Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reexamination Certificate
2003-07-15
2004-09-28
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S127000, C363S131000, C363S141000
Reexamination Certificate
active
06798677
ABSTRACT:
This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/EP01/06326 which has an International filing date of Jun. 6, 2001, which designated the United States of America and which claims priority on German Patent Application number 100 37 379.8-32 filed Aug. 1, 2000, the entire contents of which are hereby incorporated herein by reference.
FIELD OF THE INVENTION
The present invention generally relates to a power converter, constructed using a multilevel circuit, for converting direct current to alternating or vice versa. The converter may include at least one capacitor and at least two semiconductor power switches. The or each capacitor may have a connection pair for connection of the capacitor to at least two semiconductor power switches.
BACKGROUND OF THE INVENTION
Power converters using a multilevel circuit are known, for example, from U.S. Pat. No. 5,737,201. This document describes in particular the theoretical principles of a multilevel circuit. A multilevel circuit allows a power converter to be designed in a modular manner. Each of the modules includes at least two semiconductor power switches and at least one capacitor, which is arranged between the power switches. The special feature of the multilevel circuit is that the capacitors are not all of the same potential, but may be related to different potentials (so-called floating capacitors). An intermediate circuit voltage is passed to a number of floating capacitors in such a way that the voltage load on one semiconductor power switch is the result of the different between the voltage on two capacitors.
According to the prior art, the capacitors which are used in power converters have only one connection pair. The connection pair is connected to the semiconductor power switches in a first module. The semiconductor power switches in a further module are connected to the connection pair of the capacitor via an additional electrical connection. This additional electrical connection should be designed to have as low an inductance as possible, in order to reduce the load on the semiconductor power switches. U.S. Pat. No. 5,737,201 does not describe in any more detail the problem associated with the electrical connection between the capacitors and the semiconductor power switches having as low an inductance as possible.
An electrical connection with as low an inductance as possible can be achieved, in entirely general form, by special design measures. For example, a reduction in the size of an area through which a vertical component of a commutation circuit of a module flows leads to the electrical connection having a lower inductance. The inductance can be reduced considerably by using conductors that are as wide as possible between the capacitors and the semiconductor power switches, and by the distance between the forward conductor and the return conductor being as short as possible. A low-inductance electrical connection between the capacitors and the semiconductor power switches is subject to certain limits, relating to the design, accuracy and life of the power converter.
In order to allow a low-inductance electrical connection which can be physically implemented relatively easily between the capacitors and the power switches, it is known from EP 0 944 163 A1 for the capacitors in a power converter to be subdivided into two, and for one half of one capacitor to be associated with the semiconductor power switches of a first module of the power converter, and for the other half of the capacitor to be associated with the power switches of another module. Specific low-inductance electrical conductors, which are in the form of rail packs (so-called busbars) are used to connect the capacitor halves to the power switches of the associated modules. The rail packs have, for example, a laminated structure composed of two copper plates, which are used as conductors, with a plate or sheet composed of a solid insulator arranged between them. Owing to so-called partial discharges between the copper plates, the solid insulator is subject to aging, which restricts the life of the rail pack. Furthermore, low-inductance conductors in the form of rail packs cause problems in handling (mechanical loads on the rail packs can adversely affect their low inductance), and they are very expensive.
A further disadvantage of the power converter which is known from EP 0 944 163 A1 occurs with relatively low rating power converters, in which a single module with one capacitor and two power switches would be sufficient. Even in power converters such as these with only one capacitor, the capacitor is subdivided into two capacitor halves, which forces up the production costs for these known power converters.
SUMMARY OF THE INVENTION
One object of an embodiment of the present invention is to define and to develop a power converter of the type mentioned initially such that it is possible to produce an electrical connection between the capacitors and the power switches whose inductance is as low as possible, whose life is as long as possible, and which is as cost-effective as possible.
In order to achieve this object against the background of the power converter of the type mentioned initially, the an embodiment of the present invention proposes that the or each capacitor has at least one further connection pair for connection of the capacitor to at least two semiconductor power switches or to a DC network.
Thus, according to an embodiment of the present invention, at least one further connection pair is passed to the exterior on the or each capacitor. The capacitor can be connected via the one connection pair to at least two semiconductor power switches in a first module, and via each further connection pair to at least two further semiconductor power switches in further modules, or to a DC network. The connection pairs may be passed out of the capacitor at any desired point. However, it is recommended that the connection pairs be passed out where the design results in further modules of the power converter being arranged.
The capacitors which are used in power converters normally have two contact tracks, which run parallel to one another, in their interior, which contact tracks extend virtually over the entire capacitor length and between which a number of parallel-connected capacitor elements are arranged. Owing to the requirements for low inductance, these contact tracks may be in the form of low-inductance electrical conductors. Connection pairs can be passed out of the capacitor at virtually any desired points, originating from the contact tracks. Low-inductance electrical connections, which are generally provided in any case in capacitors for power converters, are thus used as low-inductance conductors for connection of the capacitors to the semiconductor power switches in the individual modules. Since there is no need for the relatively expensive rail packs (busbars) in the power converter according to the invention, the production and assembly costs of the power converter can be reduced to a major extent.
One advantageous development of an embodiment of the present invention proposes that the connection pairs of one capacitor be passed out of this capacitor on different sides of the capacitor. The capacitor may be physically arranged between the modules to whose semiconductor power switches it is intended to be connected. This allows the power converter to have a particularly compact construction.
One preferred embodiment of the present invention proposes that the or each capacitor have two connection pairs which are passed out of this capacitor on opposite sides of the capacitor. This allows the power converter to be formed from a number of modules arranged one above the other or alongside one another. This elongated structure of the power converter has the advantage that it is easy to see the individual components, so that assembly and maintenance are simplified. The elongated structure results in particular advantages when air is used to cool the semiconductor power switches, since the cooling a
Beinhold Georg
Fernahl Michael
Jakob Roland
Alstom
Riley Shawn
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