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2.23
Frequency Protection (ANSI 81)
The frequency protection function detects abnormally high and low frequencies in the generator. If the frequen-
cy lies outside the permissible range, appropriate switching actions are initiated, e.g. separating the generator
from the system.
A decrease in system frequency occurs when the system experiences an increase in real power demand, or
when a malfunction occurs with a generator governor or automatic generation control (AGC) system. The fre-
quency protection function is also used for generators which (for a certain time) operator to an island network.
This is due to the fact that the reverse power protection cannot operate in case of drive power failure. The gen-
erator can be disconnected from the power system by means of the frequency decrease protection.
An increase in system frequency occurs e.g. when large loads (island network) are removed from the
system, or on frequency control malfunction. This entails risk of self-excitation for generators feeding long lines
under no-load conditions.
Due to the use of filter functions, the frequency evaluation is free from harmonic influences and very accurate.
2.23.1
Functional Description
Frequency Increase and Decrease
Frequency protection consists of the four frequency elements f1 to f4. To make protection flexible for different
power system conditions, theses stages can be used alternatively for frequency decrease or increase sepa-
rately, and can be independently set to perform different control functions. The setting decides on the purpose
of the individual frequency stage. For the f4 frequency stage, the user can specify independently of the param-
eterized limit value if this stage shall function as decrease or increase stage. For this reason, it can also be
used for special applications, if, for example, frequency undershoot below the nominal frequency is to be sig-
naled.
Operating Ranges
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 measurement voltage drops below a settable value Umin, frequency protection is
disabled because precise frequency values can no longer be calculated from the signal.
With overfrequency protection, seal-in of the overfrequency pickup occurs during the transition to the 0 mode,
if the last measured frequency was above 66 Hz. The switch-off command drops out by a function blocking or
on transition to operational condition 1. A pickup drops out if the frequency measured last before the transition
into operational condition 0 is below 66 Hz.
With underfrequency protection, there is no precise frequency calculation on transition to the 0 mode due to a
too-low frequency. Consequently, the pickup or tripping drop out.
Time Delays/Logic
Trippings can be delayed each using an added time stage. When the time delay expires, a trip signal is gener-
ated. After pickup dropout the tripping command is immediately reset, but not before the minimum command
duration has expired.
Each of the four frequency stages can be blocked individually by binary inputs.
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
Functions
2.23 Frequency Protection (ANSI 81)
181
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