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Underfrequency-Protection Stage
6.33.3
Logic of a Stage
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Figure 6-212
Frequency-Measurement Method
Underfrequency protection is available in 2 functional configurations. These work with different frequency-
measurement methods. You select the frequency-measurement method in dependence of the application.
•
Angle-difference method (method A):
The angle-difference method determines the phasor of the positive-sequence voltage in multiphase
systems. In the case of 1-phase connection, it always processes the phasor of the connected voltage.
Since the change of angle of the voltage phasor over a given time interval is proportional to the
frequency change, the current frequency can be derived from it.
•
Filtering method (method B):
The filtering method processes the instantaneous voltage values and determines the current frequency
using a suitable combination of filters. The frequency-protection function selects automatically the
largest voltage as the measurand. In a multiphase connection, the phase-to-phase voltage is always the
largest. If in a multiphase connection the selected voltage is no longer available, the function changes
over automatically to the next maximum voltage. The function can even operate with just one voltage.
SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017
Logic Diagram of the Underfrequency-Protection Stage
Protection and Automation Functions
6.33 Underfrequency Protection
769
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