ABB Relion 670 Series Applications Manual page 281

1MRK 511 401-UEN Rev. K
Section 11
11.1 Underfrequency protection SAPTUF
11.1.1 Identification
Function description
Underfrequency protection
11.1.2 Application
Underfrequency protection SAPTUF 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 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 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.
11.1.3 Setting guidelines
All the frequency and voltage magnitude conditions in the system where SAPTUF 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:
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.
The under frequency start value is set in Hz. All voltage magnitude related settings are made
as a percentage of a global base voltage parameter. The UBase value should be set as a
primary phase-to-phase value.
Some applications and related setting guidelines for the frequency level are given below:
Bay control REC670
Application manual
Frequency protection
IEC 61850
identification
SAPTUF
© Copyright 2017 Hitachi Power Grids. All rights reserved
IEC 60617 ANSI/IEEE C37.2
identification device number
81
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SYMBOL-P V1 EN-US
Section 11
Frequency protection
IP15746-1 v3
M14865-1 v5
M13350-3 v4
M13355-3 v8
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