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2 Functions
342
7SD5 Manual
C53000-G1176-C169-1
2.18 Frequency Protection (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, ap-
propriate actions are initiated, such as load shedding or separating a generator from
the system.
Underfrequency is caused by increased real power demand or by a reduction of the
generated power, e.g. in the event of disconnection from the network, generator failure
or faulty operation of the power frequency control. Underfrequency protection is also
applied for generators which operate (temporarily) to an island network. This is due to
the fact that the reverse power protection cannot operate in case of a drive power fail-
ure. The generator can be disconnected from the power system by means of the un-
derfrequency protection. Underfrequency results also in increased reactive power
demand of inductive consumers.
Overfrequency is caused for instance in case of load shedding, system disconnection
or malfunction of the power-frequency control. This includes also a risk of self-excita-
tion for generators feeding long lines under no-load conditions.
2.18.1 Method of Operation
Frequency Stages Frequency protection consists of the four frequency elements f1 to f4. Each element
can be set as overfrequency stage (f>) or as underfrequency stage (f<) with individual
thresholds and time delays. This ensures variable matching to the application pur-
pose.
• If an element is set to a value above the rated frequency, it is automatically inter-
preted to be an overfrequency stage f>.
• If an element is set to a value below the rated frequency, it is automatically inter-
preted to be an underfrequency stage 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 Mea-
surement The largest of the 3 phase-earth currents is used for frequency measurement. This
value must be at least 6 V (secondary). Below that value frequency measurement will
not take place.
Numerical filters are employed to calculate from the measured voltage a quantity that
is proportional to the frequency which is virtually linear in the specified range (f
N
±
10 %). Filters and repeated measurements ensure that the frequency evaluation is vir-
tually free from harmonic influences and phase jumps.
An accurate and quick measurement result is obtained by considering also the fre-
quency change. When changing the frequency of the power system, the sign of the
quotient
∆
f
/
dt
remains unchanged during several repeated measurements. If, however,
a phase jump in the measured voltage temporarily simulates a frequency deviation,
the sign of
∆
f
/
dt
will subsequently reverse. Thus the measurement results corrupted by
a phase jump 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.
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