Distance Protection Phase Selection - GE MiCOM P40 Agile Technical Manual

P543i/P545i
For directional zones, the directionality element must agree with the tripping zone. Zones 1, 2, and 4 are
●
always directional whereas other zones are only directional if set as directional. In directional zones the
directionality element must agree with the tripping zone. For example, Zone 1 is a Forward Directional zone
and must not trip for Reverse faults. Therefore a Zone 1 trip is only allowed if the directionality element
issues a Forward decision. Zone 4 is reverse-looking so needs a Reverse decision by the directionality
element.
The set time delay for the measuring zone must expire, with the measured fault impedance remaining
●
inside the zone characteristic for the duration of the delay time. Typically, Zone 1 has no time delay
(instantaneous), whereas all other zones have time delays.
Where channel-aided distance schemes are used, the time delay tZ2 for overreaching Zone 2 may be
●
bypassed for some of the schemes.
4.4

DISTANCE PROTECTION PHASE SELECTION

Phase selection allows the product to identify exactly which phases are involved in a fault and enables the correct
measuring zones to trip.
Operation of the distance elements is controlled by a Superimposed Current Phase Selector. For a period of two-
cycles after pick-up of the phase selector, only elements associated with the fault type selected by the phase
selector are allowed to operate. If these elements do not operate, all elements are enabled for the following five
cycles, before the phase selector returns to its quiescent state.
Operation of an enabled distance element during the two-cycle, or five-cycle period, causes the phase selector
state to be maintained until the element resets. An exception to this is when the phase selector changes decision
while an element is operated. In this case, the selected elements are reset and the two cycle period restarts with
the new selection.
Note:
Any existing trip decision is not reset under this condition. After the first cycle following a selection, the phase selector is only
permitted to change to a selection involving additional phases.
On double phase-to-earth faults, only the phase-to-phase elements are enabled. This is because they are
generally more accurate under these conditions than earth fault elements. A biased neutral current level detector
operates to indicate the involvement of earth in the fault.
4.4.1 FAULTED PHASE SELECTION
The faulted phase or phases are selected by comparing the magnitudes of the three phase-to-phase
superimposed currents. A single phase-to-earth fault produces the same superimposed current on two of these
signals and zero on the third. A phase-to-phase or double-phase-to-earth fault produces one signal which is larger
than the other two. A three phase fault produces three superimposed currents which are the same size. The figure
below shows how the change in current can be used to select the faulted phases for a C phase-to-ground (CN)
fault.
P54x1i-TM-EN-1
Chapter 7 - Distance Protection
169