BECKWITH ELECTRIC M-3425A, Инструкция

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4–46
M‑3425A Instruction Book
21 Phase Distance
The Phase Distance function (21) is designed
for system phase fault backup protection and is
implemented as a three-zone mho characteristic.
Three separate distance elements are used to
detect AB, BC, and CA fault types. The ranges and
increments are shown in Figure 4-25. The diameter,
offset, system impedance angle (relay characteristic
angle), and definite time delay need to be selected
for each zone for coordination with the system
relaying in the specific application.
Zone 1, Zone 2 and Zone 3 may be used for backup
protection for unit transformer and transmission
faults. Zone 3 in conjunction with Zone 2 can be
used to detect an Out of Step condition and it can
be programmed to block Function 21 #1 and/or 21
#2. If Zone 3 is being used for out-of-step blocking,
it does not trip.
If Zone 1 is not set to see the transmission system,
out-of-step blocking is not recommended.
When Zone 3 is used for Out-of-step blocking, the
out of step delay is used for the detection of the
transit time of the swing between Zone 3 and Zone
2 impedances.
The load encroachment blinder function can be
set with a reach and an angle as shown in Figure
4-29. When enabled, this feature will block the
21 Function from misoperating during high load
conditions.
When the generator is connected to the system through
a delta/wye transformer, proper voltages and currents
(equivalent to the high side of the transformer) must be
used in order for the relay to see correct impedances
for system faults. By enabling the Delta-Y Transform
feature (see Section 4.2 Setup System), the relay can
internally consider the 30° phase shift (30° lead delta-ab
or 30° lag delta-ac) through the delta/wye transformer,
saving auxiliary VTs. Impedance calculations for
various VT connections are shown in Table 4-5. All
impedance settings are secondary relay quantities and
can be derived from the following formula:
Z
SEC
= Z
PRI
x (R
C
÷ R
V
)
where Z
SEC
= secondary reflected impedance, Z
PRI
=
primary impedance
,
R
C
= current transformer ratio,
and R
V
= voltage transformer ratio.
The minimum current sensitivity depends on the
programmed reach (diameter and offset). If the
current is below the minimum sensitivity current,
the impedance calculated will saturate, and not be
accurate. This will not cause any relay misoperation.
An overcurrent supervision feature can be enabled,
which will block the 21 function when all three phase
currents are below the pickup value.