GE 845 Instruction Manual page 290

PROTECTION
4–162
VOLTAGE RESTRAINT
Range: Disabled, Enabled
Default: Disabled
This setting enables or disables the Voltage Restraint function for the TOC element.
When set to "Enabled" this feature lowers the Pickup value of each individual Phase Time
Overcurrent element in a fixed relationship with its corresponding phase input voltage.
If cold load pickup, autoreclosing, or manual close blocking features are controlling the
protection, the Phase TOC Voltage Restraint does not work, even if "Enabled" is selected.
Voltage restraint is used to lower the current pickup level for TOC function in linear
proportion as shown in figure below. For example, if phase TOC PICKUP setting is set to
1.000 XCT, in case of system faults cause generator terminal voltage drops to 0.4 pu
(ratio of Phase-Phase Voltage/ VT Nominal Phase-phase voltage), the new pickup with
voltage restraint would be 1.000*0.4=0.400 xCT. During the fault condition when the
voltage drops, the overcurrent relay pickup also drops linearly and it should be verified
that for the limiting case the new voltage restraint relay pickup should be lower than
(around 50% of) the fault current. Refer IEEE C37.102-2006, Annex-A for more details.
Figure 4-53: Voltage Restraint characteristics for Phase TOC
BLOCK
Range: Off, Any FlexLogic operand
Default: Off
OUTPUT RELAY X
For details see Common
EVENTS
Range: Enabled, Disabled
Default: Enabled
TARGETS
Range: Self-reset, Latched, Disabled
Default: Self-reset
Setpoints.
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS