Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
2000-08-11
2001-09-11
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
C363S037000, C363S048000
Reexamination Certificate
active
06288915
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of power electronics, and more specifically a converter circuit arrangement.
BACKGROUND OF THE INVENTION
Converter circuit arrangements of this generic type are described, for example, in European Patent Applications EP 0 682 402 and EP 0 758 161, as well as in German Laid-Open Specification DE 196 07 201 A1. Such a converter circuit arrangement has a first converter, which can be connected, preferably via a transformer, to a three-phase voltage supply network. The first converter converts the AC voltage into a DC voltage but, at the same time, can also allow power to flow in both directions. The DC voltage supplies a DC intermediate circuit, which is essentially formed by a capacitor bank. The DC intermediate circuit is connected to a second converter which drives a load, for example a motor. The motor is, in particular, a rotating-field machine. Nowadays, frequency converters with a DC intermediate circuit (U converters) are generally used for driving rotating-field machines. Owing to the principle on which they operate, in addition to the desired balanced three-phase voltage, such converters also always produce an undesirable, but unavoidable, unbalanced, so-called common-mode voltage. This common-mode voltage produces a ground current, which is caused by the fact that the parasitic capacitances between the converter and ground (primarily caused by shielded cables) are continually harged and discharged by the common-mode voltage. The round current path is formed by: converter—ground point—ground conductor system—parasitic ground capacitances—converter. Modern systems are designed to have losses that are as low as possible. This therefore, of course, also applies to the ground current and leads to an oscillation being excited in the ground current path, and the current reaching a considerable amplitude. This represents a large additional load on the converter components. Furthermore, the superimposition of the ground current spikes on the balanced current triggers overcurrent protection thresholds. In order to avoid this, the entire converter would have to be highly overdesigned. In the past, this was not a problem since the semiconductor switches switched only very slowly, so that the additional losses were only minor. However, the problem has been exacerbated as the switching frequency and the switching flank gradient have increased. Thus, with the high switching frequencies that are possible nowadays, technical measures are required to attenuate the common-mode voltage. For this reason, in DE 196 07 201 A1, an interference protection filter is introduced into the intermediate circuit. The filter comprises a transformer and a set of filter capacitors. One winding of the transformer is connected in the positive path of the intermediate circuit, and the other in the negative path of the intermediate circuit, so that the unbalanced ground current magnetizes the transformer core, and the ground current is thus reduced.
The described converter arrangement is particularly suitable for applications in the low-voltage range, that is to say with rated voltages below 1 kV. It has the disadvantage that, in consequence, it is not possible to drive all types of rotating-field machines: The switching processes in the second converter lead to non-sinusoidal voltages at the motor terminals and, owing to the capacitive grounding via the filter capacitors, the common-mode voltage leads to voltage spikes between the motor and ground. In consequence, more complex motor designs are required, which can also be used with non-sinusoidal voltages.
SUMMARY OF THE INVENTION
Accordingly, one object of the invention is to provide a novel converter circuit arrangement which also makes it possible to feed standard motors, for example those which require a sinusoidal terminal voltage. Furthermore, it is intended to be possible to use the circuit in the medium- and high-voltage range.
This object is achieved by a converter circuit arrangement having the features of the independent claim.
According to the invention, this is achieved by a filter having a grounded star point, and which is connected between the second converter and the load. This results in a sinusoidal voltage across the load, allowing any desired rotating-field machines to be supplied.
In consequence, the converter arrangement according to the invention can also be used to retrofit existing systems, since there is no need to replace the existing machines. Such an addition of a converter system allows significant energy savings in that, for example, the drives can be operated at a variable speed.
A transformer having at least two windings is arranged in the intermediate circuit. One winding is connected in the positive path, and the other in the negative path. The windings are magnetically coupled to one another and are connected in parallel. This measure affects only the undesirable, unbalanced current components which flow in the same direction in both windings. In consequence, the core of the transformer is magnetized, and the common-mode currents are reduced. The desired, balanced current components, which flow in one direction in one winding and in the opposite direction to the second winding, are scarcely affected.
The transformer can also be connected in the current paths between the supply and the first converter, or in the current paths between the second converter and the load.
A preferred exemplary embodiment, in which the transformer can be designed more economically, is distinguished by the fact that a third winding is provided, which is likewise mechanically coupled to the first two windings, and to which an attenuating resistor is connected. The voltage induced by this in the third winding by the common-mode currents is dissipated in the resistor.
REFERENCES:
patent: 5187651 (1993-02-01), Ekstrom
patent: 5414612 (1995-05-01), Bjorklund et al.
patent: 5621628 (1997-04-01), Miyazaki et al.
patent: 2212484 (1972-09-01), None
patent: 19607201A1 (1996-08-01), None
patent: 0 135 830 (1985-04-01), None
German Search Report and partial English language translation. Mar. 24, 2000.
Stemmler Christoph
Stulz Christian
Asea Brown Boveri AG
Burns Doane Swecker & Mathis L.L.P.
Vu Bao Q.
Wong Peter S.
LandOfFree
Converter circuit arrangement having a DC intermediate circuit does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Converter circuit arrangement having a DC intermediate circuit, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Converter circuit arrangement having a DC intermediate circuit will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2536678