This text/reference presents a comprehensive, up-to-date account of computer relaying in power systems, based in part on the author's extensive experience in the field. Provides background material on current relaying practices, and covers the mathematical foundations for relaying algorithms. Each chapter contains helpful illustrations, examples, and problems.

Computer Relaying for Power Systems

An optimal measurement scheme for tracking the harmonics in power system voltage and current waveforms is presented. The scheme does not require an integer number of samples in an integer number of cycles. It is not limited to stationary signals, but it can track harmonics with time-varying amplitudes. A review is first presented of the common frequency domain techniques for harmonics measurement. The frequency domain techniques are based on the discrete Fourier transform and the fast Fourier transform. Examples of pitfalls in the common techniques are given. The authors then introduce the concepts of the new scheme. This scheme is based on Kalman filtering theory for the optimal estimation of the parameters of time-varying harmonics. The scheme was tested on simulated and actual recorded data sets. It is concluded that the Kalman filtering algorithm is more accurate than the other techniques. >

A digital recursive measurement scheme for online tracking of power system harmonics

Phasor measurement units (PMUs) are rapidly being deployed in electric power networks across the globe. Wide-area measurement system (WAMS), which builds upon PMUs and fast communication links, is consequently emerging as an advanced monitoring and control infrastructure. Rapid adaptation of such devices and technologies has led the researchers to investigate multitude of challenges and pursue opportunities in synchrophasor measurement technology, PMU structural design, PMU placement, miscellaneous applications of PMU from local perspectives, and various WAMS functionalities from the system perspective. Relevant research articles appeared in the IEEE and IET publications from 1983 through 2014 are rigorously surveyed in this paper to represent a panorama of research progress lines. This bibliography will aid academic researchers and practicing engineers in adopting appropriate topics and will stimulate utilities toward development and implementation of software packages.

Synchrophasor Measurement Technology in Power Systems: Panorama and State-of-the-Art

The wavelet transform is a powerful tool in the analysis of the power transformer transient phenomena because of its ability to extract information from the transient signals simultaneously in both the time and frequency domain. This paper presents a novel technique for accurate discrimination between an internal fault and a magnetizing inrush current in the power transformer by combining wavelet transforms with neural networks. The wavelet transform is firstly applied to decompose the differential current signals of the power transformer into a series of detailed wavelet components. The spectral energies of the wavelet components are calculated and then employed to train a neural network to discriminate an internal fault from the magnetizing inrush current. The simulated results presented clearly show that the proposed technique can accurately discriminate between an internal fault and a magnetizing inrush current in power transformer protection.

A Novel Approach to the Classification of the Transient Phenomena in Power Transformers Using Combined Wavelet Transform and Neural Network

Devices specifically dedicated to highly accurate measurement of frequency have been described for specific applications like power system stabilizers. However, in most situations the digital estimate of the frequency deviation is needed concurrently with other decision quantities. Therefore, its value is usually obtained as a by-product of a more general-purpose algorithm, based, for instance, on the extended Kalman filtering or the recursive least error squares techniques. Unfortunately, a common problem with these Kalman filters is the high computational requirements, due to transcendental functions evaluation in real-time. Therefore, the need still exists for more clever implementations of the various real-time algorithms, which could alleviate the computational burden and enhance the adaptation speed during transients. To fulfil this need to some extent, two new methods suitable for fast adaptive estimation of voltage phasor and frequency deviation are outlined. >

Fast adaptive schemes for tracking voltage phasor and local frequency in power transmission and distribution systems

Digital protection algorithms are becoming more complex as the cost of computational equipment continues to decrease. In particular, optimal response digital filters, such as Kalman filters, can be implemented on presently available devices. Optimal filters are not yet extensively applied because they appear complex and engineers have not become familiar with their use. This paper presents the basics of the Kalman filtering technique in power systems terminology and illustrates its use for estimating rotating phasors. 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 and sample results are presented.

Kalman Filtering Applied to Power System Measurements for Relaying

The authors present the design, implementation and testing of a microprocessor-based high-speed relay that correctly detects transformer winding faults. Instead of relying on the presence of harmonics to identify magnetizing inrush, it uses a nonlinear model of the transformer to determine the state of its health. One version of the relay is suitable for protecting single-phase transformers, whereas another version is for protecting three-phase transformers. A three-phase delta-wye 15 kVA transformer was used to test the relay for a variety of operating conditions. The results show that the relay performs well. >

Design, implementation and testing of a microprocessor-based high-speed relay for detecting transformer winding faults

Electric power utilities use differential relays for detecting faults in power transformers. Conventional designs of these relays use second harmonic, third harmonic or total harmonic current as a restraining quantity. This inhibits the relay operation during magnetizing inrush conditions. Initial designs of digital differential relays also used similar approaches. Most designs used the second harmonic component of the current to block trippings during the magnetizing inrush conditions. As the operating voltages and lengths of transmission lines have increased substantially, currents during internal transformer faults can contain large harmonic components. Therefore, the security of differential relays using harmonic restraint is a matter of concern. However, some recently reported algorithms do not use second harmonic restraint to block trippings during magnetizing inrush conditions. They use winding currents. In most situations, the currents in a delta winding are not readily available. The algorithms based on transformer models need equivalent circuit parameters of the transformer. The relay engineer must determine the equivalent parameters of the transformer before using these algorithms. This paper presents a new digital algorithm for detecting winding faults in single-phase and three-phase transformers. The proposed algorithm does not use harmonic component of current to block trippings during magnetizing inrush conditions. It is applicable to situations where it is and it is not possible to measure winding currents. The algorithm does not require the B-H curve data and takes hysteresis losses of the transformer into account. The algorithm was tested using a variety of operating conditions simulated on a digital computer.

A digital relaying algorithm for detecting transformer winding faults

The design, development, and implementation of a computer-aided design (CAD) tool for developing digital controllers and relays are presented. The development includes modeling A/D converters, software, and the necessary logic. Procedures for modeling A/D converters and simulating the software are described. Application of the CAD tool is demonstrated by evaluating and comparing three different designs of a digital overcurrent relay. The designs are evaluated to determine both the speed and accuracy of the relays. The effects of the size of the A/D converters, the word-size used in computations, and the truncation/rounding are included in the simulation. Also described is the algorithm used in the relay design. >

A computer-aided design tool for developing digital controllers and relays

The authors describe the design of a microprocessor-based overcurrent relay that is suitable for protecting distribution and subtransmission networks. A modified technique for modeling the time-current characteristics is described. The proposed design is implemented using a Texas Instruments TMS320C25 digital signal processor. Some details of the hardware and software of the relay are presented. A digital overcurrent relay is also described. Test results indicate that the overcurrent relay accurately emulates the desired time-current characteristics. >

H.C. Wood

Papers

Kalman Filtering Applied to Power System Measurements for Relaying

Design, implementation and testing of a microprocessor-based high-speed relay for detecting transformer winding faults

A digital relaying algorithm for detecting transformer winding faults

A computer-aided design tool for developing digital controllers and relays

Design of a microprocessor-based overcurrent relay