GE TRANSFORMER MANAGEMENT RELAY 745 Instruction Manual page 91

5 SETPOINTS
b) PERCENT DIFFERENTIAL
PATH: SETPOINTS S4 ELEMENTS
PERCENT DIFFERENTL
[ENTER] for more
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:
Restraint current calculations have been changed from average to maximum to provide better security dur-
ing external faults.
NOTE
The basic percent differential operating principle for three-winding transformers is illustrated by the following equations:
GE Multilin
Courtesy of NationalSwitchgear.com
DIFFERENTIAL
ð
PERCENT DIFFERENTIAL
ENTER
FUNCTION: Enabled
ESCAPE
PERCENT DIFFERENTIAL
ESCAPE
TARGET: Latched
MESSAGE
PERCENT DIFFERENTIAL
ESCAPE
PICKUP: 0.30 x CT
MESSAGE
PERCENT DIFFERENTIAL
ESCAPE
SLOPE 1: 25%
MESSAGE
PERCENT DIFFERENTIAL
ESCAPE
KNEEPOINT: 2.0 x CT
MESSAGE
PERCENT DIFFERENTIAL
ESCAPE
SLOPE 2: 95%
MESSAGE
PERCENT DIFFERENTIAL
ESCAPE
BLOCK: Disabled
MESSAGE
V3
V1
CT1
V2
1
Percent
Diff
Element
Figure 5–6: PERCENT DIFFERENTIAL OPERATING PRINCIPLE
( , ,
I I max I I I
= =
r restraint 1 2 3
%slope
=
745 Transformer Management Relay
PERCENT DIFFERENTIAL
Range: Enabled, Disabled
Range: Self-Reset, Latched, None
Range: 0.05 to 1.00 x CT in steps of 0.01
Range: 15 to 100% in steps of 1
Range: 1.0 to 20.0 x CT in steps of 0.1
Range: 50 to 100% in steps of 1
Range: Logc Inpt 1 to 16, Virt Inpt 1 to 16, Output Rly 2 to
8, SelfTest Rly, Vir Outpt 1 to 5, Disabled
CT3
CT2
3
2
)
; I I I I
= = + +
d differential 1 2
I
d
×
--- -
100%
I
r
5.6 S4 ELEMENTS
I
3
(EQ 5.4)
5
5-41