Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Patent
1996-10-10
1998-05-12
Wong, Peter S.
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
363 37, H02J 336
Patent
active
057515638
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an HVDC transmission with at least two converters. Each converter is adapted for connection between a three-phase alternating voltage network and a dc link common to the converters. At least a first converter has ac leads for connection of the converter to its alternating-voltage network without the use of a separate winding transformer, or "full transformer".
The concept "HVDC transmission" means in this application an electric installation or equipment for power transmission by means of high-voltage direct current. The concept comprises two main types of installations. The first of these types consists of installations adapted for power transmission between two or more spaced-apart converter stations, which are interconnected by means of dc-carrying cables or overhead lines. The second of the types consists of so-called back-to-back connections, in which two converters arranged in the same converter station are each connected to a separate alternating-voltage network, are dc-connected to each other and adapted for controllable power transmission between the alternating-voltage networks.
In the first type of installation, the dc link consists of the cable or cables or lines which connect the dc sides of the converter stations. In the second type of installation, the dc link generally consists only of a pair of busbars in the station. In both cases, however, certain devices for smoothing and filtering of the direct current, for current and voltage measurement, for protection against overvoltages, among other functions included in the dc link in a known manner.
The expression that a converter is connected to its alternating-voltage network "without the use of a separate winding transformer", or "full transformer" means that the converter is connected to its alternating-voltage network in some way other than with the aid of a separate winding transformer. A converter connected without the use of a separate winding transformer may thus have its alternating-voltage terminals galvanically connected to the alternating-voltage network, directly or via an auto-transformer, and possibly via inductors for current limitation. A converter connected without the use of a separate winding transformer may alternatively have its alternating-voltage terminals connected to the alternating-voltage network via series capacitors.
Analogously, this application uses the concept "transformerless" connection for those cases where no transformer of any kind, thus neither a separate winding transformer nor an auto-transformer, is used for connection of a converter to its alternating-voltage network. In this case, thus, the converter may have its alternating-voltage terminals connected to the alternating-voltage network galvanically, or via series capacitors.
BACKGROUND OF THE INVENTION
In an HVDC transmission, each one of the converters usually consists of two series-connected six-pulse bridges. Each bridge is connected to the alternating-voltage network via a separate winding transformer. The transformers of the bridges, or the valve windings of a common transformer, are designed with different connections, usually star- and delta-connections, in such a way that the alternating voltages of the bridges are phase shifted 30.degree.. Hence, the converter is of twelve-pulse design. HVDC transmissions of this kind are amply described in the literature, for example in Erich Uhlmann: "Power Transmission by Direct Current", Springer-Verlag Berlin Heidelberg New York 1975 (see, e.g. Fig. 2.7, page 15, or Fig. B.1, page 187).
Since the converter bridges are connected to the alternating-voltage network via transformers, a possibility of technical-economic optimization of the direct-voltage level and the dc level of the transmission is provided. By connecting the converter bridges to the alternating-voltage network via separate winding transformers, galvanic separation between the bridges and the alternating-voltage network is obtained. This means that, in the manner described above, t
REFERENCES:
patent: 4625269 (1986-11-01), Kanngiesser
patent: 5187651 (1993-02-01), Ekstrom
patent: 5414612 (1995-05-01), Bjorklund et al.
Derwent's abstract, No 94-149500/18, week 9418, Abstract of SU, 1798824 (Lenin electrotech inst.), 28 Feb. 1993.
Derwent's abstract, No H 8315D/34, week 8134, Abstract of SU, 773866 (Norilsk evening ind), 38 Oct. 1980.
Kimbark, Direct Current Transmission, vol. 1, 1971, Wiley-Interscience, New York, pp. 9-11, Constitution of EHV AC and DC Links.
Gebhardt, HGU-Kurkupplung ohne Transformatoren, 1976, pp. 4-6, Technische Hochschule Darmstadt, 1976/1977.
Uhlmann, Power Transmission by Direct Current, Springer-Verlag Berlin Heildelberg New York, 1975, pp. 14-16.
Asea Brown Boveri AB
Han Y. J.
Wong Peter S.
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