15-11-2012, 02:17 PM
High-Performance Hysteresis Modulation Technique for Active Filters
High-Performance Hysteresis.pdf (Size: 227.71 KB / Downloads: 41)
Abstract
A new, substantial improvement of the hysteresis
current control method for voltage source converters is presented.
A simple and fast prediction of the hysteresis band is added
to a linearized version of the phase-locked loop control, thus
ensuring constant switching frequency and tight control of the
position of modulation pulses. This allows high accuracy in
tracking highly distorted current waveforms and minimizes the
ripple in multiphase systems. The technique implementation is
very simple and robust, employing only a small number of
conventional inexpensive analog and logic components. It does not
require trimmings or tunings, giving the control the capability to
adjust itself to the different operating conditions. The proposed
method is compared with the most diffused modulation techniques,
demonstrating its superior performance in responding to
the most demanding conditions met in active filters. The behavior
of the method has been fully verified by simulation and by
experimental tests.
INTRODUCTION
CONVERTERS for active filters must satisfy peculiar
requirements regarding the current waveforms and their
accuracy. Indeed, in this kind of application, voltage-source
current-controlled converters are usually employed, which
should produce compensating currents characterized by high
harmonic contents and fast transients. Good tracking of the
current references has to be ensured, even in presence of
nonnegligible harmonics affecting the output line voltage.
Moreover, the line and load impedance can be time varying
and, within certain limits, unpredictable.
These requirements are much more demanding than those
typical of drive applications. Drives are characterized by sinusoidal
current waveforms with transients that are comparable
with the mechanical time constants. Voltage harmonic contents
are limited and, very often, especially in case of induction
machines, the sensitivity to current harmonics is limited. Load
impedances, even in the case where they are time-dependent,
can be estimated with good accuracy.
Feedforward/Feedback Modulation Frequency Control
The predicted bandwidth may be added to the term
resulting from the PLL control, giving a feedforward/feedback
compensated gain control of the modulation frequency according
to Fig. 3. In theory, if the value of was the exact one,
the PLL output should be zero. In practice, is reduced
to the small amount needed to correct for the inaccuracies
and the delays of the prediction of , as shown in Fig. 8.
Correspondingly, the phase error required by the PLL is
reduced too, thus resulting in a better performance and/or in
a less critical setting of the control.
It should be pointed out that as the predictive method is truly
a feedforward one, no parasitic loops are introduced (see, for
comparison, [12]). Thus, the PLL stability can be analyzed as
it was acting alone.
HYSTERESIS CONTROL OF THE ACTIVE FILTERS
In active filter applications, the inverter is used to correct for
the harmonics and distortions produced by the load [17]–[20].
In case of shunt active filters [17], [20], the load current
distortion is compensated by the inverter current according to
a general scheme like that of Fig. 16. The inverter behaves
as a current source controlled by a reference signal, which
corresponds to the difference between the current waveform
wanted for at the supply side and the actual load current
. At this purpose, every kind of inverter capable of current
control can be used. The trend is today toward the voltagesource
current-controlled inverters, using one of the many
analog or digital current control techniques available.
CONCLUSIONS
Substantial improvements are proposed to the hysteresis
current-control technique for voltage source inverters, intended
to allow not only constant frequency operation even for rapid
rates of changing of the output voltage, but also improved
control of the position of the modulation pulses. This latter
feature is of particular importance in reducing the current ripple
in insulated neutral multiphase systems where the optimal
reciprocal position of the modulation pulses of the phases is
the “centered” one.
A substantially wider bandwidth and better stability of the
PLL controlling the frequency are achieved by means of a
compensation of the variation of the loop gain with the output
voltage and by adding a prediction of the correct width of the
hysteresis band. The prediction method is much simpler and
more effective than that proposed in a previous paper [12].
Hysteresis control operation requires to sense only the output
currents and needs no adjustments or trimmings.