GE 869 Instruction Manual page 415

CHAPTER 4: SETPOINTS
NOTE:
NOTE:
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
6. Normalized dB magnitudes are computed at fault frequencies (based on formulae) as
the ratio of magnitude at fault frequency and magnitude at fundamental frequency
i.e. Magnitude in dB = 20 * Log10 (X1/Xf), where 'X1' is magnitude at fault frequency 'f1'
and 'Xf' is magnitude at fundamental frequency 'fs'.
7. Peak magnitude in dB is computed as the highest magnitude observed at all fault
frequencies and Energy in dB is computed as the ratio of an area within a +/- 0.5Hz
vicinity at a frequency corresponding to peak magnitude and at fundamental
frequency.
Energy in dB = 20 * Log10 (E1/Ef), where 'E1' is the area within the vicinity at a fault
frequency 'f1' and 'Ef' is the area within a vicinity at a fundamental frequency 'fs'.
8. During baseline mode, the dBs are computed as per steps 6 and 7 which are then
averaged over the entire configured period and then stored as averaged normalized
dB with respect to each load bin of the motor. A load bin is defined as the load interval
of 10% within the 0% to 120% range of motor load operation, a total of 12 load bins.
The relay enters baseline mode at any instant of time if the baseline data for a load bin is
not available.
9. During monitoring mode, the dBs are computed at every interval as per steps 6 and 7.
These dB levels and corresponding frequencies can be analyzed using an FFT
spectrum analyzer, as shown in the next figure, Path: Motor M&D\ Records\ ESA
Record. This spectrum tool works like the comtrade viewer available for viewing
transient records (Path: Records\ Transients\ Transient Records) with the red, black
and dark pink lines indicating fault frequencies of bearing, mechanical and stator
faults and their corresponding values.
Figure 4-115: Relative Magnitude (dB) vs. Frequency (Hz)
To support event based monitoring, an additional FFT file is captured and saved from the
Motor M&D\ Records\ ESA Record (Last PKP). This can be used to analyze the dB event
conditions.
10. Based on dBs computed in steps 8 and 9, the change in Peak magnitude and Energy
dBs are computed as the maximum difference in dB levels observed during baseline
and monitoring modes at any of the fault frequencies. The following example is
indicative of how a change in dB is computed at various fault frequencies.
MONITORING
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