Single-Phase Five-Level PWM Rectifier
N. A. Rahim, Senior Member, IEEE
J. A. Jalil
Dept. of Electrical Engineering
University of Malaya
Lembah Pantai, 50603 Kuala Lumpur.
UniKL Malaysian Spanish Institute
Kulim Hi-Tech Park,
Kulim, 09000 Kedah.
Abstract- A single-phase five level pulse width modulation
(PWM) rectifier is proposed. The control scheme is using the
sinusoidal PWM technique to improve the power factor and to
achieve a lower total harmonic distortion (THD) for the
proposed rectifier. The control method is also aimed to obtain
a nearly sinusoidal line current. Five-level PWM output is
generated at the ac terminal. The proposed topology is verified
by a software simulation.
I.
INTRODUCTION
Power converters are widely used in the industry. High
current harmonics and low power factor produced in the
distribution network contributed to the main power pollution.
Multilevel converters are introduced in recent years to attain
high power quality, low switching losses and high voltage
capability. The simple structure of the conventional diode
rectifiers or phase-controlled rectifiers contributed to the
low in cost. However, the decrease in the power factor due
to the increase in the firing angle and a relatively high on the
harmonic currents are the inherent drawbacks of the above
mentioned designed. Several topologies [1-4] of the singlephase switching mode rectifier (SMR) which offers a low
current distortion and unity power factor have been
proposed to overcome the problems.
Several control techniques were proposed to improve the
power quality in the multilevel rectifier [5-8]. Active singlephase rectifier has been proposed to draw a nearly
sinusoidal line current. [9-11]. A single-phase switching
mode multilevel rectifier with six active switches to
generate five voltage levels on the ac terminal is proposed in
[12] to achieve power factor correction and to reduce line
current harmonics. Aiming to reduce the rectifier cost,
research in decreasing the number of power switches for
multilevel rectifiers are presented in [13-16].The use of a
medium-frequency modulation strategy adapted to the
single-phase current multilevel rectifiers based on a
sinusoidal PWM technique is proposed in [17] to attain an
input current that fulfills the harmonic requirements of the
international standards.
In this paper, a single-phase five-level PWM rectifier is
proposed. Aiming in improving the power factor and
achieving a lower THD whilst reducing the rectifier cost, the
proposed topology with its control strategy shall generate
five-level of output voltages at the ac terminal. The strategy
of using the sinusoidal PWM technique to control the
proposed topology is aimed to draw a nearly sinusoidal line
current. The operation strategy and its performance
characteristics are verified by software simulations.
1
II. SYSTEM ANALYSIS
A. Circuit Configuration
A proposed topology of the multilevel rectifier is shown
in Fig.1. This configuration is modified from the
conventional two-level full-bridge rectifier and a diodebridge rectifier by adding two unidirectional power switches
connected in series, two power diodes as a substitute to the
second leg of the rectifier, a boost inductor, two fast
recovery diodes and two capacitors. The power switches
used in this configuration are the IGBT.
Figure 1. Proposed single-phase five level rectifier.
B. Operation Principles
In the first operation mode where the line current (is) is in
the positive half cycle, power switch S2 is turned on while
power switch S1, S3 and S4 are off. Diode D2 is conducting
in this operation mode whereas diode D1, D3 and D4 are in
reverse biased. Voltage vab is equal to zero and the boost
inductor voltage is equal to vs. The line current is increasing
or decreasing depending on the polarity of the mains voltage.
The two capacitors are discharging to supply the load.
In the second operation mode where the line current (is) is
still in the positive cycle, power switches S3 and S4 are both
turned on while power switches S1 and S2 are turned off.
Diodes D2 and D4 are conducting whereas diodes D1 and
D3 are in the blocking condition. The line current is
increasing if |vs| > v2 or decreasing if |vs| < v2. Thus voltage
vab is equal to vo/2.
In the third operation mode and the line current (is) is in
the positive cycle, power switch S1 is turned on while power
switches S2, S3 and S4 are turned off. Diodes D2, D3 and
D4 are conducting whereas diode D1 is off. The positive
line current (is) is charging both capacitors, thus generate
voltage vo at the ac side. The line current is decreasing since
|vs| < vdc.
Institute of Research Management & Monitoring, University
of Malaya.
The fourth operation mode is operating in the negative
half cycle of the line current (is). It generates zero voltage on
the ac side due to capacitors discharging voltage. Power
switch S1 is on while S2, S3 and S4 are off. Diode D1 is
conducting whereas diodes D2, D3 and D4 are reversed
biased.
The fifth operation mode is also operating in the negative
half cycle of the line current (is). Power switches S3 and S4
are both turned on while power switches S1 and S2 are
turned off. Diodes D1 and D3 are conducting whereas
diodes D2 and D4 are in the blocking condition. The line
current is increasing if |vs| > -v1 or decreasing if |vs| < -v1.
Thus voltage vab is equal to -vo/2.
The last operation mode is still operating in the negative
half cycle of the line current (is). Power switch S2 is turned
on while power switches S1, S3 and S4 are turned off.
Diodes D1, D3 and D4 are conducting whereas diode D2 is
in the blocking condition. The negative line current (is) is
increasing and charging both capacitors, thus voltage vab is
equal to -vo. The equivalent circuit for each operation mode
is presented in Fig.2.
Figure 2. Equivalent circuit for the operation mode
III. CONTROL SCHEME
The PWM control signals are achieved from the
sinusoidal PWM technique. In this technique, the pulse
width modulated signals are generated by comparing a high
switching frequency triangular carrier signals with a line
frequency half-sinusoidal reference voltage. The modulated
signals are then used as the input signals to the logic gates in
order to generate the switching pulses for the power
switches. This control technique is simplified by means of a
block diagram as shown in Fig.3. The corresponding
switching signals at the IGBT gates are simulated and
shown in Fig.4. The multilevel voltage generated at the ac
side of the adopted rectifier can also be observed in Fig.4.
The five output voltages (vab) generated at the ac side are vo,
vo/2, 0, -vo/2 and –vo.
Figure 3. The control block diagram
Figure 6. Simulated waveforms of the ac side voltage.
Figure 4. Corresponding switching signals and the PWM waveform on the
AC side.
Figure 7. Simulated output dc voltage.
IV. SIMULATION RESULTS
V. CONCLUSION
To verify the proposed topology and the control strategy,
computer simulations are performed. The ac mains voltage
is 220V with a 50Hz source frequency. The capacitance for
each capacitor is 2200uF. The value of the boost inductance
is 2mH. Fig.5 shows the simulated waveforms of the line
current (is) and it is observed to be a nearly sinusoidal wave.
The voltage (vab) on the ac side of the proposed rectifier
shows a five-level voltage pattern as depicted in Fig.6. The
output dc voltage is 375V as simulated in Fig.7.
This paper presents a five-level PWM single-phase
rectifier with a simple control technique to improve the
power factor and lower THD. A nearly sinusoidal line
current with high power factor is achieved on the ac side
and verified by a software simulation. The output voltage of
this rectifier can be improved by adding an ac or a dc filter.
However, the input and output filters must be designed
under the worst case condition that is under the minimum
output voltage condition.
ACKNOWLEDGMENT
I would like to acknowledge the Institute of Research
Management & Monitoring, University of Malaya for the
financial support of this research.
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Figure 5. Simulated waveforms of the line current.
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