Circutor AFQm Series, Instruction Manual

Page: 39 / 108

39 Instruction Manual
AFQm
Figure 32: Current in the load, in the filter and in the mains.
4.3.- RATED CURRENT DIMENSIONS
The purchased active filter must be sized for the harmonic currents it has to filter. The rated
current of the
AFQm
must be at least 20% higher than the maximum level of harmonics to be
filtered. This factor may be higher depending on the installation features.
The active filters can suffer from overloading when trying to cancel the harmonic currents in
high-impedance, short-circuit mains. The clearest symptom for detecting such cases is that
they originally start from a THD(V) (under voltage) of over 3%. It has been seen that the higher
the initial THD(V) , the greater the chance of filter overload.
The reason for this behaviour is that the load does not behave as a current source, but rather
the larger the harmonic current absorbed by the filter the more harmonics are produced by the
load, which can produce up to more than double what was initially measured.
To avoid this phenomenon, it is best to oversize the active filter by multiplying the initial current
of harmonics measured in the load by a safety factor ( FS
h
). In other words:
I
FILTER
(AFQm) = [FS
h
* I
LOAD
* THD(I)]
Where:
I
FILTER
(AFQm) : rated current of the active filter.
FS
h
: safety factor > 1.2.
I
LOAD
: maximum current of the load.
THD(I) : harmonic distortion of the load current.
Equation 5:Rated current of the AFQm�