Virtual Synchronous Control Using SOGI for Standalone DFIG-Based Wind Turbines with Unbalanced and Nonlinear Loads

This paper proposes a virtual synchronous control (VSynC) strategy of a stand-alone doubly-fed induction generator (DFIG)-based wind turbine with unbalanced and nonlinear loads. By adopting the proposed VSynC strategy, the stand-alone DFIG is enabled to have an inertial response, which is similar to synchronous machines. Moreover, an integration of second-order generalized integrator (SOGI) in the virtual synchronous reference frame without any sequence decomposing process has helped in simplifying the control loop, which enables DFIG to achieve the target of sinusoidal output voltage efficiently and rapidly under islanding operation. Finally, simulation has been conducted on MATLAB to prove the feasibility of the proposed control system.

I. Introduction

Recent years have witnessed a growing implementation of wind farms and the global total installation of wind energy has reached 486.8 GW at the end of 2016[1]. The doubly-fed induction generator (DFIG) is widely used because of its reduced converter capacity, reduced size, variable speed constant frequency(VSCF) feature and relatively low cost [2]. Occasionally, DFIG-based wind farms need to be disconnected from the main grid when the power system suffers from a complete or partial outage, supplying local loads all by itself in stand-alone operation. As there is no stable grid to rely on, the stand-alone DFIG should perform as a voltage source, providing a constant voltage amplitude and frequency output to the local microgrid [3]. In this case, the deteriorated stator voltage and current decrease the efficiency of DFIG and may cause some serious damages to DFIG-based wind turbines or even to the power system [4]. Therefore, maintaining a stable sinusoidal voltage and frequency output at the stator side is the most essential target for stand-alone DFIG-based wind turbines.