| Abstract |
Substitution for DC motors by PWM inverter-fed leads to high harmonics levels in the DC
side of the converters. Higher order harmonics can still be a source of electromagnetic
interference and often need to be filtered.
The conventional solution for the reduction of DC side Harmonics is the passive filter. It
is an easy solution but unfortunately the requiered reduction of harmonics leads to a filter of
excessive size and weight. It is also an expensive solution.
The use of an active filter in combination with the pasive ones has been recently
succesfully tried to reduce the higer harmonics content. This paper describes an alternative
solution to passive and active filter arrangements on the DC side of the converter [1]. It shares
with the active filter the principle of an active cancellation of harmonic content, but differs from
it in the absence of processing and amplification of signal.
The new concept that DC side harmonics elimination proposes is based on direct
reinjection of the voltage ripple in series with the DC line. The paper analyses the operation of the
proposed scheme and provides experimental verification in a six pulse scaled down model of an
Ac/DC converter link with a PWM inverter feed.
In many cases a smothing reactor is used in the output of a rectifier to eliminate the
current harmonic. It is typical in the use of this type of reactor in series between the rectifier and
a load, or a power inverter to feed a machine AC.
The concept proposed here is not pulse-related and unlike the earlier DC side harmonic
reinjection scheme, only attenuates the DC side harmonics. It is, however, considerably simpler
than the above alternatives since it does not involve any switching devices.
The proposal uses a transformer whose secondary it is connected in a similar way to the
smothing reactor mentioned previously. For this secondary one circulates the direct current and
also the harmonics current, but in opposed phase with the object of eliminating harmonics
content. In the filter, it is also necessary to include a capacitor in the transformer primary circuit
in order to block the DC voltage. |
