GE Multilin
745 Transformer Management Relay
5-41
5 SETPOINTS
5.6 S4 ELEMENTS
5
b) PERCENT DIFFERENTIAL
PATH: SETPOINTS
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S4 ELEMENTS
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DIFFERENTIAL
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PERCENT DIFFERENTIAL
This section contains the settings to configure the percent differential element. The main purpose of the percent-slope char-
acteristic of the differential element is to prevent maloperation because of unbalances between CTs during external faults.
These unbalances arise as a result of the following factors:
•
CT ratio mismatch (not a factor, since the 745 automatically corrects for this mismatch)
•
Onload tap changers which result in dynamically changing CT mismatch
•
CT accuracy errors
•
CT saturation
The basic operating principle of the percent differential element can be described by the following diagram and its associ-
ated equations:
Figure 5–6: PERCENT DIFFERENTIAL OPERATING PRINCIPLE
Restraint current calculations have been changed from average to maximum to provide better security dur-
ing external faults.
The basic percent differential operating principle for three-winding transformers is illustrated by the following equations:
(EQ 5.4)
PERCENT DIFFERENTL
[ENTER] for more
PERCENT DIFFERENTIAL
FUNCTION: Enabled
Range: Enabled, Disabled
PERCENT DIFFERENTIAL
TARGET: Latched
Range: Self-Reset, Latched, None
PERCENT DIFFERENTIAL
PICKUP: 0.30 x CT
Range: 0.05 to 1.00 x CT in steps of 0.01
PERCENT DIFFERENTIAL
SLOPE 1: 25%
Range: 15 to 100% in steps of 1
PERCENT DIFFERENTIAL
KNEEPOINT: 2.0 x CT
Range: 1.0 to 20.0 x CT in steps of 0.1
PERCENT DIFFERENTIAL
SLOPE 2: 95%
Range: 50 to 100% in steps of 1
PERCENT DIFFERENTIAL
BLOCK: Disabled
Range: Logc Inpt 1 to 16, Virt Inpt 1 to 16, Output Rly 2 to
8, SelfTest Rly, Vir Outpt 1 to 5, Disabled
ENTER
ESCAPE
ð
ð
MESSAGE
ESCAPE
MESSAGE
ESCAPE
MESSAGE
ESCAPE
MESSAGE
ESCAPE
MESSAGE
ESCAPE
MESSAGE
ESCAPE
Percent
Diff
Element
CT1
CT2
CT3
V1
V2
V3
I
1
I
2
I
3
NOTE
I
r
I
restraint
max
I
1
I
2
I
3
,
,
(
)
=
=
;
I
d
I
differential
I
1
I
2
I
3
+
+
=
=
%slope
I
d
I
r
----
100%
×
=