Frequency Protection 81 O/U; Description - Siemens 7SC80 Manual

2.10

Frequency Protection 81 O/U

The frequency protection function detects overfrequencies and underfrequencies in the power system. If the
frequency lies outside the permissible range, appropriate switching actions are initiated.
Applications
•
Underfrequency is caused by an increased real power demand of the power system or faulty operation of
the frequency regulation.
•
Overfrequency is caused, e.g. by load shedding (island network) or malfunction of the frequency regula-
tion.
2.10.1

Description

Detection of Frequency
The frequency is detected preferably from the positive sequence voltage. If this voltage is too low, the
phaseto- phase voltage V
phaseto- phase voltages is used instead.
Through the use of filters and repeated measurements, the frequency evaluation is free from harmonic influ-
ences and very accurate.
Overfrequency/Underfrequency
Frequency protection consists of four frequency elements. To make protection flexible for different power
system conditions, theses elements can be used alternatively for frequency decrease or increase separately,
and can be independently set to perform different control functions.
Operating Range
The frequency can be determined as long as in a three-phase voltage transformer connection the positive-
sequence system of the voltages, or alternatively, in a single-phase voltage transformer connection, the
respective voltage is present and of sufficient magnitude. If the measured voltage drops below a settable
value Vmin, the frequency protection is blocked because no precise frequency values can be calculated from
the signal.
Time Delays / Logic
Each frequency element has an associated settable time delay. When the time delay elapses, a trip signal is
generated. When a frequency element drops out, the tripping command is immediately terminated, but not
before the minimum command duration has elapsed.
Each of the four frequency elements can be blocked individually via binary inputs.
The following figure shows the logic diagram for the frequency protection function.
SIPROTEC Compact, 7SC80, Manual
E50417-G1140-C486-A8, Edition 07.2017
at the device is used. If the amplitude of this voltage is too small, one of the other
A-B
Functions
2.10 Frequency Protection 81 O/U
215