ELECTROMECHANICAL RELAYS:
Early relay designs utilized actuating forces that were produced by electromagnetic interaction between currents and fluxes, much as in a motor. These forces were created by a combination of input signals, stored energy in springs, and dash pots. The plunger type relays are usually driven by a single actuating quantity while an induction type relay may be activated by a single or multiple inputs (see Figs. 3.9 and 3.10.). Although existing protection is provided primarily by electromechanical relays that is because the cost and complexity of replacing them may be prohibitive; never the less, new construction and major system or station revisions are witnessing the replacing of electromechanical relays with solid state or digital relays.
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| FIGURE 3.9 Plunger type relay. |
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| FIGURE 3.10 Principle of construction of an induction disk relay. Shaded poles and damping magnets are omitted for clarity. |
SOLID STATE RELAYS:
The expansion and growing complexity of modern power systems have brought a need for protective relays with a higher level of performance and more sophisticated characteristics. This has been made possible by the development of semiconductors and other associated components, which can be utilized in many designs, generally referred to as solid-state or static relays. All of the functions and characteristics available with electromechanical relays are available with solid-state relays. They use low-power components but have limited capability to tolerate extremes of temperature, humidity, over voltage, or over current. Their settings are more repeatable and hold to closer tolerances and their characteristics can be shaped by adjusting the logic elements as opposed to the fixed characteristics of electromechanical relays. This can be a distinct advantage in difficult relaying situations. Solid-state relays are designed, assembled, and tested as a system that puts the overall responsibility for proper operation of the relays on the manufacturer. Figure 3.11 shows a solid-state instantaneous over current relay.
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| FIGURE 3.11 A possible circuit configuration for a solid-state instantaneous overcurrent delay. |
COMPUTER RELAYS:
It has been noted that a relay is basically an analog computer. It accepts inputs, processes them electromechanically or electronically to develop a torque or a logic output, and makes a decision resulting in a contact closure or output signal. With the advent of rugged, high performance microprocessors, it is obvious that a digital computer can perform the same function. Since the usual relay inputs consist of power system voltages and currents, it is necessary to obtain a digital representation of these parameters. This is done by sampling the analog signals, and using an appropriate algorithm to create suitable digital representations of the signals. The functional blocks in Fig. 3.12 represent a possible configuration for a digital relay.
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| FIGURE 3.12 Major subsystem of a computer relay. |
In the early stages of their development, computer relays were designed to replace existing protection functions, such as transmission line and transformer or bus protection. Some relays used microprocessors to make the relay decision from digitized analog signals; others continue to use analog functions to make the relaying decisions and digital techniques for the necessary logic and auxiliary functions. In all cases, however, a major advantage of the digital relay was its ability to diagnose itself; a capability that could only be obtained, if at all, with great effort, cost, and complexity. In addition, the digital relay provides a communication capability to warn system operators when it is not functioning properly, permitting remote diagnostics and possible correction.
As digital relay investigations continued another dimension was added. The ability to adapt itself, in real time, to changing system conditions is an inherent, and important, feature in the software dominated relay. This adaptive feature is rapidly becoming a vital aspect of future system reliability.