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Directional Earth Fault

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Directional Earth Fault protection” with ANSI code (67N) is widely used for detecting phase-to-ground faults

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and serves as a backup protection in transmission lines, as well as in power plants and other facilities where

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directional earth fault detection is required

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Therefore, understanding the operation of this protection function is essential

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In this protection, the Self-Polarization method is used for fault detection

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meaning the fault voltage and current are utilized to determine the fault direction

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As shown in the figure, this voltage and current include VResidual = 3V0 and IResidual = 3I0

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The reason for using -3I0 relates to the relay's rear wiring configuration

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In Siemens relays, the wiring setup is recommended as shown in the figure

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As you can see from the wiring, the zero-sequence current enters the relay’s neutral CT in the reverse direction

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To determine the MTA (Maximum Torque Angle), the angle of 3V0 is rotated by φrotation in the clockwise direction

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After determining the MTA, a zone is considered forward (as shown in the figure) that spans 90 degrees on either side of the MTA

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As a result, the MTA angle is equal to: AngleMTA = (-180 - φrotation)

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Since the fault current Isc is injected from the tester during relay testing

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the MTA and the Sector Opening range must be defined in the tester software based on the injected current

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In this case, the calculations of 3V0 and -3I0 are implemented within the relay and according to its algorithm

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. Therefore, if the MTA angle in the tester software is increased by 180 degrees, the calculations in the tester (due to the injection of Isc current)

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will be as shown in the figure: **MTA = (-180 - φrotation) + 180 -----> MTA = - φrotation**