ABB RELION REX640 Technical Manual page 929

1MRS759142 F
The measured system frequency is compared to the set value to detect the
underfrequency condition. The measured rate of change of frequency (df/dt)
is compared to the set value to detect a high frequency reduction rate. The
combination of the detected underfrequency and the high df/dt is used for
the activation of the load-shedding. There is a definite time delay between the
detection of the underfrequency and high df/dt and the activation of LSHDPFRQ.
This time delay can be set and it is used to prevent unwanted load-shedding actions
when the system frequency recovers to the normal level.
Once the frequency has stabilized, LSHDPFRQ can restore the load that is
shed during the frequency disturbance. The restoration is possible manually or
automatically.
The function contains a blocking functionality. It is possible to block function
outputs, timers or the function itself.
4.6.2.4 Analog channel configuration
LSHDPFRQ has one analog group input which must be properly configured.
Table 891: Analog inputs
Input
U3P
There are a few special conditions which must be noted with the configuration.
Table 892: Special conditions
Condition
U3P connected to real measurements
Improper analog channel configuration causes a validation error if the analog
channels are not completely configured or they do not match with certain settings.
For troubleshooting, check the contents of this chapter and also the preprocessing
blocks in this document. The configuration can be written to the protection relay
once the mismatch is corrected.
4.6.2.5 Operation principle
The function can be enabled and disabled with the
corresponding parameter values are "On" and "Off".
The operation of LSHDPFRQ can be described using a module diagram. All the
modules are explained in the next sections.
REX640
Technical Manual
Throughout this document, "high df/dt" is used to mean "a high rate of
change of the frequency in negative direction."
See the preprocessing function blocks in this document for the possible
signal sources.
Protection functions
Description
Three-phase voltages
Description
The function requires that at least one phase
or phase-to-phase voltage channel is connec-
ted.
Operation setting. The
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