基于模糊集合理论的电力系统振荡与短路的识别

将模糊集合理论中模糊隶属度的概念应用到电力系统振荡与短路的识别中,通过提取恰当的特征量并赋以一定的权值,对短路、振荡及振荡伴随短路这三种情况进行综合判断.仿真分析结果证明该方法的效果是非常明显的,识别的准确程度很高.     

用模糊集合理论识别电力系统振荡中的短路的研究

本文通过对电力系统振荡和短路所呈出的不同特征的分析,从故障特征入手,利用模糊集合理论来识别振荡过程中发生的短路,对于振荡中的三相短路的识别,本文提出一种基于振荡中心电压的波形跟踪识别方法,对于振荡中的不对称故障的识别,本文选取模变换中的Ｃｌａｒｋ变换作为数学工具,与对称分子量变换相结合,构造了Ｉα／Ｉβ判据,Ｉａ－Ｉβ与序分量组合判据,文章给出了相应的模糊数学模型,并经大量的仿真试验,获得良好的仿     

区分电力振荡与短路信号的小波算法

振荡与短路是电力系统中的两种不同现象.继电保护装置需要快速、准确地区分电力系统振荡与短路,要在继电保护误动作之前,将可能由于振荡引起的误动作的保护可靠闭锁.本文针对目前电力系统运行中出现的振荡与故障情况,依据在变电站端对三相电流检测的结果,结合小波分析的优点,提出了一种新的识别电力系统振荡与故障的模型.仿真实验表明,本文所设计的模型克服了以往方法计算效率低下、判别不准确的缺陷,并且监测具有实时性、监测变量少等优点,获得了比较满意的结果.     

应用模糊集理论识别电力系统振荡中不对称故障的新方法

根据电力系统振荡和短路所呈现出的不同特征,以模分量、三相及零序电流大小为依据,提出了一种基于模糊集理论的识别振荡中发生不对称故障的新方法,并给出了相应的模糊数学模型.动模实验和Matlab仿真计算结果验证了该方法的有效性和可靠性.     

振荡中不对称故障电流幅值判据系数的自适应修正

分析振荡过程中,利用三相电流和零序电流大小来识别不对称短路故障判别的原理.对判别式中的系数进行自适应的修正,使系数随故障点的改变而改变,从而使保护能够少带延时甚至不带延时出口动作.     

基于广域信息的振荡中再故障识别与定位

采用综合序分量构成差动电流,提出了一种差动保护原理用于识别互联电网振荡过程中再故障,能有效提高经大过渡电阻接地故障的检测灵敏度,改善振荡中心附近短路传统负序元件灵敏度低的问题.为进一步识别故障元件,利用配合关系矩阵描述智能电子设备(intelligent electronic device,IED)间的相互配合关系,并结合多点方向信息确定与故障元件直接相连的IED,这些IED保护对象的交集即为故障元件.算例分析表明,所提的广域保护原理简单易行,不受振荡频率偏移和系统运行方式影响,耐过渡电阻能力强,能很好实现振荡再故障判断及定位,作为现有振荡再故障保护的冗余补充能有效提高保护性能.     

并联碳化硅MOSFET短路振荡机理

碳化硅(SiC)金属氧化物半导体场效应晶体管(MOSFET)为高性能电力电子技术提供了技术保障,其短路承受能力是进一步提升电力电子变换器可靠性的关键;特别是在大功率场合,经常将SiC MOSFET并联使用,然而影响并联SiC器件短路振荡的关键因素并不十分明确,振荡机理有待进一步研究。此处以并联SiC MOSFET为研究对象,建立在短路工况下的等效数学模型,分析影响并联短路特性的关键因素并进行实验验证,归纳短路振荡机理。理论分析与实验结果表明,当并联SiC MOSFET发生短路故障时,栅极驱动电阻和功率回路杂散电感是导致器件并联系统振荡的主要因素,过小的栅极驱动电阻使得并联系统振荡频率和尖峰增大;过大的功率回路杂散电感导致系统振荡频率降低,而振荡尖峰增大,系统的剧烈振荡不利于SiC MOSFET稳定性提高。     

串联谐振式限流器的仿真研究

串联谐振式限流器具有工作原理、拓扑结构简单等优点,且对于高压系统,其一次回路参数能够做到合理配置;但在电网发生短路故障进入短路限流工况过程中,其电容器及并联转换开关将工作在相当恶劣的条件下。应用Matlab软件对上述情况进行了建模仿真研究,得出如下结论：①闭合转换开关短接电容器使限流器从正常运行模式切换到短路限流模式过程中,将在电容与转换开关之间产生电压（电流）的高频振荡,且振荡电压（电流）的幅值与转换开关的动作速度成振荡增幅关系;②当转换开关采用功率半导体器件构成时,要求控制系统快速响应短路故障闭合转换开关,否则极易因振荡过流（过压）而损坏;③在限流器进入短路限流模式但继电保护尚未动作于断路器切断故障回路期间,转换开关将承受全部短路电流以及振荡电流,采用电力电子型转换开关时,其功率半导体器件的容量必须按这种工况进行选择。     

串联谐振式限流器的仿真研究

串联谐振式限流器具有工作原理、拓扑结构简单等优点,且对于高压系统,其一次回路参数能够做到合理配置;但在电网发生短路故障进入短路限流工况过程中,其电容器及并联转换开关将工作在相当恶劣的条件下.应用Matlab软件对上述情况进行了建模仿真研究,得出如下结论:①闭合转换开关短接电容器使限流器从正常运行模式切换到短路限流模式过程中,将在电容与转换开关之间产生电压(电流)的高频振荡,且振荡电压(电流)的幅值与转换开关的动作速度成振荡增幅关系;②当转换开关采用功率半导体器件构成时,要求控制系统快速响应短路故障闭合转换开关,否则极易因振荡过流(过压)而损坏;③在限流器进入短路限流模式但继电保护尚未动作于断路器切断故障回路期间,转换开关将承受全部短路电流以及振荡电流,采用电力电子型转换开关时,其功率半导体器件的容量必须按这种工况进行选择.     

应用小波神经网络闭锁发电机失磁保护的研究

在发电机机端或主变高压侧发生短路以及振荡的情况下 ,发电机失磁保护都可能发生误动 ,因此在振荡与短路故障的情况下使失磁保护不误动是失磁保护所要解决的关键问题。基于小波变换和神经网络的优点 ,提出一种用小波神经网络来闭锁短路与振荡情况的方法。     

A novel power swing blocking scheme using adaptive neuro-fuzzy inference system

A power swing may be caused by any sudden change in the configuration or the loading of an electrical network. During a power swing, the impedance locus moves along an impedance circle with possible encroachment into the distance relay zone, which may cause an unnecessary tripping. In order to prevent the distance relay from tripping under such condition, a novel power swing blocking (PSB) scheme is proposed in this paper. The proposed scheme uses an adaptive neuro-fuzzy inference systems (ANFIS) for preventing distance relay from tripping during power swings. The input signals to ANFIS, include the change of positive sequence impedance, positive and negative sequence currents, and power swing center voltage. Extensive tests show that the proposed PSB has two distinct features that are advantageous over existing schemes. The first is that the proposed scheme is able to detect various kinds of power swings thus block distance relays during power swings, even if the power swings are fast or the power swings occur during single pole open conditions. The second distinct feature is that the proposed scheme is able to clear the blocking if faults occur within the relay trip zone during power swings, even if the faults are high resistance faults, or the faults occur at the power swing center, or the faults occur when the power angle is close to 180°.     

Comprehensive power swing detection by current signal modeling and prediction using the GMDH method.

Power swing is an undesirable variation in power flow. This can be caused by large disturbances in demand load, switching, disconnection or reclosing lines. This phenomenon may enter the zones of distance relays and cause relay malfunction leading to the disconnection of healthy lines and undermining network reliability. Accordingly, this paper presents a new power swing detection method based on the prediction of current signal with a GMDH (Group Method of Data Handling) artificial neural network. The main advantage of the proposed method over its counterparts is the immunity to noise effect in signals. In addition, the proposed method can detect stable, unstable, and multi-mode power swings and is capable of distinguishing them from the variety of permanent faults occurring simultaneously. The method is tested for different types of power swings and simultaneous faults using DIgSILENT and MATLAB, and compared with some latest power swing detection methods. The results demonstrate the superiority of the proposed method in terms of response time, the ability to detect power swings of different varieties, and the ability to detect different faults that may occur simultaneously with power swings.     

Improved phase selector for unbalanced faults during power swings using morphological technique

In order to prevent distance protection from tripping during power swing conditions, a power swing blocking device is often utilized. On the other hand, it has been an increasing requirement to achieve rapid clearance of internal faults during power swings. Accurate phase selection is a prerequisite to selective clearance of faults. An improved unbalanced fault phase selector during a power swing based on series multiresolution morphological gradient (SMMG) transform is proposed in this paper. As a feature extractor from raw signals, SMMG is employed to extract superimposed (fault component) current under the power swing condition in this paper. First, the operating characteristic of the sequence component fault phase selector during power swings is discussed. In order to overcome the disadvantage of the above selector, an improved fault phase selector for unbalanced faults during power swings is then proposed by using SMMG to extract superimposed components of modular current. The efficiency and feasibility of the proposed schemes are proven using a Power Systems Computer-Aided Engineering/Electromagnetic Transients including DC-based simulation on a sample power system.     

Analysis of angle stability problems: a transmission protection systems perspective

Postfault rotor angle oscillations lead to power swings. Both unstable and stable swings can induce distance relay tripping. For unstable swings, a new computational procedure to locate all of the electrical centers is developed. It simplifies the work associated with visual screening of all the R-X plots. For stable swings, a generic three-tier hierarchy of stability-related norms defined by branch norm, fault norm, and system norm is proposed. Ranking by branch norm leads to ranking of power swings. Ranking by fault norm leads to ranking of faults or contingencies. Magnitude and rate of change of system norm can be used to detect an out-of-step condition. Results on a ten-machine system and a utility system with detailed models are also presented.     

Detection of Symmetrical Faults by Distance Relays During Power Swings

For maintaining security of distance relays, power swing blocking is necessary to prevent unintended operation under power swings. To be dependable, distance relays must operate whenever a fault occurs. Therefore, detecting faults during power swings is an important issue since the relay should be able to differentiate the fault condition and not be blocked during that time. This paper presents a new method for detecting a symmetrical fault during a power swing, based on extracting components of the current waveform using the Prony method. The merit of the method is demonstrated by simulating different faults during power swing conditions using the Alternate Transients Program version of the Electromagnetic Transients Program.     

Distance Relay With Out-of-Step Blocking Function Using Wavelet Transform

Out-of-step blocking function in distance relays is required to distinguish between a power swing and a fault. Speedy and reliable detection of symmetrical faults during power swings presents a challenge. This paper introduces wavelet transform to reliably and quickly detect power swings as well as detect any fault during a power swing. The total number of dyadic wavelet levels of voltage/current waveforms and the choice of particular levels for such detection are carefully studied. A logic block based on the wavelet transform is developed. The output of this block is combined with the output of the conventional digital distance relay to achieve desired performance during power swings. This integrated relay is extensively tested on a simulated system using PSCAD/ EMTDC^reg software.     

高性能三相采样值启动元件

提出了一种反应负序电流量的三相同时刻采样值启动元件,该启动元件利用数据跨窗时的特征来放大启动元件的动作量,达到提高启动元件动作速度和动作灵敏度的目的。采用浮动门槛、正序电流制动和静稳破坏后提高动作门槛值的方法,来防止系统振荡时误启动。通过RTDS试验数据对简单故障、高阻接地和振荡等情况进行比较全面的仿真,结果表明,该启动元件性能良好,能满足高压线路保护的要求。     

A Novel Scheme to Identify Symmetrical Faults Occurring During Power Swings

A novel, fast unblocking scheme for distance protection to identify symmetrical faults occurring during power swings has been proposed in this paper. First of all, the change rate of three-phase active power and reactive power being the cosine function and the sine function with respect to the phase difference between the two power systems during power swings has been demonstrated. In this case, they cannot be lower than the threshold of 0.7 after they are normalized. However, they will level off to 0 when a three-phase fault occurs during power swings. Thereafter, the cross-blocking scheme is conceived on the basis of this analysis. By virtue of the algorithm based on instantaneous electrical quantities, the calculation of the active and reactive power is immune to the variation of the system power frequency. As the integration-based criterion, it has high stability. Finally, simulation results show that this scheme is of high reliability and fast time response. The longest time delay is up to 30 ms.     

A new principle for high resistance earth fault detection duringfast power swings for distance protection

The combined effects of a high impedance ground fault condition and a fast power swing condition cause the apparent impedance `seen' by power system distance protection schemes to vary over a wide range. Previous research work has shown that the detection and location of high impedance faults during power swings is very difficult. A new principle for high impedance fault detection during fast power swings is presented which overcomes these deficiencies. This method forms a part of a complete power swing relaying scheme and can be implemented on standard relaying hardware. Results are presented based on extensive simulation studies carried out on a typical 400 kV transmission system, using the Electromagnetic Transient program (EMTP)