P54x/EN OP/La4 Operation
(OP) 5-32
MiCOM P543, P544, P545 & P546
OP
The faulted phase impedance must appear within a tripping (measuring) zone,
corresponding to the phase selection. Five independent zones of protection are
provided. The tripping zones are mho circles or quadrilateral, and selected
independently for phase, and ground faults. The ground fault distance elements require
compensation for the return impedance, this residual compensation modifies the replica
impedance for each zone. Under conditions were a parallel line is present the relay can
compensate for the mutual coupling between the lines; this adjusts the replica
impedance in the same way as the residual compensated based on the current in the
parallel line. The reach setting Z for ground fault mho and quadrilateral elements is
determined as follows:
Z = Z
1
+ [( res / P ) x Zres ] + [( Imut / I
P
) x Zmut ]
Where:
Z
1
is the positive sequence reach setting
P
is the current in the faulted phase
res
is the residual current (= a + b + c)
Z
res
is the residual impedance (= (Z
0
-Z
1
)/3) = K
res
x Z
1
I
mut
is the residual current in the parallel line
Z
mut
is the mutual compensating impedance
For directional zones within the relay (Zone 1, P, 2, 4 and Z3 if set directional), the delta
directional line must be in agreement with the tripping zone. For example, zone 1 is a
forward directional zone, and must not trip for reverse faults behind the relay location. A
zone 1 trip will only be permitted if the directional line issues a “forward” decision. The
converse will be true for zone 4, which is reverse-looking and this needs a reverse
decision by the directional line. If the delta directional cannot make a decision then
conventional direction lines are used.
The set time delay for the measuring zone must expire, with the fault impedance
measured inside the zone characteristic for the duration. In general, Zone 1 has no time
delay (“instantaneous”), all other zones have time delays. Where channel-aided
distance schemes are used, the time delay tZ2 for overreaching Zone 2 may be
bypassed under certain conditions.
In order to achieve fast, sub-cycle operation, the phase selection, measuring zones and
directional line algorithms run in parallel, with their outputs gated in an AND configuration.
This avoids sequential measurement which would slow the operation of the relay.
1.10 Phase selection (Distance option only)
Phase
selection is the means by which the relay is able to identify exactly which phase are
involved in the fault and allow the correct measuring zones to trip.
Operation of the distance elements, is controlled by the Superimposed Current Phase
Selector. Only elements associated with the fault type selected by the phase selector are
allowed to operate during a period of two cycles following the phase selection. If no such
element operates, all elements are enabled for the following 5 cycles, before the phase
selector returns to its quiescent state.
Operation of an enabled distance element, during the two cycle or 5 cycle period, causes the
phase selector state to be maintained until the element resets. The one exception to this is
when the phase selector decision changes while an element is operated. In this case, the
selected elements are reset and the two cycle period re-starts with the new selection.
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