An equivalent instantaneous inductance-based technique for discrimination between inrush current and internal faults in power transformers

A novel equivalent instantaneous inductance (EII)-based scheme is proposed to distinguish the inrush current from internal faults in power transformers, which is derived from the inherent difference of the magnetic permeability, due to the saturation and unsaturation, in the transformer iron core between the inrush current and an internal fault. First, the EII that represents these differences is defined, and its calculation methods and the criteria to identify the internal fault in the time and frequency domains are respectively developed. A total of 98 experimental cases are used to test the performance of the technique. In all of the tests, the inrush current is characterized by the drastic variation of the EII, but the EII of the faulty phase is kept almost constant. The experimental results verify the dependability, security, and fast operation of the proposed scheme. Besides, its easy implementation in real time is another advantage of the proposed method because of its simplicity.     

电力变压器励磁涌流和故障电流仿真研究

在研究电力变压器励磁涌流和故障电流特性的基础上 ,应用电磁暂态程序 (EMTP) ,采用不同的模型和方法对变压器励磁涌流和故障电流进行了全面系统的仿真计算和分析。结果表明 ,EMTP能够根据不同的条件和要求有效地实现对变压器励磁涌流和故障电流的仿真     

A wavelet-based technique for discrimination between faults and magnetizing inrush currents in transformers

This paper presents the development of a wavelet-based scheme, for distinguishing between transformer inrush currents and power system fault currents, which proved to provide a reliable, fast, and computationally efficient tool. The operating time of the scheme is less than half the power frequency cycle (based on a 5-kHz sampling rate). In this work, a wavelet transform concept is presented. Feature extraction and method of discrimination between transformer inrush and fault currents is derived. A 132/11-kV transformer connected to a 132-kV power system were simulated using the EMTP. The generated data were used by the MATLAB to test the performance of the technique as to its speed of response, computational burden and reliability. The proposed scheme proved to be reliable, accurate, and fast.     

电力变压器励磁涌流和内部故障识别新方法

区分励磁涌流和内部故障时的短路电流,是变压器差动保护的难点技术之一,虽然目前已有多种方法,但每种方法都或多或少地存在着一些问题,空载合闸误动或内部故障时拒动(或延时动作)的情况还时有发生。在引入瞬时无功功率理论的基础上,提出了一种依据变压器两侧三相差瞬时有功功率和瞬时无功功率直流分量比值的变化关系来识别变压器励磁涌流和内部故障电流的新方法。该方法不仅简便易行,而且从平均有功和平均无功的关系出发,进一步揭示了变压器励磁涌流与内部故障本质上的不同。RTDS仿真实验结果表明:该方法简单可靠,识别效果显著。     

电力变压器励磁涌流和内部故障识别新方法

区分励磁涌流和内部故障时的短路电流,是变压器差动保护的难点技术之一,虽然目前已有多种方法,但每种方法都或多或少地存在着一些问题,空载合闸误动或内部故障时拒动(或延时动作)的情况还时有发生.在引入瞬时无功功率理论的基础上,提出了一种依据变压器两侧三相差瞬时有功功率和瞬时无功功率直流分量比值的变化关系来识别变压器励磁涌流和内部故障电流的新方法.该方法不仅简便易行,而且从平均有功和平均无功的关系出发,进一步揭示了变压器励磁涌流与内部故障本质上的不同.RTDS仿真实验结果表明:该方法简单可靠,识别效果显著.     

Current transformer performance for inrush current in power transformers

A mathematical model has been developed for a current transformer to predict the transient performance during switch-in of a power transformer with residual flux.Typical 50 - 100 - 200/1 and 300/1 current transformers have been simulated and the secondary current response has been computed for different values of the current transformer time constants. The practical results are reported.     

电磁型与电子式电流互感器传变和应涌流比较

电子式电流互感器在数字化与智能变电站中大规模应用,其特性与传统的电磁型电流互感器差异较大,需研究电磁型与电子式电流互感器传变和应涌流的特性。通过理论计算电磁型电流互感器暂态饱和后传变各次谐波的增益,证明和应涌流导致电磁型电流互感器饱和后,传变2次谐波的能力比传变基波的能力强,故2次谐波比反而升高;但分析表明经暂态饱和的电磁型电流互感器传变后和应涌流的间断角将减小。通过理论推导基于空心线圈的电子式电流互感器的传递函数,揭示了电子式电流互感器能否正确传变和应涌流主要取决于积分时间常数和衰减时间常数,和应涌流经电子式电流互感器传变后可能导致波形发生偏移,但其间断角特征和2次谐波含量不变,间断角和2次谐波制动判据不受影响。     

Computerised evaluation of the magnetising inrush current in transformers

Design of protective systems in large power transformers necessitates a detailed knowledge of the magnitude and shape of the magnetising inrush current during the switching operation. It is very difficult to evaluate the inrush current analytically owing to non-linearity in the magnetisation (B-H) curve.This paper suggests a method for the computerised evaluation of the magnetising inrush current in transformers without any drastic linearising assumption for the B-H curve.     

变压器并联与串联和应涌流对差动保护影响的比较研究

通过分析并联和应涌流与串联和应涌流等效电路的异同,结合仿真分析和理论推导,深入比较研究了并联与串联和应涌流的产生过程及其衰减特点;从电力系统规程出发,综合考虑电流互感器饱和、电流互感器参数及运行变压器负荷等多个因素,进一步比较研究了并联和应涌流与串联和应涌流对变压器差动保护的影响.研究表明:串联和应涌流出现的时间较晚,所达到的幅值较小,衰减速度明显快于并联和应涌流;和应涌流造成电流互感器饱和时,差动保护差流的间断角会消失,但二次谐波含量仍很高;在运行变压器带负荷情况下,差动保护的制动电流较大,而和应涌流减小,差动保护不易误动.     

A novel magnetizing inrush current suppressor of energized transformers

The magnetizing inrush possibly occurs when the transformer is energized in the case of no load or a light load. The reason rests with the saturation state or even deep saturation state of the transformer core due to many factors such as inception angles, residual flux, and so on. Both the transformer itself and the power quality in the power system will be adversely impacted by the magnetizing inrush. By utilizing the tertiary winding of a three‐windings transformer, a new technique called ‘magnetizing inrush suppressor’ (MIS) is proposed to eliminate the inrush. When the transformer is switched on, it allows the voltage on the tertiary winding to climb gradually and ensures that the voltages on both sides have the same phase but different magnitudes. In this case, the inrush current can be eliminated successfully. Finally, simulation tests based on PSCAD are performed and analyzed in detail, and the validity of the proposed approach is confirmed. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

A novel algorithm for discrimination between inrush current and internal faults in power transformer differential protection based on discrete wavelet transform

This paper proposes a novel methodology for transformer differential protection, based on wave shape recognition of the discriminating criterion extracted of the instantaneous differential currents. Discrete wavelet transform has been applied to the differential currents due to internal fault and inrush currents. The diagnosis criterion is based on median absolute deviation (MAD) of wavelet coefficients over a specified frequency band. The proposed algorithm is examined using various simulated inrush and internal fault current cases on a power transformer that has been modeled using electromagnetic transients program EMTDC software. Results of evaluation study show that, proposed wavelet based differential protection scheme can discriminate internal faults from inrush currents.     

一种基于差流波形特征的励磁涌流识别方法

提出了一种基于差动电流波形特征的励磁涌流和故障电流识别方法。励磁涌流的共同特征是受铁芯饱和程度的影响,差动电流是流过励磁绕组的实际电流,因而这一特征表现得更加明显。定义波形系数为磁通未饱和部分对应的电流在整个周期中所占比例,当系数大于阈值时判定为励磁涌流。为防止误动,当系数小于阈值时,引入"是否完全偏向时间轴一侧"作为辅助判据,使得该方法更加可靠。最后进行空载合闸仿真试验,证明了该方法能识别各种情况下的励磁涌流,性能优于二次谐波制动和间断角判据。     

Identifying magnetizing inrush in power transformers based on symmetry of current waveforms

This letter proposes a novel method for the discrimination between magnetizing inrush and internal fault in power transformers based on the similarity of the current waveforms. The proposed method first normalizes and sorts the sample points of the differential current in a full fundamental frequency cycle to obtain a new sequence which is independent of the magnitude and initial phase angle of the original signal. Afterward, three indices are introduced to measure the similarity between the new sequence and a template, which are used to discriminate the inrush from an internal fault. Simulation studies demonstrate the effectiveness of the proposed method and its advantages over the conventional second harmonic restraint method. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

基于小波神经网络的变压器励磁涌流和内部故障电流识别

小波神经网络 (WaveletNeuralNetwork)结合了小波变换及神经网络的优点 ,即具有良好的时频局部性质 ,又有较好的自学习能力和容错能力。针对变压器励磁涌流与内部故障电流的识别问题 ,提出了一种基于小波神经网络的解决方案 ,阐述了基本的思想方法和具体的算法过程 ,EMTP仿真实验表明了该方案的有效性与可行性。     

基于小波神经网络的变压器励磁涌流和内部故障电流识别

小波神经网络(Wavelet Neural Network)结合了小波变换及神经网络的优点,即具有良好的时频局部性质,又有较好的自学习能力和容错能力.针对变压器励磁涌流与内部故障电流的识别问题,提出了一种基于小波神经网络的解决方案,阐述了基本的思想方法和具体的算法过程,EMTP仿真实验表明了该方案的有效性与可行性.     

一种零序差动保护的辅助选相元件

针对零序差动保护不具备分相电流差动保护的选相功能,提出了一种新的零序电流差动保护辅助选相方法,利用零负序差流的关系以及各相差流的大小关系配合进行选相.通过EMTP和Matlab软件对1000 kV示范线路进行了仿真,结果表明,该选相元件在各种区内高阻故障条件下均能准确、快速地选相.     

利用二次小波变换区分变压器励磁涌流和短路电流的符号法

通过选取合适的小波基函数,利用小波变换对采样得到的差流进行一次或二次小波变换,再提取小波变换系数的局部模极大值符号,利用定义的符号识别公式即可区分对称性涌流、非对称性涌流以及内部故障电流,还能鉴别出励磁涌流伴随内部故障情况以及电流互感器饱和采样得到的励磁涌流。Matlab仿真及动模实验校验表明,该方法简单有效,不受噪声干扰,易于在工程上实现。     

Power transformers winding fault diagnosis by the on-load exciting current Extended Park's Vector Approach

This paper presents the application of the on-load exciting current Extended Park's Vector Approach to diagnose incipient turn-to-turn winding faults in operating power transformers. Experimental and simulation test results demonstrate the effectiveness of the proposed technique, which is based on the spectral analysis of the AC component of the on-load exciting current Park's Vector modulus.     

Simulation of a differential current protection scheme involvingmultiple current transformers

Differential protection schemes for busbars, generators or transformers connect multiple current transformers in parallel across a common burden. This paper describes the techniques used to simulate such an arrangement and concentrates particularly on the three current transformer case