Table of Contents
Advertisement
Functions
2.8 Frequency Protection 81 O/U
2.8
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.
2.8.1
Description
Frequency Detection
The frequency is detected preferrably from the positive sequence voltage. If this voltage is too low, the phase-
to-phase voltage V
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 at least one of the phase–to–phase voltages is present and of
sufficient magnitude. If the measured voltage sinks 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.
150
at the device is used. If the amplitude of this voltage is too small, one of the other phase–
A-B
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A5, Release date 11.2012
Advertisement
Table of Contents
