Overfrequency Protection
Overview of Functions
The Overfrequency protection function (ANSI 81O):
•
Detect overfrequencies in electrical power systems or machines
•
Monitor the frequency band and output failure indications
•
Disconnect generating units when the power frequency is critical
•
Provide additional turbine protection if the speed limiter fails
Frequency deviations are caused by an unbalance between the active power generated and consumed. Over-
frequency is caused by load shedding (island network), power system disconnection or disturbances of the
frequency controller. Overfrequency implies a risk of self excitation of machines which are connected to long
lines without load.
Overfrequency protection is available in two functional configurations (selectable from the DIGSI functions
library). The functional configurations differ in the frequency measurement method they use.
Structure of the Function
The Overfrequency protection function is used in protection function groups, which are based on voltage
measurement.
The overfrequency protection function comes with 2 factory-set stages. A maximum of 3 tripping stages can
be operated simultaneously in the function. The tripping stages have an identical structure.
The parameters Dropout differential and Minimum voltage are set for all stages.
[dwstofqp-090211-01.tif, 1, en_US]
Figure 6-209 Structure/Embedding of the Function
6.32
6.32.1
6.32.2
Protection and Automation Functions
6.32 Overfrequency Protection
SIPROTEC 5, Overcurrent Protection, Manual 763
C53000-G5040-C017-8, Edition 07.2017
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