Siemens SIPROTEC 4 7XT71 Mode D'emploi page 25

9.2 Changing the Polarity Reversal Frequency
9.2.1 Operating Conditions
Factory setting for the polarity reversal frequency
A polarity reversal frequency (frequency of the square-wave generator) of 1.5 Hz is
preset from the factory for normal operating conditions. This setting is documented
on the sticker affixed to the device.
Operating conditions that require the polarity reversal frequency to be increased:
• rotor ground capacitance too low
• rotor ground resistance too large
If the rotor ground capacitance is lower than the assumed by the presetting, it is
charged more quickly and the Multifunctional Machine Protection 7UM62 can mea-
sure earlier. The reduction of the measuring time is achieved by increasing the po-
larity reversal frequency.
Operating conditions that require the polarity reversal frequency to be reduced:
• rotor ground capacitance too large
• capacitors are switched against ground in the excitation machine
To obtain an accurate measurement, you have to make sure that the rotor ground
capacitance is charged before the charge reversal. This requires the output voltage
at the measuring output to be constant (ideally zero) for at least 100 ms when there
is no fault. For large ground capacitances this can be achieved by reducing the po-
larity reversal frequency.
.
NOTE
The SIPROTEC manual Multifunctional Machine Protection 7UM62,
Chapter 3.3.19, Rotor Ground Fault Protection (1-3 Hz)", ordering code C53000-
G1176-C149 provides a detailed description of how to determine the current oper-
ating conditions using the Multifunctional Machine Protection 7UM62.
SIPROTEC 4, Injection Unit 7XT71, Product Information
C53000-B1175-C128-1, Edition 01.2014
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