7SR210 & 7SR220 Applications Guide
2.7
Directional Earth-Fault (50/51G, 50/51N, 51/51SEF)
The directional earth-fault elements, either measure directly or derive from the three line currents the zero
sequence current (operate quantity) and compare this against the derived zero phase sequence voltage
(polarising quantity). Section 1 of the Technical Manual ‘Description of Operation’ details the method of
measurement. The required setting is entered directly as dictated by the system impedances.
Example: Expected fault angle is -45° (i.e. residual current lagging residual voltage) therefore
67G Char Angle
=
-45°
However directional earth elements can be selectable to use either ZPS or NPS Polarising. This is to allow for the
situation where ZPS voltage is not available; perhaps because a 3-limb VT is being used. Care must be taken as
the Characteristic Angle will change if NPS Polarising is used.
Once again the fault angle is completely predictable, though this is a little more complicated as the method of
earthing must be considered.
Figure 2.7-1 Earth Fault Angles
©2011 Siemens Protection Devices Limited
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