Frequency Protection; Underfrequency Protection Saptuf (81); Identification; Application - Hitachi Relion 670 Series Applications Manual

1MRK511407-UUS Rev. N
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.
The under frequency pickup 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:
Phasor measurement unit RES670
Application manual

Frequency protection

IEC 61850
identification
SAPTUF
© 2017 - 2023 Hitachi Energy. 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 v6
M13350-3 v4
M13355-3 v8
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