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Functions
2.9 Frequency Protection 81 O/U
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
182
2.9
Frequency Protection 81 O/U
The frequency protection function detects abnormally high and low frequencies in the system or in electrical
machines. If the frequency lies outside the allowable range, appropriate actions are initiated, such as load
shedding or separating a generator from the system.
Applications
• decrease in system frequency occurs when the system experiences an increase in the real power demand,
or when a malfunction occurs with a generator governor or automatic generation control (AGC) system. The
frequency protection function is also used for generators which (for a certain time) operate to an island net-
work. This is due to the fact that the reverse power protection cannot operate in case of a drive power failure.
The generator can be disconnected from the power system by means of the frequency decrease protection.
• increase in system frequency occurs e.g. when large blocks of load (island network) are removed from the
system, or again when a malfunction occurs with a generator governor. This entails risk of self-excitation for
generators feeding long lines under no-load conditions.
2.9.1
Description
Detection of Frequency
The frequency is derived from the phase–to–phase voltages V
A-B
applied to the device. If the amplitude of this
voltage is too low, one of the other phase–to–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.
Frequency Increase and Decrease
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-se-
quence system of the voltages, or alternatively, in a single-phase voltage transformer connection, the respec-
tive voltage is present and of sufficient magnitude. If the measured voltage sinks below a set 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.
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