GE 745 Instruction Manual page 145

CHAPTER 5: SETPOINTS
5.6.3.2 Percent Differential
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PERCENT [
DIFFERENTIAL
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745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
PATH: SETPOINTS
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This section contains the settings to configure the percent differential element. The main
purpose of the percent-slope characteristic 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 associated equations:
FIGURE 5–7: Percent differential operating principle
Restraint current calculations have been changed from average to maximum to
provide better security during external faults.
S4 ELEMENTS DIFFERENTIAL

PERCENT DIFFERENTIAL
FUNCTION: Enabled
PERCENT DIFFERENTIAL
RLYS (1-8): --------
PERCENT DIFFERENTIAL
TARGET: Latched
PERCENT DIFFERENTIAL
PICKUP: 0.30 x CT
PERCENT DIFFERENTIAL
SLOPE 1: 25%
PERCENT DIFFERENTIAL
KNEEPOINT: 2.0 x CT
PERCENT DIFFERENTIAL
SLOPE 2: 100%
PERCENT DIFFERENTIAL
BLOCK: Disabled
V3
V1
CT1
CT2
V2
1
Percent
Diff
Element
S4 ELEMENTS
PERCENT DIFFERENTIAL

Range: Enabled, Disabled
Range: 1 to 8
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
3
2
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