Perturbation observer-based nonlinear control of VSC-MTDC systems.

• The equivalent model of VSC stations are obtained using the feedback linearization. • The high gain observer is utilized to estimate the lumped perturbations. • Accurate model and parameters are not required by the proposed controller. The voltage source converter based multi-terminal direct-current (VSC-MTDC) system is a promising technique to integrate multiple large offshore wind farms (OWF) and to share the power between multiple ac grids. The operation and control of VSC-MTDC systems could be affected by multiple uncertainties, e.g., the stochastic power generation of OWFs, nonlinearities of VSC-MTDC systems and unknown dynamics triggered by grid faults, which may result in poor dynamic performance and stability issues. In this paper, a perturbation observer-based nonlinear control (PONC) is proposed to improve the robustness of VSC-MTDC systems with OWFs. The perturbation observer is designed to estimate the lumped perturbation terms, which include all the system uncertainties, unknown dynamics and the external disturbances together with the state estimates. Based on the estimated perturbation and states, the output feedback linearization controllers are designed to compensate the impact of the lumped perturbation and to achieve robust tracking performance, without requiring the accurate system model. The effectiveness of PONC is verified by a 5-terminal VSC-MTDC system. [ABSTRACT FROM AUTHOR]