4.27
Date Code 20150130 Instruction Manual SEL-787 Relay
Protection and Logic Functions
Basic Protection
We use this rating for all windings in the later tap calculation:
MVA = 30
Step 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:
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.
Step 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, you 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 you of the violation.
Step 6. Set the differential element characteristic. Select the settings
according to our suggestions in the earlier setting descriptions.
ICOM = Y (choose to define the CT compensation)
W1CTC=11 W2CTC=0
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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