Frequency Protection (optional) 81
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.
Functional Description
Frequency Elements
Frequency protection consists of the four frequency elements f1 to f4. Each element can be set as overfre-
quency element (f>) or as underfrequency element (f<) with individual thresholds and time delays. This
ensures variable matching to the application purpose.
•
If an element is set to a value above the rated frequency, it is automatically interpreted to be an overfre-
quency 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 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 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
Δf
/
dt
remains unchanged during several
repeated measurements. If, however, a phase jump in the measured voltage temporarily simulates a
frequency deviation, the sign of
Δf
/
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.
Operating Ranges
Frequency evaluation requires a measured quantity that can be processed. This implies that at least a suffi-
ciently high voltage is available and that the frequency of this voltage is within the working range of the
frequency protection.
The frequency protection automatically selects the largest of the phase-to-phase voltages. If all three voltages
are below the operating range of 65 % · V
N
(secondary), the frequency cannot be determined. In that case the
indication 5215
81 UnderV Blk
is displayed. If the voltage falls below this minimum value after a frequency
2.9
2.9.1
Functions
2.9 Frequency Protection (optional) 81
SIPROTEC 4, 7SD80, Manual
123
E50417-G1100-C474-A2, Edition 02.2018
Summary of Contents for SIPROTEC 4 7SD80
Page 8: ...8 SIPROTEC 4 7SD80 Manual E50417 G1100 C474 A2 Edition 02 2018 ...
Page 10: ...10 SIPROTEC 4 7SD80 Manual E50417 G1100 C474 A2 Edition 02 2018 ...
Page 18: ...18 SIPROTEC 4 7SD80 Manual E50417 G1100 C474 A2 Edition 02 2018 ...
Page 248: ...248 SIPROTEC 4 7SD80 Manual E50417 G1100 C474 A2 Edition 02 2018 ...
Page 298: ...298 SIPROTEC 4 7SD80 Manual E50417 G1100 C474 A2 Edition 02 2018 ...
Page 312: ...312 SIPROTEC 4 7SD80 Manual E50417 G1100 C474 A2 Edition 02 2018 ...
Page 322: ...322 SIPROTEC 4 7SD80 Manual E50417 G1100 C474 A2 Edition 02 2018 ...
Page 400: ...400 SIPROTEC 4 7SD80 Manual E50417 G1100 C474 A2 Edition 02 2018 ...
Page 402: ...402 SIPROTEC 4 7SD80 Manual E50417 G1100 C474 A2 Edition 02 2018 ...