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2.12
Frequency Protection (optional)
The frequency protection function detects overfrequencies or underfrequencies in the
system or in electrical machines. If the frequency is outside the permissible range, ap-
propriate actions are initiated such as load shedding or separating the generator from
the system.
Underfrequency is caused by increased real power demand of the loads or by a re-
duction 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 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 underfrequency protection. Underfrequency also results in increased
reactive power demand of inductive loads.
Overfrequency is caused for instance by load shedding, system disconnection or mal-
function of the power frequency control. There is also a risk of self-excitation for gen-
erators feeding long lines under no-load conditions.
2.12.1 Functional Description
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-
• If an element is set to a value below the rated frequency, it is automatically inter-
• 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
The largest of the 3 phase-to-phase voltages is used for frequency measurement. It
must amount to at least 65 % of the nominal voltage set in parameter 204, Unom
measurement
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
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
a phase jump in the measured voltage temporarily simulates a frequency deviation,
the sign of
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.
7SD610 Manual
C53000-G1176-C145-4
preted to be an overfrequency stage f>.
preted to be an underfrequency stage f<.
∆f
/
remains unchanged during several repeated measurements. If, however,
dt
∆f
/
will subsequently reverse. Thus the measurement results corrupted by
dt
2.12 Frequency Protection (optional)
±
N
171
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