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Protection Question No.37 – Generator protection

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  • #308

      There are 2 generators in parallel and they are coupled to a common busbar where there is a stepup transformer.

      Each generator has been earthed through an earthing transformer. I am looking for the convenient protection functions to eliminate and discriminate the earth fault in different locations:
      1) on the busbar
      2) inside generator (100% stator)
      3) between generator and busbar

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    • Author
    • #1583

        1- Did you use one step up transformer for two generators? 2-What is rated power of generators? 3-What grounding systems did you use ,high or low impedance?


          The type of grounding employed and the system configuration determine the choice of protective scheme. If a high-impedance scheme is chosen, the ground fault current is limited to a value that will not cause mechanical or thermal stress on the generator. Protective schemes applied on high impedance grounded systems are focused on sensitivity and not on speed of operation. Therefore in this case suitable discrimination is provided easily. The decision to use high-impedance grounding is based on the desire to minimize core damage by limiting the ground fault current below a damage threshold value. In theory, if the fault current is not damaging, then the protective relaying need not trip but can be applied to alarm only. This would allow generation to be manually shut down in an orderly manner, thus avoiding mechanical and thermal stresses that would accompany a trip and load rejection. For systems with damaging ground fault current rapid tripping of the generator, field breaker and, shutdown of the prime mover are recommended. On high impedance grounded systems tripping is an option. On the other hand, protection for grounding schemes that do not limit the ground fault current (low- impedance grounding systems) below damaging levels are less sensitive, but must operate quickly. An insulation fault inside the metal casing of a generating set may severely damage the generator of this set. The fault must be quickly detected and eliminated. Furthermore, if other generators are parallelly connected, they will generate energy in the fault and may cause overload tripping. Continuity of supply is no longer ensured. For this reason, a GFP device built-into the generator

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