GE SR345 Instruction Manual page 180

S3 PROTECTION
Idifferential (xCT)
897802.cdr
6–68
Courtesy of NationalSwitchgear.com
Figure 18: Differential / Restraint characteristic
OPERATE REGION
SLOPE 1
25%
PICKUP
0.30
BREAKPOINT 1
The figure above shows the differential/restraint characteristic of the main transformer
percent differential protection.
PATH:
SETPOINTS > S3 PROTECTION > S3 SETPOINT GROUP1 > XFMR PCNT DIFF
XFMR DIFF FUNCTION
Range: Disabled, Latched Alarm, Alarm, Trip
Default: Disabled
The selection of the Latched Alarm, Alarm, or Trip setting enables the Percent Phase
Differential function. Output relays #1 "R1 TRIP" and #2 "R2 TRIP" will operate when the
Trip setting is selected and the Percent Differential protection is in operation. In this
mode the "ALARM" LED does not turn on. The "ALARM" LED will flash under the phase
differential operating condition, with the function setting selected as Alarm, and will self-
reset when the condition clears. If the Latched Alarm setting is selected, the "ALARM"
LED will flash during the Phase differential operating condition, and will stay "ON" after
the condition clears, until the reset command is initiated. Output relay #1 "Trip" will not
operate when the Latched Alarm or Alarm setting is selected.
MINIMUM PICKUP
Range: 0.05 to 1.00 x CT in steps of 0.01
Default: 0.10 x CT
This setting defines the minimum differential current required for operation. The setting
is selected based on the amount of differential current that might be seen under normal
conditions. This differential current is a result of the transformer magnetizing current, CT
inaccuracies, and a transformer ratio that may change as a result of the transformer on-
load tap changer.
SLOPE 1
Range: 15 to 100% in steps of 1
Default: 25%
This setting is applicable for restraint (through) currents of zero to kneepoint 1, and
defines the ratio of differential/restraint currents, above which the element will operate.
The slope is set to ensure sensitivity of internal faults at normal transformer loading.
SLOPE 2
100%
Transition Region
(lineal)
BREAKPOINT 2
345 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 6: SETPOINTS
Irestraint (xCT)