Schweitzer Engineering SEL-387-0, Instruction Manual, Page: 72 / 556

3.22
SEL-387-0, -5, -6 Relay Instruction Manual Date Code 20170601
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Differential Element
Equation 3.9
Equation 3.10
The relay calculates these taps automatically if MVA is given.
If MVA is set to OFF, the user must calculate the taps and enter
them individually.
The relay will check to see if a violation of the maximum tap
ratio has occurred, and will notify the user of the violation.
That is, it will divide the greatest TAP
n , in this case 10.46, by
the least TAP n , here 1.74, to get a ratio of 6.01. Because this is
below 7.5, adjustment of the CT ratio is unnecessary.
Step 6. Set the differential element characteristic.
Select the settings according to our suggestions in the earlier
setting descriptions. For this example, we have selected a two
slope, variable-percentage differential characteristic for
maximum sensitivity at low currents and greater tolerance for
CT saturation on external high-current faults. The settings are
as follows:
O87P = 0.3 (Operate current pickup in multiple of tap)
SLP1 =
25
(25 percent initial slope)
SLP2 =
50
(50 percent second slope)
IRS1 = 3.0 (limit of slope 1, Restraint current in multiple of tap)
U87P = 10 (unrestrained differential Operate current level,
multiple of tap)
CT2 = 15 (block operation if second harmonic is above
15 percent)
PCT4 = 15 (block operation if fourth harmonic is above
15 percent)
PCT5 = 35 (block operation if fifth harmonic is above
35 percent)
TH5P = OFF (no fifth-harmonic alarm)
DCRB =
N
(dc ratio blocking disabled)
HRSTR =
N
(harmonic restraint disabled)
IHBL = N (no independent element blocking; any unit detecting
second or fifth harmonic above PCT2 or PCT5 will block all
units)
Remember that the O87P setting must yield an operating current value of at
least 0.1 • I
N
, at the least tap. In this case O87P
MIN
= (0.1 • I
N
)/TAP
MIN
= 0.5/
1.74 = 0.287. Therefore, the O87P setting of 0.3 is valid.
The differential unit settings are complete for this specific application. At this
point you can also choose to set backup overcurrent elements which we
discuss at the end of this section.
TAP2
100 MVA 1000
•
3 138 kV 240
• •
-----------------------------------------------
1
• =
TAP2 1.74 A
=
TAP3
100 MVA 1000
•
3 13.8 kV 400
• •
-------------------------------------------------
1
• =
TAP3 10.46 A
=