse. The control is demonstrated in a detailed phasor model of the Kundur's 4-machine systems and IEEE 16-machine system using Matlab/Simulink.