DIGITAL ALGORITHMS AND RELAYING SCHEMES FOR PROTECTION OF POWER SYSTEMS

Abstract

Digital protection relaying for power system has been a subject of wide research in many countries since the arrival of low-cost microprocessor. A primary requirement of micro computer relaying is the availability of computationally simple algorithms for digital filtering. For distance protection, the resistance and reactance seen at the relaying location, which serve as the fault dis criminants, need to be computed from the voltage and current signals at that location. Some of the algorithms presume the signals to be purely sinusoidal for this computation. Naturally, these algorithms are extremely simple and give fast convergence to post-fault steady-state values, but the results are highly inaccurate because of the presence of d.c offset and harmonics in the relaying signals in post-fault condition. Others extract fundamental frequency voltage and current components of the relaying signals and then proceed to compute resistance and reactance. Filter algorithms based on Fourier, Walsh and Haar transforms and reported in literature can provide highly accu rate estimate of fundamental frequency components, but signal processing involves some time consuming operations specially multiplication. Moreover, both the real and imaginary compo nents are extracted for computing resistance and reactance which makes the filtering process more time-consuming. There fore these algorithms are not well suited to signal process ing (which has to be done in real time) on low-cost micropro cessors.

Differential protection of transformer based on harmonicrestraint relaying has been used widely and successfully over several decades. Fundamental frequency component of the diffe rential current is used as operating quantity whereas its second and fifth harmonic components as restraining quantities to block tripping on magnetizing inrush and overexcitation conditions, respectively. The ratio of harmonics to the fundamental frequ ency component serves as the fault discriminant for harmonic restraint differential relaying. For implementing this relay ing on a microprocessor, computationally simple but accurate algorithms must be available for the extraction of these compo nents of the differential current. Most of the available filter algorithms need time consuming process of multipli cation. The present work is aimed at simplifying the available filter algorithms and developing new algorithms which are computationally simple to enable their implementation on a microprocessor for distance and differential relaying without using an arithma'tic coprocessor or multiplier and have accu racies comparable to those of Fourier-transform based algorithms. The Fourier, Walsh and Haar transform based filter algo rithm for extracting a desired frequency component of a relay ing signal contain some multiplier terms. These terms are suitably rounded off, so that multiplication can be achieved through a few binary shifts and additions/subtractions with a consequent significant reduction in processing time. The

frequency responses of the algorithms thus simplified have been studied using Z-transform and compared with those obtained for the originals. It has been found that the judicious rounding off of terms does not significantly alter the response of algorithms. The fault discriminants computed for distance relaying and harmonic restraint differential relaying from the components extracted using these simplified algorithms also match closely with those computed without involving any approxi mation. New algorithms based on cosine and Hartley transform with full-cycle window have been developed both for distance and harmonic-restraint differential relaying. These algorithms are modified to make them computationally simple for implementation on microprocessor. Frequency responses have been computed and efficacies have been studied offline for distance relaying on mainframe computer with assumed typical relaying signals and the signals obtained from power system analysis using Electro magnetic Transient package (EMTP). New definition for the fault discriminant for harmonic restraint differential relaying are proposed and their efficiencies, alongwith those of the filter algorithms meant to extract real components alone, have been tested offline. With a view to reduce tripping time, a half-cycle window cosine-transform-based algorithm has also been developed. Because of a faster convergence of the fault discriminant to its post-fault steady-state value when a half-cycle window filter is used, more detailed study has been made on this filter algorithm.

i Effect of variation of system frequency on the computed values of fault discriminants for distance relaying, using proposed filters has been studied by varying the frequency value by + 2 Hz over the nominal of 50 Hz keeping the sampling rate unchanged. Two relaying schemes, one for distance protection of transmission line and the other for harmonic-restraint diffe rential protection of transformer have been developed and implemented on Intel 80861 microprocessor without the support of any coprocessor or multiplier. Half-cycle simplified cosinetransform algorithm has been used for distance relaying and fullcycle simplified cosine-transform algorithm for differential relaying. The real time testing of the prototypes indicates a tripping time of 17.5 ms for either relay under the worst cases studied.