The invention relates to static AC to DC power converters such as can be used for AC or DC motor drive systems.
U.S. Patent Documents
“Power Electronic Converter Harmonics” by Derek A. Paice, published 1995 by the IEEE Press, ISBN 0-7803-1137-X
To meet industry needs for electrical power converters which convert AC to DC without injecting large amounts of harmonic currents into the power system, multipulse AC to DC converters are used. Industry has a wide range of requirements including 12-, 18-, and 24-pulse designs. Also, step-up and step-down of voltage may be needed. These requirements can be met with double-wound transformers of appropriate power rating, but means to reduce transformer cost and improve efficiency are continually sought. Autotransformer methods greatly help and 18-pulse systems are widely used. However, in some cases a less exacting performance is required and 12-pulse is acceptable. This invention helps overcome limitations of existing 12-pulse methods using autotransformers. In its preferred embodiment it ensures that the voltage and impedance of two phase shifted supplies are precisely balanced such that they can supply two 6-pulse converters and achieve excellent 12-pulse performance. Not only that, the invention gives a very simple means for step-up, or step-down voltage. The invention can also be applied to higher pulse number converters.
A three-phase, three-winding, wye connected autotransformer with isolated neutral and appropriate turns ratio provides a phase shifted output of the required output voltage and phase shift for one or more 6-pulse converters in an assembly designed to provide multipulse output. Such multipulse converters provide reduced distortion of the AC power source current and a smoother DC output voltage.
The invention is especially suitable for application of 12-pulse converters, but can be applied to other configurations such as 18-, 24-, 30-, and 36-pulse. It provides a very simple means for obtaining step-up and step-down of output voltage.
Two mechanically identical units of the invention provide balanced impedances such that near perfect performance of 12-pulse converters can be obtained. Use of two units also provides potential for reducing the equipment footprint. For practical values of coil resistance and leakage reactance, each transformer has an equivalent kVA rating that is about 24.8% of the total 12-pulse converter DC load kw.
The invention competes very favorably with the topology in U.S. Pat. No. 6,982,884 because it offers a simpler range of voltage adjustment and wider range of practical turns ratios. Also, it can provide a precise 30° phase shift when this is required.
U.S. Pat. No. 7,049,921 also utilizes a wye type connection to obviate the need for a zero-sequence blocking transformer, but uses a single transformer with four windings per phase. The use of an additional winding makes it more difficult to obtain perfectly balanced output impedances, also a precise phase shift is not easily obtained. Further, the footprint of a single transformer has little flexibility during installation.
Analysis of the topology in
Eq (1) shows that phase shift φ is dependent only on selection of turns n1 and n2. Also, it is noted that if n1=n2 the phase shift is exactly 30°. The general formula for output voltage is less simple, but the normal design method is to first select turns n1 and n2. Once this is done, it is relatively straightforward to determine the ratio of output to input voltage by observing the voltage divider relationship between n1 and (n1+n3). For example, output voltage/input voltage varies with n1/(n1+n3).
If a single unit of the invention is used to obtain a 12-pulse output, as in
A preferred embodiment of the invention is shown in
Significant advantages are evident from inspection of the new topology, namely:
The present invention claims the benefit of the filing date of U.S. Provisional Patent Application No. 62/008,693 filed on Jun. 6, 2014.