17-08-2012, 12:08 PM
A Novel Three-Phase Three-Leg AC/AC Converter Using Nine IGBTs
A Novel Three-Phase Three-Leg.pdf (Size: 1.27 MB / Downloads: 53)
INTRODUCTION
THREE-PHASE ac/dc/ac and ac/ac converters with variable
frequency (VF) and variable voltage operation have found
wide applications in the industry. The most popular configuration
uses voltage source inverter (VSI) with a diode rectifier as
the front end for adjustable speed drives (ASDs), uninterruptible
power supplies (UPS), and other industrial applications [1]. This
configuration features low cost and reliable operation due to the
use of a diode rectifier, but it generates highly distorted input
line currents and does not have regenerative or dynamic braking
capability. These problems can be mitigated by using a back-toback
two-level voltage source converter (B2B 2L-VSC), shown
in Fig. 1, where a pulsewidthmodulation (PWM) voltage source
rectifier is used to replace the diode rectifier [2].
NINE-SWITCH CONVERTER TOPOLOGY
the proposed three-phase nine-switch converter
topology. This converter has only three legs with three switches
installed on each of them. The novelty herein is that the middle
switch in each individual leg is shared by both the rectifier and
the inverter, thereby reducing the switch count by 33% and
50% in comparison to the B2B 2L-VSC and CMC, respectively.
The input power is delivered to the output partially through
the middle three switches and partially through a quasi-dc-link
circuit. For the convenience of discussion, we can consider that
the rectifier of the nine-switch converter is composed of the top
three and middle three switches, whereas the inverter consists
of the middle three and bottom three switches.
MODULATION SCHEMES
The reduction of the number of switches in the proposed
converter topology imposes certain switching constraints for
the switching pattern design. In the B2B 2L-VSC shown in
Fig. 1, the rectifier leg voltage vAN , which is the voltage at
node A with respect to the negative dc bus N, can be controlled
SWITCHING STATES AND CONVERTER LEG VOLTAGES
by switches S1 and S2 in the rectifier, whereas the inverter leg
voltage vXN can be controlled by S3 and S4 in the inverter.
This means that the rectifier and inverter leg voltages can be
controlled independently. The B2B 2L-VSC has four switching
states per phase, as defined in Table I.
SIMULATED SEMICONDUCTOR LOSS ANALYSIS
To further study the operating mechanism of the proposed
nine-switch converter, it would be necessary to investigate the
converter’s power loss profile. In this section, a comparative
study is presented for the nine-switch converter and the B2B
2 L-VSC. The system parameters and rated switching power of
the converters are listed in Table II. For all the converters, the
base value of the system power SR and the source/load side line
voltage are 5 kVA and 208 V.
Due to the nine-switch converter’s distinctive topological
structure, input–output currents flowing through the switches
are highly coupled and in the mean time dependent on a series
of variables such as load current magnitude, input and output
displacement factors, modulation depth, and switching frequency.
CONCLUSION
A novel nine-switch PWMac/ac converter topology was proposed
in this paper. The topology uses only nine IGBT devices
for ac to ac conversion through a quasi dc-link circuit. Compared
with the conventional back-to-back PWM VSC using 12
switches and the matrix converter that uses 18, the number of
switches in the proposed converter is reduced by 33% and 50%,
respectively. The proposed converter features sinusoidal inputs
and outputs, unity input power factor, and low manufacturing
cost. The operating principle of the converter was elaborated,
and modulation schemes for constant and VF operations were
developed. Simulation results including a semiconductor loss
analysis and comparison were provided, which reveal that the
proposed converter, while working in CF mode, has an overall
higher efficiency than the B2B 2L-VSC at the expense of uneven
loss distribution. However, the VF-mode version requires IGBT
devices with higher ratings and dissipates significantly higher
losses, and thus, is not as attractive as its counterpart.