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RESTRICTED EARTH FAULT PROTECTION (87REF)

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    System short circuits may occur across any two or even three lines, or, if the neutral Point is solidly earthed, between any one line and the earthed neutral the effect of a System short circuit is to produce over currents, the magnitude of which is Dependent on the short-circuit level of the system feeding the fault, the voltage which Has been short circuited, the impedance of the circuit up to the fault and the Impedance of the fault itself. The short-circuit currents produce very high mechanical Stresses in the equipment through which they flow, these stresses being proportional To the square of the currents. The magnitude of these short-circuit currents can be Limited by increasing the system impedance, usually incorporating this into the Supply transformers.

     

    1 -This protection shall be used for earth fault protection of solidly grounded Windings of EHV transformers/grounding transformers.

    2 -The relay shall be based on high-impedance principle and should be designed For being stable under all external faults.

     

    3-The relay shall not be affected by harmonics and DC components present in The through fault currents under CT saturation.

    4 -The relay shall be either voltage operated, current plus resistor calibrated or Voltage calibrated type and shall have a suitable and continuously adjustable Setting range.

    5 -The stabilizing/setting resistor shall be only externally mounted type with Wide range of variable flexibility setting range depending upon the type of The relay.

    6 -The current range of the relay shall be 10 to 40% of the rated current or Equivalent in voltage calibrated relays

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