| 摘要 |
The directional element, following enablement under selected input current conditions, calculates a zero-sequence impedance, in response to values of zero sequence voltage and zero sequence current. The current value is selected between two possible values, one being the value from an associated current transformer, the other being the sum of the currents IA+IB+IC. The calculated zero sequence impedance is then compared against sensitive selected threshold values, established particularly for ungrounded systems. A forward fault indication is provided when the calculated zero sequence impedance is above a first established sensitive threshold value and a reverse fault indication is provided when the calculated zero sequence impedance is below a second established sensitive threshold value.
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