Application And Setting Notes - Siemens 6MD85 Manual

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Figure 7-167
Measurand
This function uses the frequency calculated via the angle difference algorithm.
For more information, refer to chapter
The frequency difference is calculated over a settable time interval (default setting: 5 periods).
The ratio between the frequency difference and the time difference reflects the frequency change which can
be positive or negative.
A stabilization counter works to avoid overfunction. This counter is increased if the set threshold value is
exceeded. If the value drops below the threshold value, the counter is reset immediately. The counter is set to
8 internally and is activated at each half system cycle.
Undervoltage Blocking
If the measuring voltage drops below the Minimum voltage, the Rate of frequency change protection is
blocked because precise frequency values cannot be calculated anymore.
Functional Measured Value
Value
df/dt
7.21.3.2

Application and Setting Notes

Parameter: Minimum voltage
•
Recommended setting value (_:13171:101) Minimum voltage = 37.500 V
For the Undervoltage blocking, 65 % of the rated voltage of the protected object is recommended.
The method of measurement uses the phasor of the positive-sequence voltage. When determining the setting
value, keep in mind that the absolute value of the sound positive-sequence voltage is equal to the absolute
value of the phase-to-ground voltage. The default setting is referred to this value.
Parameter: Measuring window
•
Default setting (_:13171:137) Measuring window = 5 periods
You can use the Measuring window parameter to optimize the measuring accuracy or the pickup time of
the function. For information regarding pickup time and measuring accuracy, refer to the technical data.
The default setting provides maximum measuring accuracy. If you do not have specific requirements for a
decreased pickup time, Siemens recommends using the default setting.
SIPROTEC 5, High-Voltage Bay Controller, Manual
C53000-G5040-C015-9, Edition 11.2017
Logic Diagram of General Functionality
7.18.3 Overfrequency-Protection
Description
Calculated rate of frequency change
Protection and Automation Functions
7.21 Rate of Frequency Change Protection
Stage.
847