MEASUREMENT BASED CONTROL OF POWER SYSTEMS

Abstract

This chapter provides an overview of measurement-based control of power systems. The measurements are generally of fundamental frequency components of the signals in the presence of extraneous components in the power system voltage and current signals. Although measurements at such high speed are necessary in relaying applications, slower measurements have better accuracy and are therefore more desirable for power system monitoring and control tasks. As the measurements provide symmetrical components of voltages and currents, positive sequence quantities can be used in the monitoring and control tasks. The power system models used in these applications is the positive sequence model; it is therefore desirable that a consistent measurement set be used. The phasor representation of power system voltages and currents can be viewed as the Discrete Fourier Transform (DFT) of their data samples. Phasor calculations can be performed over one period, or over a fraction of a period. Longer the data window used for phasor calculation, better is the accuracy of its estimate. If the power system frequency changes from its nominal value, the phasor rotates in the complex plane at a speed proportional to the shift in frequency. Consequently, the recursive DFT offers a very sensitive and accurate frequency measuring system. There are three practical alternative techniques for synchronization of clocks to this accuracy: the first one is the navigational-aid transmission, the second alternative is the use of satellite transmissions, and the third alternative is the use of a direct communication channel between substations—preferably a fiber-optic channel.