GE 869 - Page 247

CHAPTER 4: SETPOINTS PROTECTION
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–115
differential current is below the first slope, S1, for more than 200 ms. When in the
"EXTERNAL FAULT" state, the algorithm goes into the "EXTERNAL FAULT & CT
SATURATION" state if the differential flag is set (DIF = 1). When in the "EXTERNAL FAULT &
CT SATURATION" state, the algorithm keeps the saturation flag set (SAT = 1). The state
machine returns to the "EXTERNAL FAULT" state if the differential flag is reset (DIF = 0) for
100 ms.
There are three SAT flags (one per phase). The corresponding FlexLogic operands are:
• Percent Diff Sat A
• Percent Diff Sat B
• Percent Diff Sat C
Figure 4-36: Saturation detection state machine
Phase comparison principle:
The test for direction can be summarized by the following equation:
If (|I
ts
| > B
1
or (|I
ts
| > K x I
r
and |I
ts
| > PKP)) and (|I
ns
| > B
1
or (|I
ns
| > K x I
r
and |I
ns
| > PKP))
Then DIR = abs ( <Its - <Ins ) > 90°
else DIR = 1
where:
I
r
= restraining current
DIR = flag indicating that the phase comparison principle is satisfied,
B
1
= breakpoint 1 setting,
I
ts
, I
ns
= current phasor at the terminal and neutral side banks, respectively
K = factory constant of 0.25
There are three DIR flags (one per phase). The corresponding FlexLogic operands are:
• Percent Diff Dir A
• Percent Diff Dir B
• Percent Diff Dir C
NOTE
NOTE:
That the above CT saturation (SAT) and phase direction (DIR) checking are applicable to
internal summation method only. If the Core balance method is selected, neither SAT nor
DIR check is performed, the fixed values SAT=0 and DIR = 1 are used.