Circutor AFQm, Instruction Manual, Page: 155 / 230

155
Instruction Manual
AFQm
ENGLISH
4.2.- OPERATING PRINCIPLE
Active filters are based on the following principle:
I
FILTER
= I
MAINS
- I
LOAD
Equation 5: Operating principle.
In other words, they detect the difference between the desired sine wave ( I
M
aInS
) of the current and the
signal deformed by the harmonics ( I
LOAD
). And they inject the difference between both waves ( I
FILTER
).
Figure 34
shows the wave shapes of the currents injected by the active filters.
These show the required wave (
I
M
aInS
), the existing deformed wave (
I
LOAD
) and the filter current (
I
FILTER
).
Figure 34: 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-im-
pedance, 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