Hitachi 670 Series Application Manual page 42

Section 4
Analog and binary signal transfer for line differential protection
Notes:
1
2
3
4
5
6
7
Setting example with a small tap transformer in the protected zone
A typical example is shown in Figure
36
IBase (set in GBASVAL) is the reference current of the line differential protection given in
primary Amperes. CT1 in terminal 1 has a ratio of 600/1 and that is why 600 A is chosen for
IBase in this example.
Only one physical power transformer is included in the protected circuit. However, 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 CT2, and
transformer B, which is identical to A, can be thought of as being installed at CT3. Currents
measured at CT2 and CT3 are internally separately referred to the high-voltage side of the
transformer by the multi-terminal differential algorithm using the same transformation rule. This
rule is defined by the power transformer transformation ratio and its type (Yd1 in this example).
If an in-line power transformer is included in the protected zone, 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 cause a zero sequence current that flows in the Y-
winding of the power transformer. This current does not appear outside the transformer on the
d-side, and is consequently not measured by CT2 and CT3. Thus, if Y-side earth fault is
external to the protected zone, zero sequence current that passes the neutral point of the
transformer appears as a false differential current. This can cause an unwanted trip if zero
sequence currents are not subtracted from all three fundamental frequency differential currents.
Energizing the circuit means that the power transformer is energized at the same time. It is
assumed that this is always made from the high-voltage side, and the harmonic restraint
detects the inrush current and prevents a trip. Setting IdMinHigh = 2.00 · IBase is justified in
this case since the transformer is large.
The interval when IdMinHigh is active is set to 60 s because a power transformer is included in
the protected zone. As both IEDs process the same currents, both must have the same value
set for IdMinHigh.
The unrestrained operate (differential) current limit should 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%:
×
1.2 I
×
1.2 2.75
Through
= =
Idunre
Ibase 0.6
EQUATION1424 V1 EN-US
The cross-block logic should always be active when there is a power transformer in the
protected zone.
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=
2.75 kA
kA 3.30 kA
= =
5.50
kA 0.6 kA
37
1MRK505382-UEN Rev. K
(Equation 5)
(Equation 6)
Communication set-up, 670/650 series
Application Guide