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:
- Indication of operating values on demand and thereby no need for separate indicating instruments on panel.
- A single relay can perform 10 or more different protective functions, thereby reducing number of separate relays and increasing reliability.
- 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.
- Better properties and extended range of application for generation, transmission, distribution, and industrial applications.
- The range of static relays is rapidly spreading
