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 network. 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-excita-
tion for generators feeding long lines under no-load conditions.
Functional Description
Detection of Frequency
The frequency is detected preferably from the positive sequence voltage. If this voltage is too low, the phase-
to-phase voltage V
A-B
at the device is used. If the amplitude of this voltage is too small, 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.
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.
2.8
2.8.1
Functions
2.8 Frequency Protection 81 O/U
138 SIPROTEC 4, 7SJ80, Manual
E50417-G1140-C343-A8, Edition 12.2017
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