Nonlinear Control of Inertial Response for HVDC-Connected Offshore Wind Farms

Abstract

The growing penetration of renewable energy resources could be a major concern in terms of the primary frequency response of the grid. Nevertheless, this can potentially be addressed by the inertial response of the grid-connected DFIG-based offshore wind farms through VSC-HVDC-VSC (voltage-sourced converter). However, the inherent nonlinearity of this system leads to unsatisfactory performance when controlled with linear controllers. This paper proposes a nonlinear control strategy specifically designed to enable robust inertial support under the grid frequency events. A detailed nonlinear model is developed for the entire system, including the wind turbine, DFIG converters, HVDC transmission system, and grid interface. Unlike conventional omission of nonlinear terms, the output feedback linearisation (FL) is applied to transform the coordinates of the nonlinear system into a new coordinate representing a linear structure while fully retaining the original nonlinearities, thereby ensuring accurate dynamic representation. Furthermore, a sliding mode control (SMC) strategy is proposed to ensure robust system performance in the face of load variations and uncertainties in the engaged system and grid parameters. The stability of the system's internal dynamics and the sliding surface in the SMC is demonstrated using Lyapunov's method. The performance of the proposed controller is validated through various simulations under different scenarios by employing the IEEE 39-bus test system.