Characteristics of sub-synchronous oscillation in grid-connected wind farm system

With the rapid development of wind power, wind turbines are accompanied by a large quantity of power electronic converters connected to the grid, causing changes in the characteristics of the power system and leading to increasingly serious sub-synchronous oscillation (SSO) problems, which urgently require the generalized classification and characterization of the emerging oscillation problems. This paper classifies and characterizes the emerging types of SSO caused by grid-connected wind turbines to address these issues. Finally, the impact of the typical system parameters changes on the oscillation pattern is analyzed in depth to provide effective support for the subsequent suppression and prevention of SSO.

Overview of Sub-synchronous Oscillation in Wind Power System

Nowadays with the improvement in the degree of emphasis on new energy, the wind power system has developed more and more rapidly over the world. Usually the wind plants are located in the remote areas which are far from the load centers. Generally series compensated AC transmission and high voltage DC transmission are made use of to improve the transmission capacity as two main effective ways which can solve the problem of large scale wind power transmission. The paper describes the three kinds of impact varieties and impact mechanisms in the sub-synchronous oscillation phenomena of wind power system based on doubly fed induction generator (DFIG) wind generators. At last, we point out the important problem that should be stressed in the wind power system.

Research on the Sub-synchronous Oscillation in Wind Power Connected to Series Compensated Power System and its Influencing Factors

When wind farms,which is based on double fed induction generator(DFIG),are connected to the series compensation power system,the phenomenon of sub-synchronous oscillation(SSO)may occur.In order to study the sub-synchronous oscillation in wind power connected to series compensated power system and its influencing factors,we used RT-LAB to establish a simulation model concerning wind power connected to series-compensation power system.This model take a wind power connected to series compensation power system in north China as prototype.All influence factors of wind power SSO are simulated and analyzed by using time domain analysis method.The simulation results show that the effects of wind speed,series compensation degree and proportional control coefficient of rotor side converter(RSC)are most obvious.

Connections Between Induction Generator Effect and Open-loop Modal Proximity to Cause Sub-synchronous Oscillations in Series-compensated Power System

The induction generator effect(IGE)and the openloop modal proximity(OLMP)are two different reasons why subsynchronous oscillations(SSOs)in a series-compensated power system(SCPS)may occur.The IGE attributes the growing SSOs to negative resistance,while the OLMP explains the SSO mechanism from the standpoint of modal conditions.In this paper,we investigate the connections between the IGE and the OLMP through equivalent RLC circuit and open-loop modal analysis.Our investigation is conducted for two types of seriescompensated power systems where either a synchronous generator or a DFIG is connected at the sending end.The investigation reveals the conditions,in which the IGE and the OLMP may jointly cause the growing SSOs,i.e.,both the IGE and the OLMP can explain why the growing SSOs occur.Furthermore,the investigation indicates that the IGE and the OLMP may be totally irrelevant and lead to growing SSOs separately.This implies that it is possible that in a SCPS,the growing SSOs are only due to the IGE,and the OLMP is non-existent,and vice versa.Hence,when the growing SSOs occurs in a SCPS,examination based on both the IGE and the OLMP should be carefully conducted in order to find if the oscillatory instability is due to the IGE,or the OLMP,or both of them.

Empirical Wavelet Transform Based Method for Identification and Analysis of Sub-synchronous Oscillation Modes Using PMU Data

This paper proposes an empirical wavelet transform(EWT)based method for identification and analysis of sub-synchronous oscillation(SSO)modes in the power system using phasor measurement unit(PMU)data.The phasors from PMUs are preprocessed to check for the presence of oscillations.If the presence is established,the signal is decomposed using EWT and the parameters of the mono-components are estimated through Yoshida algorithm.The superiority of the proposed method is tested using test signals with known parameters and simulated using actual SSO signals from the Hami Power Grid in Northwest China.Results show the effectiveness of the proposed EWT-Yoshida method in detecting the SSO and estimating its parameters.

Subsynchronous oscillation monitoring and alarm method based on phasor measurements

Owing to the large-scale grid connection of new energy sources, several installed power electronic devices introduce sub-/supersynchronous inter-harmonics into power signals, resulting in the frequent occurrence of subsynchronous oscillations(SSOs). The SSOs may cause significant harm to generator sets and power systems;thus, online monitoring and accurate alarms for power systems are crucial for their safe and stable operation. Phasor measurement units(PMUs) can realize the dynamic real-time monitoring of power systems. Based on PMU phasor measurements, this study proposes a method for SSO online monitoring and alarm implementation for the main station of a PMU. First, fast Fourier transform frequency spectrum analysis is performed on PMU current phasor amplitude data to obtain subsynchronous frequency components. Second, the support vector machine learning algorithm is trained to obtain the amplitude threshold and subsequently filter out safe components and retain harmful ones. Finally, the adaptive duration threshold is determined according to frequency susceptibility, amplitude attenuation, and energy accumulation to decide whether to transmit an alarm signal. Experiments based on field data verify the effectiveness of the proposed method.

双馈风电场无串补并网振荡场景及关键影响因素研究

双馈风电场经串补线路送出系统存在次同步振荡风险的问题已为共知并得到广泛深入研究,然而实际系统中也发生过双馈风电场经无串补线路送出的次超同步振荡问题,目前针对这一问题的报道和研究很少。因此,采用特征模式分析法结合电磁暂态仿真,针对双馈风电无串补接入电网的次同步振荡问题开展研究,分析了系统主导失稳振荡模式以及运行条件和风电控制参数的影响,厘清了无串补并网系统中双馈风电网侧变流器内环电流控制主导失稳,锁相控制主导失稳,以及转子侧变流器内环电流控制主导失稳等3种次超同步振荡场景,并与双馈经串补送出系统振荡场景进行了对比分析,较为全面地揭示了双馈风电并网系统振荡的不同场景及机理,可为实际工程双馈风电场并网振荡问题分析提供参考。

HVDC输电系统次同步振荡(SSO)分析的GUI实现

为提高研究效率并普及研究结果,开发了一个用于分析HVDC输电系统次同步振荡(SSO)特性的图形用户界面(GUI)软件。这个GUI软件由人机界面及相关的回调函数组成,其中人机界面是利用MATLAB的GUI工具设计的,而编写回调函数则利用了MATLAB的数值计算和数据可视化功能。实践表明在MATLAB环境下可容易、高效地开发出令人满意的GUI软件。通过GUI软件可实现SSO参数稳定域可视化分析及特征值分析和时域仿真分析,具有直观、方便的优点,适合广大技术人员对HVDC输电系统发生SSO失稳可能性的检验。

Improved Subsynchronous Oscillation Parameter Identification with Synchrophasor Based on Matrix Pencil Method in Power Systems

The subsynchronous oscillations(SSOs)related to renewable generation seriously affect the stability and safety of the power systems.To realize the dynamic monitoring of SSOs by utilizing the high computational efficiency and noise-resilient features of the matrix pencil method(MPM),this paper propos es an improved MPM-based parameter identification with syn chrophasors.The MPM is enhanced by the angular frequency fitting equations based on the characteristic polynomial coeffi cients of the matrix pencil to ensure the accuracy of the identi fied parameters,since the existing eigenvalue solution of the MPM ignores the angular frequency conjugation constraints of the two fundamental modes and two oscillation modes.Then,the identification and recovery of bad data are proposed by uti lizing the difference in temporal continuity of the synchropha sors before and after noise reduction.The proposed parameter identification is verified with synthetic,simulated,and actual measured phase measurement unit(PMU)data.Compared with the existing MPM,the improved MPM achieves better accuracy for parameter identification of each component in SSOs,better real-time performance,and significantly reduces the effect of bad data.

宁夏多直流外送系统SSO特性分析及次同步阻尼控制器设计

针对宁夏电网近电气距离耦合的火电机群与多直流输电的简化等值送端系统,利用基于复频域端口等效导纳矩阵的复转矩系数法对其进行次同步振荡(SSO)特性分析。研究表明,降低宁夏电网直流输电外送功率,加强宁夏电网送端交流系统,增强直流换流站之间电气耦合强度等均有利于改善系统SSO阻尼特性。其次,针对存在SSO风险的灵武二期电厂机组,开展直流输电附加次同步阻尼控制器设计,并通过仿真证明了其有效性。该文研究成果可为宁夏电力外送系统的运行、规划及SSO抑制提供技术支撑和参考。

多HVDC输电系统SSO抑制措施的协调控制研究

近年来,HVDC输电系统引起的次同步振荡(SSO)现象引起了众多专家学者的重视,附加次同步阻尼控制器(SSDC)是目前较为普遍的抑制措施。故以贵州电网多HVDC交直流混合输电系统为背景,对SSDC的协调控制问题进行了较为深入的探索。理论分析和大量仿真验证表明,SSDC和附加次同步励磁控制器(SEDC)提供的电气阻尼是解耦的,可以分别进行整定。而对配置在不同直流线路换流站处的多台SSDC进行整定时,协调控制是十分必要的。同时,随着通信技术的不断发展,取发电机组转速作为SSDC的输入信号会达到更理想的抑制效果。

Sub-synchronous Oscillations in Power Systems Caused by Grid-connected Wind Farms--A Survey of Mechanism Studies

Sub-synchronous oscillation(SSO)incidents caused by dynamic interaction between wind farms and power system grids can threaten safe and reliable operations.This paper investigates mechanism studies of SSOs in power systems induced by grid-connected wind farms.First,characteristics of several severe SSO incidents encountered by real power systems caused by grid-connected wind farms are introduced.Second,in terms of principles and responses,this paper illustrates the most commonly adopted mechanism investigation methods for SSOs,which include impedance-based analysis(IMA)and the openloop modal analysis(OMA)methods.Systematic reviews that summarize existing studies using the IMA and OMA methods are discussed in this paper.Furthermore,a discussion of the current research gaps in the investigation of SSOs in power systems caused by grid-connected wind farms is presented.

基于次同步模态能量的双馈风机并网系统次同步振荡能量分析与溯源方法研究

该文提出一种次同步模态能量(sub-synchronous modal energy,SSME)方法,用于双馈感应发电机(doubly-fed induction generators,DFIG)并网电力系统的次同步振荡(sub-synchronous oscillation,SSO)分析和溯源。首先,通过广义哈密顿理论构建并分析了DFIG并网电力系统的SSME结构,证明了DFIG子系统或外送线路子系统的SSME方向是判断SSO源是否在该子系统内的充分必要条件。在此基础上,提出一种基于子系统SSME的SSO溯源方法,通过监测子系统SSME的方向,可以判断SSO源是否在子系统内。最后,构造了负阻尼与强迫类型的SSO场景以证明所提出的基于SSME的溯源方法的性能,并与现有暂态能量流(transient energy flow,TEF)方法进行了比较。时域仿真结果验证提出的SSO溯源方法的有效性,并表明其比传统的TEF方法具有更好的可靠性。此外,基于该文的SSME方法有望实现风机振荡参与度评估。

适用于次同步振荡分析的直驱风电场平衡降阶方法

直驱风电场(direct-drive wind farm,DDWF)并入弱交流系统存在发生次同步振荡(sub-synchronous oscillation,SSO)的风险。由于DDWF模型阶数较高,实际分析时须降阶处理。现有平衡降阶方法在分析SSO问题时,无法在降阶过程中精确保留系统SSO模式。针对此问题,文中提出一种适用于SSO分析的平衡降阶方法。该方法基于参与因子分析,保留与SSO模式强相关的状态变量,并结合Hankel奇异值确定降阶阶数,建立与全阶系统模型SSO特性和动态特性都一致的降阶系统模型。首先,对DDWF并入弱交流系统的全阶小信号模型进行平衡变换,建立平衡系统模型。然后,对平衡系统模型进行参与因子分析,结合Hankel奇异值确定保留状态变量集合,再利用残差降阶法建立降阶系统模型。最后,对全阶系统模型和降阶系统模型进行对比验证,结果表明所提降阶方法适用于DDWF并入弱交流系统SSO问题的研究。

基于滑模控制的直驱风电场次同步振荡抑制策略

当直驱风电场(DDWF)并入弱交流系统中发生次同步振荡(SSO)时,网侧变流器(GSC)电流内环的PI控制器会放大DDWF并网电流中所含的次同步电流分量,进而形成由交流系统、GSC及其控制系统构成的次同步电流助增正反馈回路,最终导致系统SSO失稳。针对这一问题,该文提出一种基于GSC电流内环滑模控制(SMC)的直驱风电场SSO抑制策略。首先,建立DDWF并入弱交流系统的动态模型,推导由PI控制器主导的DDWF并入弱交流系统SSO的形成机理。然后,为了切断系统的次同步电流助增正反馈回路,设计基于指数趋近律的SMC控制器,并以SMC控制器替换原GSC电流内环中的PI控制器,通过理论推导证明GSC电流内环SMC控制器能有效抑制系统SSO。最后,在PSCAD/EMTDC中,以DDWF并入弱交流系统作为算例,验证所提SSO抑制策略的有效性。

基于路径分析法的直驱风电场经柔直并网系统次同步振荡机理分析

针对直驱风电场经柔性直流输电(voltage source converter based high voltage direct current,VSC-HVDC)并网系统的次同步振荡(subsynchronous oscillation,SSO)问题,目前研究在机理分析中难以揭示子系统之间的扰动传递过程和耦合关系,同时难以量化不同特性对于系统稳定的影响。因此,基于路径分析法开展系统的SSO机理研究,并以锁相环(phase-locked loop,PLL)振荡模态为例开展路径分析法的模态扩展。首先,采用模块化建模方法建立系统的线性化模型。其次,将转子运动方程的阻尼系数推广至振荡模态的主导元件动态方程,从而获取系统的稳定判据。然后,基于路径分析揭示子系统之间的扰动传递过程和耦合关系,基于阻尼分解量化风电场内部特性以及风电场与VSC-HVDC之间交互作用特性对于系统稳定的影响。最后,开展PLL控制参数对于阻尼特性的影响分析。结果表明:路径分析法可以应用于不同的振荡模态;直驱风电场与VSC-HVDC之间的扰动传递路径呈现双闭环耦合关系;通过增大直驱风电机组PLL比例系数或减小直驱风电机组PLL积分系数可以有效提高系统的总阻尼系数,从而提升系统的稳定性。

汽轮发电机组简单轴系模型扭转刚度的简易计算

研究汽轮发电机组扭振问题及次同步谐振/振荡时需使用“简单多质量块-弹簧”模型,其中扭转刚度的准确性十分重要。在已知转动惯量、固有模态频率的前提下,提出一种扭转刚度的简易计算方法,该方法根据已有资料给出扭振振型矩阵的估计值,利用模态向量的正交性求解模态扭转刚度矩阵估计值,反算出扭转刚度;考虑计算结果可能存在偏差,再采用牛顿-拉夫逊方法进行修正。给出沙角C厂1号机组的算例,解决了该机组因为缺失扭转刚度数据导致扭振问题不能详细分析、扭振保护设备不能投运的问题。所提方法计算量小、易于实现,也可以用于扭振保护在实测模态频率之后对轴系模型参数的修正。

双馈直驱风机混合接入MMC-HVDC送端系统的次同步振荡特性分析与抑制

双馈/直驱风机混合接入模块化多电平高压直流输电(modular multi-level converter based HVDC,MMC-HVDC)的次同步振荡事件频繁发生,已有双馈或直驱某一种风机与MMC-HVDC交互产生振荡的研究,但鲜有针对双馈/直驱风机混合接入MMC-HVDC的次同步振荡分析与抑制措施研究。该文首先阐述并分析了某双馈/直驱风机混合接入MMC-HVDC后发生次同步振荡实际事件的特征与原因,通过阻抗相量方法研究了双馈/直驱风机频域阻抗的交互影响方式。研究表明,在双馈或直驱风机单独接入MMC-HVDC不存在振荡风险的条件下,双馈和直驱风机并联接入后依然可能存在振荡风险。最后,根据电网调度运行的实际要求,提出了考虑机组启停数量调配的临时抑制振荡策略。

基于改进固有时间尺度分解算法的实时次同步振荡监测方法

为解决大规模风力发电并网系统中频繁发生次同步振荡的问题,需要快速准确地识别和检测次同步振荡的方法。次同步振荡发生时具有时变性和不确定性等特征,这给振荡的实时监测带来了挑战,针对该问题,首先提出了基于引入代数估计法改进的固有时间尺度分解算法(intrinsic time-scale decomposition,ITD)的解决方案。该方法不需要任何先验信息,且其性能不受振荡频率构成的影响。其次,利用合成信号、电磁暂态仿真和振荡实测数据进行了综合对比研究,结果表明该方法在信号检查的动态性能和参数估计精度等方面都取得了良好的效果。最后,通过硬件在环测试,验证了该方法的可行性。

基于模糊控制的双馈风电机组次同步振荡自适应抑制策略

双馈感应发电机与输电线路串联电容补偿之间的次同步控制相互作用(SSCI)是引发双馈风电交流并网系统次同步振荡(SSO)的主要原因。该文通过建立双馈风电交流并网系统的阻抗模型,分析风电机组不同控制参数对系统SSO的影响程度,提出一种可等效电流环控制参数调整且兼顾基频控制性能的附加阻尼控制方法。为提升对振荡模态迁徙的适应性和鲁棒性,进一步将实时测频与模糊控制相结合,形成一种可根据振荡频率自动调整附加阻尼控制参数的SSO自适应抑制策略。最后,基于Matlab/Simulink仿真平台,验证该文所提出的SSO自适应抑制策略的正确性与有效性。