Wednesday, March 23, 2011

Electromechanical Relays and Static Relays

8. Electromechanical Relays and Static Relays
  • The electromechanical relays are based on the comparison between operating torque/ force and restraining torque/force.
  • The VA burden of such relays is high.
  • The characteristics have limitations.
  • Each relay unit can perform only one protective function.
  • Such relays are used for simple and less costly protection purposes.
  • For important and costly equipment and installation, static relays are preferred
  • In static relays, sensing, comparison, and measurements are made by static (electronic) circuits having no-moving parts.
  • Static relays have versatile characteristics, offer low burden, and incorporate several protective/control/monitoring functions in one compact unit.
  • Recently (1980's), programmable static relays incorporating microprocessors have been introduced. Microprocessor based relays have several superior features such as:
  1. Indication of operating values on demand and thereby no need for separate indicating instruments on panel.
  2. A single relay can perform 10 or more different protective functions, thereby reducing number of separate relays and increasing reliability.
  3. Memory function e.g a relay which has tripped on a fault can remember and flash on the display the magnitude of current and instant of time at the time of tripping.
  4. Better properties and extended range of application for generation, transmission, distribution, and industrial applications.
  •  The range of static relays is rapidly spreading
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