Virtual Synchronous Control for Grid-Connected DFIG-Based Wind Turbines

Abstract

This paper presents a virtual synchronous control (VSynC) for doubly fed induction generator (DFIG)-based wind turbines (WTs) to provide inertia contribution in particular when integrated into weak ac grid with low short-circuit ratio (SCR). Different from the traditional vector control (VC) based on the widely used phase-locked loop synchronizing technique, VSynC is capable of synchronizing DFIG with grid directly through the active power control. Damping control and current limitation are also designed to improve the stability and dynamic response of DFIG-based WT. Eigenvalue analysis shows that the VSynC-based DFIG displays pronounced superiority in stability to the typical VC based when connected to low-SCR ac grid. In such case, the power transfer capability of VSynC-based DFIG is not limited, which means the maximum theoretical power viz., 1 pu, can be achieved even when SCR is decreased to 1. By contrast, the maximum transmittable power of VC-based DFIG is highly restricted. Moreover, VSynC makes DFIG naturally provide the desired inertial response, and as a result enhances the grid frequency stability. Comparative study between the VSynC and the typical inertia control based on VC is also performed, and simulated results demonstrate the superior inertial response of VSynC-based DFIG attached to weak ac grid.