Siemens SIPROTEC 4 7XT71 Series, Manual, Page: 93 / 122

Changing the Polarity-Reversal Frequency
Operating Conditions
Factory Settings of the Polarity-Reversal Frequency
For normal operating conditions, a polarity-reversal frequency (frequency of the square-wave generator) of
1.5 Hz is the default factory setting. This setting is documented in the label on the device.
Operating Conditions that Allow Increasing the Polarity-Reversal Frequency
•
The rotor ground capacitance is too small.
Reducing the measurement time is achieved by increasing the polarity-reversal frequency.
Operating Conditions that Require Reducing the Polarity-Reversal Frequency
•
The rotor ground capacitance is too large.
•
Capacitors are connected to ground in the excitation machine.
For a precise measurement, you must ensure that the rotor ground capacitance is charged before constant
charge reversal. In normal operation without any faults, this requires for the output voltage at the measuring
output to be constant (ideally zero) for at least 100 ms. For large ground capacitances, this can be achieved by
reducing the polarity-reversal frequency.
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NOTE
7UM62:
For a detailed description on how to determine the current operating conditions with the 7UM62, refer to
the SIPROTEC manual 7UM62, order number C53000-G1100-C149.
7UM85:
For a detailed description on how to determine the current operating conditions with the 7UM85, refer to
the SIPROTEC manual 7UM85, order number C53000-G5000-C027.
Installing and Removing the Pluggable Printed Circuit Board Assembly
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NOTE
Comply with the notes on electrostatic sensitive devices in chapter 5.1 General Notes .
Injection Unit in the Surface-Mounting Housing 7XT7100-0BA00
²
Switch off the plant providing the auxiliary voltage for the injection unit 7XT71.
²
Switch off all voltages fed to the device.
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NOTE
It is not absolutely necessary to disconnect all lines connected to the junction block from the device. You
only need to disconnect the lines from the screw terminals if the installation freedom is restricted for the
work steps described in the following. Consider the safety notes in chapter
4.4 Connection .
²
Unscrew and remove each of the 2 screws at the bottom and top Z-shaped angle brackets used to fasten
the device to the switch panel.
²
Remove the device from the switch panel.
5.2
5.2.1
5.2.2
Commissioning
5.2 Changing the Polarity-Reversal Frequency
SIPROTEC 4/5, Injection unit 7XT71, Manual 31
C53000-B1174-C128-8, Edition 05.2020