Reactive Current Constraints and Coordinated Control of DFIG’s RSC and GSC During Asymmetric Grid Condition

Hailiang Xu,Zhi Li,Rende Zhao,Yufeng Zhang,Jiabing Hu

doi:10.1109/access.2020.3029227

Abstract

Doubly fed induction generator (DFIG) based wind turbines have been found to be very sensitive to asymmetrical grid fault, wherein the harmful oscillations in the electromagnetic torque has being made a big concern. What makes things harder is that latest grid codes require grid-connected wind turbines to possess the ability of positive- and negative-sequence reactive current supporting. Hence, it becomes a guide mechanism to meet the grid codes and to ensure the operation safety of the wind turbines. Even though a tradeoff could be made between them, it is still quite difficult to coordinate the current dispatch of the DFIG’s rotor- and grid-side converters (RSC and GSC), since there are totally eight currents needing to be carefully designed, i.e., the positive- and negative-sequence active and reactive currents in each converter. To address this issue, this article analyzes the reactive current constraints in detail, based on which a coordinated reactive current control strategy is put forward for the DFIG-based wind turbines during asymmetrical grid condition. Simulation studies demonstrate the effectiveness of the proposed strategy.

Highlights

Nowadays, the doubly fed induction generators (DFIGs) have been widely utilized in wind farms, in virtue of their perfect merits, such as limited converter capacity, wide operating range, high efficiency and reliability [1]–[4]
When an asymmetric grid dip occurs, the DFIG rotor current will get seriously distorted with associated pulsations emerging in the stator active and reactive powers, and the electromagnetic power as well [6]–[9], which may lead to the disconnection of the wind turbines from the faulty grid
To meet the positive- and negative-sequence reactive current response requirement of the grid codes, the active and reactive current constraints of the DFIG-based wind turbines is derived in this article, based on which a coordinated current assignment strategy of DFIG’s rotor-side converter (RSC) and GSC during asymmetric grid conditions is proposed

Summary

INTRODUCTION

The doubly fed induction generators (DFIGs) have been widely utilized in wind farms, in virtue of their perfect merits, such as limited converter capacity, wide operating range, high efficiency and reliability [1]–[4]. Given that the control target is reasonably determined, it is still complicated on how to dispatch the positive- and negative-sequence reactive current of the back-to-back converter in the DFIG based system To settle such problem, this article puts forward a coordinated control strategy that can simultaneously satisfy the positive- and negative-sequence reactive current requirement by the grid codes and eliminate the harmful oscillations in the electromagnetic torque, with the maximum active power delivery capability being fully considered. In order to restrain the oscillations of electromagnetic torque, the sine and cosine terms of twice the grid frequency in (4) should be eliminated, which are related to the positive- and negative-sequence active and reactive rotor currents

DIFFICULTY IN SIMULTANEOUSLY ACHIEVING THE TWO TARGETS

COMPREHENSIVE FEASIBLE REGION BASED ON THE CONSTRAINTS OF THE RSC AND GSC

Findings

CONCLUSION