Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
2001-08-17
2002-10-01
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
C363S089000, C363S127000
Reexamination Certificate
active
06459596
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to multilevel switching component rectifiers, and more particularly to a multilevel rectifier with fewer switching components than known multilevel rectifiers. The invention is valid for any number of phases and at least three levels.
The general trend in power electronics is to switch power semiconductors at increasingly high frequencies in order to minimize harmonics and reduce passive component sizes. However, the increase in switching frequency increases switching losses, which become significant at high power levels. Two methods for decreasing switching losses, and at the same time improving power quality, that have been proposed are constructing resonant converters and multi-level converters.
Resonant converters avoid switching losses by adding an LC resonant circuit to the hard switched inverter topology. The inverter transistors can be switched when their voltage or current is zero, thus mitigating switching losses. Examples of this type of converter include the resonant DC link, and the Auxiliary Resonant Commutated Pole inverter (ARCP). One disadvantage of resonant inverters is that the added resonant circuitry will increase the complexity and cost of the converter control. Furthermore, high IGBT switching edge rates can create switch level control problems.
Multi-level converters offer another approach to providing high power quality. One of the primary advantages of multilevel converters are the high number of switching states offered so that the output voltage can be “stepped” in smaller increments thereby producing better voltage waveforms. This allows mitigation of harmonics at low switching frequencies thereby reducing switching losses. In addition, EMC concerns are reduced through the lower common mode current facilitated by lower dv/dt's produced by the smaller voltage steps as well as reducing the switching dv/dt stresses allowing for potentially longer component life. One significant disadvantage of these techniques is that they require a high number of switching devices thereby increasing the cost and complexity of these circuits. Although the devices are rated at a lower voltage, gate drive and control circuitry must still be provided. Another disadvantage of multi-level inverters is that they must be supplied from isolated DC voltage sources or a bank of series capacitors with balanced voltages. In systems where isolated DC sources are not practical, capacitor voltage balancing becomes the principal limitation for multi-level converters. One possible solution to the voltage balancing problems inherent in multilevel converters is described in U.S. Pat. 6,031,738, Lipo et al.
U.S. Pat. No. 5,644,483, Peng et al., which is hereby incorporated by reference, describes a multilevel converter system. The number of switching devices needed in the converter is determined by the equation m
s
=2(m−1), where m
s
is the number of switching devices needed for each phase leg of the converter and m is the number of converter voltage levels. This equation is valid for both the rectifier and the inverter of the converter system. Indeed, the symmetrical configuration of the converter system described allows it to accept bi-directional power flow. However, as can be readily seen by the equation the number of switching devices increases significantly as the number of voltage levels increases, thereby increasing the cost and complexity of the system.
An example of a three-level rectifier with reduced numbers of switching devices was presented in Y. Zhao, Y. Li, and T. A. Lipo, “Force Commutated Three Level Boost Type Rectifier,”
IEEE Transactions on Industry Applications,
vol. 31, no. 1, pp. 155-161, January-February 1995. This three level reduced parts rectifier is illustrated in and will be discussed with reference to FIG.
1
A. Therein, one rectifier phase (phase x) and the capacitor bank are shown. The other phases have an identical structure. The general idea behind this topology was a re-arrangement of IGBT's
102
and
103
and diodes
104
and
106
in a standard thee-level rectifier to obtain the circuit shown in FIG.
1
A. The performance of this circuit and that of the present invention is identical for their respective three level topologies. However, there is an important difference when the number of voltage levels is increased beyond three.
Although not disclosed by Zhao et al., the reduced parts count topology in
FIG. 1A
may be extended to rectifiers with a higher number of voltage levels such as the four-level topology illustrated in FIG
1
B. For the four-level circuit shown in
FIG. 1B
, the inner most IGBT's
103
require a rating of (2/3)v
c
, where v
c
is the sum of all of the voltage level capacitor
108
voltages represented by the equation v
c
=v
c1
+v
c2
+vc
c3
. Whereas the outer most IGBT's
102
require a voltage rating of (1/3)v
c
. This imbalance of voltage ratings precludes the use of dual IGBT modules in this topology. For low voltage applications this imbalance does not pose a significant problem. However, for high-voltage applications, it may be necessary to use two IGBT's in series for the high-voltage IGBT's. However, this approach increases the parts count to that of a fully active four-level rectifier. Thus, for low voltage applications this method can result in a simpler lower cost rectifier. However, the performance is limited as the number of voltage levels increases and several different component ratings are required yielding no parts savings in high voltage applications.
It is desirable to provide a multilevel rectifier with a reduced number of switching devices to reduce the cost and complexity of converter systems with a high number of voltage levels and phases that is suitable for high voltage applications.
SUMMARY OF THE INVENTION
A multilevel uni-directional power converter system including a multilevel rectifier that has a reduced number of switching devices is provided. The system comprises an input to a multilevel rectifier with at least one phase leg. The rectifier is composed of switching device and anti-parallel diode pairs and clamping diodes. The number of switching devices required depends on the given number of voltage levels according to the equation 2(n−1) where n is the number of voltage levels. This number of required devices is then reduced in accordance with the present invention by removing the top and the bottom switching devices from the rectifier circuit. The anti-parallel diodes remain in the rectifier circuit. There is no performance degradation from this reduction in the number of switching devices to the rectifier. The clamping diodes separate the switching devices from the output nodes. The output nodes are joined through a group of series connected capacitors that serve as the input source for a multilevel inverter. The multilevel inverter has the requisite number, 2(n−1), of switching devices for the given number of voltage levels. The multilevel inverter has a set of outputs that supply the conditioned power to a load. Control of the switching for the rectifier is achieved through hysteresis current control and redundant state selection with feedback from the capacitor bank. Control of the multilevel inverter is achieved through redundant state selection regulated by duty-cycle modulation.
Additionally a method for reducing the required number of switching devices in a multilevel rectifier is provided for any given number of levels and phases. The method comprises designing a traditional n-level rectifier, where n is the number of voltage levels, and then removing the top and bottom switching devices from each phase leg of the rectifier circuit. This method does not result in any appreciable performance loss for the circuit. An important design consideration is that this method will only work for uni-directional power flow (AC to DC).
Additional advantages of the invention will be set forth in the description which follows, and will in part be obvious from the description
Crabb Steven W.
Sterrett Jeffrey
The United States of America as represented by the Secretary of
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