Abstract:
Current distortion of 12-pulse rectifier loads is significantly
lower compared to six-pulse rectifier loads. However, in
passive filtering of the lowest and dominant characteristic 11th and
13th harmonics the use of 5th and 7th filters is often required in
order to prevent possible parallel and series resonance between
passive filter and source impedance which can be excited by source
background distortion or by load current residual noncharacteristic
harmonics at the 5th and 7th harmonic frequencies. In hybrid
filter systems, an active filter (AF) can be added in series with
the passive filter in order to isolate the source and load. In most
proposed hybrid filter systems, AF control is based on the detection
of total current distortion and high-frequency inverters.With
a selective AF control system and voltage-controlled inverter, the
AF can be controlled to isolate the load at the critical frequencies
only while at all other frequencies the passive filter function
is preserved so that lower switching frequency and AF rating is required.
In this paper, we present a selective AF filter control system
and simple hybrid filter topology suitable for the compensation of
high-power 12-pulse rectifier loads. Harmonic current controllers
based on the second-order infinite-impulse response digital resonant
filters are used, as they can be considered as simple digital
algorithms for more complex double cascaded synchronous-reference-
frame-based proportional plus integral controllers. They are
centered to the targeted harmonic frequencies by using an adaptive
fundamental frequency tracking filter. This approach gives
good results, even if the reference waveform (in our case, a load
voltage) is highly distorted or unbalanced and no separate phaselocked
loop is required. Test results for a laboratory model of this
system and stability analysis are presented and the importance of
delay-time compensation is discussed.
