ABB Relion 670 Series Applications Manual page 237

1MRK 511 401-UUS F
Section 10
10.1 Underfrequency protection SAPTUF (81)
10.1.1 Identification
Function description
Underfrequency protection
10.1.2 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.
10.1.3 Setting guidelines
All the frequency and voltage magnitude conditions in the system where SAPTUF (81) performs its
functions should be considered. The same also applies to the associated equipment, its frequency
and time characteristic.
There are two specific application areas for SAPTUF (81):
1. to protect equipment against damage due to low frequency, such as generators,
transformers, and motors. Overexcitation is also related to low frequency
2. to protect a power system, or a part of a power system, against breakdown, by shedding load,
in generation deficit situations.
Bay control REC670
Application manual
Frequency protection
IEC 61850
identification
SAPTUF
© Copyright 2017 ABB. All rights reserved
IEC 60617 ANSI/IEEE C37.2
identification device number
81
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SYMBOL-P V1 EN-US
Section 10
Frequency protection
IP15746-1 v3
M14865-1 v5
M13350-3 v4
M13355-3 v8
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