Coordinated Frequency and Voltage Optimal Control of Wind Farm With Nonlinear Power Constraints

Abstract

In the high-voltage direct current transmission system, if converter blocking occurs at the sending-end converter station, a serious power imbalance takes place in the sending-end grid, which causes severe over-frequency and over-voltage problems. To address this issue, a double-layer coordinated optimal control strategy based on model predictive control (MPC) is proposed in this article. By cooperatively regulating the active and reactive power of the wind farm at the sending end, it achieves coordinated frequency and voltage control. On the basis of MPC and moving horizon estimation, the upper-layer controller computes the optimal active and reactive power references of the wind farm. Subsequently, the optimized power references are fed into the lower-layer wind farm local controller for execution. A key feature of this strategy lies in its handling of nonlinear power constraints. Specifically, for a wind farm, coordinated frequency and voltage regulation faces a critical challenge of coupled nonlinear power constraints, which are formed by the coupled relationship between the active and reactive power outputs of the power converters. To overcome this challenge, the double description method is introduced to approximate and linearize the coupled nonlinear power constraints. Simulation results verify the effectiveness of the proposed strategy.