New Algorithms for Computer Based Power System Measurements for Relaying

Abstract

The paper presents two new algorithms for power system measurements for computer relaying. The algorithms involve the design of a discrete-time digital filter which interpolates the signal samples and generates Fourier Co-efficients either by using spectral observations or a functional expansion technique. Sample designs are presented for voltage and current phasor estimation during system transients. A method for including decaying dc and harmonic frequency components in the filter design is also described in the paper. Both computational and on-line test results using an LSI-11/23 microcomputer for power system phasor estimation are presented in the paper.     

Digital impedance protection of power transmission lines using aspectral observer

A recursive discrete-time filter for calculating the impedances from digitized voltage and current samples at relay location is derived. From an interpolating signal model comprising a delaying DC component and components of fundamental harmonic frequencies, an interpolating spectral observer is constructed to give the desired Fourier coefficients recursively. The interpolators are also useful for sampling rates other than Nyquist rate and unevenly spaced samples. The proposed algorithm is tested using fault data recorded at the Saskatchewan Power Corporation, resulting in fast and reliable tripping conditions. The performance of the spectral observer is enhanced by suitable placement of poles of the observer, producing a fast operation of the digital relay. The performance of the protection scheme with evenly spaced data samples is presented     

High Speed Digital Directional Comparison Relaying

ABSTRACT

A high speed directional comparison relay based on the evaluation of the locally measured deviations of the voltage and current from their prefault values is described in this paper. Signal processing techniques are used to damp out the exponentially decaying dc component and high frequency transient components so that the operation of the relay depends on the power frequency components of the voltage and current deviation signals. The direction to a fault is determined by detecting the first transgression of the fault trajectories across threshold boundaries in the deviation plane. Studies on a three phase power system model show that the direction to a fault is determined within a first few milliseconds following the inception of a fault.     

微机保护中卡尔曼滤波模型参数的选择

卡尔曼滤波,是线性、无偏、最小方差的实时递推滤波,是一种高效、优化的数据处理方法。以输电系统的二状态电压模型和三状态电流模型的滤波模型参数的确定,进行了分析研究,以具体线路(山东邹县——潍坊500kV数学仿真线路)为例作了短路过程基频电压和电流分量的最优估计,获得了一套适合于各种故障情况的模型参数。此模型参数之滤波精度和收敛速度俱称满意。本法实时计算简单,特别适合于在高压输电线路的微机保护上应用。     

Recursive algorithm for real-time measurement of electrical variables in power systems

In this paper, a novel complex bandpass filter is presented which overcomes the pitfalls of the techniques in common use. This complex bandpass filter can correctly extract the phasor of the fundamental component and symmetrical components in voltage or current waveforms and then accurately estimate their instantaneous amplitude, phase angle, and frequency, even encountering various power disturbances. Further, a recursive algorithm is also developed for the complex bandpass filtering that updates current filtering output only using several previous sample values and filtering outputs. This attribute greatly reduces the computational complexity of complex bandpass filtering, which is the weakness of the continuous wavelet transform based on the well-known Morlet Wavelet. Thus, this recursive algorithm is highly desirable for real-time applications. The performance of the proposed technique is ascertained by using both simulated and practical power disturbance waveforms.     

Removal of DC offset in current and voltage signals using a novelFourier filter algorithm

Protecting transmission lines frequently involves adopting distance relays. Protective relays must filter their inputs to reject unwanted quantities and retain signal quantities of interest. Accuracy and convergent speed of the filter algorithm are essential for protective relays. A widely applied filter algorithm, the discrete Fourier transform (DFT) can easily erase harmonics using simple calculation. However, the voltage and current signals contain large harmonics and DC offset during the fault interval. The DC offset heavily influences the precision and convergence speed of the fundamental frequency signal from DFT. In this investigation, the authors present a novel Fourier algorithm to remove the DC offset in a voltage or current signal. Applying a full-cycle DFT (FCDFT) requires one cycle plus two samples to calculate and compensate for the DC offset. Half-cycle DFT (HCDFT) only requires half of a cycle plus two or three samples to accomplish the algorithm when the input signal has no even order harmonics. Adopting the proposed algorithm in distance relays effectively suppresses the DC offset and quickly decomposes the accurate fundamental frequency components     

A hybrid fault locator for three-terminal lines based on wavelet transforms

This paper presents a hybrid fault location algorithm for three-terminal transmission lines based on wavelet transforms (WT). In this work, the WT were used to analyze the low and high frequency components of the current and/or voltage signals generated by an electric fault. A simple routine was used to determine which method should be used for each case. The methodology is called “hybrid” as it uses either traveling waves or impedance calculation methods to locate the fault. The traveling wave method analyses the current and/or voltage high frequency signals extracted by WT to determine the fault location. On the other hand, the fundamental frequency components of the voltage and current signals, also extracted by WT, can be used for the same purpose. The hybrid algorithm was tested for different fault conditions by simulations using alternative transients program (ATP) software. The results obtained are promising and demonstrate a highly satisfactory degree of accuracy and reliability of the proposed method.     

A Digital Measurement of Earth Fault Loop-Impedance Using a Parameter Estimation Algorithm

ABSTRACT

In electrical low voltage power system, a neutral earthing is used for protection against electrical shock hazards. Testing of the effectiveness of the earthing system could be achieved by measuring the system fault loop impedance Z. This paper presents a new application of least error squares (LES) parameter estimation algorithm for measurement of earth fault impedance, the proposed method uses the digitized samples of the instantaneous voltage across the fault loop impedance, effects of data window size, sampling rate and the voltage frequency drift on the accuracy of the impedance measurements are studied. Simulated test results are reported in this paper, which from the basis of our conclusion.     

Digital Application of Second Order Generalized Integrator Based Grid Estimator Under Unbalanced and Distorted Voltage Conditions

Abstract

Frequency, amplitude, and phase information of the grid voltage are the main constraints for constructing a robust controller algorithm for grid connected applications under unbalanced and distorted voltage conditions. This paper narrates a simple, robust, straight forward method to estimate the instantaneous positive and negative sequence voltage components under unbalanced and distorted voltage circumstances. A second order generalized integrator (SOGI) is encapsulated to filter out the distorted voltage as well as to generate orthogonal voltage components for the three phases of AC grid. Furthermore, these filtered and orthogonal components are accounted for the calculation of instantaneous symmetrical components. Developed technique is more frequency adaptive compared to conventional phase locked loop (PLL) techniques. A set of test outcome results are provided in this paper based on MATLAB/Simulink simulations with real grid data captured from an industrial plant. Moreover, SOGI based estimator is digitally implemented by using dSPACE ds1103 digital controller to validate the numerical simulation results in accordance with the developed theoretical prediction.     

SOLUTIONS INCORPORATING MAGNETIC HYSTERESIS TO THE SUBHARMONIC PROBLEM IN POWER SYSTEMS

ABSTRACT

One possibility of taking care of the subharmonics in high speed protection is to filter the fundamental frequency components from the fault transients and use them for fault detection and discrimination. This is correct only if the phases and amplitudes of fundamental components during and post-subharmonic periods remain the same in transient waveforms. An attempt is therefore made to establish this aspect by a detailed study of a typical electrical network presenting subharmonics. The Dual Input Describing Function is adopted to show the subharmonic boundaries and the influence of system parameters on subharmonic oscillations. Magnetic hysteresis is incorporated in the analysis. A fifth order nonlinearity and one-third subharmonic are assumed. The theory could be easily extended to any order of nonlinearity and to any subharmonic frequency and also to a combination of subharmonics. The analysis gives a set of nonlinear algebraic equations. These equations are solved by Newton's technique using a digital computer. The solution gives a direct indication of the system susceptibility to subharmonics.     

Recursive functional expansion technique for computer-based impedance and differential protection

The development of a recursive functional expansion algorithm for extracting the desired frequency components from transient power system relaying signals is presented. The applications of this algorithm to impedance detection in transmission line protection and to harmonic restraint in transformer differential protection are discussed. The recursive algorithm generates fast fault detection timings for transmission lines and does not have restrictions on sample rate, data window or spacing of samples with respect to time. For power transformer differential protection, the combined second- and fifth-harmonic amplitude of the differential current is compared with the fundamental amplitude to arrive at a trip decision.     

Removal of decaying DC in current and voltage signals using amodified Fourier filter algorithm

Protecting transmission lines frequently involves applying distance relays. Protective relays must filter their inputs to reject unwanted quantities and retain signal quantities of relevant interest. Accuracy and convergent speed of filter algorithm are essential for protective relays. A widely applied filter algorithm, the discrete Fourier transform (DFT) can easily remove integer harmonics using simple calculation. However, the voltage and current signals contain serious harmonics and decaying DC during the fault interval. In addition, the decaying DC and higher order harmonics seriously decrease the precision and convergence speed of fundamental frequency signal from DFT. In this investigation, the authors derive a novel algorithm which combines the appropriate analog low pass filter and modified full cycle DFT (FCDFT) or half cycle DFT (HCDFT) algorithm to remove the decaying dc in a voltage or current signal. Using the Electromagnetic Transients Program (EMTP) simulates the transient responses of transmission lines during the fault period. Applying the proposed algorithm in distance relays effectively suppresses the decaying DC and quickly decomposes the accurate fundamental frequency components     

交流1000kV特高压线路三相故障暂态过程分析

通过理论分析和仿真验证得到特高压线路三相故障后相关暂态电气量的特点:特高压线路故障电流中除有一很大的衰减直流分量外还存在高频分量:故障电压中也含有较大的高频分量但没有直流分量;故障高频分量的初始值与短路角(以电压角为基准)有关.当故障角为90°时.高频分量的初始值最大:高频谐波的频率与故障距离和系统运行方式有关.系统方...     

有源电力滤波器的双闭环串级控制

针对在有源电力滤波器中,补偿电流变化比直流电压快很多的特点,提出一种采用以电流环为内环、电压环为外环的串级控制策略。由于补偿电流是快速变化的正弦量,引入递推积分算法来消除电流环的稳态误差。通过检测电网电压和直流电压的控制直接获得期望的电网电流,简化了检测电路。利用串级控制的思想,对电流环和电压环的参数进行设计,为有源滤波器控制器参数设计提供一定的理论指导。该方法的物理意义明确,且算法的复杂度小、实时性强和容易工程实现。实验结果证明了该文方法的可行性。     

A fault location technique for rural distribution feeders

This work presents a digital fault location technique for rural distribution feeders, using the voltage and current data at a single location. Rural distribution feeders include single-phase, two-phase, and three-phase laterals off a main three-phase primary distribution feeder. The fault location scheme presented here attempts to account for the multiphase laterals, the unbalanced conditions, and the unsymmetrical nature of distribution feeders by continually updating voltage and current vectors at set locations within the system. The updated voltage and current vectors are the estimates of the 60-Hz phasor quantities obtained using a recursive optimal estimation algorithm. The distance to the fault is then estimated using a method based on the apparent impedance approach and the updated voltage and current vectors. Another consideration is the ability to determine the fault location on a lateral. A simulation of an actual rural distribution feeder using the Electromagnetic Transients Program (EMTP) is used to test the approach     

A novel fast distance relay for long transmission lines

The measuring accuracy and the measurement stability of conventional distance relay will be influenced by complex and remarkable harmonic components due to the large capacitance of the line when it is used for a long line. Correspondingly, the tripping speed will be delayed to some extent. To solve this problem, a fast distance relay for long transmission lines is presented, which is on the basis of the differential equation algorithm using π transmission line model and the theory of Equal Transfer Process of Transmission Lines (ETPTL). The shortcomings of π model differential equation algorithm due to the impact of high frequency components can be overcome by using a low-pass filter. The problem resulting from the difference between the transfer feature of the voltages used by the distance protection and that of the currents due to the transient characteristic of coupling capacitor voltage transformers (CCVT) can be solved by using virtual digital CCVT. Then, the new distance relay can trip quickly by re-structuring the voltage at the fault point and iterative calculations. A variety of ATP simulation tests show that the new relay has fast tripping speed and high reliability when applied to the long transmission lines.     

远距离输电线路等传变瞬时值差动保护

研究表明线路分布电压、电流经过相同的线性电路传变后，其相互关系仍符合原线路分布参数模型，工频附近的电压信号在线路上接近线性分布。在此基础上，提出一种用于远距离输电线路保护的等传变瞬时值差动保护方法，该方法从线路一次电气量到用于差动保护计算的瞬时值，电压和电流经过的传变环节基本相同。首先将电流采样值经虚拟数字CVT处理，然后对电压和电流采样值通过相同的低通滤波处理。由线路电压瞬时值分别实时计算出流入并联电抗器和流入线路Π型等值电路两侧电容的电流瞬时值，再从线路每侧电流除去这两部分电流，用得到的新电流瞬时值作常规采样值差动和故障分量采样值差动保护计算。ATP仿真表明，等传变瞬时值差动保护可靠性高、动作速度快、耐过渡电阻能力强、具有选相能力。     

基于递归VMD的有源滤波器谐波检测

针对有源滤波器谐波检测问题,将递归变分模态分解(VMD)应用于有源滤波器中谐波检测部分。对谐波电流进行递归VMD分解,得到一系列带宽受限的固有模态函数(BIMF),不同的BIMF代表了不同的频率分量,即得到基波和各次谐波BIMF分量,从而完成谐波的检测。在分解中,通过限制带宽估计中心频率,重构模态函数。仿真结果表明,基于递归VMD的谐波检测方法,可以准确有效地提取电力系统中的谐波。     

Removal of dc offset and subsynchronous resonance in current signals for series compensated transmission lines using a novel Fourier filter algorithm

In this paper, a novel Fourier filter algorithm for high-accuracy and high-convergence-speed fundamental frequency component calculation of series compensated transmission line is proposed. The performance of digital relaying is highly reliant on the algorithm of digital filtering, especially involving the dc offset and subsynchronous resonance. Input signals of protective relays must be filtered in advance to reject unwanted quantities and retain signal quantities of interest. Accuracy and convergence speed of filter algorithms are essential for protective relays. However, the voltage and current signals contain large harmonics and dc offset during fault intervals. A widely applied filter algorithm, the Discrete Fourier Transform (DFT), can easily eliminate harmonics via simple calculations. The dc offset heavily influences the precision and convergence speed of fundamental frequency component from DFT. Under high resistive fault conditions, the current signals will have subsynchronous resonance. The subsynchronous resonance current and voltage signals influence the performance of DFT much more than the dc offset. Although capable of erasing high order harmonics, the low pass filter cannot remove the dc offset and subsynchronous resonance. The Alternative Transient Program (ATP) of the Electromagnetic Transients Program (EMPT) is utilized to simulate the transient responses of series compensated transmission lines during the fault period. Adopting the proposed algorithm in distance relays effectively locks the dc offset and subsynchronous resonance signals and quickly estimates the accurate fundamental frequency components.     

Research on the protection coordination ofpermanent magnet synchronous generatorbased wind farms with low voltage ridethrough capability

This paper proposes a pattern recognition based differential spectral energy protection scheme for ac microgrids using a Fourier kernel based fast sparse time-frequency representation (SST or simply the sparse S-Transform). The average and differential current components are passed through a change detection filter, which senses the instant of fault inception and registers a change detection point (CDP). Subsequently, if CDP is registered for one or more phases, then half cycle data samples of the average and differential currents on either side of the CDP are passed through the proposed SST technique, which generates their respective spectral energies and a simple comparison between them detects the occurrence and type of the fault. The SST technique is also used to provide voltage and current phasors and the frequency during faults which is further utilized to estimate the fault location. The proposed technique as compared to conventional differential current protection scheme is quicker in fault detection and classification, which is least effected from bias setting, has a faster relay trip response (less than one cycle from fault incipient) and a better accuracy in fault location. The significance and accuracy of the proposed scheme have been verified extensively for faults in a standard microgrid system, subjected to a large number of operating conditions and the outputs vindicate it to be a potential candidate for real time applications