Twelve-pulse rectifiers including separate six-pulse...

Details Twelve-pulse rectifiers including separate six-pulse... Twelve-pulse rectifiers including separate six-pulse...

1999-12-20

2001-10-23

Wong, Peter S. (Department: 2838)

Electric power conversion systems

Current conversion

Having plural converters for single conversion

Reexamination Certificate

active

06307763

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to rectifier systems and methods, and more particularly to twelve-pulse rectifier systems and methods.
BACKGROUND OF THE INVENTION
Twelve-pulse rectifiers are widely used to convert alternating current (AC) to direct current (DC). Twelve-pulse rectifiers can draw AC input current with low Total Harmonic Distortion (THD). As is well known to those having skill in the art, a twelve-pulse rectifier generally is produced by coupling the outputs of two six-pulse rectifiers. The two six-pulse rectifiers may be connected via Y-&Dgr; and Y—Y isolation transformers that are 30 degrees out-of-phase. Each six-pulse rectifier may include a rectifier bridge that is arranged in a secondary circuit of a 3-phase transformer.
The design and operation of six- and twelve-pulse rectifiers are described in the following publications: P. C. Sen,
Evaluation of Control Schemes for Thyristor
-
Controlled DC Motors,
IEEE Transactions on Industrial Electronics and Control Instrumentation, Vol. IECI125, No. 3, August, 1978, pp. 247-255; M. H. Rashid and M. Aboudina,
Analysis of Forced
-
Commutated Techniques for AC
-
DC Converters
, First European Conference on Power Electronics and Applications, Brussels, Oct. 16-18, 1985, pp. 2.263-2.266; and M. H. Rashid and A. J. Maswood,
Analysis of
3-
Phase AC
-
DC Converters Under Unbalanced Supply Conditions,
IEEE Industry Applications Conference Record, 1985, pp. 1190-1194. The design and operation of six- and twelve-pulse rectifiers also are described in U.S. Pat. No. 4,164,013 to Kaufhold entitled Six-Pulse Rectifier Circuit and U.S. Pat. No. 5,903,066 to Enjeti entitled Active Interphase Reactor for 12-Pulse Rectifier, respectively. Accordingly, the design and operation of six- and twelve-pulse rectifiers need not be described in detail herein.
Twelve-pulse rectifiers also generally include a controller that controls the operation of the twelve-pulse rectifier. The controller also preferably controls the current balance between the two six-pulse rectifiers so that the difference between the currents in the first and second six-pulse rectifiers may be controlled and preferably minimized. For example, as described in a publication entitled
Twelve
-
Pulse Converter With Auto
-
Balance and Integrated Magnetics Cuts THD
by Bourbeau, PCIM November 1999, pg. 94, a controller with an “auto-balance” feature can reduce the difference in the two currents to typically 10 percent or less of the total DC output current.
Other controllers may provide load-sharing between the 30 degree out-of-phase transformers that constitute the twelve-pulse rectifier using “droop control”. See, for example, the publication entitled
Fault Tolerant Power Supply System Uses the Droop Method of Current Sharing
by Frank DiJoseph, PCIM Magazine, 1998. Load sharing between two 30 degree out-of-phase transformers also may be achieved using analog signal comparison.
Unfortunately, the controller may be a source of reliability problems for twelve-pulse rectifiers because the twelve-pulse rectifier generally ceases to operate upon failure of the controller. Moreover, conventional controllers may not be able to balance the currents between the two six-pulse rectifiers sufficiently to reduce the input current total harmonic distortion as desired for high performance applications.
SUMMARY OF THE INVENTION
The present invention can provide rectifier systems including a first controller that controls a first six-pulse rectifier, a second controller that controls a second six-pulse rectifier and a data network that couples the first controller and the second controller to exchange data therebetween. The first and second controllers preferably have a common architecture and preferably operate independently. In other words, they preferably do not operate in a master-slave relationship. By providing the first and second controllers that communicate via a data network, failure of one controller can allow a load to be supported by the other controller and its associated six-pulse rectifier to thereby improve reliability. Moreover, digital controllers that communicate over a data network can accurately control the load balancing between the six-pulse rectifiers, for example, within one percent or less, to thereby allow low distortion AC input current to be drawn for high performance applications.
Preferred embodiments of rectifier systems according to the present invention comprise a first six-pulse rectifier including a first six-pulse rectifier output and a second six-pulse rectifier including a second six-pulse rectifier output. The first and second six-pulse rectifier outputs are coupled together to provide a twelve-pulse rectifier output. A first controller controls the first six-pulse rectifier. A second controller controls the second six-pulse rectifier. A data network couples the first controller and the second controller. The first and second controllers preferably have the same architecture and more preferably are identical. They preferably are separate from one another and preferably operate independently, rather than in a master-slave relationship.
Each controller preferably controls its associated six-pulse rectifier as a function of the current and/or voltage output of the associated six-pulse rectifier, preferably using conventional feedback control techniques. Moreover, the first controller also preferably controls the first six-pulse rectifier as a function of a second current at the second six-pulse rectifier output, and the second controller preferably controls the second six-pulse rectifier as a function of a first current at the first six-pulse rectifier output. Thus, each six-pulse rectifier may be controlled as a function of the current at the other six-pulse rectifier output. The first controller preferably receives second data representing the second current at the second six-pulse rectifier output via the data network, and the second controller preferably receives first data representing a first current at the first six-pulse rectifier output via the data network.
More specifically, the first controller preferably comprises a first feedback loop that controls the first six-pulse rectifier as a function of the voltage and/or current at the first six-pulse rectifier output and independent of the second six-pulse rectifier output, and a second feedback loop that also controls the first six-pulse rectifier as a function of the current at the second six-pulse rectifier output. Similarly, the second controller preferably comprises a first feedback loop that controls the second six-pulse rectifier as a function of the voltage and/or current at the second six-pulse rectifier output and independent of the first six-pulse rectifier output, and a second feedback loop that also controls the second six-pulse rectifier as a function of current at the first six-pulse rectifier output. Analog and/or digital feedback control loops may be used.
By providing separate first and second controllers, improved reliability of the twelve-pulse rectifier may be obtained. In particular, the first controller preferably continues to control the first six-pulse rectifier upon failure of the second controller to thereby provide a first output current from the first six-pulse rectifier at the twelve-pulse rectifier output. Similarly, the second controller preferably continues to control the second six-pulse rectifier upon failure of the first controller to thereby provide a second output current from the second six-pulse rectifier at the twelve-pulse rectifier output. When the first and second controllers are operational, the first and second six-pulse rectifiers may be controlled by their respective first and second controllers. As was described above, the first six-pulse rectifier may be controlled by the first controller as a function of the second current at the output of the second six-pulse rectifier output, and the second six-pulse rectifier may be controlled by the second controller as a function of a first current at the output of the first six-pulse re

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