A High-Order Fully Actuated Control Strategy for Frequency Support in VSC-HVDC Systems

Voltage source converter-based high-voltage direct current (VSC-HVDC) systems can provide frequency support to island grids, of which the dynamic characteristics are susceptible to fluctuations in load and generation. This paper proposes a high-order fully actuated sliding mode control (HOFASC) strategy to provide frequency support for low inertia grids via VSC-HVDC systems. The proposed approach utilizes feedback linearization to transform a primitive underactuated inverter model into an equivalent high-order fully actuated model. A high-gain disturbance observer is designed to estimate the lumped disturbance, including the random power fluctuations, unknown nonlinear dynamics, and parameter error. Then the sliding mode feedback controller is designed to compensate for the lumped disturbance and restore the grid frequency to the nominal value with the help of estimations. Simulation results demonstrate the HOFASC can effectively improve the frequency stability of power systems with high renewable energy penetration, and has superior disturbance rejection compared to conventional methods.