Hitachi Relion 670 Series Applications Manual page 197

1MRK506375-UEN Rev. N
Section 10
10.1 Underfrequency protection SAPTUF
10.1.1 Identification
Function description
Underfrequency protection
10.1.2 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.
10.1.3 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:
Railway application RER670
Application manual
Frequency protection
IEC 61850
identification
SAPTUF
© 2017 - 2023 Hitachi Energy. All rights reserved
IEC 60617 ANSI/IEEE C37.2
identification device number
81L
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SYMBOL-P V1 EN-US
Section 10
Frequency protection
IP15746-1 v3
M14865-1 v6
M13350-3 v4
M13355-3 v8
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