Comprehensive UHS Distance Protection Via Knowledge-based Approach

Abstract

A knowledge-based decision system is developed for EHV transmission line distance protection. This paper describes the two major aspects of the system, (1) Knowledge representation, (2) logic process, for fault distance calculation. Different knowledge representation techniques are discussed with their relative performances, The developed system is shown to be reliable and stable under different fault condition, including zero fault inception angle. Computing burden is quite reasonable and ultra high speed (UHS) performance can be easily achieved with commercial microprocessors. The need and significance of UHS relaying for EHV transmission lines are by now well known. It improves power system reliability, transient stability and reduces the damagre caused to the equipments. There are caused to the equipments. There are apparently two distinct approaches in distance protection taken to achieve high speed performance. One is based on fast estimation of power frequency components using Kalman filters symmetrical. components etc, The other approach relier on the information content in the high frequency part of the fault induced components. The later is popularly called as traveling wave (TW) based distance rela ying. Both the schemes have their own advantage and dioadvantages. The initial position of the fault induced voltage and currents are heavily dominated by travelling waves. So for the initial period of a few milliseconds, signal to noise ratio is higher for TW-based relaying and lower for power frequency based relaying. However, for near zero-fault inception angles the magnitudes of the travelling waves are very low and it is very difficult to detect these faults using TW-baoed distance relaying. The problem can be solved by combining both the methods with greater computing burden and algorithm complexity. A novel approach is presented here to solve this problem using knowledge-based system. The beauty of this system is that the whole spectrum of the information -ontalned In the fault Induced signals (limited by the sampling frequency) is considered in a very simple manner. The idea of the knowledge-based system came from the fact that post fault voltage and current waveforms are dependent on fault distance. The authors, for a time, were engaged in applying different signal processing techniques to develop TW-based distance relaying. It is observed that fault distance can be estimated from the fault-induced signals using various signal processing techniques e.g. correlations of the signals, power spectrum of the signals signal cepstrum . The decission process related to all the algorithms are more or less same : detecting some strongest component and its position. The information in the other part of the power spectra on correlation functions cannot be easily treated or related to fault distance always, thus motivating the application of knowledge-based system. The informations in fault induced signals are stored as a function of fault distance with the help of digital simulation. The information may be the raw post fault signals or some pre- processed versions of it. However, fault-generated waveforms are also dependent on various system parameters, other than fault distance. Some of the system parameter variations are taken account of in the knowledge representation part. Even then the searching procedure will be too much time consuming and some amount of higher level knowledge has to be applied to cut down the searching procedure. This technique helped the proposed relay achieving UHS performance with commercial microprocessors.