06-08-2013, 02:41 PM
Three-Phase PWM Rectifier with Constant Switching Frequency
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Abstract
In this paper, a novel adaptive hysteresis
on predictive
reasoning for three-phase PWM rectifier is presented. A
simple and quick prediction of the hysteresis band is
added to a phase-locked-loop control to ensure constant
switching frequency and synchronization of modulation
pulses independently on the system parameters. The
proposed technique allows the advantages of quick
response, robustness, good current tracking accuracy
and minimal supply current ripples. The three-phase
PWM rectifier has the characteristic of drawing nearly
sinusoidal current with stable switching frequency and
tight control of the position of modulation pulses. The
simulation results show that the designed technique can
improve three-phase PWM rectifier performance
noticeably.
INTRODUCTION
IODE rectifiers are widely employed in industrial fields
and consumer products thanks to advantages of low
cost, simple structure, robustness and absence of control.
However, this type of converters results in only
unidirectional power flow, low input power factor, high
level of harmonic input currents, malfunction of sensitive
electronic equipment, increased losses and also contributing
to inefficient use of electric energy. To maintain supply
quality at acceptable levels, various standards and guidelines
(i.e. IEEE 519 and IEC 61000-3) specify limits of current
harmonic content for certain types of applications. Recently,
many promising power factor correction (PFC) techniques
[1-5] have been proposed for rectifiers. Apart from
application of active and passive filters, the best solution is
in using pulse width modulated (PWM) rectifiers. Research
interest in three-phase PWM rectifiers has grown rapidly
over the last few years due to some of their important
advantages, such as power regeneration capabilities, control
of dc-bus voltage over a wide range, and low harmonic
distortion of input currents. Since the converter has abilities
to control the input currents in sinusoidal waveforms, unity
power factor operation can be easily performed by
regulating the currents in phase with the power-source
voltages.
Switching pulses regulation
Now we make an additional improvement by introducing
a synchronization of modulation pulses. This is based on
adding a modification to equation (21), where the phase
error between an external clock and the control signal
contributes to ensure a good regulation of modulation pulses
position. The solution is to implement the PLL, generally,
this is the effective solution to synchronize two signals if the
stability conditions are respected [10, 11].
The PLL is a device which causes one signal to track
another one, it keeps the output signal synchronized with a
reference input signal in frequency as well as in phase. More
precisely. The PLL is simply a servo system, which controls
the phase of its output signal in such a way that the phase
error between output phase and reference phase reduces to a
minimum. The basic fundamental block diagram of PLL
consists of a phase detector, a loop filter and a voltage
controlled oscillator (VCO).
CONCLUSION
This paper has presented a considerable improvement
which offers all the advantages of the hysteresis technique
for three-phase PWM rectifier, where a novel adaptive
HBCC based on predictive reasoning has been developed
which ensures a good regulation of switching frequency and
a tight control of the position of modulation pulses.The
technique performs by creating an adaptive hysteresis band
envelope, and then compensating for the interaction between
the phases that occurs when the midpoint of dc bus
capacitors is floating. The three-phase PWM rectifier shows
excellent features such as unity power factor, high reliability
and simple robust control.