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。     

A novel digital directional transformer protection technique based on wavelet packet

This paper presents a novel digital technique for transformer protection. The technique is based on deriving a directional quantity proportional to the fault current signal and the prefault voltage signal. Standard fast wavelet transform (FWT) schemes may not be as effective for data that has chiefly oscillatory features. An effective solution to discrimination involves examining the signal in both the time and frequency domains simultaneously. The wavelet packet transform is an extension of the FWT that allows for finer characterization of signal content for both time and frequency together. A 11/132-kV transformer connected to a 132-kV power system was simulated using Alternative Transient Program/Electromagnetic Transient Program (ATP/EMTP). Results indicate that the proposed technique is stable, reliable, and fast during the discrimination between internal and external faults, magnetizing inrush currents, and internal faults, ratio-mismatch, and saturation of current transformers (CTs).     

ANN-Based Protection Scheme for Power Transformer

The implementation of a pattern recognizer for power system diagnosis can provide great advancement in the protection field. The work reported in this paper demonstrates the use of an Artificial Neural Network (ANN) as a pattern classifier for differential relay operation in the protection scheme for power transformer protection. The proposed relay takes care of maloperation due to inrush current, overexcitation, and ensures operation during internal fault. The off-line experimental result presented in this paper shows that a FeedForward Neural Network (FFNN) can be considered as an attractive alternative method to make the discrimination among normal, magnetizing inrush, overexcitation, and internal fault currents in a digital relay implementation. Results showing the performance of the protection scheme indicate that it is fast and reliable.     

Wavelet based transformer protection using high frequency power directional signals

This paper proposes a novel wavelet transform based relaying scheme for power transformer protection. The relay logic consists of two parts: disturbance detection based on first level high frequency details of the voltage signals only and fault discrimination using a power based directional signal derived from the first level high frequency details of both voltage and current signals. The logic is deterministic, computationally efficient, fast, secure and highly reliable. The operating time is 6 ms, about 1/3rd of power frequency cycle (20 ms). The scheme uses only the sign of the directional signals, rather than the difference in their magnitudes, hence it can work reliably in the presence of transformer tap variation, fault resistance and CT saturation. The validity of the proposed logic was exhaustively tested by simulating various types of internal and external faults, energization conditions and load variations on a 132 kV system modeled in ATP/EMTP with a 31.5 MVA, 132/33 kV, Y–Δ transformer. The proposed logic was able to correctly discriminate between internal faults, external faults and non-fault disturbances for all the 880 test cases.     

A Morphological Scheme for Inrush Identification in Transformer Protection

This paper presents a novel morphological scheme for the identification of transformer magnetizing inrush. The scheme decomposes a current signal into multiresolution levels based on synthesis and analysis operators of mathematical morphology. It is able to discriminate between inrush and internal fault currents even in the case of an inrush with a low second harmonic component and an internal fault current with a high second harmonic component. Simulation studies have been undertaken to evaluate the scheme based on a three-phase high-voltage power transformer operating under different conditions, which is simulated with an Alternative Transients Program. The testing results show that the proposed scheme is able to identify inrush currents reliably and provides an alternative solution for transformer differential protection.      

基于最小二乘矩阵束的励磁涌流识别方案

准确、快速地切除变压器故障直接影响电力系统能否持续安全稳定运行,励磁涌流的鉴别正是变压器保护中的重要一环。为了有效辨识励磁涌流与故障电流,避免变压器差动保护的不正确动作,利用变压器励磁涌流与短路故障电流频域信息的差异,提出了一种基于最小矩阵束算法的励磁涌流识别新方案,以变压器两侧的差动电流作为信号量,通过矩阵束算法分析采样信号中不同频率的分量,计算含衰减因数的电流能量信息熵识别励磁涌流。仿真试验证明了新判据具有原理清晰、抗干扰能力强等优点,能够正确区分励磁涌流和故障电流,为后续研究及工程应用提供借鉴和参考。     

基于波形正弦度特征的变压器励磁涌流判别算法

提出一种基于波形正弦度判别变压器励磁涌流的方法。利用变压器励磁涌流和内部故障电流波形特征的不同，即变压器正常运行、内部短路、外部短路时波形具有正弦函数特征，而励磁涌流因其包含非周期分量、间断角，不具有正弦特征。比较波形的正弦度，形成区别涌流和短路故障的判据。通过变压器各种运行情况的大量动模试验验证了所提方法的可行性和判据的正确性。研究表明，所提方法具有特征明显，判断精确，同时对TA饱和有制动作用等特点。     

基于波形正弦度特征的变压器励磁涌流判别算法

提出一种基于波形正弦度判别变压器励磁涌流的方法。利用变压器励磁涌流和内部故障电流波形特征的不同，即变压器正常运行、内部短路、外部短路时波形具有正弦函数特征，而励磁涌流因其包含非周期分量、间断角，不具有正弦特征。比较波形的正弦度，形成区别涌流和短路故障的判据。通过变压器各种运行情况的大量动模试验验证了所提方法的可行性和判据的正确性。研究表明，所提方法具有特征明显，判断精确，同时对TA饱和有制动作用等特点。     

特高压变压器的阻抗快速后备保护

研究了由三个单相变压器组成的特高压三相变压器组的后备保护,提出了用阻抗纵联保护作为特高压大容量变压器相间短路的快速后备保护的方案。分析了采用超范围允许式和超范围闭锁式阻抗纵联保护方案的可行性和优越性。说明了阻抗保护受励磁涌流影响小的原理,并用数字仿真证明了此论断的正确性。建立了特高压变压器的仿真模型,仿真结果证明在变压器空投和外部故障切除时产生的励磁涌流不会使阻抗保护误动,而在变压器出口相间短路时能够快速切除故障。采用所提出的保护方案可提高特高压变压器保护的可靠性。     

电力变压器励磁涌流判别的自适应小波神经网络方法

励磁涌流识别一直是电力变压器差动保护中比较关注的问题。文中提出了一种基于自适应小波神经网络实现变压器励磁涌流判别的新方法。结合励磁涌流和内部故障电流的特点，构建了一个四层的自适应小波神经网络模型，并对其具体的实现方法进行了详细的分析；利用ATP—EMTP程序进行仿真计算生成训练样本和测试样本，对所构建的网络进行了训练和测试，结果表明自适应小波神经网络能准确、可靠地识别出变压器的励磁涌流状态。     

基于磁制动原理的特高压变压器励磁涌流快速识别

针对特高压大容量三相分体变压器差动保护，提出一种基于磁特性的励磁涌流快速识别新方案，该方案充分考虑了特高压变压器端部接长距离输电线路的实际特点，配合自适应数字低通滤波器计算变压器等效瞬时磁阻，有效解决了内部故障时由于变压器模型简化造成的计算误差问题。根据内部故障和励磁涌流时等效瞬时磁阻的不同变化特征，能够在10 ms识别出励磁涌流，同时还考虑了内部故障TA饱和的影响，能够保证在TA严重饱和情况下做出正确判断。该方案识别速度快、可靠性高、整定方便，具有广阔的工程应用前景。实验结果验证了其快速性和可靠性。     

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.     

Reducing the magnetizing inrush current by means of controlled energization and de-energization of large power transformers

Energizing unloaded transformers may result in magnetizing inrush current of high amplitude. These currents have many unfavorable effects including differential protection maloperation, deterioration of the insulation material and mechanical support structure of windings, voltage sag and other power quality issues on the HV, as well as on lower voltage terminal(s). If no point-on-wave controller is applied the energization of the transformer may occur at any time on the sinusoidal wave producing high inrush current peaks if the transformer core is driven into saturation. The point-on-wave control methodology presented in this paper has been elaborated to eliminate the inrush currents of 132/15 kV, Y/delta connected generator step-up transformers, which are switched very often in quick start, gas-turbine power stations. A new controller has been elaborated to operate the mechanically staggered, common drive type circuit breakers which are characterized by three-pole operated spring drive and fixed time delay between the operating poles. The paper proposes a new method to minimize the residual flux in the core by means of controlled de-energization. The new concept has been proved by several field tests and the new controllers have been put into service in several power stations in Hungary.     

基于瞬时励磁电感频率特性判别变压器励磁涌流

基于变压器瞬时励磁电感基频分量的有无，提出了判别励磁涌流与内部故障的新方法。涌流时变压器铁心必然经历饱和与非饱和过程，瞬时励磁电感是时变、交替变化的，具有较大的基频分量；内部故障时，变压器铁心工作于线性区，瞬时励磁电感恒为常数，无基频分量。据此，可实现励磁涌流与内部故障的有效判别。但瞬时励磁电感很难精确求取，为方便计算，提出了等效瞬时电感的概念，论证了其基频特性的等效性。动模实验有力地证实了所提方法的可靠性与安全性。     

用模糊贴近度识别变压器故障电流和励磁涌流的研究

提出了用波形对称及模糊集贴近度原理来辨识故障电流和励磁涌流的新方法。数字仿真及实验结果表明：本方法可准确识别故障电流和励磁涌流;即使在空载合闸于内部短路故障时,保护继电器也能迅速可靠动作,避免了采用二次谐波制动导致的拒动问题。     

基于有功无功直流分量比值的变压器涌流新判据

针对变压器差动保护存在的区分励磁涌流和内部故障电流的难题,在引入瞬时无功功率理论的基础上,提出一种依据变压器各侧三相差有功功率和差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.该方法原理简单,易于实现,具有数据采样方便、计算量小、动作可靠等特点.该方法从能量守恒的角度及有功功率和无功功率的关系出发,揭示了变压器励磁涌流和故障状态的本质不同.对变压器各种运行状态进行了实时数字仿真(RTDS)实验,仿真实验结果表明:该方法简单可靠,识别效果显著.     

基于瞬时功率的变压器励磁涌流和内部故障电流识别新方法

在分析瞬时功率频谱特性的基础上，提出了一种基于瞬时功率的变压器励磁涌流和内部故障电流识别新方法。该方法主要依据变压器两侧三相差瞬时功率幅频特性中直流分量和基频分量的相对关系来识别变压器励磁涌流和内部故障电流。该方法具有传统变压器电流差动保护简便易行的特点，并从能量守恒的角度出发，进一步揭示了变压器励磁涌流与内部故障电流本质上的不同。HYBRISIM混合仿真实验结果表明该方法简单可靠、识别效果明显。     

基于差动电流正弦曲线拟合波形的变压器保护原理

以变压器的磁化特性曲线为基础,对比分析了变压器差流为涌流和非涌流时的波形特点,进而提出了一种基于差动电流正弦曲线拟合波形的变压器保护原理。为了更能体现励磁涌流的特征和减少拟合波形幅值的计算误差,建议取拟合角度为差动电流峰值时的角度最为合理。给出了拟合波形幅值的计算方法,并讨论了减小计算误差的方法。大量数字仿真结果表明,所提出的原理可以快速、有效地区分励磁涌流和内部故障电流。     

用相关分析法识别变压器励磁涌流的研究

提出了用相关分析法和模糊集理论来识别故障电流及励磁涌流的新方法。论证了该原理与谐波制动原理之间的对应关系,证明了辨识对称涌流的有效性。模拟实验结果表明：本方法可准确识别故障电流和励磁涌流;即使在空载合闸于内部短路故障时,保护继电器也能迅速可靠动作,避免了采用二次谐波制动导致的拒动问题。