ABB RELION 650 Series Applications Manual page 100

Section 6
Differential protection
Setting
a
b
c
Remarks:
1
2
3
4
5
6
7
Setting example for a small Tap-off transformer
94
IED 1
0.14
1.00
1.00
IBase (set in the Global base values function (GBASVAL).) is the reference
The parameter
current of Line differential protection given in primary Amperes. CT1 in terminal 1 (at end
1) has ratio 600/1 and, based on that, we chose
In this case, only one physical power transformer is included in the protected circuit.
However, in order to handle the situation with two CTs on the low-voltage side of the
transformer, one more fictitious power transformer is introduced. Thus, transformer A
can be thought of as being installed at the current terminal (end) 2, and transformer B,
which is identical to A, can be thought of as being installed at the current terminal (end)
3. The currents, measured at current terminals (current sources) 2 and 3, are internally
separately referred by the multi-terminal differential algorithm to the high-voltage side of
the transformer, using one and the same transformation rule. This rule is defined by the
power transformer transformation ratio and its type, which is Yd1 in this example. If an
in-line power transformer is included in the protected zone, then the protected power
lines are usually on the high-voltage side of the in-line power transformer. The differential
algorithm always transforms the low-voltage side currents to the high-voltage side.
Earth faults on the Y-side of the transformer will cause a zero sequence current that will
flow in the Y-winding of the power transformer. This zero sequence current will not
appear outside the transformer on the d-side, and will consequently not be measured by
CT 2 and CT 3. Thus, in case a Y-side earth fault is external to the protected zone, the zero
sequence current that passes the neutral point of the transformer will appear as false
differential current. This could cause an unwanted trip if the zero sequence currents are
not subtracted from all three fundamental frequency differential currents.
Energizing the circuit means that the power transformer will be energized at the same
time. This is assumed to be made always from the high-voltage side, and the harmonic
restraint will detect the inrush current and prevent a trip. Setting
is motivated in this case as the transformer is large.
IdMinHigh is active is set to 60 s because a power transformer is
The interval when
included in the protected circuit. As both IEDs process the same currents, both must have
IdMinHigh .
the same value set for
The unrestrained operate differential current value shall be greater than the highest
through fault current. This current appears at a three-phase short circuit on the 33 kV
side of the transformer and can be calculated as:
220
=
I
Through
× + +
3 (7.0 15.0 24.2)
EQUATION1423 V1 EN-US
With a safety margin of 20% we get:
×
1.2 I
×
1.2 2.75
Through
= =
Idunre
Ibase 0.6
EQUATION1424 V1 EN-US
The cross block logic shall always be active when there is a power transformer in the
protected zone.
IED 2
0.14
1.00
1.00
IBase to 600 A in this case.
=
2.75 kA
kA 3.30 kA
= =
5.50
kA 0.6 kA
1MRK 505 363-UEN A
Remarks
Not applicable in this
case (Default)
Not applicable in this
case (Default)
Not applicable in this
case (Default)
IdMinHigh = 2.00 · IBase
(Equation 21)
(Equation 22)
Application manual