Análise de estimadores de frequência sob condições transitórias

Abstract

With the advances and the integration of digital relays in electrical power systems, major developments occurred in the protection systems. Through a microprocessor, digital relays use algorithms based on the processing of voltage and current signals, which are becoming more complex and are performing various functions beyond the basic protection ones. In this context, more and more the electric companies are looking for the best performance in the operation of the networks, which also depends on the protection systems. Therefore, it is necessary to conduct studies on the behavior of protection devices for the different conditions which the electric power system is subject. This includes to analyze the reliability, the speed of response, the robustness and the security offered by such protective elements to the Electric Power Systems. Thus, an important characteristic and one of the monitored quantities in a power system is the electrical frequency of the provided voltage signal. Such parameter should be constant since it directly influences in the functioning of the system equipment and in the quality of industrial processes. Conducted in an agile and accurate way, the frequency estimation assists in the protection, control and monitoring of the power quality in order to ensure the fullness of the system. To perform the frequency estimation, various methodologies have been proposed, since the most of them do not have good performance when the signal presents harmonic distortion or noise. Therefore, this work presents an evaluation of some of the methods already proposed for this purpose. Such techniques are compared using indices such as the convergence time, the overshoot and the estimation transient error of the response in order to verify which technique has the best performance. For this comparison, tests were carried out using signals with sudden frequency variation and frequency varying in ramp, in exponential and in damped variation. Besides, tests including signal amplitude variation, power swings and sub-harmonic and harmonic distortion were carried out too. The comparison results are shown in graphs and tables, in order to distinguish between the advantages and disadvantages of each method against the different variations, in order to determine the best options to be used in the protection of electric power systems.