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2.17
Frequency Protection (ANSI 81)
The frequency protection function detects abnormally high and low frequencies of the
generator. If the frequency lies outside the admissible range, appropriate actions are
initiated, such as separating the generator from the system.
in real power demand, or when a frequency or speed control malfunction occurs. The
frequency decrease protection is also used for generators which (for a certain time)
function on an island network. This is due to the fact that the reverse power protection
cannot operate on drive power failure. The generator can be disconnected from the
power system using 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.
Through the use of filters measurement is practically independent of harmonic influ-
ences and very accurate.
2.17.1 Functional Description
Frequency Increase
Frequency protection consists of four frequency elements f1 to f4. To make protection
and Decrease
flexible for different power system conditions, theses stages can be used alternatively
for frequency decrease or increase separately, and can be independently set to
perform different control functions. The parameter setting decides for what purpose
the particular stage will be used. For the f4 frequency stage, the user can instead
specify independently of the parametrized limit value whether this stage shall function
as decrease or increase stage. For this reason, it can also be used for special appli-
cations, if, for example, frequency undershoot below the nominal frequency is to be
signaled.
Operating Ranges
The frequency can be determined as long as there is a sufficiently strong positive se-
quence system of voltages. 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 under these conditions.
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 opera-
tional 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 ex-
pires, a trip signal is generated. After pickup dropout the tripping command is imme-
diately reset, but not before the minimum command duration has elapsed.
Each of the four frequency stages can be blocked individually by binary inputs.
Figure 2-33 shows the logic diagram for frequency protection.
7UM61 Manual
C53000-G1176-C127-3
A decrease in system frequency occurs when the system experiences an increase
2.17 Frequency Protection (ANSI 81)
109
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