Frequency Protection 81 (Optional); Description - Siemens SIPROTEC 7SD80 Manual

2.9

Frequency Protection 81 (Optional)

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.
Underfrequency is caused by increased real power demand of the loads or by a reduction of the generated
power, e.g. in the event of disconnection from the power system, generator failure or faulty operation of the
power/frequency regulation. Underfrequency protection is also applied for generators which operate (tempo-
rarily) 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 underfre-
quency protection. Underfrequency results also in increased reactive power demand of inductive loads.
Overfrequency is caused e.g. by load shedding, system disconnection or malfunction of the power/frequency
control. There is also a risk of self-excitation for generators feeding long lines under no-load conditions.
In order that the frequency protection can work, you must connect the voltages to the device.
2.9.1

Description

Frequency Elements
Frequency protection consists of the four frequency elements f1 to f4. Each element can be set as overfrequen-
cy element (f>) or as underfrequency element (f<) with individual thresholds and time delays. This ensures vari-
able matching to the application purpose.
• If an element is set to a value above the rated frequency, it is automatically interpreted to be an overfrequen-
cy element f>.
• If an element is set to a value below the rated frequency, it is automatically interpreted to be an underfre-
quency element f<.
• If an element is set exactly to the rated frequency, it is inactive.
Each element can be blocked via binary input and also the entire frequency protection function can be blocked.
Frequency Measurement
The largest of the 3 phase-to-phase voltages is used for frequency measurement. It must have an amount of
at least 65 % of the rated voltage, which is set in parameter 204, Vnom SECONDARY. Below that value frequen-
cy measurement will not take place.
Numerical filters are employed to calculate from the measured voltage a quantity that is proportional to the fre-
quency which is virtually linear in the specified range (f
that the frequency evaluation is virtually free from harmonic influences and phase jumps.
An accurate and quick measurement result is obtained by considering also the frequency change. When
changing the frequency of the power system, the sign of the quotient
repeated measurements. If, however, a phase jump in the measured voltage temporarily simulates a frequency
deviation, the sign of
are quickly discarded.
The dropout value of each frequency element is approximately 20 MHz below (for f>) or above (for f<) of the
pickup value.
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
Δf
/ will subsequently reverse. Thus the measurement results corrupted by a phase jump
dt
2.9 Frequency Protection 81 (Optional)
± 10 %). Filters and repeated measurements ensure
N
Δf
/ remains unchanged during several
dt
Functions
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