Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
2000-09-19
2001-12-25
Vu, Bao Q. (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
C323S207000, C323S217000
Reexamination Certificate
active
06333863
ABSTRACT:
This application claims priority under 35 U.S.C. §§119 and/or 365 to Application Ser. No. 199 44 917, filed in Germany on Sep. 20, 1999; the entire content of which is hereby incorporated by reference.
FIELD OF THE INVENTION
The present invention relates to a method as claimed in the pre-characterizing clause of patent claim
1
, and to an apparatus for carrying out the method.
BACKGROUND OF THE INVENTION
Electrical power is mainly and preferably supplied via balanced three-phase systems. Unequal loads between the individual phases result in such three-phase systems becoming unbalanced, which furthermore results in power losses or damage in electrical machines which are connected to the three-phase system. Unbalanced three-phase systems are in this case distinguished not only by unbalanced currents and phases but, in particular, also by unbalanced voltages, to be precise owing to the different voltage drops in the individual paths.
The said unbalanced voltages in particular have a negative effect on electrical machines which are connected to the three-phase system. Depending on the extent of the unbalance, increased power losses occur first of all, then reduced life, and, finally, machine failure. For this reason, International Standards and Recommendations have been issued in which the maximum permissible level of unbalance in a three-phase system has been defined (Engineering Recommendation, IEC—The Electrical Council, London, June 1975, page 16).
An unbalance in a three-phase system may occur, for example, due to a single-phase induction furnace, due to a single-phase propulsion system which is connected directly to the three-phase system, or due to an arc furnace, which in principle admittedly represents a three-phase load, but in which an unbalance can occur briefly when one or two arcs are quenched.
One known method to compensate for unbalanced loads was proposed by Ch. P. Steinmetz (L. Gyugyi et. al, Principle and Applications of static, thyristor-controlled Shunt Compensators, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-97, No. 5, September/October 1978, pages 1935to 1945 and, in this case, in particular page 1936, left-hand column). Steinmetz was able to show that a resistive load contained between two phases of a three-phase system can be compensated for by connecting a capacitance of suitable size and an inductance of suitable size between the phase to which the resistive load is connected and the phase which is not loaded by the resistive load. In this case, Steinmetz assumed that this was a pure resistive load. If this condition is not satisfied, then additional complications are involved in order to make it possible to correct the power factor as well.
The aim of a compensating load may theoretically be achieved firstly by eliminating the so-called negative-sequence system (L. Gyugyi et. al, Principle and Applications of static, thyristor-controlled Shunt Compensators, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-97, No. 5, September/October 1978, page 1937, right-hand column), and secondly by correcting the power factor. This can be achieved in a known manner by providing a compensation circuit in triangular form, comprising reactive elements such as capacitors and inductances.
With the availability of power switches—such as thyristors—the power factor can be corrected as required, that is to say it has even been possible to balance three-phase systems with widely varying loads. A circuit developed on this principle is described and explained in the abovementioned article by Gyugyi et. al. (page 1942, right-hand column, FIG.
14
).
This known circuit for compensating for unbalanced loads has the disadvantage, however, that no volt-ampere optimization is provided. For this reason, the losses are correspondingly high.
SUMMARY OF THE INVENTION
The present invention is thus based on the object of specifying a method which does not have the abovementioned disadvantages.
The invention has the following advantages: since the compensation currents produced in the compensation unit may have arbitrary phases and arbitrary amplitudes, the invention means that the volt-ampere rating can be considerably reduced in order to achieve the same level of compensation as is possible by means of the known compensation circuit by Steinmetz.
The method according to the invention works optimally particularly when the amplitudes of the compensation currents are of equal magnitude. The compensation circuit paths are thus equally loaded.
Furthermore, the method according to the invention is distinguished by exchanging real power between at least two phases, to be precise via an energy buffer store, which is preferably in the form of a capacitor.
REFERENCES:
patent: 5627742 (1997-05-01), Nakata et al.
patent: 5864474 (1999-01-01), Jang
patent: 6172488 (2001-01-01), Mizutani et al.
patent: 19737590C1 (1998-10-01), None
März, Von G., “Die Blindleistungsschwankungen bei Lichtbogenöfen und ihre Kompensation mit Hilfe elektronisch regelbarer Phasenschieber”, in Elektrowärme International 30, 1972, B1, Feb., S.B40-B44.
Sonnenschein, M., et al, “Shunt-Connected Power Conditioner for Improvement of Power Quality in Distribution Networks,” in Int. Conf. on Harmonics and Quality of Power, Las Vegas, Nev., U.S.A., Oct. 16-18, 1996.
L. Gyungyi et al., “Principles and Applications of Static, Thyristor-Controlled Shunt Compensators”, IEEE Transactions on Power Apparatus and Systems, Vo. PAS-97, No. 5, Sep./Oct. 1978.
Y. Sundberg, “On the balancing of single-phase loads on power networks”, ASEA Journal, vol. 52, No. 5, 1979.
Hillberg Claes
Linhofer Gerhard
ABB (Schweiz) AG
Burns Doane Swecker & Mathis L.L.P.
Vu Bao Q.
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