Application And Setting Notes - Siemens 6MD85 Manual

Protection and Automation Functions
7.18 Overfrequency Protection
Both methods of measurement are characterized by a high measuring accuracy combined with a short pickup
time. Disturbance values such as harmonics, high frequency disturbances, phase-angle jumps during
switching operations and compensation processes due to power swings are effectively suppressed.
Functional Measured Value
The angle-difference method provides the following measured value:
Measured Value
f
Behavior on Leaving the Operating Range
The sampling-frequency tracking makes a wide frequency operating range possible. If the stage has picked up
before leaving the frequency operating range and the measuring voltage is higher than the set minimum
voltage, the pickup is maintained. A dropout of the pickup is only possible by means of a blocking.
Blocking the Stage
In the event of blocking, a picked-up stage will be reset. The following blocking options are available for the
stage:
•
Externally or internally via the logical binary input
•
Internally when the voltage drops below the Minimum voltage

Application and Setting Notes

7.18.4
Frequency-Measurement Method
The frequency measurement method is selected with the functional configuration from the DIGSI functions
library. The letter at the end of the function name describes the method of measurement.
Redundant solutions can be implemented by combining 2 different methods of frequency measurement. You
can implement a 2-out-of-2 decision by connecting the operate indications of both functions in a CFC with a
logical AND gate. This increases the reliability of protection.
Frequency-Measurement
Method
Angle difference method
(method A)
Filtering method (method B)
Parameter: Threshold
•
Recommended setting value (_:31:3) Threshold = 51.50 Hz for f
The Threshold parameter is used to define the pickup threshold of the overfrequency protection stage in
dependence of the application. 51.50 Hz is a typical warning threshold in 50-Hz systems.
Parameter: Operate delay
•
Recommended setting value (_:31:6) Operate delay = 10 s
The Operate delay parameter is used to set the stage so that overfunctions due to disturbing influences
(for example, switching operations) are avoided. Fairly high time delays are sufficient for warning indications.
Parameter: Minimum voltage
•
Recommended setting value (_:2311:101) Minimum voltage = 37.5 V
For the Undervoltage blocking, 65 % of the protected object's rated voltage is recommended.
826
Description
Frequency calculated with the angle-difference method
>Block stage
Description
Select this method of measurement if the frequency protection stage is
used for the protection of machines.
Select this method of measurement if the frequency protection stage is
used in an electrical power system.
= 50 Hz
rated
SIPROTEC 5, High-Voltage Bay Controller, Manual
C53000-G5040-C015-9, Edition 11.2017