ABB RELION RET670, Applications Manual

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Section 11 Frequency protection
11.1 Underfrequency protection SAPTUF (81)
IP15746-1 v3
11.1.1 Identification
M14865-1 v5
Function description IEC 61850
identification
IEC 60617
identification
ANSI/IEEE C37.2
device number
Underfrequency protection SAPTUF
f <
SYMBOL-P V1 EN-US
81
11.1.2 Application
M13350-3 v4
Underfrequency protection SAPTUF (81) is applicable in all situations, where reliable
detection of low fundamental power system frequency is needed. The power system
frequency, and the rate of change of frequency, is a measure of the unbalance between
the actual generation and the load demand. Low fundamental frequency in a power
system indicates that the available generation is too low to fully supply the power
demanded by the load connected to the power grid. SAPTUF (81) detects such
situations and provides an output signal, suitable for load shedding, generator boosting,
HVDC-set-point change, gas turbine start up and so on. Sometimes shunt reactors are
automatically switched in due to low frequency, in order to reduce the power system
voltage and hence also reduce the voltage dependent part of the load.
SAPTUF (81) is very sensitive and accurate and is used to alert operators that
frequency has slightly deviated from the set-point, and that manual actions might be
enough. The underfrequency signal is also used for overexcitation detection. This is
especially important for generator step-up transformers, which might be connected to
the generator but disconnected from the grid, during a roll-out sequence. If the
generator is still energized, the system will experience overexcitation, due to the low
frequency.
1MRK 504 163-UUS A
Section 11
Frequency protection
Transformer protection RET670 2.2 ANSI 637
Application manual