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Relays and circuit breakers

  • Relays and circuit breakers

    Faults in electrical systems are of two main types; symmetrical (with three-phases) and unsymmetrical (with one or two phases) faults. Lightning, which is a form of excessive charge, is the number one cause of electrical faults, as it interrupts normal current flow at high level transmission, and provides a short circuit path for these charges through the ground rather than the normal line current flow. Permanent damage to TL insulators and/or transformers can be avoided by taking some detective measures (through relays), and protective actions (through circuit breakers) against these very short but big impact faults. Relays, which are considered to be the brain of protection systems, are designed to send an “OPEN” control signal to high–speed switching circuit breakers, in order to isolate the faulty part from the rest of the system, until the fault clears. A few cycles after the transient dies out, the relay sends a “CLOSE” control signal to the circuit breakers for normal steady-state operation to resume.

    this topic is highly important for relay and circuit breaker selection and ratings, and is covered in many references.
    Protection equipment includes; instrument transformers as sensing devices, relays, and circuit breakers, which are installed along the electrical system in various locations and can be illustrated as in

    Dependent on the sensed values of instrument transformers and compared to nominal values, output of the relay sent to circuit breakers can be one of the three possible control decisions:

    1. Normal operation detected: keep flow (stay ON).
    2. Fault detected: disconnect flow “OPEN” (open circuit breaker).
    3. Fault cleared: transient died out “CLOSE” (reclose for normal flow).

     
    Reference: Electric Renewable Energy Systems
     

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