ABB REL650 series, Applications Manual, Page: 275 / 516

Section 9 Frequency protection
9.1 Underfrequency protection SAPTUF (81)
9.1.1 Identification
Function description IEC 61850
identification
IEC 60617
identification
ANSI/IEEE C37.2
device number
Underfrequency protection SAPTUF
f <
SYMBOL-P V1 EN
81
9.1.2 Application
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 506 334-UUS A
Section 9
Frequency protection
269
Application manual