GE 869 Instruction Manual page 243

CHAPTER 4: SETPOINTS
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
Where: I is the current phasor of the terminal side CTs (J1-CT bank), in per unit normalized
ts
to the reference CT nominal; I
per unit normalized to the reference CT nominal; They are calculated based on the
following equations:
Where:
The Percent Differential element with the internal summation method selected uses dual
slope dual break point differential/restraint characteristic. The main purpose of the
percent-slope characteristic is to prevent improper operation caused by unbalances
between CTs during external faults. CT unbalances arise as a result of the following factors:
1. CT accuracy errors
2. CT saturation
The characteristic allows for very sensitive settings when fault current is low and less
sensitive settings when fault current is high and CT performance may produce incorrect
operate signals. The dual slope characteristic is shown in the following figure.
Figure 4-35: Percent Differential Characteristic
The horizontal axes value of the percent differential characteristic is the Restraining
current (I ) and the vertical axes value is the Differential current (I
r
is the current phasor of the neutral side CTs (K1-CT bank), in
ns
PROTECTION
Eq. 3
).
d
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