GE 889 Instruction Manual page 295

CHAPTER 4: SETPOINTS
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
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.
The steady-state system fault current may result in generator current magnitudes less
than the full-load current of the generator (generally TOC delay is higher than transient
& sub-transient overcurrent time constants of the machine). This function relies on the
fact that the system fault will cause generator terminal voltage to drop, which can be
used to lower the pickup values.
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 4-57: Voltage Restraint characteristics for Phase TOC
VOLTAGE LOWER LIMIT
Range: 0.1 to 0.6 pu in steps of 0.1 pu
Default: 0.1 pu
This setpoint is effective only when the VOLTAGE RESTRAINT setpoint is set to Enabled.
The VOLTAGE LOWER LIMIT setpoint enhances coordination of this system back function
with system protections, and prevents very rapid generator tripping prior to primary
system protection operation. The typical value for this setting is between 0.1 pu to 0.3
pu, and it is calculated such that under extreme emergency conditions (the lowest
expected system voltage), the function will coordinate with system protection to prevent
misoperation, but still provide backup in case system faults are not cleared.
BLOCK
Range: Off, Any FlexLogic operand
Default: Off
OUTPUT RELAY X
For details see Common
EVENTS
Range: Enabled, Disabled
Default: Enabled
Setpoints.
PROTECTION
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