Virtual Synchronous Grid Interface of Distributed Wind Turbines Without PLL

Abstract

This paper proposes a virtual synchronous grid interface for Distributed Doubly-Fed Induction Generator (DFIG)-based wind turbines. On the basis of the proposed excitation control model orientated by rotor flux, and the P-f, Q-U droop control models which emulate the governor and exciter of synchronous machine, both grid-side converter (GSC) and rotor-side converter (RSC) can be mathematically equivalent to the conventional synchronous generator. This control strategy can provide DFIG-based wind turbines with self-synchronization mechanism without using the phase-locked loop (PLL). Considering the power limitation under certain wind speed, the proposed adaptive power-based droop control method realized the maximum power point tracking (MPPT) of wind turbines in steady state. Simulation results demonstrate the feasibility and effectiveness of the presented virtual synchronous control for grid-connected DFIG. This control strategy not only provide the possibility to unify grid-connected wind turbines as standard synchronous power, but also decrease negative effect on grid frequency and voltage in system with high penetration of wind energy.