Underfrequency Protection
Overview of Functions
The Underfrequency protection function (ANSI 81U) is used to:
•
Detect underfrequencies in electrical power systems or machines
•
Monitor the frequency band and output failure indications
•
Decouple power systems
•
Load shedding to ensure power system stability and protect motors
•
Disconnect generating units when the power system frequency is critical (for example, f < 0.95 f
rated
)
Frequency deviations are caused by an unbalance between the active power generated and consumed. Under-
frequency is caused by an increase of the consumers' active power demand or by a decrease of the power
generated. These conditions occur in the case of power system disconnection, generator failure, or distur-
bances of the power and frequency controller.
Underfrequency protection is available in 2 functional configurations (selectable from the DIGSI functions
library). The functional configurations differ in the frequency measurement method they use.
Structure of the Function
The Underfrequency protection function is used in protection function groups, which are based on voltage
measurement.
The Underfrequency protection function comes with 3 factory-set stages. A maximum of 5 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.
[dwstufqp-090211-01.tif, 1, en_US]
Figure 7-158
Structure/Embedding of the Function
7.19
7.19.1
7.19.2
Protection and Automation Functions
7.19 Underfrequency Protection
SIPROTEC 5, High-Voltage Bay Controller, Manual
829
C53000-G5040-C015-9, Edition 11.2017
Summary of Contents for 6MD85
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