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Inability of Earth fault protection system

  1. Apr 7, 2008 #1
    As you know, for earth fault protection of electrical system, the residual current measurement is often performed using the three line CTs. But in this case the protective relay must be slightly time-delayed to avoid spurious trips caused by circulation of a false zero sequence current following a brief period of saturation in the current transformers (magnetizing current, switching surge or down stream faulty). This limitation leads to too high a setting compared to the maximum available earth fault current, and consequently to a loss of sensitivity.
    This difficulty can be largely overcome by using a ring current transformer in neutral point of power transformer in order to achieve zero-sequence current measurement. In this case the earth fault relay can simultaneously provide both rapid and sensitive earth fault protection.
    This technical recommendation is applied to a new island electrical system with power auto transformer by a design group. They used the earth fault relay and related NCT in neutral point of auto transformer. But they had not suitable response from earth fault protection against the occurrence of ground faults.

    How can you explain the reason of protection system inability in this case?


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  2. jcsd
  3. Apr 7, 2008 #2


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    Staff: Mentor

    Your quiz questions are usually out of my area of expertise, MSJ, but I'll venture a guess on this one. Since you specifically mentioned "island", I'll guess that the higher soil conductivity associated with islands in salt water oceans was causing the ground fault currents to still trip the GFCI circuit too quickly. Am I close?
  4. Apr 7, 2008 #3
    No my friend, the island electrical system concept is not important here.

    How can we address experts and how can we adverties for our web site?

    Last edited: Apr 7, 2008
  5. Apr 24, 2008 #4
    Because of electrical and magnetically coupling between primary and secondary windings of auto transformers, the primary and secondary earth fault currents are mixed in neutral point of transformer. The difference phase angle between secondary current and induced primary current is approximately 180 degree, therefore the amount of through earth fault current is the difference of Ip and Is , where the Ip and Is are the primary and secondary earth fault current respectively. So the earth fault protection system in neutral point can not detect actual primary or secondary earth fault currents.

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