Twelve-phase transformer configuration

Electric power conversion systems – Phase conversion without intermediate conversion to d.c. – By induction-type converter

Reexamination Certificate

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Reexamination Certificate

active

06198647

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates to transformers for converting three-phase power to nine-phase power, and more particularly to transformers for providing reduced harmonics on the AC and minimizing ripple on the DC side of an AC to DC rectifier.
Rectifiers are used to rectify AC voltages and generate DC voltages across DC buses. A typical rectifier includes a switch-based bridge including two switches for each AC voltage phase which are each linked to the DC buses. The switches are alternately opened and closed in a timed fashion that, as the name implies, causes rectification of the AC voltage. As well known in the energy industry the global standard for AC power distribution is three phase and therefore three phase rectifier bridges are relatively common.
When designing a rectifier configuration there are three main considerations including cost, AC line current harmonics and DC bus ripple. With respect to AC current harmonics, when an AC phase is linked to a rectifier and rectifier switches are switched, the switching action is known to cause harmonics on the AC lines. AC line harmonics caused by one rectifier distort the AC voltages provided to other commonly linked loads and therefore should generally be limited to the extent possible. In fact, specific applications may require that large rectifier equipment be restricted in the AC harmonics that the equipment produces.
With respect to DC link ripple, rectifier switching typically generates ripple on the DC bus. As with most hardware intensive configurations cost can be minimized by using a reduced number of system components and using relatively inexpensive components where possible.
It is well known in AC to DC rectification that AC current harmonics and DC ripple may be improved by increasing the number of AC phases that are rectified by the rectifier. These AC phases are phase-shifted from each other. For example, by rectifying twelve-phase AC current instead of three-phase harmonics and ripple are reduced appreciably. Where AC harmonic restrictions are placed on rectifier systems such restrictions are often satisfied by employing a 24-pulse rectifier that requires a twelve-phase source of AC power. As the global standard for AC power distribution is three phase, 24-pulse rectifiers require three-to-twelve phase power converters between utility supply lines and rectifier switches.
Isolation transformers for converting three-phase AC power to twelve-phase AC power are known in the art but have several shortcomings. First isolation transformers must be rated for the full power required. Second, isolation transformers are typically relatively large as separate primary and secondary windings are required for isolation purposes.
Where isolation between a utility supply and a rectifier is not required, employing an autotransformer including a plurality of series and common windings may advantageously reduce the size and weight of a three-to-twelve phase converter that consists of an autotransformer and a rectifier unit. Exemplary three-to-twelve phase autotransformers are described in U.S. Pat. No. 5,148,357 (the “'357 patent”); and U.S. Pat. No. 4,876,634 (the “'634 patent”), each of which is incorporated herein for the purpose of describing the prior art.
The '634 patent teaches the general concept of providing three phase autotransformer coils in a plurality of series connected windings which are arranged to form a hexagon. Three phase AC input lines are linked to three input nodes and twelve output nodes provide voltages to rectifier bridges. Phase shift between the output voltages is accomplished by providing differently sized windings between the input nodes and the output nodes. Importantly, the '634 patent teaches that, for each autotransformer input phase, the phase shift between four corresponding output voltages should be 15 degrees and accomplishes 15 degree phase shift by providing short windings between each two adjacent output nodes corresponding to the same input phase. Long windings are provided between adjacent output nodes corresponding to different input phases. In the '634 patent the twelve output voltages are provided to several bridges.
Unfortunately, there are at least two problems with the 24-pulse autotransformer described in the '634 patent (hereinafter the '634 topology). First, there is an inherent impedance mismatch in the '634 topology which results in looping currents among the bridges and which requires additional hardware to correct. For example, if the outputs and inputs to the '634 24-pulse autotransformer are linked to provide unity gain one of the bridges would be fed directly from the input power source while the other bridges would be fed through transformer windings which each are characterized by a certain amount of leakage inductance. This means that there would be different impedances for each of the bridges and the different impedances would cause disparate DC output voltages and hence looping currents among the bridges. A similar impedance disparity would results when the '634 patent 24-pulse autotransformer is linked for step-down transformation.
The '634 topology teaches use of two inter-phase transformers to reduce the looping currents. As an initial matter Applicant believes the inter-phase transformers provided in the '634 topology are erroneously specified and that more than the two specified inter-phase transformers would be required to reduce the looping currents. While additional inter-phase transformers can be provided, inter-phase transformers are required to carry DC bus currents. Therefore, inter-phase transformers are relatively bulky and increase system size appreciably. Moreover, additional inter-phase transformers are relatively expensive and increase system costs.
Second, the '634 topology would result in current sharing problems among the bridges due to enclosed electrical circuits formed by the multi-phase shift bridges. The current sharing problems are exacerbated when AC line harmonics occur as different source harmonics substantially change bridge current sharing. Because AC line harmonics are often irregular and unpredictable it is impossible to balance the impedance mismatch via addition of resistance elements. While the inter-phase transformers may ease current harmonics to the power source, the inter-phase transformers are not effective as a solution for the current sharing problem.
Because of the current sharing problem described above all of the bridges in the '634 topology have to be capable of handling over-rated current conditions as high as 150% of the current level required to be handled if the bridges were able to share current equally. This is because from time to time each bridge is forced to operate close to its rated current level while the other bridges only operate at 50% of their rated level. This drastic current difference among bridges also forces the windings of the '634 topology to carry appreciably disparate current magnitudes. For this reason, in addition to the bridges having high current ratings, the autotransformer also must be rated to handle high current value and therefore results in inefficient material utilization.
One solution to the looping and sharing current problems associated with the '634 topology is to provide an autotransformer that equally spaces output voltages in phase. For example, where there are twelve outputs the outputs can be phase shifted from each other by 30 degrees each. In the '357 patent this is accomplished by providing an autotransformer having three coils that are phase shifted from each other by 120 degrees. Each coil forming first through sixth coil specific windings. The windings are arranged to form six separate legs, the first coil first through third windings forming a first leg and the fourth through sixth windings forming a fourth leg, the second

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