Transient Characteristics and Quantitative Analysis of Electromotive Force for DFIG-based Wind Turbines during Grid Faults

For doubly-fed induction generator(DFIG)-based wind turbines(WTs),various advanced control schemes have been proposed to achieve the low voltage ride through(LVRT)capability,whose parameters design is significantly reliant on the rotor electromotive force(EMF)of DFIG-based WTs.However,the influence of the rotor current on EMF is usually ignored in existing studies,which cannot fully reflect the transient characteristics of EMF.To tackle with this issue,this study presents a comprehensive and quantitative analysis of EMF during grid faults considering various control modes.First,the DFIG model under grid faults is established.Subsequently,the transient characteristics of EMF are analyzed under different control modes(that is,rotor open-circuit and connected to converter).Furthermore,the EMF transient eigenvolumes(that is,accessorial resistance item,transient decay time constant,and frequency offset)are quantitatively analyzed with the typical parameters of MW-level DFIG-based WT.The analysis results contribute to the design of the LVRT control scheme.Finally,the analysis is validated by the hardware-in-the-loop experiments.

A novel control solution for improved trajectory tracking and LVRT performance in DFIG-based wind turbines

This paper presents a new control strategy for the rotor side converter of Doubly-Fed Induction Generator based Wind Turbine systems,under severe voltage dips.The main goal is to fulfill the Low Voltage Ride Through performance,required by modern grid codes.In this respect,the key point is to limit oscillations(particularly on rotor currents)triggered by line faults,so that the system keeps operating with graceful behavior.To this aim,a suitable feedforward-feedback control solution is proposed for the DFIG rotor side.The feedforward part exploits oscillation-free reference trajectories,analytically derived for the system internal dynamics.State feedback,designed accounting for control voltage limits,endows the system with robustness and further tame oscillations during faults.Moreover,improved torque and stator reactive power tracking during faults is achieved,proposing an exact mapping between such quantities and rotor-side currents,which are conventionally used as controlled outputs.Numerical simulations are provided to validate the capability of the proposed approach to effectively cope with harsh faults.

Low-voltage ride-through capability improvement of Type-3 wind turbine through active disturbance rejection feedback control-based dynamic voltage restorer

Disconnections due to voltage drops in the grid cannot be permitted if wind turbines(WTs)contribute significantly to electricity pro-duction,as this increases the risk of production loss and destabilizes the grid.To mitigate the negative effects of these occurrences,WTs must be able to ride through the low-voltage conditions and inject reactive current to provide dynamic voltage support.This paper investigates the low-voltage ride-through(LVRT)capability enhancement of a Type-3 WT utilizing a dynamic voltage restorer(DVR).During the grid voltage drop,the DVR quickly injects a compensating voltage to keep the stator voltage constant.This paper proposes an active disturbance rejection control(ADRC)scheme to control the rotor-side,grid-side and DVR-side converters in a wind–DVR integrated network.The performance of the Type-3 WT with DVR topology is evaluated under various test conditions using MATLAB®/Simulink®.These simulation results are also compared with the experimental results for the LVRT capability performed on a WT emulator equipped with a crowbar and direct current(DC)chopper.The simulation results demonstrate a favourable transient and steady-state response of the Type-3 wind turbine quantities defined by the LVRT codes,as well as improved reactive power support under balanced fault conditions.Under the most severe voltage drop of 95%,the stator currents,rotor currents and DC bus voltage are 1.25 pu,1.40 pu and 1.09 UDC,respectively,conforming to the values of the LVRT codes.DVR controlled by the ADRC technique significantly increases the LVRT capabilities of a Type-3 doubly-fed induction generator-based WT under symmetrical voltage dip events.Although setting up ADRC controllers might be challenging,the proposed method has been shown to be extremely effective in reducing all kinds of internal and external disturbances.

面向系统电压动力学演化过程分析的电力电子并网设备内电势幅值运动机制研究

系统电压和频率演化机制认识一直是开展电力系统稳定、控制和保护等研究的重要基础。针对系统频率演化问题,已有研究以转子运动方程所刻画的内电势频率运动机制为核心形成了频率同步过程的直观认识。而针对系统电压演化问题,由于不明确驱使内电势幅值运动的激励力和支撑内电势幅值运动的物质特性,没有形成与内电势频率运动机制相对应的内电势幅值运动机制认识,更未能结合网络特性开展物理概念明确的电压动力学演化过程研究。随着电力电子设备持续大规模并网,电压问题愈加凸显,亟需更加深入地认识系统电压演化过程。因此,文中首先从设备调节内电势幅值/频率的基本原理出发,明确无功不平衡是驱使内电势幅值运动的主要激励力,并以双馈风机(doubly-fedinduction generator-based wind turbine,DFIG-based WT)为例构建无功/端电压不同控制目标下内电势幅值/频率响应模型。然后,基于三相系统动态过程中无功功率的物理内涵,建立支撑内电势幅值运动的无功不平衡存储特性的直观认识,从而通过类比频率运动机制,指出内电势幅值是反映无功不平衡存储水平的状态。最后,在含DFIG-basedWT单机系统中分析考虑不同控制目标时的电压动力学演化过程,并初步探讨多机系统电压动力学演化过程的研究思路。

Comparative study of three types of controllers for DFIG in wind energy conversion system

This paper presents an enhanced control strategy for Wind Energy Conversion System(WECS)using Doubly-Fed Induction Generator(DFIG).A robust Super-Twisting(STW)sliding mode control for variable speed wind turbine is developed to produce the optimal aerodynamic torque and improve the dynamic performance of the WECS.The electromagnetic torque of the DFIG is directly tracked using the proposed control to achieve maximum power extraction.The performance and the effectiveness of the STW control strategy are compared to conventional Sliding Mode(SM)and Proportional-Integral(PI)controllers.The proposed STW algorithm shows interesting features in terms of chattering reduction,finite convergence time and robustness against parameters variations and system disturbances.