CHAPTER 4: SETPOINTS MONITORING
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–273
Frequency OOB
Accumulation (81A)
A generator turbine is designed to operate within a specific frequency and speed range
(around the nominal), typically ±5%. During load rejection or coast down, the generator
turbine may operate at a higher speed (frequency) or lower speed (frequency) respectively,
which is outside of the normal operation frequency band. In these conditions, the turbine
will pass through operating frequencies where the turbine blades find stimulus at their
natural frequencies. Blade vibrations at these natural frequencies result in stress on the
blades that is accumulated, and can result in cracking in some parts of the blade structure
such as the tie wires or blade covers. The Frequency Out-Of-Band (OOB) Accumulation
feature provides diagnostic information and alarms based on the accumulated off-
nominal (out-of-band) frequency operation time of a turbine over several frequency bands.
This alarm can be used to schedule maintenance or other actions as desired. The available
setting of an upper and lower frequency for each band enables the user to overlap bands
of frequencies in the accumulation function. The accumulation function can be blocked as
desired. For example, accumulation may be blocked during an out-of-service condition or
during testing as set via an input to the relay.
The following General settings are available to all accumulators.
Path: Setpoints > Monitoring > Generator > Frequency OOB Accumulation > General
MINIMUM VOLTAGE
Range: 0.10 to 1.25 x VT in steps of 0.01 x VT
Default: 0.10 x VT
This setting specifies the minimum positive-sequence voltage level required to allow the
frequency out-of-band accumulators to operate. This threshold is used to prevent an
incorrect accumulation when operating quantity is too low.
RESET ACCUMULATORS
Range: Off, Any FlexLogic operand
Default: Off
Frequency OOB accumulators can be reset to zero using any FlexLogic operand.
NOTE:
Using Virtual Input with the Self-reset option is not recommended. Any FlexLogic operand
selected as a reset input should remain high for at least two power cycles.
BLOCK
Range: Off, Any FlexLogic operand
Default: Off
This setting selects a FlexLogic operand to block out-of-band frequency accumulation
ina ll bands/accumulators. This is typically required to avoid unnecessary accumulation
when relay testing is being performed.
EVENT
Range: Enabled, Disabled
Default: Disabled
TARGET
Range: Self-reset, Latched, Disabled
Default: Self-reset
The following individual settings are available for each of the seven frequency out-of-band
accumulators.
Path: Setpoints > Monitoring > Generator > Frequency OOB Accumulation > Accum 1(7)
FUNCTION
Range: Disabled, ALarm, Latched Alarm, Configurable
Default: Disabled
This setting enables and disables a specific frequency band accumulator.
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