Date Code 20020129
Differential, Restricted Earth Fault,
3-25
Overcurrent, Voltage, and Frequency Elements
SEL-387E Instruction Manual
ICOM
= Y (choose to define the CT compensation)
W1CTC = 11
W2CTC
= 11
W3CTC = 0
The relay will multiply the wye CT currents from the wye transformer windings by the
matrix [CTC(11)] to give the same results as the physical DAC CT connection. Using
the flexibility of the current compensation feature, another logical setting choice might
have been to use the autotransformer windings as a reference and to adjust the delta
tertiary currents. However, considering the need to remove zero-sequence current from
the autotransformer CT outputs to the relay, the settings for this approach would be
W1CTC = 12, W2CTC = 12, and W3CTC = 1. This is more burdensome,
computationally, than the default settings indicated above.
5. Enter winding line-to-line voltages. The relay needs these voltages for the tap
calculation. Voltages are in units of kV. For this example we enter the following values:
VWDG1 = 230
VWDG2
= 138
VWDG3 = 13.8
The relay now calculates each tap current, using the formula stated previously:
C
•
CTRn
VWDGn
•
3
1000
•
MVA
TAPn
¼
=
(C = 1 for wye CTs)
Thus, we have the following:
1
•
120
•
kV
230
•
3
1000
•
MVA
100
1
TAP
=
TAP1 = 2.09 A
1
•
240
•
kV
138
•
3
1000
•
MVA
100
2
TAP
=
TAP2 = 1.74 A
1
•
400
•
kV
8
.
13
•
3
1000
•
MVA
100
3
TAP
=
TAP3 = 10.46 A
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 TAPn, in this case
10.46, by the least TAPn, here 1.74, to get a ratio of 6.01. Because this is below 7.5,
adjustment of the CT ratio is unnecessary.
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)
