ABSTRACT
The switching frequency is an important control
parameter of PWM rectifier to reduce switching losses and EMI noise. This paper
proposed a variable switching frequency PWM (VSFPWM) strategy for DC-link
voltage ripple control in two-level rectifier. DC-link voltage ripple is
determined by the DC-link current directly, and can be predicted synchronously
with PWM signals. A real-time prediction model of DC-link voltage ripple is
derived for a common voltage oriented control (VOC) PWM rectifier. Then, VSFPWM
control is introduced, which changes the switching frequency cycle to cycle with
a restriction of DC-link voltage ripple peak value. Furthermore, the dynamic
behavior is also observed when the proposed VSFPWM control scheme is adopted.
Detail simulation and experimental comparisons are carried out between VSFPWM
and normal constant switching frequency PWM (CSFPWM), which demonstrate the advantages
of the proposed method.
KEYWORDS
1.
Voltage ripple
2.
Prediction
3.
Variable
switching frequency
4.
PWM rectifier
5.
Switching
losses
6.
EMI
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
Fig.1 Control structure of AFE
rectifier
EXPECTED SIMULATION
RESULTS
Fig.2 Comparison between the
prediction and the simulation results of the
DC-link voltage ripple in one
line-cycle
Fig.3 DC-link voltage ripple
comparison
Fig.4 Switching frequency comparison
Fig.5 AC-side current
Fig.6 Spectrum comparison (a)
AC-side (2) DC-link
Fig.7 Step response (a) Step
response of DC-link voltage (f) The change of
switching frequency with VSFPWM
CONCLUSION
The contribution of this paper is
develop the VSFPWM strategy for DC-link voltage ripple control. Different from
the previous work on the AC-side current ripple or torque ripple, the DC-link
voltage ripple is nearly not affected by the PWM current ripple of AC-side. In
a rectifier system, the DC-link voltage ripple is determined by the PWM method
and load current, and the peak value of it is important for DC-link capacitor
design or selection. The proposed VSFPWM fully utilizes the freedom of
switching frequency, which is often neglected in the PWM module. However, the
proposed VSFPWM is different from the random PWM [24], which changes the
switching frequency based on the statistics and no prediction model is used. It
should be noted that the proposed technique can be applied to a different power
factor than the unitary one and not can be applied direct to the rectifier with
neutral wire (four wire). Few conclusions can be derived as follows:
(1) DC-link voltage ripple
prediction model can be built in the time-based-domain. With the three-phase
duty cycles, AC-side current and load current measured by the current sensors,
the DC-link voltage ripple peak can be predicted for updating the switching
frequency in next cycle. The prediction method also applies to other PWM
methods, and also be used for design and analysis of DC capacitors and DC
battery reliability.
(2) In a whole line period, the
switching frequency of VSFPWM continuously varies below the designed constant switching
frequency, keeping the DC-link voltage ripple always under the requirement.
Using the proposed VSFPWM strategy, the switching losses decrease
significantly, and EMI noise reduces markedly.
(3) The dynamic property of VSFPWM
is firstly investigated in a typical closed-loop control system. In fact,
VSFPWM still has a good dynamic response, without nearly impairing the tracking
performance shown in common CSFPWM. The open-loop Bode plot indicates the
VSFPWM methods just decrease a little bit of bandwidth of both voltage control
loop and the current in CSFPWM because of the reduction of average switching
frequency.
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