9291_c005 - 5 Digital Relaying 5.1 5.2 5.3 5.4 5.5 5.6...

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5 Digital Relaying Jame s S. Thorp Virginia Polytechnic Institute 5.1 Sampling . ............................................................................. 5 -2 5.2 Antialiasing Filters. .............................................................. 5 -2 5.3 Sigma-Delta A = D Converters. ............................................ 5 -2 5.4 Phasors from Samples. ........................................................ 5 -4 5.5 Symmetrical Components . ................................................. 5 -5 5.6 Algorithms . .......................................................................... 5 -7 Parameter Estimation . Least Squares Fitting . DFT . Differential Equations . Kalman Filters . Wavelet Transforms . Neural Networks Digital relaying had its origins in the late 1960s and early 1970s with pioneering papers by Rockefeller (1969), Mann and Morrison (1971), and Poncelet (1972) and an early field experiment (Gilcrest et al., 1972; Rockefeller and Udren, 1972). Because of the cost of the computers in those times, a single high-cost minicomputer was proposed by Rockefeller (1969) to perform multiple relaying calculations in the substation. In addition to having high cost and high power requirements, early minicomputer systems were slow in comparison with modern systems and could only perform simple calculations. The well-founded belief that computers would get smaller, faster, and cheaper combined with expectations of benefits of computer relaying kept the field moving. The third IEEE tutorial on microprocessor protection (Sachdev, 1997) lists more then 1100 publications in the area since 1970. Nearly two thirds of the papers are devoted to developing and comparing algorithms. It is not clear this trend should continue. Issues beyond algorithms should receive more attention in the future. The expected benefits of microprocessor protection have largely been realized. The ability of a digital relay to perform self-monitoring and checking is a clear advantage over the previous technology. Many relays are called upon to function only a few cycles in a year. A large percentage of major disturbances can be traced to ‘‘hidden failures’’ in relays that were undetected until the relay was exposed to certain system conditions (Tamronglak et al., 1996). The ability of a digital relay to detect a failure within itself and remove itself from service before an incorrect operation occurs is one of the most important advantages of digital protection. The microprocessor revolution has created a situation in which digital relays are the relays of choice because of economic reasons. The cost of conventional (analog) relays has increased while the hardware cost of the most sophisticated digital relays has decreased dramatically. Even including substantial software costs, digital relays are the economic choice and have the additional advantage of having lower wiring costs. Prior to the introduction of microprocessor-based systems, several panels of space and considerable wiring was required to provide all the functions needed for each zone of transmission line protection. For example, an installation requiring phase distance protection for phase-to-phase and three-phase faults, ground distance, ground-overcurrent, a pilot scheme, breaker failure, and reclosing logic demanded redundant wiring, several hundred watts of power, and a lot of panel space. A single
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This note was uploaded on 03/03/2010 for the course POWER 332 taught by Professor Dr during the Spring '10 term at Ain Shams University.

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9291_c005 - 5 Digital Relaying 5.1 5.2 5.3 5.4 5.5 5.6...

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