A Corrective Controller for Improved Ratio-Based Frequency Support through Multiterminal High-Voltage Direct Current Grids

A recently proposed droop controller design achieved approximate ratio-based frequency support through Multiterminal High-Voltage Direct Current (MTDC) grids connecting asynchronous AC areas. The design was performed via a reduced-order model, which neglects system losses. In this paper, to achieve improved tracking, a model-reference-estimation-based corrective control approach is presented, which estimates the differences between the reduced and actual models and compensates for the same. The sufficient condition for perfect ratio tracking by the proposed controller in the presence of modeling uncertainty is established. It is shown that a continuous sliding-mode controller that is robust to bounded modeling uncertainty takes the form of the PI compensator in the proposed corrective control. The region of stability of the proposed controller is established through root locus and numerical eigenvalue analyses. Finally, the effectiveness of this corrective strategy is illustrated through time-domain simulations performed in a full-order model, which is a detailed phasor-based differential-algebraic representation of the AC-MTDC grid.