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the power grid voltage drops symmetrically,the generator approximate equation under steady-state conditions is no longer applicable.Considering the dynamic process of stator current excitation,according to the change of stator flux and the depth of voltage drop,the system can dynamically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.

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.

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.

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.

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.

Conjugate Vectors Method Applied to Asymmetrical Fault Analysis of Power Electronized Power Systems

With the wide application of power electronized resources(PERs),the amplitude and frequency of voltages show significant time-varying characteristics under asymmetrical faults.As a result,the traditional phasor model,impedance model,and symmetrical components method based on the constant amplitude and frequency of voltages are facing great challenges.Hence,a novel asymmetrical fault analysis method based on conjugate vectors is proposed in this paper which can meet the modeling and analysis requirements of the network excited by voltages with time-varying amplitude/frequency.Furthermore,asymmetrical fault characteristics are extracted.As an application,a faulted phase identification(FPI)strategy is proposed based on the fault characteristics.The correctness and superiority of the asymmetrical fault analysis method and FPI strategy are verified in time-domain simulations and a real-time digital simulator.

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.

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。

模块化多电平矩阵换流器接入下的工频故障特性分析

基于模块化多电平矩阵换流器(M3C)的柔性低频输电系统在中远海风电外送、异步电网互联等方面优势突出。然而,有别于模块化多电平换流器(MMC)等其他电力电子换流设备,M3C的控制策略将导致系统故障特性发生显著变化。现有将电力电子设备负序部分等效为开路的故障等值与响应特性计算方法无法适用。文中计及M3C正序定电容电压总平均值控制、负序电流注入控制以及环流控制分层构成的桥臂电容电压均衡策略,推导了工频侧线路故障以后,M3C输入输出功率均衡、桥臂功率均衡(电容电压均衡的必要条件)与正负序电压电流间的映射关系,由此构建了工频侧线路故障(不对称、对称)后的M3C输出电压电流响应特性的等效模型。在此基础上,提出了M3C接入下工频侧线路故障后电压电流响应特性的精确计算方法。最后,基于PSCAD/EMTDC仿真平台,搭建了海上风电柔性低频送出系统仿真模型,通过大量仿真算例充分验证了所提故障特性分析方法的正确性和精确性。

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。

基于双层负序差值与负序测距的风电场集电线不对称故障定位

针对风电场集电线上复杂电源密集接入使得现有定位方法应用困难的问题,提出一种基于双层负序差值与负序测距的风电场集电线故障定位方案。首先,深入分析风机故障电流特性,从负序分量出发实现风机建模,同时计及风电场结构特点对其负序网络实施等效。然后,引入故障模拟策略,通过负序电压差值表征模拟故障与实际故障的差异以识别故障区域第一节点,进一步探究从实际故障场景到特定模拟故障场景的负序电压变化趋势,并定义负序电压比消除故障点电流影响,由此提出负序电压比差值判据以实现故障区域第二节点识别。最后,针对故障区域利用双端负序量推导测距公式确定故障位置。PSCAD/EMTDC验证表明,所提方案能够以较少测点计及含线路分支集电线的适用性准确定位集电线各类不对称故障,且定位效果不受故障位置、过渡电阻、风机、风速分布及相量不同步等因素影响。

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。

基于二阶线性自抗扰的柔性直流电网控制策略

针对柔性直流电网中某换流站交流侧发生不对称故障时产生负序电流,引起瞬时功率和直流侧电压、电流较大波动继而影响系统稳定运行的问题,提出了一种基于线性自抗扰控制(linear active disturbance rejection control,LADRC)的柔性直流电网换流站级控制策略。该策略采用比例积分(proportional-integral,PI)控制器和二阶LADRC控制器分别进行外环控制和内环电流控制,并以模块化多电平换流器(modular multilevel converter,MMC)为被控对象,针对二阶LADRC控制器进行频域分析,确定控制器参数的合理取值。最后,利用PSCAD/EMTDC平台搭建了四端柔性直流电网模型进行算例分析。结果表明:相比于传统PI控制,所提控制策略有效抑制了负序电流,大幅减小了有功、无功功率和直流侧电压的二倍频波动,提高了系统的故障穿越能力和动态响应速度。

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。

不对称短路故障下多风电场电压控制方法研究

由于风电场往往处在电网末端,与负荷中心呈现逆分布特点,系统在不对称短路故障条件下具有较差的电压支持能力,风电机组的低电压运行适应性能受到严峻挑战。因此,有必要进一步结合风电并网导则,研究提升电网故障下多风电系统低电压运行适应能力的控制方法。论文首先建立了多风电系统的正负序等效电路,分析了影响系统公共连接点电压的因素。其次,分析了不对称故障下单风电场电压控制的限制条件,然后以改善不同运行场景下风电系统低电压运行适应能力为目标,提出了多风电系统暂态电压协同控制方法。最后,建立了多风场暂态电压控制仿真计算模型,仿真结果表明所提方案可有效改善公共连接点暂态电压水平,并灵活实现最大风能跟踪,为有效提升多风电系统的低电压运行适应能力奠定基础。

Transient synchronous stability analysis and enhancement control strategy of a PLL-based VSC system during asymmetric grid faults

The stability of a voltage source converters(VSC)system based on phase-locked loop(PLL)is very important issue during asymmetric grid faults.This paper establishes a transient synchronous stability model of a dual-sequence PLL-based VSC system during low voltage ride-through by referring to the equivalent rotor swing equation of syn-chronous generators.Based on the model,the synchronization characteristics of the VSC system under asymmetric grid faults are described,and the interaction mechanisms,as well as the transient instability phenomena of positive and negative sequence PLL during asymmetric faults are explained.Using the equal area criterion,the influences of sequence control switching action,detection delay,and interaction between the positive and negative sequence PLL on the transient synchronous stability of the VSC system are analyzed,respectively.In addition,a transient stabil-ity assessment criterion based on the critical fault clearance angle and time and an enhancement control strategy based on the improved positive and negative sequence PLL are proposed.Finally,the analytical results are validated through simulation and experiments.