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I.L. 40-386.3
1)
If the calculated Zone1 impedance is 0.5 ohms (secondary) or less the line per-
centage, used for the calculation, should be 70-75%.
2)
If the Source Impedance Ratio or SIR, (ratio of positive sequence source imped-
ance to positive sequence line impedance) is in the range of 3-5 the line percent-
age, used for the calculation, should be no more than 75%.
3)
Circuit fault impedance angles in the range of 80 degrees produce dc time con-
sonant of about one cycle. One cycle time constants result in maximum over-
reach error of about 16%. Hence the line percentage used should be no more
than 70 - 75%. If the total fault impedance angle is greater than 86 degrees, the
dc time constant is greater than 2.3 cycles, and the overreach error is reduced
to 10 percent or less. The same is true if the fault impedance angle is less than
75 degrees. If system fault impedance angles are known to be either above 86
degrees or below 75 degrees, the line percentage, used for the Zone1 calculation,
can be increased by 5 percent. (All angles are based on 60 Hz systems.)
4)
If CCVT's, of the low-capacitance type, (e.g. 1960's vintage PCA-5 and PCA-8 de-
signs) are in use, the line percentage, used for the Zone1 calculation, should be
70-75%. Severe subsidence-transient related overreach has been noted in cases
where low-capacitance CCVT's are used in "short line" applications. An alterna-
tive to reducing the Zone1 setting, is to introduce a Zone1 time delay (T1) of one
or two cycles and using the 80 percent Zone1 reach calculation.
3.2.3
Distance Unit Reverse Reach Settings
Ground distance units in the REL 301/302 have a reverse reach setting as well as the
forward reach setting discussed in the Zone1 ground distance setting example above.
The reverse reach setting is intended to change the diameter of the ground mho char-
acteristic, which increases coverage along the resistive (R) axis, while holding the
reach in the "maximum torque" direction fixed. (See Figure 2-2). The reverse reach set-
ting does not provide reverse tripping capability since the distance units are direction-
ally supervised.
As an example, if the Zone1 forward reach (Zone1 G) is set for 10 ohms and the Zone1
reverse reach (Zone1G R) is set to the minimum setting of 0.01 ohms, the resistive
reach will be approximately 2.6 ohms. If Zone1 G remains set for 10 ohms and
Zone1G R setting is increased to 10 ohms, the resistive reach will now be 10 ohms.
Increasing the reverse reach further will increase the coverage in the resistive "direc-
tion" while not increasing the reach in the forward direction. The following formula can
be used to determine the intercept of the ground characteristic with the resistive axis,
in both the positive (Rpos) and negative (Rneg) R direction.
.
,
Rpos
Rneg
Where ZG and ZGR represent the Zone1, Zone2, Zone3 or Pilot (REL 302) ground dis-
tance forward and reverse reach settings respectively. Ang Pos is the positive sequence
Impedance angle. The factory default setting, for the reverse reach(s) is 0.01 ohms.
3-4
(
)
ZG ZGR
–
cos
–
Ang Pos
=
------------------------------------------------------------------------------------------------------------------------------- -----------------------------------------------------------------------------
2
2
±
∠
((ZG ZGR)
–
cos
AngPos)
2
+
4 ZG ZGR
(10/94)
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