296
FUNCTION CHARACTERISTICS
Use
As described by two-generator model previously introduced, out of step draws on the RX plane (refe-
rence Fig. C) a sliding of measured impedance Z from half-plane R> 0 to half-plane R <0 intersecting
points locus identified with Z
TOT
. Other transient phenomena (e.g. power swing) draw initially similar
trajectories but the subsequent results are different because they don’t cross Z
TOT
points locus and do
not run in R<0 half-plane.
It is possible to declare out of step occurs close points where δ = 180°, where Z measurement falls on
Z
TOT
and near to R-X plane ordinates axis.
78 protection detectsout of step conditions from Z trajectory on R-X plane checking δ angle and Z
real component sign (resistive component).
The protection provides for an alarm emission (identified as ALARM 1) in case of detection of a po-
tential OOS trajectory, Z lies on half-plane R> 0 and δ angle exceeds alarm angle δAL, moving close to
scroll zone. ALARM 1 detects both power swing and OOS events, therefore it is not always followed
by the protection trip.
Out of step events represent a full slip impedance in R-X plane and can be classified as (reference
Fig. E1):
• Internal – when out of step draws a sliding trajectory that falls in generator/elevator
transformer area
• External – when out of step draws a sliding trajectory that falls in outside generator/
elevator transformer area
In case of internal out of step event, it is recommended an immediate generator discon-
nection in order to minimize generator damages. On the contrary, in the case of external
out of step event, it is recommended the generator disconnection after 3 or 4 slides, in
order to allow correct protection coordination.
78 Protection allows to discriminate sliding typology by counting of detected slides in
each zone:
• All (Internal and External) detected out of step sliding are counted as “ZONE A” slides
and increase relative counter A. The “ZONE A” trip command is issued when the
counter reaches NA set value
• Detected Internal sliding are counted as “ZONE B” slides and increase relative counter
B. The “ZONE B” trip command is issued when the counter reaches NB set value.
ALARM 2 is provided if the value of one of two counters is greater than 1.
Sliding counters of both zones are reset in case of trip command emission or when no
further sliding events occur for a user programmable time.
Out of step protection must disconnect the generator from the network avoiding gene-
rator circuit breaker opening action when the generator is out-of-phase with electrical
system; in this state, angle δ is close to 180°and there are the less favorable conditions for
generator switch opening operation (maximum currents to be interrupted and maximum
transient recovery voltage between the breaker poles).
XMR-D EQUIPMENT MANUAL
Ed. 2.9 - 02/2021
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