GE 845, Instruction Manual

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4–232 845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
PROTECTION CHAPTER 4: SETPOINTS
Overfrequency (81O)
The 845 relay provides two identical Overfrequency (OVERFREQ) elements per protection
group, or a total of 12 elements.
A significant overfrequency condition, likely caused by a breaker opening and
disconnecting load from a particular generation location, can be detected and used to
quickly ramp the turbine speed back to normal. If this is not done, the over speed can lead
to a turbine trip, which would then subsequently require a turbine start up before restoring
the system. If the overfrequency turbine ramp down is successful, the system restoration
can be much quicker. The overfrequency monitoring feature of the relay can be used for
this purpose at a generating location.
The Overfrequency feature is inhibited from operating unless the magnitude of the positive
sequence or auxiliary voltage rises above a threshold. When the supply source is
energized, the overfrequency delay timer is allowed to start timing only when the
threshold is exceeded and the frequency is above the programmed Pickup level. In the
same way, when an overfrequency condition starts the overfrequency delay timer and the
voltage falls below the threshold before the timer has expired, the element resets without
operating.
The Overfrequency element may be set as an instantaneous element with no time delay,
or as a definite time delayed element. The Overfrequency element has a fixed minimum
operating threshold to prevent undesired operation during periods of unavailable voltage.
The input voltages are the three phase-to-phase voltages from delta connected VTs (PTs),
three phase-to-ground voltages from wye connected VTs (PTs), or single phase auxiliary
voltage.
The settings of this function are applied to each source to produce Pickup and Operate
flags. The Overfrequency Pickup flag is asserted when the measured frequency of the
specified source is above the PKP value and the voltage is above the threshold. The
Overfrequency Operate flag is asserted if the element stays picked up for the time defined
by the Pickup time delay. The element drops from Pickup without operation if the
measured frequency decreases below 0.03 Hz of the Pickup value and stays dropped out
for the defined time delay before the time for operation is reached.
The minimum operating voltage is set as a threshold below which the element is blocked.
Path :
Setpoints > Protection > Group 1(6) > Frequency > Overfrequency 1(X)
FUNCTION
Range: Disabled, Trip, Alarm, Latched Alarm, Configurable
Default: Disabled
PICKUP
Range: 20.00 to 65.00 Hz in steps of 0.01 Hz
Default: 60.5 Hz
PICKUP DELAY
Range: 0.000 to 6000.000 s in steps of 0.001 s
Default: 2.000 s
DROPOUT DELAY
Range: 0.000 to 6000.000 s in steps of 0.001 s
Default: 2.000 s
MINIMUM VOLTAGE
Range: 0.000 to 1.250 x VT in steps of 0.001 x VT
Default: 0.700 x VT
The setting sets the minimum voltage for Overfrequency element operation specified
per times VT.