Coordinated Rotor-Side Control Strategy for Doubly-FedWind Turbine under Symmetrical and Asymmetrical Grid Faults

In order to solve the problems of rotor overvoltage,overcurrent and DC side voltage rise caused by grid voltage drops,a coordinated control strategy based on symmetrical and asymmetrical low voltage ride through of rotor side converter of the doubly-fed generator is proposed.When the power grid voltage drops symmetrically,the generator approximate equation under steady-state conditions is no longer applicable.Considering the dynamic process of stator current excitation,according to the change of stator flux and the depth of voltage drop,the system can dynamically provide reactive power support for parallel nodes and suppress the rise of DC side voltage and rotor over-current.When the grid voltage drops asymmetrically,the positive and negative sequence components are separated in the rotating coordinate system.The doubly fed generator model is established to suppress the rotor positive sequence current and negative sequence current respectively.At the same time,the output voltage limit of the converter is discussed,and the reference value is adjusted within the allowable output voltage range.In order to adapt to the occurrence of different types of power grid faults and complex operating conditions,a fast switching module of fault type detection and rotor control mode is designed to detect the type of power grid faults and voltage drop depth in real time and switch the rotor side control mode dynamically.Finally,the simulation model of the doubly fed wind turbine is constructed in Matlab/Simulink.The simulation results verify that the proposed control strategy can improve the low-voltage ride through performance of the system when dealing with the symmetrical and asymmetric voltage drop of the power grid and identify the power grid fault type and provide the correct control strategy.

Power Network Asymmetrical Faults Analysis Using Instantaneous Symmetrical Components

Although the application of Symmetrical Components to time-dependent variables was introduced by Lyon in 1954, for many years its application was essentially restricted to electric machines. Recently, thanks to its advantages, the Lyon transformation is also applied to power network calculation. In this paper, time-dependent symmetrical components are used to study the dynamic analysis of asymmetrical faults in a power system. The Lyon approach allows the calculation of the maximum values of overvoltages and overcurrents under transient conditions and to study network under non-sinusoidal conditions. Finally, some examples with longitudinal asymmetrical faults are illustrated.

Asymmetrical Fault Current Calculation Method and Influencing Factors Analysis of Droop-Controlled Inverters

Since the fault dynamic of droop-controlled inverter is different from synchronous generators (SGs), protection devices may become invalid, and the fault overcurrent may damage power electronic devices and threaten the safety of the microgrid. Therefore, it is imperative to conduct a comprehensive fault analysis of the inverter to guide the design of protection schemes. However, due to the complexity of droop control strategy, existing literatures have simplified asymmetric fault analysis of droop-controlled inverters to varying degrees. Therefore, accurate fault analysis of a droop-controlled inverter is needed. In this paper, by analyzing the control system, an accurate fault model is established. Based on this, a calculation method for instantaneous asymmetrical fault current is proposed. In addition, the current components and current characteristics are analyzed. It was determined that fault currents are affected by control loops, fault types, fault distance and nonlinear limiters. In particular, the influences of limiters on the fault model, fault current calculation and fault current characteristics were analyzed. Through detailed analysis, it was found that dynamics of the control loop cannot be ignored, the fault type and fault distance determine fault current level, and part of the limiters will totally change the fault current trend. Finally, calculation and experimental results verify the correctness of the proposed method.

Influence of fault asymmetric dislocation on the gravity changes

A fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement along the fractures as a result of earth movement. Large faults within the Earth＇s crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, energy release associated with rapid movement on active faults is the cause of most earthquakes. The relationship between unevenness dislocation and gravity changes was studied on the theoretical thought of differential fault. Simulated observation values were adopted to deduce the gravity changes with the model of asymmetric fault and the model of Okada, respectively. The characteristic of unevenness fault momentum distribution is from two end points to middle by 0 according to a certain continuous functional increase. However, the fault momentum distribution in the fault length range is a constant when the Okada model is adopted. Numerical simulation experiments for the activities of the strike-slip fault, dip-slip fault and extension fault were carried out, respectively, to find that both the gravity contours and the gravity variation values are consistent when either of the two models is adopted. The apparent difference lies in that the values at the end points are 17.97% for the strike-slip fault, 25.58% for the dip-slip fault, and 24.73% for the extension fault.

DC System Modeling and AC/DC System Harmonic Calculation under Asymmetric Faults

The accurate DC system model is the key to fault analysis and harmonic calculation of AC/DC system. In this paper, a frequency domain analysis model of DC system is established, and based on it a unified fundamental frequency and harmonic iterative calculation method is proposed. The DC system model is derived considering the dynamic switching characteristic of converter and the steady-state response features of dc control system synchronously. And the proposed harmonic calculation method fully considers the AC/DC harmonic interaction and fault interaction under AC asymmetric fault condition. The method is used to the harmonic analysis and calculation of CIGRE HVDC system. Compared with those obtained by simulation using PSCAD/EMTDC software, the results show that the proposed model and method are accurate and effective, and provides the analysis basis of harmonic suppression, filter configuration and protection analysis in AC/DC system.

Seismic Responses of Asymmetric Base-Isolated Structures under Near-Fault Ground Motion

An inter-story shear model of asymmetric base-isolated structures incorporating deformation of each isolation bearing was built, and a method to simultaneously simulate bi-directional near-fault and far-field ground motions was proposed. A comparative study on the dynamic responses of asymmetric base-isolated structures under near-fault and far-field ground motions were conducted to investigate the effects of eccentricity in the isolation system and in the superstructures, the ratio of the uncoupled torsional to lateral frequency of the superstructure and the pulse period of near-fault ground motions on the nonlinear seismic response of asymmetric base-isolated structures. Numerical results show that eccentricity in the isolation system makes asymmetric base-isolated structure more sensitive to near-fault ground motions, and the pulse period of near-fault ground motions plays an import role in governing the seismic responses of asymmetric base-isolated structures.

改进卷积胶囊网络的滚动轴承故障诊断方法

目前许多基于卷积网络的滚动轴承故障诊断方法受噪声信号以及负荷变化的影响,存在诊断效果不佳、泛化能力差的问题。针对此问题提出一种改进卷积胶囊网络的滚动轴承变工况故障诊断方法。该方法设计了多尺度非对称卷积模块,其中采用不同尺度的非对称卷积层对输入数据进行特征提取,在实现最大化提取数据中的特征信息的同时,还能够有效减少参数量;在该模块中引入通道注意力机制,能更好地提取有用的通道特征,提高该方法特征提取的能力;通过将网络中的全连接层改进为胶囊全连接层,使得胶囊在输出向量特征信息时,避免了特征信息在空间中的丢失。使用凯斯西储大学轴承数据集和东南大学变速箱数据集来验证所提方法的诊断性能,并与其他深度学习方法进行了比较。实验结果表明,与其他深度学习方法相比,具有较好的泛化性,效果更佳。

不对称短路故障下永磁直驱风电机组并网控制策略研究

电网侧发生不对称故障时,永磁直驱风电机组并网电流会发生三相不平衡的问题。针对这一问题,提出一种基于自适应陷波器的网侧换流器控制策略。该控制策略基于解决机组并网电流二倍频分量的思想,在电网侧发生不对称短路故障时,将进入逆变器的dq坐标系下的二倍频电压分量去除,从而使得网侧换流器输出的电流达到三相平衡。本文首先分析了电网侧发生不对称短路故障后永磁直驱风力发电机组的动态响应;然后建立了机侧和网侧换流器的模型,并分别采用了双闭环矢量控制技术和基于自适应陷波器的控制策略;最后进行仿真分析,验证了改进后的控制策略相较于传统控制策略,有效解决了不对称短路故障下并网电流三相不平衡问题。

光储联合发电系统不对称故障穿越控制策略

针对电网不对称故障下光伏发电系统输出功率与直流侧电压二倍频波动问题,提出了一种光储联合发电系统低电压穿越控制策略。在低电压穿越期间,充分利用光伏并网逆变器和储能变流器容量向电网输送有功和无功功率;在此基础上令储能变流器采取基于负序虚拟电导的负序电压补偿控制策略;同时,分析了单台以及多台储能变流器参与负序电压补偿的原理,并对负序虚拟电导进行了自适应优化设计。仿真结果表明,所提控制策略能够有效削弱光伏发电系统输出功率与直流侧电压的二倍频波动。

基于改进MobileNet的串联电弧故障检测方法

针对非线性负载和复杂组合负载发生串联电弧故障时检测效果不理想,提出一种基于改进轻量型网络MobileNet的串联电弧故障检测方法。根据磁通不对称分布将中性线和相线同时穿过电流互感器获取高频电流耦合信号,通过在实验室模拟大量电弧故障实验,获得国标规定7种负载与常见家用电器5种负载的各类电弧故障样本,运用数据增强生成数据集进行训练并测试,最后将串联电弧故障检测网络模型搭载于电弧故障保护样机上,进行在线检测测试。实验结果表明,所提方法能有效识别串联电弧故障,且有利于实现电弧故障保护器的产品化。

网压故障下光储系统安全域及控制策略研究

电网侧电压发生不对称跌落时,为保证光储系统的并网安全,基于逆变器运行约束构建安全运行域,并提出输出控制策略。阐述正、负序分离的控制原理,在满足并网标准规定的无功电流基础上,以相电流幅值、相电压幅值和有功波动幅值为约束,构建各工况下的安全运行域;提出正、负序有功电流固定比例控制和最大化输出有功控制策略,满足并网要求且提升系统的稳定性。基于Matlab/Simulink仿真平台,构建光储系统仿真模型,验证所提策略的有效性。

不对称故障下低电压穿越的多目标解耦控制策略

在传统不对称故障低电压穿越控制中,由于控制自由度有限,并网逆变器控制存在无法同时实现输出电流负序分量和直流侧电压二倍频波动抑制的问题。对此,文中提出一种不对称故障下两级式光伏并网系统低电压穿越的多目标解耦控制策略。该策略将逆变器的控制目标设置为输出电流负序分量抑制,给出了综合考虑逆变器输出电流限幅和无功输出需求的逆变器电流内环控制参考值计算方法;通过双向Buck-Boost变换器将超级电容接入直流母线电容两端维持其电压稳定,并将直流侧电压二倍频波动转移至超级电容输入侧进行抑制。仿真结果表明,相比传统控制方法,所提控制策略有效降低了逆变器三相间的不平衡度,改善了输出电流畸变,减小了直流侧电压二倍频波动。

电力系统中非对称接地和断线故障分析

将对称分量法应用到中性点不接地电网非对称故障分析中,用于计算电网各位置的故障后电流和电压,并首次应用零序序网图解释中性点偏移的物理含义。然后本文系统介绍了中性点接地和不接地电网非对称故障中负序和零序分量的分布问题,最后对接地短路和断线故障进行了对比分析。

网压不对称下MMC多目标控制参数优化选择研究

为解决多目标保护控制中调节参数难以确定的问题,引入有功功率波动和交流电流波动的权重因子,构建线性加权表达式;为获取最优权重因子,深入分析MMC(模块化多电平换流器)系统安全运行条件对权重选取的限制,建立权重优化选取模型。在此基础上提出一种MMC多目标协调控制策略,以抑制有功功率波动与协调交流电流不平衡度为目标,考虑系统最大允许电流、桥臂电容电压允许最大波动,不需要为了抑制过流、桥臂电压过压而额外增加控制环节。仿真实验表明了所提策略的可行性与有效性。

海上直驱永磁风电系统非对称故障主动识别方法

传统风电系统故障识别方法容易受到线电压误差影响出现误判,导致故障数据识别结果精度不高,因此提出海上直驱永磁风电系统非对称故障主动识别方法。分析非对称故障下风电系统的运行特性。根据连续时间信号,采用主动学习方法消除信号基频。使用逆向扩散法调整主动学习参数,判断故障态、预警态事件。利用电压幅值和时间宽度双重标准分析线电压误差,采用数据互校验方法确认故障事件可信性,完成故障主动识别。实验结果表明,所提方法能够主动发现风电系统中的电流异常情况和谐波畸变程度,直流分量最大误差仅为0.05 A。

基于Hexverter的AC/AC系统交流不对称故障控制策略研究

六边形模块化多电平换流器(hexagonal modular multilevel converter,Hexverter)作为一种新型的AC/AC功率变换器,在未来海上风电场并网及低频输电技术中有着广阔的应用前景。为了解决Hexverter在电力系统不对称故障工况的稳定运行,文章首先对Hexverter的拓扑结构和工作原理简要介绍,并对其支路电流、支路功率进行分析。然后,研究其AC/AC系统交流侧不对称故障工况下的控制策略,基于理论推导部分得出的Hexverter六个支路间功率分布的规律(即不相邻的三个支路功率分别相等的特征),设计了子模块电容电压均衡控制方法,共包含两部分控制,分别为六个支路间子模块电容电压均衡控制和每个支路内部n个子模块电容电压之间的均衡控制。考虑到支路电流含有两侧交流系统不同频率成分,基于比例谐振控制器,提出了一种对支路电流直接进行控制的方法。最后,通过算例中稳态工况与故障工况研究分别对其工作原理和文中所提出的控制策略有效性进行验证。结果表明,在二次侧交流系统发生BC相短路故障期间,整个系统保持稳定运行,各个子模块电压始终保持均衡,仿真结果验证了文中所提控制策略的有效性。

Fault Ride-Through(FRT)Behavior in VSC-HVDC as Key Enabler of Transmission Systems Using SCADA Viewer Software

The world’s energy consumption and power generation demand will continue to rise.Furthermore,the bulk of the energy resources needed to satisfy the rising demand is far from the load centers.The aforementioned requires long-distance transmission systems and one way to accomplish this is to use high voltage direct current(HVDC)transmission systems.The main technical issues for HVDC transmission systems are loss of synchronism,variation of quadrature currents,amplitude,the inability of station 1(rectifier),and station 2(inverter)to either inject,or absorb active,or reactive power in the network in any circumstances(before a fault occurs,during having a fault in network and after a fault cleared),and the variations of power transfer capabilities.Additionally,faults impact power quality such as voltage dips and power line outage time.This paper presents a method of overcoming the aforementioned technical issues using voltage-source converter(VSC)based HVDC transmission systems with SCADA VIEWER software and dynamic grid simulator.The benefits include having a higher capacity transmission system and proposed best method for control of active and reactive power transfer capabilities.Simulation results obtained using MATLAB validated the experimental results from SCADA Viewer software.The results indicate that the station’s rectifier or inverter can either inject or absorb either active power or reactive power in any circumstance.Also,the reverse power flow under different modes of operation can ride through faults.At a 100.0%power transfer rate,the rectifier injected 775.0 W into the network.At a 0.0%power transfer rate,the rectifier injected 164.0 W into the network.At a-100.0%rated power,the rectifier injected 1264.0 W into the network and direction was also changed.

Research on Formation Mechanism of Asymmetric Basins, Taking the Weihe Basin as an Example

Asymmetry of the Weihe Basin is discussed in the paper,and also the master control fault,secondary control fault of asymmetric basin is proposed in the paper.The asymmetry of the Weihe Basin is expressed as follows:(1) its shape of the cross sections is asymmetrical;(2) the tectonic activity of the southern margin fault and the northern margin one is apparently different;(3) its deep tectonics is asymmetrical.Finally,we use the Weihe Basin as an example to establish a "cantilever-beam" model for calculations.The results show that:(1) flexure leads to stress accumulation and forming extensional fractures;(2) fractures slope steeply towards the free end;(3) when the length of beam becomes longer,it is possible that new extensional fractures will occur in the fixed end.

三相分布参数不对称配电线路接地故障检测与消弧技术综述

配电线路普遍存在具有随机差异性的对地分布参数不对称,现有接地故障保护原理往往在配电线路三相分布参数严格对称的基础上提出,如何提升不对称配电线路接地故障检测与消弧的可靠性是当前故障保护应重点解决的问题。该文立足于国内外配电网接地故障保护的研究进展,首先阐述了配电线路三相分布参数不对称形成的因素,探讨了配电线路三相分布参数不对称对系统零序等效模型、零序电压轨迹和绝缘参数测量的影响。然后,论证了配电线路三相分布参数不对称对零序电流和接地电流的影响,梳理归纳了在不对称影响下故障检测与消弧的研究现状。最后,对计及配电线路三相分布参数不对称随机差异性的单相接地故障检测与消弧发展趋势进行了总结与展望,以期为未来接地故障的研究方向提供参考和建议。

基于零序分量的风电场集电线不对称接地故障定位

风电场集电线上并接电源多、电源故障特性复杂,其故障定位问题持续困扰风电运营商。为此提出一种基于零序分量的集电线不对称接地故障新型定位方案。在定义无分支区域的基础上,充分考虑风机与风电场零序分量之间的特殊关系,分层实现风机无关的故障无分支区域识别与故障测距。首先,引入无分支区域状态信息的概念以反映零序电流主流通路径,通过状态信息的优化匹配识别故障无分支区域。然后,从零序阻抗出发推导故障无分支区域测距模型,并将其转化为优化问题实现故障距离求解。所提方案在较少测点下即可完全消除风机影响,并能有效适用于含分支集电线,同时无需数据严格同步,测距不会出现多解。最后,还设计了线路参数在线计算方案应对线路参数变化问题。仿真结果表明,所提方案较为准确,性能不受风机、故障位置、故障类型、过渡电阻以及数据不同步影响,整体性能优于现有方案。同时也验证了线路参数在线计算的有效性。