4.24
SEL-787 Relay Instruction Manual Date Code 20081022
Protection and Logic Functions
Basic Protection
4. Decide whether to use internal CT compensation and determine
compensation settings. Because there are both wye and delta
transformer windings but only wye CTs, we must adjust for the
phase angle shift. In the “traditional” differential relay
connection the wye transformer windings would have their CTs
connected in delta to produce a shift in the same direction as
that produced in the transformer. In this case a “DAC” or “30-
degree lagging” connection would have been used. This would
not only shift the currents, but it would remove the zero-
sequence current component by physically subtracting the
appropriate phase currents via the delta connection. We achieve
the same effect within the relay by using the selected
compensation. The settings are:
ICOM = Y (choose to define the CT compensation)
W1CTC = 11 W2CTC = 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.
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:
The relay now calculates each tap current, using the formula
stated previously:
Thus, we have the following:
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.
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
VWDG1 = 138 VWDG2 = 13.8
TAPn
MVA 1000•
3VWDGn CTRn••
---------------------------------------------------------
C•=
where
C 1 for wye CTs=
TAP1
30MVA 1000•
3 138kV 40••
-----------------------------------------=
TAP1 3.14=
TAP2
30MVA 1000•
3 13.8kV 400••
----------------------------------------------=
TAP2 3.14=
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