Underfrequency Restoration - GE Multilin 850 Instruction Manual

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Underfrequency Restoration

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4–350
Upon voltage recovery (i.e. measured voltage above programmed setpoint), and "Close"
output relay selected as Self-Reset type, the UF Restore operating condition and hence the
Close output relay, will reset automatically after the breaker is detected closed. If the
"Close" output relay is selected as Pulsed type, the output relay will stay closed, until the
programmed Seal-In time expires.
The 8 Series relays provide up to two Underfrequency Restoration (UF Restore) elements.
When the relay is ordered with support for two UF Restoration elements, the UF Restore 1
is always associated with Breaker 1, and UF Restore 2 is always associated with Breaker 2.
Other breaker combinations are not possible. The UF Restore element will send close
command only to the output relay that is selected under the Close Relay Select setpoint of
the corresponding breaker.
This scheme is initiated by user-defined operands, which can be any operand of the
underfrequency elements or frequency rate of change in a decreasing direction. Once
initiated, it monitors the composite voltage level and frequency, and sends a command
when the frequency and the composite voltage has risen above the programmed
minimum level for a selected time interval. Synchrocheck supervision may be applied for
this command to operate Close Relay #2. The scheme is equipped with an Incomplete
Sequence timer, so it does not remain initiated for an indeterminate time, but
automatically resets if the voltage and frequency do not recover during the programmed
interval. Initiation of the scheme can be cancelled by a Reset command. A "Block
Restoration" logic input is available to prevent both initiation and operation. It is
recommended that if automatic Underfrequency Restoration is to be used, the Cold Load
Pickup feature is also enabled in order to prevent the breaker from tripping shortly after it
is automatically closed.
The settings of this function produce Pickup and Close flags. Once Underfrequency
Restoration is initiated by the user-selected operand, the Underfrequency Restoration
Pickup flag is asserted when the measured frequency and voltage of the specified source
are above the MINIMUM value. The Underfrequency Restoration Close flag is asserted if
the element stays picked up for the time defined by the restore time delay.
The incomplete sequence timer automatically resets the element if the voltage and
frequency do not recover during the programmed interval after the initiation.
The use of the Underfrequency (UF) Restoration feature must be carefully accessed
regarding the application of the breaker in the power system.
If the "Synchrocheck Supervision" setpoint is not available (per relay's order code) to
program in the UF Restoration menu, or it is available, but is set to "Bypass", the application
of the feature must be limited to the use of breakers associated with de-energized feeders,
lines, buses.
If the Synchrocheck function is selected in the order code, the UF Restoration can be
safely used for breakers connecting two live lines, or two live buses, providing the setpoint
Synchrocheck Supervision is set to Synchrocheck Breaker Close Permission (Sync1 Close
Perm).
Path:
Setpoints > Control > Underfrequency Restoration 1(X)
FUNCTION
Range: Disabled, Close, Alarm, Latched Alarm, Configurable
Default: Disabled
Output relay #2 "Close" operates only when the Close function is selected, the element
operates, and Synchrocheck supervision is applied. The "ALARM" LED does not turn on if
the element operates when set to function Close.
850 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS