【作者】 王波；

【导师】 胡家兵；

【作者基本信息】 华中科技大学 ， 电气工程， 2018， 博士

【摘要】 并网导则要求风电场在故障持续期间响应无功电流支撑电网电压以协助电网恢复。风机并网系统在故障持续期间的稳定性既是可靠响应无功电流的内在要求,也是故障穿越后续阶段逐步恢复有功出力的基础,因而十分重要。并网系统的稳定性与电网条件密切相关,然而现有风机装备故障穿越控制策略的设计和研究一般基于并网点为强电网的条件,未充分考虑大规模集中式开发、远距离输送等风电开发模式下长输电线路具有显著阻抗这一实际因素。随着风电渗透率进一步提高,现有风机并网系统在故障持续期间的失稳风险未能得到充分评估,且已有研究未考虑一般性的不对称故障工况。故障持续期间可长达数百毫秒乃至数秒,并网风机系统电磁暂态量已近于完全衰减,可采用小信号方法研究其稳定性。相比同步发电机,基于电力电子技术的风机装备可提供更快的电流控制动态,该特性须在稳定性建模和分析中得到重视。本文基于深度对称/不对称故障场景,围绕高阻型弱电网条件研究计及电流动态双馈风力发电机并网系统在电网故障持续期间发生小信号中低频(约10-100 rad/s)失稳问题的机理,重点探索转子侧变换器控制参数的影响,努力为工程实践中故障穿越控制系统设计和进一步研究提供参考。具体内容包括:(1)基于一般化建模思路分别建立了故障持续期间双馈风力发电机装备和不对称故障网络计及电流动态的小信号模型。从外特性出发定义了双馈发电机装备计及电流动态的内电势和内电抗,在此基础上建立了双馈发电机装备的频域小信号模型;建立了不对称故障工况下故障网络计及电流动态的频域小信号模型;建立了不对称故障网络和双馈发电机装备计及电流动态的状态方程时域模型;基于并网系统频域模型指出了弱电网故障持续期间并网系统可能的小信号稳定问题的两个主要类型和相应研究思路。(2)基于弱电网对称故障场景,分析了深度故障持续期间双馈发电机并网系统受正序网络阻抗引入的正序锁相环-转子电流环相互作用影响在中低频段因阻尼不足发生第一类振荡失稳问题的机理。首先阐述了正序阻抗在并网系统内引入的正序锁相环-转子电流环相互作用;鉴于模态分析结果表明并网系统的弱阻尼模态是锁相环主导的,为量化该相互作用对锁相环的影响,基于复转矩系数法概念将该相互作用对锁相环相位运动的影响用阻尼系数和恢复系数量化;分析了各关键因素如何影响阻尼系数和恢复系数从而影响并网系统稳定性,揭示了弱电网深度对称故障期间并网系统因正序锁相环-转子电流环相互作用减弱了正序锁相模态的阻尼而发生第一类失稳问题的机理。(3)基于弱电网不对称故障场景,在确保正序子系统自身和负序子系统自身均充分稳定的前提下,分析了深度不故障期间双馈发电机装备受序间耦合网络阻抗引入的序间相互作用影响在中低频段因阻尼不足发生第二类失稳问题的机理。首先研究了发电机装备和故障网络的输入输出特性,明确了影响序间相互作用的关键因素;然后基于模态分析方法,得到了序间相互作用影响正、负序子系统的不同规律,并完成了序间耦合支路往负序的等效归算;接下来研究了双馈发电机装备和故障网络基本单元的输入输出特性与并网系统稳定性间的内在联系,并基于复转矩系数法概念定义了弱阻尼的正、负序耦合系统的阻尼系数和恢复系数,将序间相互作用对负序子系统稳定性的影响用阻尼系数和恢复系数的变化量予以量化。在此基础上,分析了不对称故障工况下影响序间相互作用的关键因素,揭示了弱电网深度不对称故障持续期间因序间相互作用减弱了负序锁相模态的阻尼而发生第二类失稳问题的机理。更多还原

【Abstract】 Common to most grid codes,wind farms are required to inject reactive current during riding through grid faults.Stability of grid-connected wind turbine during grid fault is not only the precondition of reliable reactive current response during grid fault but also the basic of active power gradual recovery after grid fault is cleared,and thus is of importance.It is known that the stability of grid-connected equipment is related to the grid condition closely.However,existing control strategies of fault riding-through are designed at the assumption that the grid at connection point is strong enough,and the high grid impedance resulted by the large-scale centralized development and long-distance transmission of wind power has not been fully considered yet.Therefore,with the increasing penetration of wind power,the risk of instability of grid-connected wind turbines during grid faults has not been fully evaluated yet,and the existing studies do not cover the asymmetrical fault condition of more common.Since the duration of grid faults can be as long as several hundred milliseconds or even several seconds,electromagnetic transient quantity will be almost completely attenuated and wind turbine will be in quasi steady state during grid faults,and thus small signal method can be employed to investigate the stability of grid-connected wind turbine.With power electronic technology employed,wind turbine has fast current dynamics and this characteristic should be considered in the stability modeling and analyzing.In this thesis,based on deep symmetrical/asymmetrical fault scenario of high-impedance grid and with the current dynamics considered,the mechanism of low-frequency instability(i.e.,10-100 rad/s approximately)for doubly-fed induction generator(DFIG)based wind turbine during grid faults is explored.With special attention paid to the rotor-side converter’s parameter,the objective of this work is to provide suggestion for the designing of grid fault riding-through control strategy in engineering practice and the possible further research.Details are as follows:(1)Based on the general modeling idea,the DFIG equipment and the grid during general asymmetrical grid fault are modeled by means of small-signal method with current dynamics considered,respectively.Based on the external characteristics of DFIG equipment,the internal voltage and reactance of DFIG equipment with current dynamics considered are defined.Then the small-signal model of DFIG in frequency domain is developed.And the small-signal model of grid under general asymmetrical fault condition in frequency domain with current dynamics considered is developed.Moreover,the state equations of the fault grid and the DFIG equipment during general asymmetrical grid fault with current dynamics considered in time domain are developed.According to the developed small-signal model in frequency domain,the main two categories of instability issue of grid-connected DFIG during faults of weak AC grid can be identified.(2)Based on the scenario of deep symmetrical fault of weak AC grid,the mechanism of the first-category low-frequency instability issue of grid-connected DFIG system during grid fault,which is induced by the interaction between phase-locked loop(PLL)and rotor current controller(RCC)inside positive-sequence subsystem,is analyzed.Firstly,the intra-sequence interaction in positive sequence subsystem resulted by the impedance of positive-sequence grid is elaborated.As indicated by the modal analysis results,the poor damping modes are dominated by PLL,thus the complex torque coefficient method is generalized to investigate the phase motion of PLL and the damping and restoring coefficients are obtained to quantify the influence of the PLL-RCC interaction on phase locked loop.Based on these coefficients,key factors related with the stability are analyzed and thus the mechanism of the first-category instability issue induced by the intra-sequence interaction in positive sequence subsystem during deep symmetrical fault of weak AC grid is revealed.(3)Based on the scenario of deep asymmetrical fault of weak AC grid and under the precondition that both positive-and negative-sequence subsystem itself are stable enough,the mechanism of the second-category low-frequency instability issue,which is induced by the interaction between positive-and negative-sequence subsystems,is analyzed for gridconnected DFIG system during grid fault.Firstly,the transfer characteristics of the DFIG equipment and the fault grid,in the form of vector,are studied and the key factors affecting the sequence coupling are made clear.As indicated by modal analysis results,the influences of the inter-sequence interaction on the positive-and negative-sequence subsystem are different,thus the inter-sequence networks are equivalently referred to negative sequence subsystem.Then the relationship between the stability of grid-connected system and the transfer characteristics of basic units is researched.Based on the concept of complex torque coefficient method,the stability of the sequences-coupling system can be evaluated by the damping and restoring coefficients,and the influence of inter-sequence grid on the stability of negative-sequence subsystem can be quantified by damping and restoring coefficients.Finally,the key factors affecting the inter-sequence interaction under asymmetrical grid fault condition are analyzed,and thus the mechanism of the second-category instability issue induced by the inter-sequence interaction during deep asymmetrical fault of weak AC grid is revealed.更多还原