Siemens SENTRON Manual page 60

Notes on installation and use
Frequency components in the residual current of a frequency converter
The frequency components in the residual current must be taken into account in
addition to the tripping characteristics of the RCCB in order to assess the protective
effect of this RCCB in conjunction with a frequency converter. The following critical
frequency components occur at fault location 3:
• Clock frequency of the frequency converter (a few kHz)
• Motor frequency (normally 0 to 50 Hz, maximum frequency 1 kHz)
• 3. harmonic of 50 Hz (150 Hz if the frequency converter has a three-phase
connection)
Fig. 22 shows a typical example of the frequency components that can flow across
a fault impedance of 1 kΩ in the area of fault location 3 (see Fig. 21).
The clock frequency accounts for a smaller percentage of the total residual current
as the motor frequency increases, while the motor frequency accounts for a
correspondingly higher percentage.
This behavior is characteristic of many different frequency converter versions.
Frequency converter with 3-phase supply
Frequenzumrichter mit 3-phasiger Versorgung
250
200
150
100
50
0
0
Fig. 22: Frequency components in the residual current based on the example of a
frequency converter
60
Residual currents with R = 1 kΩ
Fehlerströme mit R =1k
10 20 30 40 50
Motor frequency (Hz)
Motorfrequenz (Hz)
RMS value components of
motor frequency
RMS value component 150Hz
RMS value component of
clock frequency (16 kHz)
RMS value: Total
60