04-05-2011, 03:46 PM
3-Phase Power Factor Correction, Using Vienna Rectifier Approach and Modular Construction for Improved Overall Performance, Efficiency and Reliability
1.0 Abstract:
While applications for 1-Phase PFC are now familiar and prevalent, the same is not the
case with 3-Phase PFC. Many equipments using kilowatts of power from 3-Phase mains
should be candidates of 3-Phase power factor correction, because several advantages
ensue, both to the user of the equipment and to the utility. The Vienna Rectifier approach
to achieve 3-Phase power factor correction offers many advantages and convenient,
user-friendly features as compared to the two-level, six-switch boost PWM Rectifier.
Amongst them are: continuous sinusoidal input currents with unity power factor and
extremely low distortion; no need for a neutral wire; reduction in voltage stress and
switching losses of power semiconductors by almost 40%; immunity towards variation or
unbalance in mains 3-Phase voltages or absence of one of the phases; wide mains
voltage range: 320VAC to 575 VAC; very low conducted common-mode EMI/RFI; very
high efficiency of the order of 97.5%, say, for power levels of 10 KW and input line
voltage of 400 VAC and short circuit immunity to failure of control circuit. The paper
describes the Vienna Rectifier’s power stage and control techniques, with particular
emphasis on modular construction. What is proposed in this paper is a new approach of
employing Fuzzy Logic for building controller for Vienna Rectifier DCB Modules for 3-
Phase AC to DC power conversion.
2.0 Overview
In the past decade, there is growing awareness about line pollution and deteriorating
power factor due to all pervading inductive and non-linear loads. Utilities are as much
concerned as the users. Passive power factor correction techniques are neither
convenient nor economical; they need bulky components and are not adaptive to
changing needs. Although many solutions were offered for 1-Phase power factor
correction, 3-Phase active power factor correction was seldom considered. As all high
power equipments derive electrical power from 3-Phase mains, incorporating an active 3-
Phase PFC front end can contribute significantly in improving overall power factor and
reducing line pollution.
In addition to lowering power bill to the consumers, improved power factor also
contributes towards conservation of energy and helps in reducing air pollution, by virtue of
less fossil fuel required for generating same amount of electrical power. Other resultant
1 Paper originally presented at the 2003 Power Electronics Conference in Long Beach, CA.
effects are lower I2R losses, steadier terminal Voltages, released system capacity and
reduced cable & switchgear sizes. Active PFC front ends also help meet the IEEE 519-
92, IEC-555 and European EN 61000-3-2 standards for allowable harmonic contents of
mains.
2.1 Advantages
Although there are some passive and active 3-Phase PFC solutions, Vienna Rectifier is
unique by virtue of its embodiment of several advantages:
1. It is a 3-Phase, three level PWM rectifier, utilizing three MOSFETS, with controlled
output Voltage; three wire input, not requiring any connection to Neutral.
2. It is a dual boost type PFC with continuous sinusoidal input current and
unidirectional power flow
3. It needs only three active switches, i.e. MOSFETS
4. It is Operational even in presence of unbalanced mains or only two phases.
5. Total switching losses are reduced by a factor of six, assuming switching
frequency below 50 KHz.
6. Any malfunction in control circuit does not manifest itself in short circuit of output or
PFC front end.
7. Sinusoidal input currents with Power Factor = 0.997, THD<5% and overall
efficiency > 97% are obtainable with current designs.
As can be seen, Vienna Rectifier is the 3-Phase PFC solution that needs to be fully
exploited. Readymade Vienna Power Semiconductor modules, embodying MOSFETS+
FREDS to make a stand alone Direct Copper Bonded Power Circuit, make it quite
convenient and user-friendly. Powerful, yet inexpensive, microcomputers and DSPs make
it attractive to design compact controllers for the Vienna Rectifiers.
A new approach of employing Fuzzy Logic for building embedded controller for Vienna
rectifier for 3-Phase AC to DC power conversion is proposed here.
3.0 Applications
A large number of industrial, telecom and computing equipments now use 3-Phase mains
power. Salient amongst them are:
1. A.C. and D.C Drives
2. Telecommunication Power Supplies
3. Uninterruptible Power Supplies
4. Air Conditioning Units
5. Large Computer Installations
6. Power supplies for all industrial uses such as welding, surface treating, motion
control, large appliances and process control
7. R.F. Transmitters and Radar Transmitters and repeater stations
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8. Regulated and tracking +350V and –350VDC, for high power bidirectional servo
amplifiers for driving heavy-duty critical loads such as Antenna Tracking and
Positioning systems.
All the above and many more such systems are good candidates for using Vienna
Rectifiers for achieving performance improvements as per the advantages listed in 2.1.
4.0 Vienna Rectifier
4.1 Modus Operandi
Vienna Rectifier as shown in Figure 1, was originally developed at the Technical
University Vienna. It comprises a semiconductor switch, say, a MOSFET in each phase
leg of a 3-Phase diode bridge. By adjusting the width of the pulse that turns ON the
MOSFET, corresponding line current is forced to be sinusoidal and in phase with the
Voltage. When the MOSFET is turned ON the corresponding phase is connected, via the
line inductor, to the center point between the two output capacitors. The phase current
rises, through the MOSFET, during that pulse period, charging the capacitor. When the
MOSFET is turned off, current tapers through the diode half bridge (upper or lower
depending on direction of the current flow).
4.2 Objective of Vienna Rectifier
It is a highly efficient method of high current, 3-Phase AC to DC conversion and is
particularly attractive for achieving unity power factor operation. In figure-1 ACR, ACY
and ACB are 440V, 50Hz, 3-Phase sinusoidal line Voltages. -Vdc and +Vdc are the DC
outputs connected to load. There are three semiconductor switches, corresponding to
each phase T1, T2 and T3. These are switched continuously at around 25 KHz.
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